Daniel R S Caon
email: caon@telecom-paristech.fr

Asmaa Amehraye
email: asmaa.amehraye@esigetel.fr

Joseph Razik
email: joseph.razik@univ-tln.fr

Gérard Chollet
email: chollet@telecom-paristech.fr

Rodrigo V Andreão
email: rodrigova@ifes.edu.br

Chafic Mokbel
email: chafic.mokbel@balamand.edu.lb

Experiments on Acoustic Model supervised adaptation and evaluation by K-Fold Cross Validation technique

Keywords: speech recognition, maximum likelihood linear regression, maximum a priori, k-fold cross validation

This paper is an analysis of adaptation techniques for French acoustic models (hidden Markov models). The LVCSR engine Julius, the Hidden Markov Model Toolkit (HTK) and the K-Fold CV technique are used together to build three different adaptation methods: Maximum Likelihood a priori (ML) , Maximum Likelihood Linear Regression (MLLR) and Maximum a posteriori (MAP). Experimental results by means of word and phoneme error rate indicate that the best adaptation method depends on the adaptation data, and that the acoustic models performance can be improved by the use of alignments at phoneme-level and K-Fold Cross Validation (CV). The very known K-Fold CV technique will point to the best adaptation technique to follow considering each case of data type.

I. INTRODUCTION

The speech recognition systems have faced many challenges. The bigger ones were the hardware capacities for data processing. Nowadays, the main problem is adaptation to the speaker [START_REF] Goronzy | A Combined MAP + MLLR Approach for Speaker Adaptation[END_REF], to the speech type (spontaneous conversations, read speech, debates etc), to the channel (microphone), to the environment and to the dialect. This work is conducted within the CompanionAble Project 1 , with the aim of acquiring the best acoustic models for very specific scenarios. The objective of this work is to find the best adaptation technique ( a common research in the area of automatic speech recognition, e.g. [START_REF] Wang | Comparison of acoustic model adaptation techniques on non-native speech[END_REF]) for the French Acoustic Models (HMM's) trained on ESTER [START_REF] Galliano | Corpus description of the ester evaluation campaign for the rich transcription of french broadcast news[END_REF] broadcasting news database. The French acoustic models are the adaptation targets in this paper. In section II the objective of this work, the acoustic models and all databases used in the experiments are described. Different databases are tested and the adaptations are supervised. The use of a K-Fold CV aims to provide more reliable evaluation of the results. The experiments are conducted in a lower level language unit using forced phoneme alignment by Viterbi. This is known as a good tool for identifying the actual pronunciation [START_REF] Young | The HTK Book, version 3.4[END_REF] contained in the utterances (although the best matching pronunciation must be previously listed in the lexicon). The Maximum Likelihood (ML) reestimation is used with two different configurations: The first updates transitions, means, variances and weights (ML tmvw). The second configuration doesn't update the transitions (ML mvw). The Maximum a posteriori (MAP) and The Maximum Linear Likelihood Regression (MLLR) adaptation methods are the other tested techniques. It is known that HTK's MAP implementation [START_REF] Young | The HTK Book, version 3.4[END_REF] does not update transition probabilities while the means (m), the variances (v) and the weights (w) are acceptable options. The explanations about the HTK's implementations are documented in [START_REF] Young | The HTK Book, version 3.4[END_REF]. MLLR is used in a static two-pass adaptation approach, which is described further. A K-Fold CV is used to evaluate the experiments. More details about this technique will be introduced later in subsection IV-A. As it is not only a matter of adaptation, but also to have a good evaluation method, the phone or the word aligned comparison with references are studied in V-A. The Section III explains the main features of MAP and MLLR adaptation approaches. The experimental protocol is described in section IV and section V opens a discussion towards the level of alignments (recognition output) for the validation of supervised adaptations, also showing the experimental results. The conclusion comes in section VI.

II. DATABASES, MODELS AND OBJECTIVES

Three different types of adaptation databases are employed in this work: Readings, Interviews and Distress Situations. The ESTER [START_REF] Galliano | Corpus description of the ester evaluation campaign for the rich transcription of french broadcast news[END_REF] database is recorded on French broadcasting news and around 40 hours were used to train the acoustic models (hidden Markov models composed by monophones containing 5 states and 256 mixture components per state). The language model is based on 3-gram probabilities from large newspaper data ("Le Monde") and the dictionary composed of 65k words.

For the adaptation databases, the speakers are non-native French, which permits some analysis on non-native speech adaptation. The Speaker Dependent Interviews (SDI) are recorded by two non-native speakers (SDIm by one male speaker and SDIf by one female

III. MAP versus MLLR

A. MAP

The MAP adaptation has a capability of achieving performance near to speaker dependent) systems [START_REF] Goronzy | A Combined MAP + MLLR Approach for Speaker Adaptation[END_REF]. It uses effective combination of prior knowledge, i.e. the initial model parameters, and ML estimates obtained on the adaptation data [START_REF] Goronzy | A Combined MAP + MLLR Approach for Speaker Adaptation[END_REF] [START_REF] Zavaliagkos | Maximum a posteriori adaptation for large scale hmm recognizers[END_REF]. The MAP adaptation can also deal with foreign accents where often some phonemes differ a lot from the usual pronunciation while other phonemes don't [START_REF] Goronzy | A Combined MAP + MLLR Approach for Speaker Adaptation[END_REF] [START_REF] Zavaliagkos | Maximum a posteriori adaptation for large scale hmm recognizers[END_REF]. The main disadvantage of MAP is the large amount of adaptation data needed before all phonemes can be updated. This adaptation may be hard for those phonemes which do not appear very frequently in the adaptation database.

B. MLLR

Unlike MAP, the MLLR adaptation updates many (or even all) gaussians at once. One global regression class concerning all gaussians can be used (the fast case of MLLR). The basic principle is to calculate one or several transformation matrices from the adaptation data. Clustering several gaussians into regression classes that share the same transformation or regression matrix creates the possibility to update even the not observed parameters (i.e. a parameter that is not observed will join the group of an observed one, the one which has nearer acoustic features, and use the same transformation). The MLLR can quickly adapt the acoustic model to new speakers, environments, channel etc requiring only few adaptation data. One limitation of MLLR is about the foreign accents: Some phonemes will differ a lot from the usual pronunciation and the use of a transformation matrix in the specific regression class of this phoneme may not be appropriated for acquiring better results.

IV. EXPERIMENTAL PROTOCOL

A. Validation Technique

Validation Techniques have two main problems in the pattern recognition research area: Model selection and performance validation. This paper aims to validate the performance (recognition accuracy) of adapted acoustic models by taking into consideration the transcription level of references (detailed in V-A). A K-Fold CV was implemented. The variable K is equal to 20 to make the experiments, which means that every time about 5% of the adaptation data is tested while 95% (the other K-1 parts) are used for adaptation. The K-Fold CV technique is commonly used to give more accurate evaluation results. K has to be chosen accordingly to the database size due to the desired computational time issues and the expected bias for the true error rate.

B. Two-Pass MLLR

The MLLR adaptation can be supervised (using labeled adaptation data) or not (labeling adaptation data by recognition before adapting the target). In this work, we used a supervised MLLR, with two-pass static adaptation by means of HERest (HTK tool) 3 . The first one of the two-pass steps builds a global transformation class, while the second pass builds a (multiple) regression transformation class [START_REF] Young | The HTK Book, version 3.4[END_REF].

V. EXPERIMENTAL RESULTS

The experiments start by analyzing the best forced alignment options. The alignment type will directly affect the observed results A 20-Fold CV Technique (for the chosen forced alignment option) is done for the adapted acoustic models. The figure 1 explains how the experiments use a supervised K-Fold CV adaptation.

A. Forced Alignment Options

To choose the best language unit level to evaluate the adaptation, the SDR database is used to do a ML re-estimation on the ESTER's acoustic models. The other databases could be used to evaluate the forced alignment options too, but SDR was chosen because it is not as large as FDE and not as short as the SDI. Also, a ML re-estimation should give good results with the SDR database, due to the utterance's repetitions and the sufficient data to cover all the acoustic model set. For building the phoneme reference transcriptions from word reference transcriptions, the procedure is simple: The first occurrence of possible pronunciation for each word found in the dictionary is chosen. This makes the reference files not always compatible with what will be truly spoken. For example the French liaisons are spoken in an random mood (the speaker sometimes makes the liaisons, and sometimes it doesn't). This problem could be solved by a manual review of the reference transcriptions. In this work, the experiments employ an adaptation procedure of transcriptions which permits to fix some mispronunciations (if provided in the lexicon, the best pronunciation can be chosen by HTK's Viterbi tool), but the reference file construction (considering only the first pronunciation on the lexicon of each word) is not very flexible. This way, the adaptation is conducted with no problems unlike the evaluation results which may be affected by the variability of the lexicon's pronunciations for a specific word. At the same time, for evaluating results with a higher language unit level like words, the chance of error is higher in the hypothesis, due to the impact of the language model probabilities (n-grams) to make the system fail when putting the phonemes together to compose the words, even if it recognizes the right phonemes before word aligning. If an utterance contains "there for" (spoken fasten as "therefore", with no short silence interval between the words), the pronunciation will be "DH EH R F AO R", the same for the word "therefore". Then, the hypothesis depends on the language model probabilities for choosing if the recognized output (word aligned) will be "there for" or "therefore". The results considering word and phoneme forced alignments are presented in figure 2. The Phoneme Error Rate(PER) is used for phoneme alignment and the Word Error Rate(WER) is used for the word alignment. The results confirm a better recognition rate for phoneme aligning (PER is lower than WER). It cannot prove that phone alignment is better than word alignment for evaluation though. The last column shows the error rate mean of all folds together. The goal is to provide a more reliable information by the use of the K-Fold CV, instead of taking just one random fold to validate. Although the inflexibility of the reference transcriptions at phoneme level (as described before) the experiments results are given in Phone Error Rates (PER) and should provide good observations about adaptation gains. The PER makes us more independent from the language model probabilities. This is a very good aspect as we assume to analyze only the acoustic model adaptation.

