Olivier Christmann
email: olivier.christmann@loria.fr

Noëlle Carbonell
email: noelle.carbonell@loria.fr

Browsing through 3D Representations of Unstructured Picture Collections: an Empirical Study

Keywords: H.4.3 [Communications Applications]: Information browsers H.5.2 [User Interfaces]: Graphical user interfaces. I.3.6 [Methodology and Techniques]: Ergonomics, Interaction techniques Design, Experimentation, Human Factors 3D visualization, immersive virtual reality, manipulation of 3D objects, picture browsing, photograph viewers, usability studies

The paper presents a 3D interactive representation of fairly large picture collections which facilitates browsing through unstructured sets of icons or pictures. Implementation of this representation implies choosing between two visualization strategies: users may either manipulate the view (OV) or be immersed in it (IV). The paper first presents this representation, then describes an empirical study (17 participants) aimed at assessing the utility and usability of each view. Subjective judgements in questionnaires and debriefings were varied: 7 participants preferred the IV view, 4 the OV one, and 6 could not choose between the two. Visual acuity and visual exploration strategies seem to have exerted a greater influence on participants' preferences than task performance or feeling of immersion.

CONTEXT AND MOTIVATION

Entertainment and commercial Web-sites, information kiosks and public terminals tend to display a growing number of pictures simultaneously: video and movie stills, CD sleeves, book covers, etc. Personal electronic archives and directories are increasingly cluttered with collections of photographs, scanned documents, videos. It is a standard practice for designers of picture browsers, to display visual information items grouped in 2D arrays that users browse through, using horizontal and vertical scrollbars. Current products (e.g., ACDSee, PhotoSuite or ThumbsPlus) make general use of scrollable 2D arrays of file icons or miniatures for displaying folder contents. Research prototypes of multimedia news summaries [START_REF] Maybury | News on demand: Introduction[END_REF] or "zoomable" image browsers [START_REF] Bederson | PhotoMesa: a Zoomable Image Browser Using Quantum Treemaps and Bubblemaps[END_REF] also use 2D array presentations. However, browsing through 2D rectangular representations of fairly large collections of 2D icons or pictures (in the form of miniatures) is slow and tiring. In particular, skimming through a whole collection involves two tedious gestural actions which may divert visual attention from the collection, namely, careful horizontal and vertical mouse drags along narrow scrollbars. In addition, searching for a specific item often makes it necessary to go over the whole collection several times, hence to reverse the direction of mouse moves whenever reaching a border of the representation. Such changes of direction may increase users' workload and decrease scanning performances, thus reducing the usability and efficiency of interactive standard 2D representations of graphical objects.

We propose a 3D representation meant to facilitate browsing through fairly large unstructured collections of graphical objects displayed in the form of icons or miniatures. To implement this generic representation, interface designers may choose between two interaction or navigation metaphors: direct manipulation of a virtual 3D object representing the whole collection, and interaction with 2D representations of the collection items; or, immersion in a virtual 3D environment representing the whole collection, and interaction with this virtual space and the 2D objects that represent the collection items in it.

The empirical study reported here attempts to compare the efficiency and usability of these two interactive visualization paradigms with a view to providing picture browser developers with effective design recommendations. The next section presents the 3D generic representation we propose for visualizing, and interacting with, collections of graphical items. The following sections describe the empirical study we performed in order to assess the respective efficiency and usability of the two interaction metaphors available to Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. AVI'06, May, 2006, Venice, Italy designers for implementing this representation in actual user interfaces.

GENERIC 3D REPRESENTATION 2.1 Description

The proposed representation seems most appropriate for browsing collections including a number of graphical objects in the region of one thousand, on standard PC displays (19'' or 21'' screen size). Graphical objects in collections of this size should be in the form of icons or miniatures.

For any such collection, our proposal is to arrange graphical items in rows and columns and to plaster them on the inner or outer wall of a vertical cylinder; see figure 1. Rotation of the cylinder to the left or to the right is activated by means of two arrows at the bottom of the display. Therefore, repeated surveys of the entire collection are performed without changing the direction of the cylinder motion. Clicking on one arrow launches the rotation; clicking on the right button of the mouse stops it. Rotation speed can be controlled by means of the mouse scroll wheel. Two other arrows are available for adjusting the apparent distance between the user and the virtual cylinder to their visual capabilities. One sixth of the cylinder lateral surface only is displayed at once, in order to reduce the negative effects of perspective distortions on visual search without loosing the useful focusing effects of such distortions, namely, enlargement of pictures in the lateral columns for the inner view of the cylinder, and in the central column(s) for the outer view. Standard zooming facilities (fixed size of the zoomed image) are available for enlarging icons or miniatures.

Figure 1. Inner and outer views (same number of photos).

A vertical "slit" in the cylinder (i.e., an empty column) helps users to keep track of navigation progress; they may use it as a full turn completion indicator. Additionally, at the bottom of the display, a 3D miniature cylinder with a vertical slit reproduces the motion of the representation of the collection. The current position of the column displayed at the centre of the screen is materialized on the miniature cylinder by a red vertical line. The motions of both cylinders are synchronised, thus providing users with continuous navigation feedback.

Comments

The inner and outer cylindrical representations of a collection are not equivalent; they differ from each other in their visual properties and their possible influence on users' subjective judgements. See figure 1. Firstly, for the same number of displayed rows and columns, the size of items in the inner view increases from the centre of the display towards its right and left boundaries, while it decreases from the centre towards side boundaries in the outer view. In addition, enlarged items are less distorted in the inner view than in the outer view. These differences may influence users' visual exploration strategies.

Secondly, the convexity of the outer view may induce users to view interaction with this view as manipulation of virtual objects, while the concavity of the inner view will make them feel immersed in the virtual representation space. This difference in system image is likely to influence subjective judgements.