B. PER 20-Fold Cross Validation

The tests are conducted using the original HMMs trained on the ESTER database (Not-Adapted) and the HMMs adapted with different techniques (ML, MAP and MLLR). The results are shown by means of the 20 folds (results added and divided by 20, the total fold number) for easier visualization. Due to the high variation of the recognition accuracy results for each part, the mean of the K recognition tests is an information more useful than taking only a random part which explains the use of K-Fold CV. It can be noticed from figure 3 that ML has a better impact in the SDR and FDE databases than MAP or MLLR. This is explained by the fact that the SDR and the FDE have many utterance's repetitions (around 5 for SDR and 21 for FDE). For the SD Interview databases , a better accuracy is observed when doing MAP or MLLR than when doing ML re-estimation.

VI. CONCLUSION

The use of phoneme's alignments is recommended for the evaluation of acoustic models adaptation. It is still better than considering, for example, the comparison of two words with the same pronunciation as being mismatched. The adaptation method should be chosen according to the data available. If the data is sufficient for covering the acoustic space or there are mispronunciations (like in foreign accents), MAP is better. With enough statistical information about the acoustic space and not too much mispronunciations, MLLR is effective even with short adaptation data. The K-Fold CV points to the best technique to use in each case and solves the choice of method for further adaptation iterations.

Fig. 1 .

 1 Fig. 1. Supervised K-Fold CV adaptation. D is the adaptation database, D(k) is the k-th data subset, M(k) is the adapted acoustic model obtained using all the adaptation data available in D except the k-th part D(k) which is a test database.

Fig. 2 .

 2 Fig. 2. SDR's comparison of error rates after ML re-estimation with phoneme or word alignment

Fig. 3 .

 3 Fig. 3. Adaptation results for different techniques.

  ). The FDE contains French Distress Expressions 2 recorded by 19 native and 2 nonnative speakers in distress situations. More information are summarised in table I.

	Database Utterances Words Speakers Speech Type
	SDR	162	1572	1	Read Text
	SDIf	103	521	1	Interview
	SDIm	103	521	1	Interview
	FDE	2646	10080	21	Distress Exp.

TABLE I DATABASE INFORMATION

 IINFORMATION

http://www.companionable.net/

Authors thank ESIGETEL ( http://www.esigetel.fr/ ) for providing the French Distress Expressions database.

HERest is the main HTK's training tool. It performs a single re-estimation of all HMMs, simultaneously. It uses the Forward-Backward algorithm to store statistics of state occupation, means, variances etc.

The 20-Fold CV is used to give the mean of recognition accuracy for the not-adapted (original models trained on ESTER corpus) and the adapted acoustic models (with ML, MLLR and MAP options).

ACKNOWLEDGMENT

This work was conducted in the framework of the IST, FP7 Integrated Project CompanionAble (http://www.companionable.net/).


Jean-François Ponge
email: ponge@mnhn.fr

Bernard Jabiol

Jean-Claude Gégout

Geology and climate conditions affect more humus forms than forest canopies at large scale in temperate forests

Keywords: EcoPlant database, humus form, Humus Index, parent rock, soil type, climate, geography, RDA

We investigated by linear gradient analysis (RDA) the relationships between forest humus forms (9 humus forms and the Humus Index) and 148 variables describing geology, climate, soil type, geography and the floristic composition of forest canopies, using 3441 plots of the EcoPlant database covering the whole French territory. Among these variables, geology (alkaline vs acidic substrate) and climate (warm/dry vs cold/rainy) were the major determinants of humus forms, scaling mull humus forms from eumull to dysmull and opposing them to mor/moder, while the contribution of tree canopies was negligible. This trend was verified by partial RDA with environment or abundance of tree species from forest canopy as co-factors. The original position of amphi was confirmed: it was the only humus form not included in the gradient of increasing biological activity ordinated according to climate and geology. Results and possible forecasts of humus forms according to global warming were discussed to the light of existing knowledge.

Introduction

The humus form, i.e. the vertical arrangement of organic matter in topsoil horizons, is known to be influenced by biotic (litter amount and quality, soil-dwelling microbial and animal communities) and abiotic factors (climate, parent rock, soil type) according to a variety of key processes which have been reviewed by [START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF]. While abiotic factors such as regional climate and geology cannot be back-influenced by humus forms, at least in the short-term (but see [START_REF] Van Breemen | Soils as biotic constructs favouring net productivity[END_REF][START_REF] Marland | The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy[END_REF], biotic factors are tightly linked to humus forms according to feed-back loops [START_REF] Perry | Self-organizing systems across scales[END_REF][START_REF] Wardle | Linkages between soil biota, plant litter quality and decomposition[END_REF][START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF], making distal and proximal causes hard to discern and thus any predictions hardly questionable. In forests, past land use [START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF][START_REF] Dupouey | Irreversible impact of past land use on forest soils and biodiversity[END_REF] and present-day management options [START_REF] Hölscher | Effects of coppicing in temperate deciduous forests on ecosystem nutrient pools and soil fertility[END_REF][START_REF] Godefroid | Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium[END_REF][START_REF] Van Calster | Management driven changes (1967-2005) in soil acidity and the understorey plant community following conversion of a coppice-with-standards forest[END_REF] are known to influence humus forms at the scale of management units. At a more local scale individual trees [START_REF] Boettcher | Single-tree influence on soil properties in the mountains of eastern Kentucky[END_REF][START_REF] Kuuluvainen | A zone-of-influence model to predict the effect of trees on the spatial distribution of understorey vegetation and soil properties in a boreal Scots pine stand[END_REF][START_REF] Peltier | Humus forms in Mediterranean scrublands with Aleppo pine[END_REF], forest vegetation [START_REF] Bernier | Comparative study of soil organic layers in two bilberryspruce forest stands (Vaccinio-Piceetea): relation to forest dynamics[END_REF][START_REF] Emmer | Humus form characteristics in relation to undergrowth vegetation in a Pinus sylvestris forest[END_REF][START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF], microtopography [START_REF] Dwyer | Infleunce of topographic heterogeneity on deciduous litter decomposition[END_REF] and animal/microbial populations [START_REF] Gourbière | Pourriture blanche de la litière d'Abies alba Mill. II. Répartition spatiotemporelle et activité annuelle des Basidiomycètes du genre Collybia[END_REF][START_REF] Wilcox | Soil carbon and nitrogen dynamics in Lumbricus terrestris L. middens in four arable, a pasture, and a forest ecosystems[END_REF] stem in a pronounced variation of humus forms and ground floor thickness [START_REF] Arp | The forest floor: lateral variability as revealed by systematic sampling[END_REF][START_REF] Riha | Spatial variability of soil pH and organic matter in forest plantations[END_REF][START_REF] Aubert | Variability and heterogeneity of humus forms at stand level: comparison between pure beech and mixed beech-hornbeam forest[END_REF].

Soil types and humus forms do not vary at the same scale of time [START_REF] Crocker | Soil development in relation to vegetation and surface age at Glacier Bay, Alaska[END_REF][START_REF] Switzer | Successional development of the forest floor and soil surface on upland sites of the East Gulf coastal plain[END_REF][START_REF] Turk | Soil morphology of a debris flow chronosequence in a coniferous forest, southern California, USA[END_REF], making their causal relationships highly variable in space and time, more especially in forest environments [START_REF] Kuuluvainen | Factors related to seedling growth in a boreal Scots pine stand: a spatila analysis of a vegetation-soil system[END_REF][START_REF] Ponge | The forest regeneration puzzle: biological mechanisms in humus layer and forest vegetation dynamics[END_REF][START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF]. It has been shown that the thickness of forest floor and the structure of organo-mineral horizons, which are under the paramount influence of ecosystem engineers such as earthworms [START_REF] Hoogerkamp | Effect of earthworms on grassland on recently reclaimed polder soils in the Netherlands[END_REF][START_REF] Bernier | Earthworm feeding activity and development of the humus profile[END_REF][START_REF] Wironen | Exotic earthworm invasion increases soil carbon and nitrogen in an old-growth forest in southern Quebec[END_REF]; but see [START_REF] Burghouts | Spatial heterogeneity of element and litter turnover in a Bornean rain forest[END_REF], can vary according to the age of trees [START_REF] Aubert | Effect of tree mixture on the humic epipedon and vegetation diversity in managed beech forests (Normandy, France)[END_REF][START_REF] Godefroid | Variation in the herb species response and the humus quality across a 200-year chronosequence of beech and oak plantations in Belgium[END_REF][START_REF] Chauvat | Humus structure during a spruce forest rotation: quantitative changes and relationship to soil biota[END_REF], plant successional processes [START_REF] Leuschner | Soil profile alteration and humus accumulation during heathland-forest succession in NW Germany[END_REF][START_REF] Emmer | Temporal and vertical changes in the humus form profile during a primary succession of Pinus sylvestris[END_REF][START_REF] Scheu | Secondary succession, soil formation and development of a diverse community of oribatids and saprophagous soil macro-invertebrates[END_REF] and undergo cycles at the scale of centuries in naturally regenerating late-successional forests [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF][START_REF] Sagot | Earthworm distribution and humus forms in the development of a semi-natural alpine spruce forest[END_REF][START_REF] Salmon | Relationships between soil fauna communities and humus forms: reponse to forest dynamics and solar radiation[END_REF]. Litter quality, resulting from the species composition of forest vegetation [START_REF] Wolters | Allium ursinum litter triggering decomposition on a beech forest floor: the effect of earthworms[END_REF][START_REF] Loranger | Leaf decomposition in two semi-evergreen tropical forests: influence of litter quality[END_REF][START_REF] Wardle | Determinants of litter mixing effects in a Swedish boreal forest[END_REF] and conditions of tree growth [START_REF] Northup | Intraspecific variation of conifer phenolic concentration on a marine terrace soil acidity gradient: a new interpretation[END_REF][START_REF] Hättenschwiler | Polyphenols in litter from tropical montane forests across a wide range in soil fertility[END_REF], is known to influence and be influenced by humus forms and associated soil trophic networks [START_REF] Davies | Polyphenols in plant, humus, and soil. IV. Factors leading to increase in biosynthesis of polyphenols in leaves and their relationship to mull and mor formation[END_REF][START_REF] Nicolai | Phenolic and mineral content of leaves influences decomposition in European forest ecosystems[END_REF][START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF]. At last, forest vegetation is locally selected (filtered out from regional pools of species) by forest floor and topsoil properties, combined with species interactions [START_REF] Daniel | Lethal and nonlethal effects of the organic horizons of forested soils on the germination of seeds from several associated conifer species of the Rocky Mountains[END_REF][START_REF] Goldberg | The distribution of evergreen and deciduous trees relative to soil type: an example from the Sierra Madre, Mexico, and a general model[END_REF][START_REF] Falkengren-Grerup | Soil chemical properties excluding field-layer species from beech forest mor[END_REF] and in turn it influences the activity of soil organisms, and thus the development of humus forms, through litter and rhizosphere effects [START_REF] Emmer | Humus form characteristics in relation to undergrowth vegetation in a Pinus sylvestris forest[END_REF][START_REF] Bradley | Interactions between tree seedling roots and humus forms in the control of soil C and N cycling[END_REF][START_REF] Milleret | Impact of roots, mycorrhizas and earthworms on soil physical properties as assessed by shrinkage analysis[END_REF].

While it is difficult to disentangle this network of multiple and often symmetrical causal relationships in the variety of situations observed in a region [START_REF] Klinka | Relationships among humus forms, forest floor nutrient properties, and understory vegetation[END_REF][START_REF] Kindel | Humus form in ecosystems of the Atlantic Forest, Brazil[END_REF][START_REF] Galvan | Humus components and soil biogenic structures in Norway spruce ecosystems[END_REF], attempts can be made to discern a limited number of strategies by which plants, microbes and animals can adapt themselves to their physical environment, and adapt it to their own requirements [START_REF] Read | The mycorrhizal fungal community with special reference to nutrient mobilization[END_REF][START_REF] Northup | Polyphenols as regulators of plant-litter-soil interactions in northern California's pygmy forest: a positive feedback?[END_REF][START_REF] Wardle | Ecological linkages between aboveground and belowground biota[END_REF], stemming in the view of humus forms as ecological attractors [START_REF] Beisner | Alternative stable states in ecology[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF][START_REF] Graefe | Humus forms as tool for upscaling soil biodiversity data to landscape level? Mitt[END_REF]. In this respect, we suggest that multivariate methods using a wide array of sites would be better able to discern a limited array of realistic patterns than comparative studies based on a limited number of selected couples. For instance, it is current tenet that conifers impoverish the topsoil and thus change locally the humus form. However, most comparative studies on existing (non-experimental) stands [START_REF] Nihlgård | Pedological inflrunce of spruce planted on former beech forest soils in Scania, South Sweden[END_REF][START_REF] Gauquelin | Physico-chemical heterogeneity of superficial soil layers in conifer plantations versus original broad leaf forests in Ariege (Pyrenees, France)[END_REF][START_REF] Augusto | Effects of tree species on understory vegetation and environmental conditions in temperate forests[END_REF] are flawed by the facts (not controlled by the sampling procedure) that (i) the replacement of a deciduous by a coniferous species is often (although not always) dictated by poor growth of the previous crop, due to low soil fertility or water availability, unfavourable aspect or bad management practices, (ii) the growth rate of timber species and the duration of stand rotation (much shorter for conifers, but decided by the forester) influence the balance between aboveground nutrient immobilisation and below ground nutrient recycling through litter decomposition and mineral weathering [START_REF] Schlesinger | Carbon balance in terrestrial detritus[END_REF][START_REF] Vitousek | A general theory of forest nutrient dynamics[END_REF][START_REF] Ulrich | Stability, elasticity, and resilience of terrestrial ecosystems with respect to matter balance[END_REF].

Studies on well-designed experimental plantations with homogeneous initial conditions conclude to an influence of tree species composition on humus form and litter decomposition [START_REF] Ovington | Studies of the development of woodland conditions under different trees. II. The forest floor[END_REF][START_REF] Chapman | Metabolic and faunal activity in litters of tree mixtures compared with pure stands[END_REF], while studies on the same tree or litter species in a variety of soil and climate conditions conclude to a strong influence of abiotic factors [START_REF] Vitousek | Litter decomposition on the Mauna Loa enviromental matrix, Hawai'i: patterns, mechanisms, and models[END_REF][START_REF] Coûteaux | Decomposition of standard plant material along an altitudinal transect (65-3968 m) in the tropical Andes[END_REF][START_REF] Titeux | Experimental study of DOC, nutrients and metals release from fores floors developed under beech (Fagus sylvatica L.) on a Cambisol and a Podzol[END_REF]. But what when both biotic and non-biotic factors vary together?

We aimed here at investigating the respective influence of abiotic conditions and tree forest species on the spatial variability of humus forms at the scale of the French continental territory. The use of a large array of forest sites of varying tree composition (including coniferous and deciduous species), soil type, climate, and parent material may help to reveal distal factors acting on humus forms, making realistic predictions possible despite of the ground noise to be expected from the variety of ecological conditions thus analysed in the same data matrix [START_REF] Bednorz | Humus forms in the forestalpine tundra ecotone at Stillberg (Dischmatal, Switzerland: spatial heterogeneity and classification[END_REF]. We used 3441 plots from the phytoecological database EcoPlant [START_REF] Gégout | EcoPlant : a forest site database linking floristic data with soil and climate variables[END_REF], covering the whole forested French territory, to investigate relationships between humus forms and environmental conditions. Plots were characterized by:  Humus form  Composition of the forest canopy  Soil type  Geological substrate  Climate (rainfall and temperature indices)  Geography (latitude and longitude, phytoecological region) Data about soil chemical and physical analyses were discarded, being too fragmentary in the database and thought redundant with soil type and geological substrate [START_REF] Chaplot | Soil spatial distribution in the Armorican massif, western France: effect of soil-forming factors[END_REF]. In order to better relate variations of humus forms to environmental factors, we used a Humus Index as a mean to transform the qualitative assessment of humus forms, based on a typology of morphological diagnostic characters [START_REF] Green | Towards a taxonomic classification of humus forms[END_REF][START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], into a numerical scale easier to be treated statistically [START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF][START_REF] Ponge | Humus Index as an indicator of forest stand and soil properties[END_REF][START_REF] Lalanne | Opposite responses of vascular plant and moss communities to changes in humus form, as expressed by the Humus Index[END_REF]. We used total (Van den [START_REF] Van Den Wollenberg | Redundancy analysis: an alternative for canonical correlation analysis[END_REF][START_REF] Kenkel | On selecting an appropriate multivariate analysis[END_REF] and partial redundancy analysis [START_REF] Borcard | Partialling out the spatial component of ecological variation[END_REF][START_REF] Ter Braak | On the statistical analysis of vegetation change: a wetland affected by water extraction and soil acidification[END_REF] to measure the respective contribution to humus form variation of tree canopy composition vs environmental data. Our hypothesis was that the correlation between aboveground vegetation and topsoil properties, which has been often described at local scales [START_REF] Emmer | Humus form characteristics in relation to undergrowth vegetation in a Pinus sylvestris forest[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF][START_REF] Kounda-Kiki | Humus profiles under main vegetation types in a rock savanna (Nouragues inselberg, French Guiana)[END_REF] does not hold at a more regional or even continental scale, where the influence of long-lasting distal factors such as geology and climate prevail over that of short-duration proximal factors such as forest stand composition [START_REF] Augusto | Effects of tree species on understory vegetation and environmental conditions in temperate forests[END_REF][START_REF] Sadaka | Climatic effects on soil trophic networks and the resulting humus profiles in holm oak (Quercus rotundifolia) forests in the High Atlas of Morocco as revealed by correspondence analysis[END_REF][START_REF] Zhang | Rates of litter decomposition in terrestrial ecosystems: global patterns and controlling factors[END_REF], while both factors combine in a nested manner [START_REF] Coûteaux | Litter decomposition, climate and litter quality[END_REF][START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Aerts | Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship[END_REF]. Consequences of present-day anthropogenic global changes [START_REF] Walther | An ecological 'footprint' of climate change[END_REF][START_REF] Vetter | Partitioning direct and indirect human-induced effects on carbon sequestration of managed coniferous forests using model simulations and forest inventories[END_REF][START_REF] Rosenzweig | Attributing physical and biological impacts to anthropogenic climate change[END_REF] for protracted trends in the evolution of humus forms will be discussed to the light of present results and existing knowledge.

Material and methods

Data matrix

The complete data matrix crossed 3441 rows (sites) and 148 columns (variables). Sites were adult forest stands distributed over the whole French continental territory, with a sampling density roughly representing the distribution of forested areas (Fig. 1). No attempt was made to compensate for the over-or under-representation of some regions, given that plots were near always located several kilometres apart, thereby alleviating the risk of autocorrelation for humus profiles [START_REF] Aubert | Variability and heterogeneity of humus forms at stand level: comparison between pure beech and mixed beech-hornbeam forest[END_REF].

Humus forms were classified in nine types according to the French classification system [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF][START_REF] Jabiol | L'Humus sous toutes ses Formes[END_REF], which allowed the calculation of the Humus Index or HI [START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF] according to the following scale of decreasing incorporation of organic matter to the mineral soil, ranging from 1 to 8:

1. Eumull (crumby A horizon, OL horizon absent, OF horizon absent, OH horizon absent) 2. Mesomull (crumby A horizon, OL horizon present, OF horizon absent, OH horizon absent) 3. Oligomull (crumby A horizon, OL horizon present, OF horizon < 1 cm, OH horizon absent) 4. Dysmull (crumby A horizon, OL horizon present, OF horizon ≥ 1 cm, OH horizon absent) 5. Amphimull (crumby A horizon, OL horizon present, OF horizon present, OH horizon present) and Hemimoder (compact A horizon, OL horizon present, OF horizon present, OH horizon absent): these humus forms are quite different in their diagnostic A horizon but both exhibit intermediate characters between dysmull and eumoder, hence same HI value) 6. Eumoder (compact A horizon, OL horizon present, OF horizon present, OH horizon ≤ 1 cm) 7. Dysmoder (compact A horizon, OL horizon present, OF horizon present, OH horizon > 1 cm) 8. Mor (no A horizon, OL horizon present, OF horizon present but with few animal faeces)

In the following text amphimull will be referred to amphi according to [START_REF] Graefe | Humus forms as tool for upscaling soil biodiversity data to landscape level? Mitt[END_REF], [START_REF] Graefe | Gibt es in Deutschland die Humusform Amphi? Mitt[END_REF], [START_REF] Galvan | Humus components and soil biogenic structures in Norway spruce ecosystems[END_REF] and [START_REF] Zanella | Toward a European humus forms reference base[END_REF]. Humus forms were coded as dummy variables (taking the value 1 or 0 according to presence or absence in a given site of the corresponding humus form). Clearly, the assignment of humus forms to the system of 9 humus forms used for the calculation of HI, was sometimes imperfect. Depending on the time at which data were collected, some older classification systems were used by field collectors. In a few instances, the distinction between hemimoder, dysmull and amphimull, which is based on the presence/absence of an OH horizon and on the structure of the A horizon [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], could not be done because these forms were confused under the old-fashioned name mull-moder. In these cases, they were arbitrarily assigned to dysmull. In most cases field collectors have been trained to the recognition of diagnostic biological processes (fragmentation of litter, deposition of animal faeces, mixing of mineral with organic matter) by using the field identification booklet by [START_REF] Jabiol | L'Humus sous toutes ses Formes[END_REF].