The diversity and scientific interest of the ergonomic issues stemming from these differences are the main research motivations underlying the study presented in the following sections. The simplicity of the proposed generic representation and its originality also count among our motivations. Design simplicity facilitates software implementation and ergonomic evaluation. As for simplicity of usage, it is a major asset for ensuring the success of a software product intended for the general public. Originality here refers to the absence of published research on this representation. Up to now, research on visualization techniques has mainly focused on interactive 2D representations of very large structured data sets, such as hyperbolic trees or treemaps [START_REF] Pirolli | The Effect of Information Scent on Searching Information Visualizations of Large Tree Structures[END_REF]. In addition, the use of such techniques for visualizing collections of graphical objects has motivated but a few studies, such as [START_REF] Bederson | PhotoMesa: a Zoomable Image Browser Using Quantum Treemaps and Bubblemaps[END_REF] which reports the use of treemaps for presenting photographs. A few 3D visualization techniques only have been proposed. Cone trees [2] are meant for representing structured data while Card's 3D wall [3] can be used for presenting unstructured and structured sets of textual, digital or graphical items. It is composed of a front wall parallel to the screen and flanked by two side walls. This 3D representation has obvious similarities with the outer view of the representation we propose. However, it is devoid of the advantages provided by a cylindrical representation with a current position indicator.

EVALUATION PROCEDURE 3.1 Design and implementation of both views

The representation described in the preceding section has been implemented as follows. The chosen application domain was photograph browsing. We used colour photographs pulled from popular Web sites. Collections included 869 photographs, arranged along 11 rows and 79 columns (80 with the empty column). 14 columns or so were visible at once. Maximum rotation speed was 7 times the default minimum speed. The size of zoomed images was 300 x 225 pixels on a screen with a 1280 x 1024 pixel resolution. Computer displays were projected on a wall screen (size: 1.00 x 0.75 m; distance between participant and screen: 2.20 m). Interaction facilities with the inner (IV) and outer (OV) views were identical; see figure 1. The virtual reality environment AReVi was used for all software developments.

Tasks

Participants performed two types of visual search tasks which are frequently used while browsing through collections of pictures:

-T1: looking for pictures matching a set of criteria; these pictures and their locations in the collection are unknown. -T2: looking for a visually familiar picture; the location of the picture in the collection may be known or unknown. Participants performed 10 tasks of each type for each of the two views. All T1 tasks (20) were carried out first, and processing order was counterbalanced between participants as follows:

-T1[IV then OV], T2[IV then OV] for half the participants, -T1[OV then IV], T2[OV then IV] for the other half. Each T1 task series was preceded by a short training (2 tasks). To achieve a T1 task scenario, participants had to search for and select a unique target photograph in the currently displayed collection, using an unambiguous written description of the target's topic, spatial layout, dominant colours and specific details. The structure and information content of target descriptions were standardized in order to homogenize task difficulty and help users to develop efficient reading and searching routines. Following target selection, the system demonstrated the correct solution, so that all participants had the opportunity to watch the target at least once. Each T2 task consisted in first displaying one of the targets used for T1 at the centre of the screen for three seconds. Then, participants started searching for it on the 3D cylinder and, when they had found it, selected it. T1 targets were visually familiar to participants since they had already seen them once or twice during T1. Many participants remembered the locations of some targets more or less precisely, especially the row they belonged to. Search time was bounded (3 min.) for both tasks.

Participants

17 experienced computer users, 7 female and 10 male graduate or PhD students, participated in the experiment. The number of participants being rather small, we chose a narrow age range (from 21 to 30 years) in order to limit the heterogeneity of the group regarding task execution performance, interests and preferences. All participants had normal sight according to the Bioptor test kit (Stereo Optical Company, Inc.

Data collection

Participants' interactions with both views were logged up. In addition, after the experimentation, all participants filled in a questionnaire meant to elicit their reactions, subjective judgements, and comments, and each of them participated in a debriefing interview.

FIRST RESULTS

Participants' preferences and performances are detailed in table 1. Preferences have been established from questionnaires and debriefing transcripts. Performance measures, computed from participants' logs for T1 and T2 tasks, include: the number of search failures for each view, IV and OV, the difference between IV and OV search times, and the number of cylinder rotations for each view.

The order in which participants interacted with the two views had no significant influence on their performances. Due to a technical incident, data from participant 8 have been ignored. Some participants expressed judgements during the debriefings that differed slightly from those in questionnaires; both judgements have been reported in table 1 According to the debriefings, all participants save one judged both 3D views positively, and preferred them to 2D representations. Contrastingly, their attitudes towards each of the 3D views disagreed. Seven participants preferred the inner view, four favoured the outer view, and six could not choose between the two views; the last group includes participants who expressed different preferences in the questionnaire and during the debriefing. A first tentative conclusion can be drawn from the diversity of participants' preferences: in order to meet with general user acceptance, the inner and outer views of the proposed representation should be both implemented in picture browsers.

Participants' performances, questionnaires and debriefings were further analysed so as to gain an insight into the factors that may have influenced their subjective judgements. To refine the three user profiles detected, we analysed the results of the vision tests, and elicited participants' visual exploration strategies using debriefings and interaction logs (especially the positions of zoomed pictures on the display). Our aim was to determine whether participants' preferences had been influenced by the specific visual properties of each view, mainly the location of enlarged 1 pictures which varies from one view to the other (see subsection 2.2). Participants were found to use two main visual exploration strategies: they focused their gaze either on incoming columns (In strategy) or on the central column(s) of the display (C strategy). Synthesized data are presented in table 2, grouped by user profile; GI designates the group of participants who preferred the inner view, GO the participants who liked the outer view better, and GH those who hesitated between the two views. Task performance may have influenced the attitudes of some participants. However, visual acuity and exploration strategies are factors which seem to have exerted a greater influence on preferences. 5 participants (out of 7) in the GI group had normal visual acuity while 3 (out of 4) participants in the GO group had lower visual acuity. Similarly, 5 GI participants used the In strategy consistently versus only one in the GO group.

1 Due to 3D perspective rules.

Contrastingly, performances in both groups vary from one type of tasks to the other, and from one criterion to another. Participant 17 in the GI group even achieved better performances using the OV view with respect to all criteria. As expected, data from participants in the GH group show a much greater diversity. Interestingly, the immersive feature of the IV view seems to have exerted a limited influence on judgements. Four participants only alluded to it during the debriefings: S9 in the GO group mentioned it as a negative feature, saying that she felt ill at ease when using the IV view, while S5 and S11 in the GI group and S18 in the GH group referred to it as an attractive feature. Designers of adaptable or adaptive browsers of graphical objects could take advantage of these findings. The effectiveness of static and dynamic user models could be greatly improved by taking these factors into account in their design.

CONCLUSION

We have presented an empirical study meant to assess the utility and usability of a generic cylindrical 3D representation of fairly large unstructured sets of graphical objects. This representation is meant to facilitate browsing. Two metaphors are available for designing interaction with this representation: users may either manipulate the cylinder (outer view) or feel immersed in it (inner view). 17 participants interacted with both views successively. 7 participants preferred the inner view, 4 the outer one, and 6 could not decide between the two. Analysis of performances, satisfaction questionnaires and debriefings suggests that participants' preferences were less influenced by task performance and subjective perceptual 3D effects than by visual acuity and visual exploration strategy. Participants who had normal visual acuity and focused their visual attention on incoming columns seem to have been biased towards the inner view, while participants who had lower visual acuity and focused their gaze on columns at the centre of the display may have been biased towards the outer view. 4 participants only mentioned the feeling of immersion induced by the inner view to explain their subjective judgements; the specific visual characteristics of each view, especially picture size and distortion, location of enlarged pictures (in the centre or on the sides of the display) were alluded to much more often for justifying preferences.

We are currently preparing an experimental study meant to compare the usability of both 3D views with standard 2D displays for carrying out T1 and T2 type tasks. As visual exploration strategy seems to have a significant influence on user preferences, the eye movements of a few participants will be recorded and analysed.

Table 1 . Participants' preferences and performances.

 1 .

	Part.: Participant numbers.
	Pref.: Preferences from questionnaires (Q) and debriefings (D) for
	the inner (I) or outer (O) view. "<" denotes a weak preference,
	and "H" hesitation between the two views.
	Diff. Err.: Differences* in error numbers between IV and OV.
	Diff. T: Differences* in cumulated task completion times between
	IV and OV (percentages being computed over the shortest time).
	Diff. Rot.: Differences* in numbers of full rotations between IV
	and OV.
	* i.e., OV_value -IV_value.
	Part.	Q	Pref.	D	Diff. Err. T1 T2	Diff. T. % T1 T2	Diff.. Rot. T1 T2
	1	<I		-	1	-	38	13	0	-
	2	<O	H	-1	0	-55	39	1	3
	3	H		I	4	-1	33	-29	2	-5
	4	H		O	1	0	-5	32	0	1
	5	<I		I	0	0	19	-41	0	-1
	6	<<I	<I	-1	1	-5	7	2	3
	7	O		-	3	-1	57	-60	1	-7
	9	O		-	0	0	-19	-9	0	-4
	10	O		-	0	0	-11	-26	-3	-3
	11	I		-	1	2	-11	147	-3	1
	12	H		-	1	0	74	66	0	1
	13	H		-	1	0	22	-29	-4	-6
	14	<I		I	3	0	83	52	0	2
	15	<I		I	2	0	51	6	2	-3
	16	O		-	0	0	-23	78	-8	2
	17	<I		-	-1	-2	-51	-15	-5	-3
	18	<O	H	2	0	104	12	1	-1

Table 2 . Performances and visual exploration strategies of participants grouped by preferences, GI, GO, GH.

 2

	Vis. Ac.: Visual acuity; normal (N), low (L).
	I, O: inner (O) and outer (I) views.
	Err., T., Rot.: View (I or O) for which participants achieved best
	results regarding error number (Err.), task completion time (T),
	number of full rotations (Rot), respectively.
	Strat. (T1): Visual exploration strategies for T1 tasks.
	In, C: Observation of the display focused on incoming/central
	column(s).

Table 2 .1. GI group

 2

	Participant	1	5	6	11	14	15	17
	Vis. Ac.	N	N	L	N	N	N	L
	Err.	T1 T2	--	--	--	-I	I -	I -	-O
	T.	T1 T2	I I	I O	--	O I	I I	I -	O O
	Rot.	T1 T2	--	--	I I	O -	-I	I O	O O
	Strat.	IV	In	In	In	In	C	C	In
	(T1)	OV	In	In	In	In	C	C	In

Table 2 .2. GO group and GH group

 2

	Participant	7	GO group 9 10 16	2	3	GH group 4 12 13 18
	Vis. Ac.	N	L	L	L	N	N	L	N	N	N
	Err.	T1 T2	I -	--	--	--	--	I -	--	--	--	I -
	T.	T1 T2	I O	O -	O O	O I	O I	I O	-I	I I	I O	I I
	Rot.	T1 T2	-O	-O	O O	O I	-I	I O	--	-I	O O	--
	Strat.	IV	?	?	In	C	C	In	?	?	In	In
	(T1)	OV	C	C	In	In	?	C	?	?	C	In


Matteo Grella
email: matteogrella@gmail.com

Taking Advantage of BiLSTM Encoding to Handle Punctuation in Dependency Parsing: A Brief Idea

In the context of the bidirectional-LSTMs neural parser (Kiperwasser and Goldberg, 2016), an idea is proposed to initialize the parsing state without punctuation-tokens but using them for the BiLSTM sentence encoding. The relevant information brought by the punctuation-tokens should be implicitly learned using the errors of the recurrent contributions only.

I assume the reader is familiar with the formal framework of transitionbased dependency parsing originally introduced by Nivre (2003) [START_REF] Nivre | An efficient algorithm for projective dependency parsing[END_REF] and the most recent deep-learning techniques applied to the natural language processing; see [START_REF] Goldberg | Neural network methods for natural language processing[END_REF] [START_REF] Goldberg | Neural network methods for natural language processing[END_REF] for an introduction.

Traditional data-driven transition-based dependency parsers analyze punctuation as words. Experimental results showed that parsing accuracy drops on sentences which contain higher ratios of punctuation. The problem is that punctuation is not as consistently annotated in tree-banks as words, and this makes the learning and therefore the parsing processes harder.