The composition of forest canopies was described by 88 variables (Appendix) measuring the percent cover of tree species according to a scale from 0 to 6. The environment (geography, climate, geology, soil type) was described by 36 variables (Table 1) to which were added 11 geographical regions, thus totalling 47 variables, 38 of them being dummy variables. Table 1 summarizes the main characteristics of the 11 regions covering the whole continental French territory.

A soil profile was dug under a clump of adult trees with a near complete canopy cover (≥ 80%) at more than 2 m from the nearest trunk. Soil types were classified according to the last version of the French Référentiel Pédologique (AFES, 2009) and main correspondences with WRB classification (IUSS, 2006) are given in Table 1.

Data analysis

Data were analysed by RDA (Redundancy Analysis). A total RDA was performed, using both environmental and floristic variables as independent variables (n = 171) and variables describing humus forms (9 dummy variables) and the Humus Index (discrete variable) as dependent variables.

All variables were centred and reduced (mean = 0, variance = 1) prior to analysis. Partial RDAs were performed using either floristic or environmental variables as co-variables in order to estimate their respective influence on humus forms (and their interaction) and to decompose the total variance according to [START_REF] Borcard | Partialling out the spatial component of ecological variation[END_REF]. Some particular trends depicted by RDA were verified by classical statistics such as t-and  2 -tests and simple linear regression. All statistical treatments were performed with XLSTAT ® (Addinsoft, Paris, France) under EXCEL 7 ® (Microsoft Corporation, Redmond, WA).

Results

The first three axes of RDA (F1, F2 and F3) extracted 83% of the explained variance (60% for F1, 15% for F2 and 8% for F3). The variance explained by floristic and environmental data represented 21% of the total variance, but permutation tests (500 permutations of rows and columns) showed that the explanatory value of independent variables was highly significant (p<0.0001). The projection of humus variables in the plane of the first and third canonical axes (Fig. 2) showed that humus forms were distributed along F1 from most active (mull) to less active (moder, mor) forms.

Humus forms belonging to the mull group (eumull, mesomull, oligomull, dysmull) were discriminated in the order of increasing accumulated organic matter while those of the moder-mor group (eumoder, dysmoder, mor) were imperfectly discriminated, pointing to other factors than geology, pedology, climate and woody flora that might influence them. Hemimoder was not fully discriminated from dysmull along F1, both humus forms occupying an intermediate position. The position far from the origin of the Humus Index indicated a good correlation of this numerical variable with the first canonical axis, which was verified by regression analysis (Fig. 3): the Humus Index explained by itself 57% of the variance of F1. Amphi, which occupied an intermediate position along F1, like hemimoder and dysmull, was fully discriminated by the third canonical axis, pointing to a distinct factor explaining this humus form. The second canonical axis did not correspond to any interpretable factor, the parabolic arrangement of sites in the F1-F2 plane (not shown) suggesting a horseshoe (Guttman) effect.

The projection of independent variables in the F1-F3 plane (Fig. 4) showed that points corresponding to tree species were projected near the origin while most environmental variables were projected far from the origin, more especially along F1. Tree canopies had a feeble explanatory value for humus forms, compared with environmental variables, to the exception of Abies alba Mill. for amphi. Most prominent relationships with the humus form were displayed by geology: 'alkaline' and 'acid' geological substrates were projected far from the origin on opposite sides of F1 (on 'mull' and 'mor-moder' sides, respectively), while the 'neutral' substrate was projected in an intermediate position, i.e. not far from the origin. Soil types scaled accordingly along F1, from calcisols (hypereutric cambisols) and calcosols (calcaric cambisols) on the 'mull' side to alocrisols (hyperdystric cambisols) and podzosols (podzols) on the 'mor-moder' side. This might suggest a gradient of soil acidity and nutrient availability along F1, but climatic and geographical variables scaled also along the same canonical axis, in particular 'July temperature' and 'Northern continental plains', suggesting that mull (in particular eumull) was associated with milder climate, too, without any discrimination between the respective influences of geology and temperature. This was verified by studying the distribution of humus forms and of the Humus Index according to geological substrates (Fig. 5a) and temperatures of the warmest month (Fig. 5b). The distribution of humus forms, and accordingly the value of the Humus Index (H.I.), changed significantly between 'alkaline' and 'neutral' substrates (H.I. = 2.3 and 4.1, respectively), while they did not differ between 'neutral' and 'acid' substrates (H.I. = 4.1 and 4.3, respectively), which were thus equivalent for the distribution of humus forms but not for other environmental variables, as suggested by F1 scores of environmental variables (Fig. 4).

Among climate variables, July temperature, and to a lesser extent mean annual temperature, were projected on the 'mull' side while annual rainfall and Thornthwaite Aridity Index (which increases with rainfall) were projected on the 'mor-moder' side of F1, indicating a combined influence of temperature and precipitation on humus forms, temperature having a prevailing influence as indicated by respective F1 scores. The influence of climate was verified by comparing the distribution of humus forms and the value of the Humus Index in three groups differing by July temperature (>19°C, 15-19°C and <15°C): all differences were significant, indicating that there is a continuous gradient of decreasing presence of mull and increasing presence of mor and moder and a higher Humus Index when the environment becomes colder (Fig. 5b).

The poor influence of tree canopy on humus forms (at the scale of the whole French territory) was verified by classifying the sites in deciduous and coniferous forests (Fig. 5c): differences in the distribution of humus forms between these two groups of contrasted canopy composition were only marginally significant (P = 0.05) and their Humus Index did not differ. Mixed forests were not taken into account, being too weakly represented in the EcoPlant database (5%).

The third canonical axis F3 showed that 'amphi' was mostly associated with high mountain environments of the Alps and Pyrenees and to a lesser extent to Mediterranean and Aquitaine basins, and with poorly developed soils such as organosols (folic umbrisols), dolomitisols (dolomitic cambisols), peyrosols (episkeletic soils), lithosols (leptosols) and arenosols (in decreasing order of association with amphi according to F3 scores). The position of the variable 'altitude' on both positive sides of F1 and F3 (Fig. 4) indicated that higher elevation favours either 'amphi' or 'mor-moder' according to the geological substrate being 'alkaline' or 'acid', respectively (as shown by the respective projection of these two variables on F3). The projection of the variable 'latitude' on the negative side of F3 (i.e. opposed to 'amphi') can be explained by the southern localisation of high altitude and Mediterranean sites across the French territory. The respective position of 'amphi' and 'hemimoder' along F3 also showed that they differed according to ecological factors, although being given the same Humus Index value (5).

Partial RDA showed that tree species composition and abiotic variables did not contribute to the same extent to the distribution of humus forms. When the composition of tree canopy was kept constant, environmental variables explained a significant part (19%) of humus form variation (Monte-Carlo permutation test, Pseudo-F = 0.23, P<0.0001), while when only tree species variables were allowed to vary, a much lower(3%) and insignificant part of humus form variation was explained (Pseudo-F = 0.03, P = 1). The interaction between environment and tree species composition was negligible (0.2%). Residual (unexplained) variation amounted to 79%.

Discussion

We showed that at the scale of the French continental territory (1000 km from West to East and 1000 km from North to South, elevation from 1 to 2500 m) the distribution of humus forms and the value of the Humus Index were but poorly explained by the composition of the forest canopy, but were affected by geology, climate and associated soil types. This contradicts current tenets about the detrimental influence of coniferous litter on soil biological activity, which has been always demonstrated at a scale at which geology and climate could not vary and interfere with it. Our results point to a decisive influence of large-scale factors such as parent rock, temperature and to a lesser extent rainfall. We are more cautious about the influence of soil conditions on humus forms, shown by the distribution of soil types along the first canonical axis of RDA, given that the development of the soil profile is strongly influenced by soil biology [START_REF] Bullinger-Weber | Influence of some physicochemical and biological parameters on soil structure formation in alluvial soils[END_REF][START_REF] Frey | Weathering-associated bacteria from the Damma glacier forefield: physiological capabilities and impact on granite dissolution[END_REF], pointing to symmetric influences during the common development of humus forms and soils [START_REF] Van Breemen | Soils as biotic constructs favouring net productivity[END_REF][START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF].

The influence of parent rocks on the development of humus forms may be at least partly explained by biogeochemical cycles: the nature of the subsoil influences (1) the quality of litter, in particular its mineral content [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF][START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF], and (2) the access of soil-dwelling animals to nutrients through their direct use of mineral matter [START_REF] Bernier | Earthworm feeding activity and development of the humus profile[END_REF]. Earthworms, when digging the soil, ingest mineral particles and may contribute to mineral weathering [START_REF] Carpenter | Earthworm induced mineral weathering: preliminary results[END_REF], quite to the same extent as plant roots and their symbionts [START_REF] Van Breemen | Do plants drive podzolization via rockeating mycorrhizal fungi?[END_REF]. In the sandy context of the Fontainebleau forest, it has been shown that the presence of a limestone table, at a depth not exceeding that available to beech roots, allowed the maintenance of an active mull-forming earthworm population through a litter enriched in calcium [START_REF] Ponge | Interactions between earthworms, litter and trees in an old-growth beech forest[END_REF]. Other studies showed that earthworms were able to modify mineral assemblages, thereby increasing the available nutrient pool of the topsoil [START_REF] Rangel | Ion exchange properties of casts of the anecic earthworm (Martiodrilus carimaguensis Jiménez and Moreno) in a Colombian savanna Oxisol[END_REF].