Indeed, certain aspects of punctuation are merely stylistic, and punctuation dependencies are a bit hard to defend compared to word-to-word dependency, so that in the standard CoNLL evaluation the arcs leading to the punctuation tokens just do not count.

However, it is out of the question that punctuation (especially commas) plays an essential role in the syntactic analysis as it is vital to resolve many attachment ambiguities. Furthermore, it can completely alter the meaning of a sentence.

In the context of dependency parsing, Ma et al. (2014) [START_REF] Ma | Punctuation processing for projective dependency parsing[END_REF] suggest treating punctuation as properties of its neighboring words, without involving the punctuation itself directly in any attachment. In their model, before parsing starts, a pre-processing step is used to first set the punctuation marks as properties of their neighboring words, and then remove them from the sentence. During parsing, each transition that creates a dependency relation causes these properties to be propagated from a token to its governor, producing additional features to guide the parser to build the dependency graph. 1 and improves parsing performance of about 0.90% UAS over a greedy baseline parser on the English Penn Tree-bank. [START_REF] Grella | Notes about a more aware dependency parser[END_REF] [START_REF] Grella | Notes about a more aware dependency parser[END_REF] obtained comparable results for the Italian language.

Ma et al. show that their method reduces errors-propagation

I think that a similar strategy could be implemented efficiently in the scheme for dependency parsing based on bidirectional-LSTMs proposed by [START_REF] Kiperwasser | Simple and accurate dependency parsing using bidirectional LSTM feature representations[END_REF] [START_REF] Kiperwasser | Simple and accurate dependency parsing using bidirectional LSTM feature representations[END_REF]. In that model, each sentence token is associated with a BiLSTM vector representing the token with its surrounding context. The next best transition is obtained by using a multi-layer perceptron whose input features are constructed by concatenating a few of these BiLSTM vectors, based on the current configuration of the parsing state.

The gist of my idea is to initialize the parsing state without punctuationtokens but using them for the BiLSTM sentence encoding. 2 In this way, the punctuation will never be directly involved in either the features construction or any attachment, but since the BiLSTM is trained jointly with the parserobjective, the relevant information brought by the punctuation-tokens should be implicitly learned using the errors of the recurrent contributions only.

Besides, it might be possible to let the BiLSTM indicate the best head of a punctuation-token, using a sort of attention mechanism to evaluate on which previous/next word it had the overall strongest influence. 3Future thoughts can be made to exploit this idea to perform parsing and punctuation prediction jointly.

I encourage researchers to try to implement this technique inside the opensource BIST-parsers. 4

A wrong attachment in punctuation may require non-projective transitions to continue the analysis correctly. Even for transition systems that support non-projective, crossing-links remain more subject to errors due to their relative rarity.

In the arc-standard model the initial parsing state would be an empty stack and the input buffer filled with all the sentence tokens except for punctuation marks. The oracle shall consider that some tokens will never be reduced.

It might make sense to replace the LSTM[START_REF] Hochreiter | Long short-term memory[END_REF] with the RAN[START_REF] Lee | Recurrent additive networks[END_REF] to take advantage of its highly interpretable outputs.

https://github.com/elikip/bist-parser


Frédéric Chaux

Xenie Johnson

Pascaline Auroy

Audrey Beyly-Adriano

Isabelle Te

Stéphan Cuiné

Dr Gilles Peltier
email: gilles.peltier@cea.fr

PGRL1 and LHCSR3 Compensate for Each Other in Controlling Photosynthesis and Avoiding Photosystem I Photoinhibition during High Light Acclimation of Chlamydomonas Cells

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Dear Editor,

In natural environments, photosynthetic organisms experience frequent changes in the light supply, requiring modulation of light harvesting and electron transfer reactions to avoid mismatch between light conversion and metabolic reactions which may result in the production of harmful reactive oxygen species. They have thus evolved photoprotective regulatory mechanisms including the dissipation of excess energy (or non-photochemical quenching, NPQ), which relies on specific light harvesting antennae, such as PSBS and Light-Harvesting Complex Stress-Related 3 (LHCSR3) [START_REF] Peers | An ancient light-harvesting protein is critical for the regulation of algal photosynthesis[END_REF], and is triggered by low pH in the thylakoid lumen. The photosynthetic control, which consists of a down-regulation of electron flow between PSII and PSI by the slowing effect of trans-thylakoidal proton gradient on cytochrome b 6 f electron transfer, also contributes to photoprotection [START_REF] Suorsa | PROTON GRADIENT REGULATION5 is essential for proper acclimation of Arabidopsis photosystem I to naturally and artificially fluctuating light conditions[END_REF]. Cyclic electron flow around PSI (CEF), that involves Proton Gradient Regulation 5 (PGR5) and PGR5-Like Photosynthetic Phenotype 1 (PGRL1), generates a component of the proton gradient critical for the induction of both NPQ [START_REF] Munekage | PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis[END_REF] and photosynthetic control [START_REF] Suorsa | PROTON GRADIENT REGULATION5 is essential for proper acclimation of Arabidopsis photosystem I to naturally and artificially fluctuating light conditions[END_REF][START_REF] Tolleter | Control of hydrogen photoproduction by the proton gradient generated by cyclic electron flow in Chlamydomonas reinhardtii[END_REF]. In the present study, we questioned whether NPQ strictly depends on CEF, and whether these two mechanisms may work independently. We also explored to what extent these regulatory mechanisms could limit the maximal capacity of CO 2 assimilation and biomass productivity by excessively down-regulating the photosynthetic electron flow, as it was recently reported that biomass productivity was increased in the C. reinhardtii npq4 mutant [START_REF] Berteotti | Increased biomass productivity in green algae by tuning non-photochemical quenching[END_REF]).

To answer these questions, we grew C. reinhardtii pgrl, npq4 and pgrl1npq4 mutant strains in controlled photobioreactors (PBRs) operated as turbidostats and measured CO 2 exchange rates and the specific growth rate during a transient from non-saturating light to saturating light under non-limiting CO 2 conditions. In this cultivation mode, the cell density of the culture is maintained constant by dilution with fresh medium, thus ensuring a wellcontrolled light supply. CO 2 -enriched air was bubbled at a constant flow rate and CO 2 concentration was measured in the outflow to determine CO 2 uptake rates (Supplemental Figure S1 A,B). Based on CO 2 assimilation saturation curves (Supplemental Figure S1C) we selected two light intensities, a non-saturating low light (LL) level (40 µmol photons m -2 s -1 ) and a saturating high light (HL) level (200 µmol photons m -2 s -1 ) to further investigate on the different C. reinhardtii strains during a LL to HL switch (Figure 1A,B). No significant difference in CO 2 uptake rates was observed between the four genotypes under LL. Upon transition to HL, CO 2 uptake rates increased in a similar manner in the WT and in single mutants, but dramatically decreased after 2 hours HL in the double pgrl1npq4 mutant. The CO 2 uptake rate thereafter increased in the double mutant and displayed an oscillatory mode indicating the involvement of long-term acclimation mechanisms (Supplemental Figure S2). Similar effects were observed on specific growth rates (Supplemental Figure S3). All four strains showed similar growth rate increases during transition to a lower, non-saturating light intensity (Supplemental Figure S4). The strong inhibition of CO 2 uptake observed in the C. reinhardtii double pgrl1npq4 mutant in comparison to the WT upon HL exposure, and the absence of inhibition in single mutants (pgrl1 or npq4), show that PGRL1 and LHCSR3 can compensate for each other in the acclimation of photosynthesis to saturating light. This result obtained in non-limiting CO 2 and light conditions is in line with a recent report showing a stronger growth delay in the double mutant pgrl1npq4 in comparison to single mutants under mixotrophic conditions [START_REF] Kukuczka | Proton Gradient Regulation5-Like1-mediated cyclic electron flow is crucial for acclimation to anoxia and complementary to nonphotochemical quenching in stress adaptation[END_REF], but contrasts with experiments performed under CO 2 limitation, in which a strong PSI inhibition was observed in both pgrl1 [START_REF] Dang | Combined increases in mitochondrial cooperation and oxygen photoreduction compensate for deficiency in cyclic electron flow in Chlamydomonas reinhardtii[END_REF] and pgrl1npq4 [START_REF] Bergner | STATE TRANSITION7-dependent phosphorylation is modulated by changing environmental conditions, and Its absence triggers remodeling of photosynthetic protein complexes[END_REF] mutants. This suggests that the functional redundancy between PGRL1 and LHCSR3 may be modulated by carbon avaibility. PSII and PSI measurements were then performed to determine at which level photosynthesis is affected in the double mutant. The maximum PSII yield (ΦPSIImax) measured by chlorophyll fluorescence remained almost constant in the WT, pgrl1 and npq4, while a slight decrease was observed in pgrl1npq4 (Figure 1C). As expected, NPQ was slightly lower in pgrl1 than in the WT, but much lower in npq4 and in pgrl1npq4 mutants (Supplemental Figure S6). PSI photochemistry, measured as the maximum oxidizable P700, was slightly affected in the WT and in single pgrl1 and npq4 mutants, but severely dropped in the double mutant after 4 hours of HL (Figure 1D). Interestingly, the decrease in PSI lagged 1-2 hours after the drop in CO 2 uptake (Figure 1G), indicating that PSI inhibition was not the cause, but rather the consequence of the CO 2 uptake drop. The decrease in PSI activity was not accompanied by a decrease in PsaA subunit amounts even after 24h (Supplemental Figure S5), indicating that PSI inhibition in pgrl1npq4 results from an acceptor side limitation rather than from PSI degradation [START_REF] Chaux | A security network in PSI photoprotection: regulation of photosynthetic control, NPQ and O2 photoreduction by cyclic electron flow[END_REF]. This effect therefore sharply contrasts with previous observations performed under limiting CO 2 conditions, in which the PSI activity decrease observed in pgrl1 or pgrl1npq4 mutants was accompanied by a decrease in PSI subunit amounts [START_REF] Bergner | STATE TRANSITION7-dependent phosphorylation is modulated by changing environmental conditions, and Its absence triggers remodeling of photosynthetic protein complexes[END_REF][START_REF] Dang | Combined increases in mitochondrial cooperation and oxygen photoreduction compensate for deficiency in cyclic electron flow in Chlamydomonas reinhardtii[END_REF]. LHCSR3 was not detected under LL in the WT but transiently accumulated upon exposure to high light, peaking at around 2-4 hours of light exposure (Figure 1E). In pgrl1, LHCSR3 was detected in LL and accumulated in HL to much higher amounts than in the WT. Since the NPQ intensity has been correlated to LHCSR3 accumulation [START_REF] Bonente | Acclimation of Chlamydomonas reinhardtii to different growth irradiances[END_REF], it seems likely that higher LHCSR3 amounts accumulating in pgrl1 contribute to compensate for the lack of cyclic electron flow. We therefore conclude that LHCSR3 is active in pgrl1 despite the absence of a proton gradient component generated by cyclic electron flow. The water-water cycle, which is enhanced in pgrl1 and pgr5 C. reinhardtii mutants [START_REF] Dang | Combined increases in mitochondrial cooperation and oxygen photoreduction compensate for deficiency in cyclic electron flow in Chlamydomonas reinhardtii[END_REF][START_REF] Johnson | Proton gradient regulation 5-mediated cyclic electron flow under ATP-or redox-limited conditions: a study of DeltaATPase pgr5 and DeltarbcL pgr5 mutants in the green alga Chlamydomonas reinhardtii[END_REF] may contribute to generate the extra proton gradient needed for the induction of NPQ. On the other hand, PGRL1 accumulated to similar levels in the WT and npq4 both at LL and HL (Figure 1F), indicating that absence of LHCSR3 is not compensated by increased amounts of PGRL1.