The influence of climate (mull associated with more heat) can be understood through the stimulatory influence of temperature on most chemical processes, in particular those involved in organic matter decomposition [START_REF] Aerts | Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship[END_REF] and mineral weathering [START_REF] Turner | Effects of temperature on silicate weathering: solute fluxes and chemical weathering in a temperate rain forest watershed, Jamieson Creek, British Columbia[END_REF][START_REF] Williams | The temperature dependence of feldspar dissolution determined using a coupled weathering-climate model for Holocene-aged loess soils. Geoderma 156, 11-19. humus type as indicators of soil nutrient regime for an ecological site classification of British forests[END_REF]. In addition to benefits of heat for their own activity of cold-blooded animals [START_REF] Briones | Soil biology and warming play a key role in the release of 'old C' from organic soils[END_REF], mull-forming invertebrates may benefit from the more rapid recycling of elements authorized by an increase in temperature [START_REF] Zhang | Rates of litter decomposition in terrestrial ecosystems: global patterns and controlling factors[END_REF], at least in the absence of any limitation by drought [START_REF] Cortez | Field decomposition of leaf litters: relationships between decomposition rates and soil moisture, soil temperature and earthworm activity[END_REF] which is often marginal in OF and OH horizons. The association of mor and moder with heavy rainfall can be explained by (1) the intensity of leaching, which favours nutrient losses and thus topsoil impoverishment [START_REF] Turner | Effects of temperature on silicate weathering: solute fluxes and chemical weathering in a temperate rain forest watershed, Jamieson Creek, British Columbia[END_REF], and (2) the association of heavy rainfall with altitude and low temperatures [START_REF] Roe | Orographic precipitation[END_REF].

The poor influence of tree species on humus forms, compared to that of geology and climate, can be explained by the influence of parent material and climate on litter quality. For a given tree species, the composition of leaf or needle foliage may vary according to substrate [START_REF] Nicolai | Phenolic and mineral content of leaves influences decomposition in European forest ecosystems[END_REF][START_REF] Hättenschwiler | Polyphenols in litter from tropical montane forests across a wide range in soil fertility[END_REF] and to a lesser extent according to climate [START_REF] Aerts | Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship[END_REF], making palatability and nutritional quality of tree litter more dependent on the environmental context than on the taxonomic level. [START_REF] Bernier | Earthworm feeding activity and development of the humus profile[END_REF] showed that the anecic earthworm Lumbricus terrestris L. consumed spruce needles from a coniferous selection forest (not planted) and mixed them with the mineral soil, stemming in the building of mull humus. Such results point on possible confusion of negative coniferous effects with the detrimental influence of (1) plantation forests (whether deciduous or not) to which local animal and microbial communities needs time to become, if any, adapted [START_REF] Hansen | Red oak litter promotes a microarthropod functional group that accelerates its decomposition[END_REF], and (2) associated ground vegetation [START_REF] Wolters | Allium ursinum litter triggering decomposition on a beech forest floor: the effect of earthworms[END_REF]. In the present database most measurement plots were installed in indigenous forest stands, avoiding young plantations. However, it must be noticed that we found a marginally significant negative correlation between the Humus Index and the species richness of the tree canopy (r = -0.035, P = 0.041), which could be explained by either or both (1) the favourable effect of tree mixture on litter decomposition and [START_REF] Chapman | Metabolic and faunal activity in litters of tree mixtures compared with pure stands[END_REF][START_REF] Aubert | Variability and heterogeneity of humus forms at stand level: comparison between pure beech and mixed beech-hornbeam forest[END_REF]; but see [START_REF] Wardle | Determinants of litter mixing effects in a Swedish boreal forest[END_REF], and (2) distal effects of environmental factors on both humus forms and plant diversity [START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF][START_REF] Wardle | Ecological linkages between aboveground and belowground biota[END_REF]. A much better negative correlation (the higher the Humus Index the lower the species richness) was observed with ground flora but this relationship held true at the local scale only [START_REF] Lalanne | Local and regional trends in the ground vegetation of beech forests[END_REF].

The fact that moder (eumoder and dysmoder) and mor could not be discriminated by the first canonical axis of RDA points to the existence of other possible environmental influences, not taken into account in our data matrix, such as soil chemistry, known to discriminate moder and mor in addition to morphological and biological characters (Wilson et al., 2001). In addition, it must be pointed out that in the French context the mor humus form is rare and never typical.

The particular case of amphi is worth to notice. This humus form, which has been previously confused with dysmull, hemimoder or moder [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF], is associated with highly seasonal mull-forming activity in an otherwise favourable geological context [START_REF] Tagger | Characteization of an amphimull under Mediterranean evergreen oak forest (Quercus ilex): micromorphological and biodynamic descriptions[END_REF][START_REF] Galvan | Humus components and soil biogenic structures in Norway spruce ecosystems[END_REF]: the shorter the season during which invertebrate (and microbial) activity is possible, the thicker will be the organic horizon (of moder type) overlying a crumby A horizon (of mull type). This may happen both at higher elevation and in Mediterranean landscapes when the richness of the substrate (limestone, dolomitic rocks) allows the activity of subterranean animals during less favourable seasons. Although still badly understood and imperfectly described, amphi does not seem in line with the temperature/acidity gradient corresponding to the contrast between mull and mor.

A large part of the variation of humus forms was not explained by the abovementioned factors. The bulk of unexplained variation (79%) could be ascribed to local factors, the influence of which has been demonstrated in particular cases (detailed in the Introduction), such as ground vegetation, previous land use, management options, canopy density, and the mosaic patchwork of animal and microbial populations. Nonetheless, the fact that geology and climate may explain by themselves a fifth of the total variation of this highly fluctuating compartment of the forest ecosystem may help to discern possible trends in the actual context of global warming. Linear regression analysis (F = 182, P<0.0001) showed that the Humus Index (H.I.) was related to July temperature (J) according to the following equation: H.I. = 9 -0.3 J According to this linear relationship any increase of 1°C would correspond to a decrease of the Humus Index averaging 0.3 units. This is highly speculative, given what we know about the counteracting influences of litter recalcitrance due to fertilisation with CO 2 [START_REF] Cotrufo | Elevated CO 2 reduces field decomposition rates of Betula pendula (Roth.) leaf litter[END_REF]; but see [START_REF] Coûteaux | Influence of increased atmospheric CO 2 concentration on quality of plant material and litter decomposition[END_REF] and anthropogenic acidification and eutrophication [START_REF] Thimonier | Simultaneous eutrophication and acidification of a forest ecosystem in North-East France[END_REF].

Nonetheless, in the present state of our knowledge, we can hypothesize in accordance with [START_REF] Graefe | Gibt es in Deutschland die Humusform Amphi? Mitt[END_REF] that the combined influence of increased litter recalcitrance and acidification (favouring the accumulation of litter) and warming trends and eutrophication (favouring the activity of soil organisms) will extend the domain of amphi, a humus form not fitted with the mull-mor gradient of decreasing litter quality and biological activity associated with climate/trophic constraints [START_REF] Ponge | Humus forms in terrestrial ecosystems: a framework to biodiversity[END_REF]. Environmental variables are described in Table 1. Tree species (listed in the Appendix) were represented as black dots, given their position not far enough from the origin. A more detailed view of this zone, showing plant codes (see Appendix), was inserted for the sake of clarity.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .Figure 3 .

 23 Figure 2. Total Redundancy Analysis (RDA): projection of dependent variables (9 humus forms as

Figure 4 .

 4 Figure 4. Total Redundancy Analysis (RDA): projection of independent variables (148 variables

Figure 5 .

 5 Figure 5. Distribution of humus forms and average values of the Humus Index according to geology

  Fig. 1

Table 1 .

 1 Average values of quantitative variables (geography, climate) and percent occurrence of dummy variables (geology, pedology) used in Redundance Analysis (RDA) in each geographical region covered by the study.Geographical regions were also included as dummy variables (refer to text for further details)