Since the rate of PGRL1-CEF is regulated in vivo by the redox poise of molecular intermediates (Alric, 2010), it seems likely that up-regulation of CEF activity compensates for the deficiency of thermal dissipation in npq4 by increasing the photosynthetic control.

In order to better understand the HL-induced inhibition observed on the double mutant, the different strains were then exposed to a slow and gradual (30 hours) increase in light intensity (from 40 to 200 µmol photons m -2 s -1 ). A strong decrease of CO 2 assimilation was observed in the double mutant, but only when the light intensity reached the saturation level (Supplemental Figure S7). This experiment shows that PSI inhibition in the double mutant does not result from a HL stress effect, but rather results from an inability of the double mutant to maintain a maximal level of photosynthesis under saturating light. Neither the CO 2 uptake rate nor the biomass productivity was improved in pgrl1 and npq4 mutants in comparison to the WT, showing that none of these mechanisms actually limits biomass productivity under our experimental conditions. This result sharply contrasts with a recent work reporting an increase in biomass productivity in the C. reinhardtii npq4 mutant in comparison to the WT [START_REF] Berteotti | Increased biomass productivity in green algae by tuning non-photochemical quenching[END_REF]. Such a contrast is rather intriguing and may be explained by different experimental conditions and set-ups. For instance, our experiments were performed at constant biomass by operating PBRs in the turbidostat mode, whereas the biomass increase was recorded in batch cultures in [START_REF] Berteotti | Increased biomass productivity in green algae by tuning non-photochemical quenching[END_REF]. Also, our experiments were performed under saturating CO 2 concentration and non-limiting light, resulting in a doubling time around 4-5 hours, while CO 2 supply was likely limiting in [START_REF] Berteotti | Increased biomass productivity in green algae by tuning non-photochemical quenching[END_REF] since air bubbled through PBRs resulted in a much higher doubling time (estimated at around 24 hours).

We conclude from this work that photoprotection can be promoted by LHCSR3 despite the absence of PGRL1 cyclic electron flow, and that PGRL1 and LHCSR3 can work together or independently and compensate for each other during high light acclimation. The absence of both mechanisms neither improves CO 2 uptake nor biomass productivity under non-limiting light and CO 2 , but compromises both CO 2 uptake and biomass productivity.

PGRL1 and/or LHCSR3, by contributing to the regulation of electron flow upstream of PSI, limit the accumulation of electrons on the PSI acceptor side, thus avoiding PSI photoinhibition (Supplemental Figure S8).

SUPPLEMENTAL INFORMATION

Supplemental information is available at Molecular Plant Online.

FUNDINGFFigure 1 .

 1 Figure 1. Time course of CO 2 uptake and specific growth rates measured in Chlamydomonas wild-type cells and in pgrl, npq4 and pgrl1npq4 mutants during a rapid transition from LL to HL. Cells were grown photoautotrophically in PBRs operated as turbidostats in the presence of 1.8% CO 2 in air under non-saturating light (LL, 40 µmol photons m -2 s -1 ). After at least 24 hours stabilization under LL, light intensity was suddenly increased to 200 µmol photons m -2 s -1 (HL). (A) Representative measurements of CO 2 uptake rates performed on a set of 4 PBRs. (B) Means values (± SD) from 3 biological replicates of CO 2 uptake (left panel) and specific growth rates (right panel) measured under LL, and upon 1 hour or 4 hours HL exposure. Differences between strains were not significant (t-test, p>0.05), except for the pgrl1npq4 after 4 hours HL. (C, D, E, F, G) At different time points, aliquots were harvested from PBR cultures for PSII, PSI measurements and immunodetection analysis. (C) Maximal PSII yield was measured after 5 min dark acclimation as (F M -F 0 )/F M . Shown are mean values (± SD, n=3). (D, G) PSI activity was measured as the oxidizable P700 content calculated from light-minus-dark absorbance difference at 705nm in the presence of DCMU and methylviologen. Shown (D) are mean values (± SD, n=3) for LL and after 8 hours HL measurements. (E) LHCSR3 and (F) PGRL1 protein accumulation in C. reinhardtii wild-type and pgrl1, npq4 and pgrl1npq4 mutant strains upon a LL to HL transient. Samples were loaded at equal total proteins amounts based on Coomassie blue staining (Control). (G) CO 2 uptake rates (closed symbols) and maximum oxidizable P700 content (open symbols) measured during a LL to HL transient in three independent pgrl1npq4 strains.

ACKNOWLEDGMENTS

We thank Pierre-Louis Lucas for technical support, Pr. Michael Hippler (IBBP, WWU Munster, Germany) for the kind supply of pgrl1npq4 strain, Dr. Jean Alric for fruitful discussions, and Léo Dubus for improvement of PBR-monitoring software. No conflict of interest declared.

Supplemental Information

Materials and Methods

Strains. C. reinhardtii pgrl1 and npq4 mutants have been previously described [START_REF] Peers | An ancient light-harvesting protein is critical for the regulation of algal photosynthesis[END_REF][START_REF] Tolleter | Control of hydrogen photoproduction by the proton gradient generated by cyclic electron flow in Chlamydomonas reinhardtii[END_REF]. The pgrl1npq4 double mutant was generated by serial backcrosses [START_REF] Kukuczka | Proton Gradient Regulation5-Like1-mediated cyclic electron flow is crucial for acclimation to anoxia and complementary to nonphotochemical quenching in stress adaptation[END_REF]. Reference wild-type strains (WT) CC-124 (mt-nit1 nit2) BV, The Netherlands). CO 2 concentration was measured in the gas outflow by infrared spectroscopy using a CO 2 gas analyzer (LI-840A, LI-COR, USA). An automatically controlled gas chromatography valve (Cheminert C25F, VALCO Instruments Inc. Co., the USA) was used to sequentially analyze the CO 2 concentration of the gas outflow of each PBR. CO 2 measurements were performed on a 5 min period for each PBR, so that measurements on a given PBR were performed every 20 min. For accurate CO 2 uptake calculation, one of the PBRs was kept cell-free and served as a CO 2 control for the three other PBRs. CO 2 uptake rates (µmol CO 2 h -1 L -1 ) were calculated as:

where V PBR is the volume of the PBR (1 L), [CO 2 ] is the molar content (ppm) of carbon in the gas from the reference PBR ( NO CELL ) or from the cultured ones ( CELLS ), D V the volumetric flow rate (30 L h -1 ) and v M the molar volume (24.055 L mol -1 at atmospheric pressure and 20°C). Specific growth rates (h -1 ) were calculated as dilution rates (averaged on 10 min period of time) multiplied by the PBR volume (1L).

Chlorophyll fluorescence measurements. Cells grown in PBRs were quickly harvested into a glass cuvette and introduced in pulse amplitude modulated fluorimeter (Dual PAM-100, Walz, Germany) where it was continuously stirred to avoid sedimentation. Detection light is blue and very weak (~10 µmol photons m -2 s -1 ) while actinic light is red and its level (photosynthetically active radiation, PAR) was adjusted as follows. After 10 min in the dark, basal fluorescence (F 0 ) and maximal fluorescence (F M ) were measured in absence of actinic light. Actinic light was switched on to 500 µmol photons m -2 s -1 for 5 minutes (steady state fluorescence, F) and a saturating flash (6,000 µmol photons m -2 s -1 during 600 ms) was superimposed every minute (F M '). NPQ values was determined from chlorophyll fluorescence measurements as (F M -F M ')/F M 'as (F M -F M ')/F M ' upon 4 min of high light exposure (490 µmol photons m -2 s -1 ).

Absorption change measurements.

Cell suspensions were harvested from PBRs cultures, centrifuged at 3,000 rpm for 2 min and resuspended in 3 mL 20% Ficoll buffered with 20 mM HEPES (pH 7.2). PSII was inhibited by adding 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) at a 10 µM final concentration. P700 absorption was measured at 705 nm using a JTS-10 (BioLogic, France). Measurements were carried our in the dark to reach a baseline level and under saturating light (1,170 µmol photons m -2 s -1 ) where P700 + was accumulated within 1s and represented the total amount of oxidizable P700. Concentration of oxidizable P700 was calculated from steady-state (~5 s) relative absorbance levels and molar extinction coefficient: (A light -A dark )/( -50 * 2,3 * 5) (Hiyama and Ke, 1972;Joliot and Joliot, 2005). P700 absorption changes were also measured in the presence of the PSI electron acceptor methylviologen (MV) at final concentration 10 mM and in the presence of 10 µM DCMU with similar results similar to that obtained with DCMU alone (not shown).

Immunodetection. Cells were harvested from PBR cultures, centrifuged at 3,200 g for 2 min, frozen in liquid nitrogen then stored at -20°C until use. Protein samples were prepared as in [START_REF] Dang | Combined increases in mitochondrial cooperation and oxygen photoreduction compensate for deficiency in cyclic electron flow in Chlamydomonas reinhardtii[END_REF]) and loaded on one large 10% PAGE Bis-Tris SDS gel buffered with MOPS (pH 7.7). Protein contents were estimated from Coomassie blue staining using an Odyssey IR Imager (LICOR, USA) in order to load equal protein amounts for immunoblot analysis. Proteins were transferred to nitrocellulose membranes by semi-dry transfer (Biorad, CA, USA). Membranes were blocked in milk for at least 30 min and the primary antibody added according to supplier recommendation (Agrisera, Sweden). As an exception, LHCSR3

was detected after dry i-Blot transfer (Thermofischer, MA USA). After 4°C overnight incubation, primary antibody was removed by several rinsing in TBS and a peroxidasecoupled secondary antibody was added for at least 1 hour before detection with a Gbox imaging system (Syngene, UK).

Statistics. Statistical analyses were performed with GraphPad Prism software (La Jolla (CA), USA) using t-test with fewer assumptions (consistent SD not assumed). Outcomes were equal whether data where corrected for multiple comparisons using the Holm-Sidak method (recommended) or not. Error bars on graph account for standard deviation. In Figure 1D, linear regression was drawn for the WT and simple pgrl1 and npq4 mutants whereas sigmoidal function was fitted to pgrl1npq4 dataset. Values were fitted as well to sigmoidal functions for pgrl1npq4 in Figure 1G. ). After at least 24 hours stabilization, the light intensity was increased to 100 µmol photons m -2 s -1 . Specific growth rates were averaged over 10 min, with t0 corresponding to the light increase (dotted vertical line). A B

Supplemental

Supplemental


Suzanne Kieffer
email: suzanne.kieffer@loria.fr

Noëlle Carbonell
email: noelle.carbonell@loria.fr

Oral messages improve visual search

Keywords: H.5.2 [User Interfaces]: Ergonomics, Evaluation/Methodology, Graphical User Interfaces, Natural Language, Voice I/O. I.3.6 [Methodology and Techniques]: Interaction Techniques Performance, Design, Experimentation, Human Factors Visual search. Multimodal system messages. Speech and graphics. Usability study. Experimental evaluation. Visual target spotting

Input multimodality combining speech and hand gestures has motivated numerous usability studies. Contrastingly, issues relating to the design and ergonomic evaluation of multimodal output messages combining speech with visual modalities have not yet been addressed extensively. The experimental study presented here addresses one of these issues. Its aim is to assess the actual efficiency and usability of oral system messages including some brief spatial information for helping users to locate objects on crowded displays rapidly and without effort. Target presentation mode, scene spatial structure and task difficulty were chosen as independent variables. Two conditions were defined: the visual target presentation mode (VP condition) and the multimodal target presentation mode (MP condition). Each participant carried out two blocks of visual search tasks (120 tasks per block, and one block per condition). Scene target presentation mode, scene structure and task difficulty were found to be significant factors. Multimodal target presentation mode proved to be more efficient than visual target presentation. In addition, participants expressed very positive judgments on multimodal target presentations which were preferred to visual presentations by a majority of participants. Besides, the contribution of spatial messages to visual search speed and accuracy was influenced by scene spatial structure and task difficulty. First, messages improved search efficiency to a lesser extent for 2D array layouts than for some other symmetrical layouts, although the use of 2D arrays for displaying pictures is currently prevailing. Second, message usefulness increased with task difficulty. Most of these results are statistically significant.