	Variable	Meaning	Brittany	Northern	Vosges	Jura	Paris	Massif	Alps	Pyrenees	Rhone	Aquitaine	Mediterranean
	name			continental			basin	Central			valley	basin	basin
				plains
			(n = 99)	(n = 654)	(n = 472)	(n = 171)	(n = 971)	(n = 229)	(n = 417)	(n = 151)	(n = 18)	(n = 247)	(n = 15)
	Geography:
	Longitude	Lambert II extended X	281307	900896	952636	893364	614624	673735	906434	494659	821386	414911	783533
		coordinates (m)
	Latitude	Lambert II extended Y	2397578	2356856	2377920	2227127	2359891	2044778	2067732	1771497	2053854	1907341	1876533
		coordinates (m)
	Climate:
	Mean	Mean annual temperature	11°C	10°C	8°C	9°C	10°C	9°C	7°C	10°C	11°C	12°C	13°C
	temperature
	January	Mean January temperature	5°C	1°C	-0.2°C	0.4°C	3°C	1.5°C	-1.3°C	4°C	2.4°C	6°C	5°C
	temperature
	July	Mean July temperature	17°C	19°C	17°C	18°C	18°C	17°C	16°C	18°C	21°C	20°C	22°C
	temperature
	Altitude	Elevation (m)	96	252	617	596	161	743	1277	834	290	157	280
	Rainfall	Mean annual rainfall (mm)	894	854	1279	1307	744	1202	1386	1101	938	969	855
	Water balance	Rainfall:potential	-175	-124	28	69	-183	-30	122	-39	-119	-154	-251
		evaporation (May to
		September) Thornthwaite
		index
	Aridity index	Rainfall:temperature	43	43	71	70	37	65	84	55	44	43	38
		Martonne index
	Parent rock:
	Acid	Acid silicate rocks	52%	41%	91%	9%	66%	74%	30%	57%	39%	73%	13%
	Neutral	Weakly acidic silicate	46%	0%	6%	0%	2%	19%	5%	8%	0%	0%	0%
		rocks
	Alkaline	Calcareous rocks	2%	59%	3%	91%	31%	4%	65%	35%	61%	26%	87%
	Soil type	Soil type (WRB):
	(AFES):
	Alocrisol	Cambisol (Hyperdystric)	37%	7%	27%	4%	8%	31%	14%	12%	0%	12%	0%
	Andosol	Andosol	0%	0.0%	0%	0%	0%	0.4%	0%	0%	0%	0%	0%
	Arenosol	Arenosol	1%	1%	0%	0%	0.1%	0%	0%	0%	0%	6%	0%
	Brunisol	Cambisol	19%	21%	36%	25%	17%	31%	23%	31%	17%	21%	7%
	Calcisol	Cambisol (Hyperautric)	0%	8%	1%	17%	8%	1%	12%	15%	11%	13%	27%
	Calcosol	Cambisol (Calcaric)	0%	10%	0.4%	14%	6%	0.4%	10%	3%	6%	6%	13%
	Colluviosol	RSG (Colluvic)	1%	3%	2%	7%	2%	3%	6%	0%	28%	0.4%	0%
	Dolomitisol	Cambisol (Dolomitic)	0%	0%	0%	0%	0%	0%	0%	1%	0%	0%	7%
	Fluviosol	Fluviosol	0%	15%	1%	0%	1%	1%	2%	1%	17%	3%	0%
	Histosol	Histosol	0%	0.2%	0%	0%	0.1%	0%	0%	0%	0%	0%	0%
	Lithosol	Leptosol	0%	0.3%	1%	0%	0.1%	2%	3%	3%	0%	0%	13%
	Luvisol	Luvisol	2%	6%	0.2%	4%	14%	3%	2%	6%	11%	6%	0%
	Neoluvisol	Cambisol (Luvic)	9%	9%	3%	9%	11%	2%	3%	12%	6%	9%	0%
	Organosol	Umbrisol (Folic)	0%	0.3%	0.2%	6%	0%	1%	6%	4%	0%	0%	0%
	Pelosol	Regosol (Clavic)	0%	3%	0%	1%	2%	0%	0%	0%	0%	0%	0%
	Peyrosol	suffix qualifier Episkeletic	0%	0.3%	3%	1%	1%	0%	1%	5%	0.0%	0.4%	0%
	Planosol	Planosol	0%	1%	0%	0%	6%	0%	0%	0%	0%	0.4%	0%
	Podzosol	Podzol	17%	3%	22%	1%	14%	16%	7%	5%	0%	11%	0%
	Rankosol	Leptosol	5%	0.3%	3%	0%	0.2%	6%	2%	2%	0%	0%	7%
	Redoxisol	Stagnosol	7%	3%	0.4%	0%	5%	1%	1%	0%	0%	4%	0%
	Reductisol	Gleysol	1%	3%	1%	1%	1%	0.4%	1%	0%	0%	4%	0%
	Regosol	Regosol	0%	0.3%	0%	0%	0%	0%	0.2%	0%	0%	1%	7%
	Rendisol	Leptosol (Rendzic)	0%	1%	0.2%	6%	1%	0.4%	3%	1%	6%	2%	0%
	Rendosol	Leptosol (Rendzic)	0%	5%	0.2%	6%	3%	0%	5%	1%	0%	2%	20%

Acknowledgements

EcoPlant is a phytoecological database financially supported by the Paris Institute of Technology for Life, Food, and Environmental Science (AgroParisTech), the Environment and Energy Management Agency (ADEME), and the National Office of Forests (ONF) which are greatly acknowledged.

F1

Fig. 4


Adrien Merer
email: merer@lma.cnrs-mrs.fr

Sølvi Ystad
email: ystad@lma.cnrs-mrs.fr

Richard Kronland-Martinet

Mitsuko Aramaki
email: aramaki@incm.cnrs-mrs.fr

On the potentiality of abstract sounds in perception research

Keywords: abstract sound, stimuli selection, acousmatic

Recognition of sound sources and events is an important process in sound perception and has been studied in many research domains. Conversely sounds that cannot be recognized are not often studied except by electroacoustic music composers. Besides, considerations on recognition of sources might help to address the problem of stimulus selection and categorization of sounds in the context of perception research. This paper introduces what we call abstract sounds with the existing musical background and shows their relevance for different applications.

Introduction

How do sounds convey meaning? How to identify acoustic characteristics that convey the relevant information in sounds? These questions interest researchers within various research fields such as cognitive neuroscience, musicology, sound synthesis, sonification, etc. Recognition of sound sources, identification, discrimination and sonification deal with the problem of linking signal properties and perceived information. In several domains (linguistic, music analysis), this problem is known as "semiotics" [START_REF] Jekosch | 8. In: Assigning Meaning to Sounds -Semiotics in the Context of Product-Sound Design[END_REF]. The analysis by synthesis approach [START_REF] Risset | Exploration of timbre by analysis and synthesis[END_REF] has permitted to understand some important features that characterize the sound of vibrating objects or interaction between objects. A similar approach was also adopted in [START_REF] Dessein | Free classification of vocal imitations of everyday sounds[END_REF] where the authors use vocal imitations in order to study human sound source identification with the assumption that vocal imitations are simplifications of original sounds that still contain relevant information. Recently, there has been an important development in the use of sounds to convey information to a user (of a computer, a car, etc.) within a new research community called auditory display [START_REF] Hermann | Taxonomy and definitions for sonification and auditory display[END_REF] which deals with topics related to sound design, sonification and augmented reality. In such cases, it is important to use sounds that are meaningful independently of cultural references taking into account that sounds are presented through speakers concurrently with other audio/visual information. As a function of research topics, authors focused on different sound categories (i.e. speech, environmental sounds, music or calibrated synthesized stimuli). In [START_REF] Gaver | What in the world do we hear ? an ecological approach to auditory source perception[END_REF], the author proposed a classification of everyday sounds according to physical interactions from which the sound originates. Such classification contains any sounds except synthesized sounds that do not imitate everyday sounds. When working within synthesis and/or sonification domains, the aim is often to reproduce the acoustic properties responsible for the attribution of meaning and thus, sound categories can be considered from the point of view of semiotics i.e. focusing on what information can be gathered in sounds. In this way, we considered a specific category of sounds that we call "abstract sounds". This category includes all the sounds that cannot be associated with an identifiable source. It includes environmental sounds that cannot be easily identified by listeners or that give rise to many different interpretations depending on listeners and contexts. This also includes synthesized sounds, and laboratory generated sounds if they are not associated with a clear origin. For instance, alarm or warning sounds cannot be considered as abstract sounds. In practice, recordings with a microphone close to the sound source and some synthesis methods like granular synthesis are especially efficient for creating abstract sounds. Note that in this paper, we mainly consider acoustically complex stimuli since they best meet our needs in the different applications (as discussed further).

Various labels that refer to abstract sounds can be found in the literature: "confused" sounds [START_REF] Bonebright | Data collection and analysis techniques for evaluating the perceptual qualities of auditory stimuli[END_REF], "strange" sounds [START_REF] Tanaka | Study of onomatopoeia expressing strange sounds : Cases of impulse sounds and beat sounds[END_REF], "sounds without meaning" [START_REF] Fastl | Neutralizing the meaning of sound for sound quality evaluations[END_REF]. Conversely, [START_REF] Smalley | Defining timbre -refining timbre[END_REF] uses the term "source-bonded" and the expression "source bonding" for the "The natural tendency to relate sounds to supposed sources and causes". Chion introduced "acousmatic sounds" [START_REF]Audio-vision, Sound on Screen[END_REF] in the context of cinema and audiovisual applications with the following definition: "sound one hears without seeing their originating cause -an invisible sound source" (for more details see section 2). The most common expression is "abstract sounds" [START_REF] Ricard | Morphological sound description computational model and usability evaluation[END_REF][START_REF] Dubois | A cognitive approach to urban soundscapes: Using verbal data to access everyday life auditory categories[END_REF][START_REF] Peeters | Automatic morphological description of sounds[END_REF] particularly within the domain of auditory display, when concerning "earcons" [START_REF] Bonebright | Most earcons do not interfere with spoken passage comprehension[END_REF]. "Abstract" used as an adjective means "based on general ideas and not on any particular real person, thing or situation" and also "existing in thought or as an idea but not having a physical reality"1 . For sounds, we can consider another definition used for art "not representing people or things in a realistic way" 1 . Abstract as a noun is "a short piece of writing containing the main ideas in a document" 1 and thus share the ideas of essential attributes which is suitable in the context of semiotics. In [START_REF] Dack | Abstract and concrete[END_REF], the author discussed the different uses and meanings of the word "concrete", and report that "concrete" has acquired a particular meaning in English regarding the use of recognizable sounds sources". In [START_REF] Bergman | Perceptual and emotional categorization of sound[END_REF], authors wrote: "Edworthy and Hellier (2006) suggested that abstract sounds can be in-terpreted very differently depending on the many possible meanings that can be linked to them, and in large depending on the surrounding environment and the listener." In fact, there is a general agreement for the use of the adjective "abstract" applied to sound that expressed both the ideas of source recognition and different possible interpretations. This paper will first present the existing framework from electroacoustic music composers and researchers for the use of abstract sounds. We will then discuss some important aspects that should be considered when conducting listening test with a special emphasis on the specificities of abstract sounds. Finally, three practical uses of abstract sounds in different domains will be presented.

The acousmatic approach

Even if the terms "abstract sounds" was not used in the context of electroacoustic music, it seems that this community was one of the first to consider the question of recognition of sound sources and to use such sounds. In 1966, P. Schaeffer who was both a musician and a researcher published the Traité des objets musicaux [START_REF] Schaeffer | Traité des objets musicaux[END_REF], which contains more than ten years of research on electroacoustic music. With a multidisciplinary approach, he intended to carry out fundamental music research that include both "Concrète"2 and traditional music. One of the first concepts he introduced was the so called "acousmatic" listening, related to the experience of listening to a sound without paying attention to the source or the event. The word "acousmatic" is at the origin of many discussions, and is now mainly employed in order to describe a musical trend. Discussions about "acousmatic" listening was kept alive due to a fundamental problem in concrete music. Indeed, for music composers the problem is to create new meaning from sounds that already carry information about their origins. In compositions where sounds are organized according to their intrinsic properties, thanks to the acousmatic approach, information on the origins of sounds is still present and interacts with composers' goals. There was an important divergence of points of view between Concrete and Elektronische music (see [START_REF] Cross | Electronic music, 1948-1953[END_REF] for a complete review), since the Elektronische music composers used only electronically generated sounds and thus avoided the problem of meaning [START_REF] Eimert | What is electronic music[END_REF]. Both Concrete and Elektronische music have developed a research tradition on acoustics and perception but unfortunately only Schaeffer adopted a scientific point of view. In [START_REF] Dack | Abstract and concrete[END_REF], the author wrote: "Schaeffers decision to use recorded sounds was based on his realization that such sounds were often rich in harmonic and dynamic behaviors and thus had the most potential for his project of musical research". This work is of importance for electroacoustic musicians but almost unknown by auditory perception researchers since there is no published translation except for concomitant works [START_REF] Schaeffer | Solfège de l'objet sonore[END_REF] and Chion's Guide des objets musicaux3 . As reported in [START_REF] Dack | Translating pierre schaeffer: Symbolism, literature and music[END_REF], translating Schaeffer's writing is extremely difficult since he used neologisms and very specific meanings of certain words. However, recently, there was a growing interest in this book and in particular in the domain of music information retrieval, for the morphological sound description [START_REF] Ricard | Morphological sound description computational model and usability evaluation[END_REF][START_REF] Peeters | Automatic morphological description of sounds[END_REF][START_REF] Bloit | Towards morphological sound description using segmental models[END_REF]. Authors indicated that in the case of what they call "abstract" sounds, classical approaches based on sound source recognition are not relevant and thus base their algorithms on Schaeffer's morphology an typology classifications. Morphology and typology have been introduced as analysis and creation tools for composers as an attempt to construct a music notation that includes electroacoustic music and therefore any sound. Typology classification (cf. figure 1) is defined by a combination of spectral (mass) and dynamical (facture4 ) "profiles" with considerations of how complex they are. There are nine central categories of "balanced" sounds where "balanced" means that the variations are neither too rapid and random nor too slow or nonexistent. Those nine categories are combinations of three facture profiles (sustained, impulsed or iterative) and three mass profiles (tonic, complex and varying). On both sides of the "balanced objects" there are additional categories for which mass and facture profiles are too simple/repetitive or vary too much. In all, the typology classification consists of twenty-eight categories. Note that some automatic classification methods are available [START_REF] Peeters | Automatic morphological description of sounds[END_REF]. In [START_REF] Thoresen | Spectromorphological analysis of sound objects: an adaptation of pierre schaeffer's typomorphology[END_REF] the authors proposed an extension of Schaeffer's typology that includes graphical notations.

Since the 1950s, electroacoustic music composers have addressed the problem of meaning of sounds and provided an interesting tool for classification of sounds with no a priori differentiation on the type of sound. For sound perception research, those categories may be useful since they are suitable for all sounds. The next section will detail the use of such classification for the design of listening tests.

Design of listening tests using abstract sounds

Listening tests are at the basis of many studies in sound perception. The design of listening tests implies considerations of different aspects of perception that interact with intended measurement. For instance, it is important to design calibrated stimuli and experimental procedures to best control the subjects' evaluations. We propose to discuss such aspects in the context of abstract sounds.

Stimuli

It is common to assume that perception differs as a function of sound categories (i.e. speech, environmental sounds, music). Even more, these categories are underlying elements defining a research area. Consequently, it is difficult to determine a general property of human perception by collecting results obtained from different studies. For instance, results from loudness studies based on elementary synthesized stimuli (sinusoids, noise, etc.) cannot be directly adapted to complex environmental sounds as reported by [START_REF] Schlauch | 12 -Loudness[END_REF]. Furthermore, the judgment of listeners could differ for sounds belonging to a same category. For instance, in the environmental sound category, [START_REF] Dubois | A cognitive approach to urban soundscapes: Using verbal data to access everyday life auditory categories[END_REF] have shown specific categorization strategies for sounds that involve human activity. When there is no hypothesis regarding the signal properties, it is important to gather sounds that present a large variety of acoustic characteristics as discussed in [START_REF] Shafiro | How to select stimuli for environmental sound research and where to find them[END_REF]. Schaeffer's typology offers an objective selection tool than can help the experimenter to construct a very general sound corpus representative of most existing sounds. Besides, abstract sounds may constitute a good compromise in terms of acoustic properties between elementary (sinusoids, noise, etc.) and ecological (speech, environmental and music) stimuli. Furthermore, abstract sounds cover all the typology categories by contrast with environmental sounds for example that do not concern certain rows of the classification table (mainly the "balanced" objects).

A corpus of abstract sounds can be constituted in different ways. Many databases available for audiovisual applications contain such sounds (see [START_REF] Shafiro | How to select stimuli for environmental sound research and where to find them[END_REF]). Different synthesis techniques (like granular or FM synthesis, etc.) are also ef-ficient to create abstract sounds. In [START_REF] Fastl | Neutralizing the meaning of sound for sound quality evaluations[END_REF] and further works [START_REF] Zeitler | Significance of meaning in sound quality evaluation[END_REF][START_REF] Zeitler | Methodological approaches to investigate the effects of meaning, expectations and context in listening experiments[END_REF], the authors presented some techniques to transform any recognizable sound into an abstract sound, preserving several signal characteristics. Conversely, many transformations drastically alter the original environmental or vocal sounds by modifying some important acoustic attributes. For instance, [START_REF] Moore | Perceived naturalness of spectrally distorted speech and music[END_REF] has shown that high and low-pass filtering influence the perceived naturalness of speech and music sounds. Since abstract sounds do not convey precise meaning, it is possible to use them in different ways according to the needs of the experience. For instance, a same sound corpus can be evaluated in different contexts in order to study specific aspects of the information conveyed by the sounds. In particular, we will see how the same set of abstract sounds was used in 2 different studies described in sections 4.3 and 4.1.

Procedure

In the case of abstract sounds, it is important to verify that they are actually "abstract" for most listeners. In a musical context, D. Smalley [START_REF] Smalley | Space-form and the acousmatic image[END_REF] has introduced the expression "surrogacy" level (or degree) to quantify the ease of source recognition. This level is generally evaluated by using identification tasks. In [START_REF] Bonebright | Data collection and analysis techniques for evaluating the perceptual qualities of auditory stimuli[END_REF], the authors describe three methods: 1) Free identification tasks that consists of associating words or any description with sounds [START_REF] Ballas | Common factors in the identification of an assortment of brief everyday sounds[END_REF]. 2) Context-based ratings, which are comparisons between sounds and other stimuli. 3) Attribute rating, which is a generalization of the semantic differential method. The third method may be the most relevant since it provides graduated responses on an unlimited number of scales. In particular, we will see in section 4 that we evaluated the degree of recognition of abstract sounds ("the sound is easily recognizable or not") by asking listeners to use a non graduated scale from "not recognizable" to "easily recognizable". There are many evidences for interaction between auditory and visual information (see [START_REF] Kohlrausch | Audio-visual interaction in the context of multi-media applications[END_REF] for a review). By definition, abstract sounds are not easily associated with a source (and to the corresponding label). Nevertheless, they can be attributed to several meanings that may depend on the type of experimental procedure and task. In particular, we will see that it is possible to take advantage of this variability to highlight for example differences between groups of listeners as described in section 4.1.

Type of listening

In general, perception research distinguishes analytic and synthetic listening. Given a listening procedure, subjects may focus on different aspects of sounds since different concentration and attention levels are involved. From a different point of view, [START_REF] Gaver | How do we hear in the world ? explorations of ecological acoustics[END_REF] introduced the terms "everyday listening" (as opposed to "musical listening") and argued that even in the case of laboratory experiences, listeners are naturally more interested in sound source properties than in intrinsic properties and therefore use "everyday listening". [START_REF] Schaeffer | Traité des objets musicaux[END_REF] also introduced different types of listening ("hearing", "listening", "comprehending", "understanding") and asserted that when listening to a sound we switch from one type of listening to another. Even if different points of view are used to define the different types of listening, they share the notions of attention direction and intention when perceiving. Abstract sounds might help listeners to focus on intrinsic properties of sound and thus to adopt musical listening. An other aspect that could influence the type of listening and therefore introduce variability in responses is the coexistence of several streams in a sound 5 . If a sound is composed of several streams, listeners might focus on different elements without possible control from the experimenter. Since abstract sounds have no precise meaning to be preserved, it is possible to proceed to transformations that select one stream (and alter the original meaning). This is not the case for environmental sound recordings for instance, since transformations can make them unrecognizable. Note that classification of sounds with several streams according to Schaeffer's typology might be difficult since they present concomitant profiles associated with distinct categories.

Potentials of abstract sounds

As described in section 2, potentials of abstract sounds was initially shown in a musical context. In particular, their ability to evoke various emotions was fully investigated by electroacoustic composers. In this section, we present some applications in which we used abstract sounds in three different research domains, i.e. sound synthesis, cognitive neuroscience and clinical diagnosis. Note that we only aim at giving an overview of some experiments that use abstract sounds, in order to discuss the reasons that motivated their use. Details of the material and methods can be found in the articles referred in the following sections.

Stimuli used in the experiments

The three experiments partially shared the same stimuli. We collected abstract sounds provided by electroacoustic composers. Composers constitute an original resource of interesting sounds since they have thousands of specially recorded or synthesized sounds, organized and indexed to be included in their compositions. From these databases, we selected a set of 200 sounds6 that was representative of the typology proposed by Schaeffer (cf. tab 1). A subset of sounds was then selected according to the needs of each experiment: In [START_REF] Micoulaud-Franchi | Bizzare and familiar recognition troubles of auditory perception in patient with schizophrenia[END_REF] (section 4.1), we chose 20 sounds from the previous set of 200 sounds by a pre-test on seven subjects and selected sounds that best spread in the space of measured variables (the perceptual dimensions). This procedure is similar to experiment design methods, for protocols where stimuli are synthesized. This preselection was validated by a second pre-test on fourteen subjects that produced similar repartition of the sounds along the perceptual dimensions. In [START_REF] Schön | The evocative power of sounds: Conceptual priming between words and nonverbal sounds[END_REF] (section 4.2), only sounds from the nine "balanced" (see section 2) categories were used. Abstract sounds were presented successively to listeners who were asked to write the first words that came to their mind after listening. A large variety of words were given by listeners, for instance a sound obtained the following responses: "dry, wildness, peak, winter, icy, polar, cold". Nevertheless, for most sounds, it was possible to find a word that was accepted as coherent by more than 50% of the listeners. This step allowed us to validate the abstract sounds since no label referring to the actual source was given. In [START_REF] Merer | Semiotics of sounds evoking motions: Categorization and acoustic features[END_REF][START_REF] Merer | Perceptual evaluation of sound source trajectories for synthesis[END_REF] (section 4.3), we selected 40 sounds among the 200 using the procedure described above (pre-test and optimization of the repartition of sounds). We then conducted an evaluation test of the 40 sounds on 29 listeners through 2 questions rated on linear scales:

• "Is the sound source recognizable?" (rated on a non graduated scale from "not recognizable" to "easily recognizable")

• "Is the sound natural?" (rated from "natural" to "synthetic") When the sources were judged "recognizable", listeners were asked to write a few words to describe the source.

We found a correspondence between responses of the two questions: the source is perceived natural as long as it is easily recognized (R=.89). Note that abstract sounds were judged as "synthesized" sounds even if they actually were recordings from vibrating bodies. Obviously, for each experiment, loudness of sounds was equalized and the sounds' onset and offset were smoothed to avoid clicks.

Bizarre and familiar sounds

Abstract sounds are not often heard in our everyday life and could even be completely novel for listeners. Thus abstract sounds might be considered as "strange" or "bizarre". As mentioned above, the judgements of abstract sounds are highly subjective for listeners. In some cases, it is possible to use this subjectivity to investigate some specificities of human perception and in particular, to highlight differences of sound evaluation between groups of listeners. From those considerations, [START_REF] Micoulaud-Franchi | Bizzare and familiar recognition troubles of auditory perception in patient with schizophrenia[END_REF] has explored the perception of bizarre and familiar sounds in patients with schizophrenia. Indeed, the concept of "bizarre" is one important element from standard classification of mental disorders (DSM -IV) for schizophrenia [START_REF] Association | The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV)[END_REF] pp. 275. An other frequently reported element is the existence of auditory hallucinations7 , i.e. perception without stimulation.

The study employed both environmental and abstract sounds to explore individual differences in the judgement of sounds. The procedure was equivalent to semantic differential protocol, which consisted in rating sounds on continuous scales defined by an adjective (by contrast, classical differential semantic uses an adjective and an antonym to define each scale). Sounds were evaluated on six dimensions along linear scales: "familiar", "reassuring", "pleasant", "bizarre", "frightening", "invasive"8 . Preliminary results showed that there was a significant difference between patients with schizophrenia and control groups along the familiar/bizarre dimensions: abstract sounds were judged more familiar by patients than by control subjects. Further analysis and testing (for instance brain imaging techniques) will be conducted in order to better understand these differences and in particular, the tendency of patients with schizophrenia to hyper-familiarize the abstract sounds.

Reduction of linguistic mediation and access to different meanings

Within the domain of cognitive neuroscience, a major issue is to determine whether similar neural networks are involved in the allocation of meaning for language and other sounds. A well-known protocol largely used to investigate semantic processing in language, i.e. the semantic priming paradigm [START_REF] Bentin | Event-related potentials, lexical decision and semantic priming[END_REF], has been applied to other stimuli such as pictures, odors and sounds. One difficulty that occurs when considering non-linguistic stimuli, is the potential effect of linguistic mediation. For instance watching a picture of a bird or listening to the song of a bird might automatically activate the verbal label "bird"). In this case, the conceptual priming cannot be considered as purely non-linguistic because of the implicit naming induced by the stimulus processing. Abstract sounds are interesting candidates to weaken this problem, since they are not easily associated with a source and therefore may reduce the automatic and immediate verbal labelling. However, when listeners are asked to label abstract sounds, we assume that multiple evocations are enabled. In other words, a given abstract sound can be associated with different labels by listeners depending on the task and the context. In [START_REF] Schön | The evocative power of sounds: Conceptual priming between words and nonverbal sounds[END_REF], the goals were to determine how a sense is attributed to a sound and whether there are similarities between sound and word brain processing. For that, a priming protocol was used with word/sound pairs and the degree of congruence between the prime and the target was manipulated. In a first experiment a written word (prime) was visually presented before a sound (target) and subjects had to decide whether or not the sound and the word fit together. In a second experiment, presentation order was reversed (i.e. sound presented before word). Results showed that participants were able to evaluate the semiotic relation between the prime and the target in both sound-word and word-sound presentations with relatively low inter-subject variability and good consistency (see [START_REF] Schön | The evocative power of sounds: Conceptual priming between words and nonverbal sounds[END_REF] for details on experimental data and related analysis). This result indicated that abstract sounds are suitable for studying conceptual processing. Moreover, their contextualization by the presentation of a word reduced the variability of interpretations and led to a consensus between listeners. The study also revealed similar electrophysiological patterns on both abstract sounds and word targets, supporting the assumption that similar processing is involved for sounds and words.

Sound synthesis

Intuitive control of synthesizers through high-level parameters is still an open problem in virtual reality and sound design. Both in industrial and musical contexts, the challenge consists of creating sounds from a semantic description of their perceptual correlates. For this purpose, we proposed a general methodology based on evaluation and analysis of abstract sounds. Indeed, as discussed formerly, abstract sounds can be rich from an acoustic point of view and allow testing different spectro-temporal characteristics at the same time. Thus they might help to identify general signal properties that might be valid for different categories of sounds. They are particularly designed for restitution through speakers (as this is the case for synthesizers). Given a set of desired control parameters and a set of sounds, the proposed method consists of asking listeners to evaluate the sounds on scales defined by control parameters. Sounds with same/different values on a scale are then analyzed in order to identify signal correlates. Finally, using feature based synthesis [START_REF] Hoffman | Feature-based synthesis: Mapping acoustic and perceptual features onot synthesis parameters[END_REF], signal transformations are defined which may correspond to targeted control parameters.

In [START_REF] Merer | Semiotics of sounds evoking motions: Categorization and acoustic features[END_REF], we addressed the control of perceived movement evoked by monophonic sounds. Note that in the case of movement, we are aware that the recognition of the physical sound source can introduce a bias in the evaluation. If the source can be easily identified, the corresponding movement is more likely to be established: a car sound only evokes horizontal movement and cannot fall or go up. We conducted a free categorization task asking subjects to group sounds that evoke a similar movement and to label each category. This method aims at identifying sound categories and represents therefore a first step towards the identification of perceptually relevant sound parameters. Based on subjects' responses, we identified six main categories of perceived movements: "rotate", "fall down", "approach", "pass by", "go away"and "go up"and determined a set of sounds representative of each category. This study is still in progress and continues in [START_REF] Merer | Perceptual evaluation of sound source trajectories for synthesis[END_REF], where we improve the perceptual characterization of movements through a new methodology. Movements are evaluated with a drawing interface that allows non-exclusive categorization (sounds can rotate and go-up at the same time) and where drawing parameters will correspond to control parameters of the synthesizer. Preliminary results showed that it is possible to determine the relevant control parameters and the needed precision. Abstract sounds allowed us to test a wide variety of acoustic characteristic that may be responsible of the perception of movement.

Conclusion

In this paper, we have discussed the problem of sound stimulus selection and interaction between information they convey and information that actually is of interest in an experiment. A review of studies that used abstract sounds has clarified the underlying notions. Abstract sounds can be considered as "unrecognizable", "synthetic" and "bizarre" depending on context and task. We also discussed that several meanings can be attributed to a same abstract sound and might be used either to orient the type of listening adopted by listeners or to study individual differences. Several arguments in favor of the use of abstract sounds are still to be validated. In particular the reduction of interaction between source recognition and dimension measured in a listening test should be addressed in further research. The richness of signal characteristics of abstract sounds and the thoughts on recognition of sound sources might open some perspectives in sound perception studies. For instance abstract sounds might help in proposing a general categorization of sounds for research purpose. Indeed, it is known that categorization is supposed to help reducing the complexity of a problem. In auditory perception research there are implicit categories of sounds that distinguish different domains (verbal/non verbal sounds, environmental sounds, music). Therefore, it is possible to propose a categorization based on information available in sounds with a particular emphasis on the context of use. For instance, it might be interesting to consider different levels of accordance between listeners, in tasks where they are told to label sounds. Such levels might help to define continuous distinctions of verbal and non-verbal sounds.

Another perspective is the constitution of a database regrouping both results from different perceptual studies and signal analysis. Such database might facilitate the setting up of listening tests and increase efficiency of research as proposed in the case of music application [START_REF] Mckay | A large publicly accessible prototype audio database for music research[END_REF]. Abstract sounds are used in musical context, in cinema and in other audiovisual productions. Their signal characteristics can be closer to environmental sounds than to sounds produced by classical musical instruments. This indicates that abstract sounds can be considered as musical as well as environmental sounds.

Fig. 1 .

 1 Fig. 1.Schaeffer's typology. Note that some columns are redundant since the tab must be read from center to borders. For instance, "No evolution" in the right part of the tab correspond to endless iterations whereas "No evolution" in the left part correspond to sustained sound (with no amplitude variations). Translation from[START_REF] Dack | Translating pierre schaeffer: Symbolism, literature and music[END_REF]

Definitions from[START_REF]Oxford Advanced Learner's Dictionary[END_REF]

The term "concrete" is related to composition method which is based on concrete material i.e recorded or synthesized sound, in opposition with "abstract" music which is composed in an abstract manner i.e from ideas written on score, and become "concrete" afterwards.

Translation by J.Dack available at http://www.ears.dmu.ac.uk/spip.php?page= articleEars&id_article=3597

As discussed in[START_REF] Dack | Translating pierre schaeffer: Symbolism, literature and music[END_REF] even if facture is not a common English word, there is no better translation from French

Auditory streams have been introduced by Bregman[START_REF] Bregman | Auditory Scene Analysis[END_REF], and describe our ability to group/separate different elements of a sound

Some examples from[START_REF] Merer | Semiotics of sounds evoking motions: Categorization and acoustic features[END_REF] are available at http://www.sensons.cnrs-mrs.fr/ CMMR07_semiotique/

"[...] auditory hallucinations are by far the most common and characteristic of Schizophrenia."[START_REF] Association | The Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV)[END_REF] pp. 275

These are arguable translations from French adjectives: familier, rassurant, plaisant, bizarre, angoissant, envahissant