CONTEXT AND MOTIVATION

Input multimodality combining speech and hand gestures has motivated numerous usability studies. Contrastingly, to our knowledge, issues relating to the design and ergonomic evaluation of multimodal output messages combining speech with visual modalities, mainly 2D or 3D graphics, have not yet been addressed extensively. Until recently, main research efforts have been focused on the implementation of speech either as a substitute for text in the design of multimedia documents, or as a useful alternative (or supplementary) output medium for both blind (or ill-sighted) users and mobile users of PDAs, wearable computers or embedded systems. Speech and graphics appear as useful output modalities. First, speech is the most natural human communication modality. Second, most current interactive applications use graphics as their main output modality. Recent scientific advances in the area of conversational user interfaces [START_REF] Glass | A framework for developing conversational user interfaces[END_REF] are liable to stimulate research aimed at endowing interactive systems with human-like multimodal communication capabilities. In particular, numerous prototypes of human-like embodied conversational agents (ECAs) have been developed, ranging from talking heads to real robots. The main aim of the work presented here is to assess the actual efficiency and usability of speech as a supplementary output modality to standard visual presentations. We performed an experimental study with a view to determining whether oral messages including coarse information on the locations of graphical objects on the current display may facilitate visual search tasks sufficiently for making it worth while to integrate such messages in graphical user interfaces. In addition, we varied display spatial layout in order to test the influence of visual presentation structure on the contribution of these messages to facilitating visual search on crowded displays. Objectives, working hypotheses, methodology and experimental setup are described in the two following sections. Results are then presented and discussed. Conclusions and future work direction are summed up in the last section.

OBJECTIVES

Results of an earlier experimental study [START_REF] Carbonell | Do oral messages help visual search? Advances in Natural Multimodal Dialogue[END_REF] indicate that coarse spatial information presented orally facilitates visual search for visually familiar components of realistic scenes, compared to situations where the target is visually familiar but its location in the scene is unknown. In addition, they suggest the possible influence of the scene spatial structure on the effectiveness of spatial information messages. The experiment reported here contributes to validating these preliminary results. It is focused on the search for "(visually) familiar" targets, that is, pre-viewed targets (or "items"). Its main objectives are to ascertain that:

(i) providing coarse information on target location facilitates visual search for familiar targets on crowded graphical displays significantly; (ii) the resulting improvement in search efficiency (i.e., target selection time and accuracy) is sufficient for motivating designers to integrate oral messages including such spatial information into graphical user interfaces; (iii) information on target location is useful whatever the layout (or spatial structure) of the scene components, and whatever the difficulty of the search task; (iv) multimodal system messages associating speech with graphics will meet with general user acceptance. These objectives are grounded on two working hypotheses inferred from general knowledge on visual perception: A. By narrowing visual search space, oral messages including information on target location (TL) will sensibly reduce target selection time and improve target spotting accuracy.

If the size of the reduced search space is inferior or equal to the size of the human visual field, selection time may be drastically reduced without loss of accuracy, due to the possible occurrence of target "pop out" effects [START_REF] Shneiderman | A photo history of SIGCHI: evolution of design from personal to public[END_REF]. B. Assuming that visual search strategies are influenced by display layout, the effects of scene spatial structure on users' scan paths may interfere with those of TL information; therefore, search efficiency might vary with scene spatial structure.

METHOD

The overall experimental protocol is first presented. Then, the description focuses on the design of the visual material. Target presentation mode, scene spatial structure and task difficulty were chosen as independent variables. Two dependent variables were used to assess participants' performances: target selection time (from scene display onset until first mouse click), and accuracy (i.e., mouse click on the target vs elsewhere); in addition, participants' subjective judgements were elicited through post-session questionnaires and debriefing interviews. To assess hypothesis A, two conditions were defined: the VP condition (target visual presentation) and the MP condition (target multimodal presentation). Each participant carried out two blocks including 120 visual search tasks each, one block per condition.

In addition, the order of blocks was counterbalanced between participants so as to neutralize possible task learning effects. To assess hypothesis B, we had to create specific visual material, due to the great structural diversity and complexity of real objects and scenes. To control spatial structure variations, scenes were build from sets of photographs, each scene including 30 photographs arranged along one out of four standard symmetrical structures (see figure 1):

• Matrix-like, the layout most frequently used for presenting collections of pictures [START_REF] Shneiderman | A photo history of SIGCHI: evolution of design from personal to public[END_REF]; • Elliptic (two concentric ellipses), a useful structure for displaying several sets of pictures simultaneously; • Radial (8 radii along the screen medians and diagonals), another possible structure for visualizing sets of pictures, especially ordered sets; \item • Random (random placing of items), the reference layout. We used the same 120 scenes for the VP and MP conditions in order to eliminate target visual and semiotic characteristics as possible factors of influence on participants' performances. We used 3600 different photographs collected from popular sites on the Internet to build the required 120 scenes (30 photographs per scene), so as to enhance the realism and attraction of the experimental visual search tasks as well as to obtain useful results for the design of picture browsers. Photographs were sorted out into 40 themes (e.g., sport, monuments, animals) and sub-themes (e.g., snakes or cats, for animals), then formatted (125x95 pixels, i.e., 4/3). Scenes were exclusively composed of photographs belonging to the same theme or sub-theme so as to reduce intrascene diversity in visual salience and subjective appeal. They were presented to participants in random order, regardless of their structure, their visual properties and those of the target. In addition, a different order was assigned to each subject so as to neutralize possible sequence effects. Target position varied from one scene to the other. For each scene, participants had to locate a pre-viewed photograph in the scene, and to select it as fast as they could, using the mouse. In the VP condition, the isolated target was displayed in the centre of the screen during 3 seconds. In the MP condition, a short oral message containing information on the TL was played simultaneously with the target visual presentation. Messages were composed of one or two short spatial phrases, for instance, "On the left (of the screen)" or "At the bottom (of the screen), on the right". Following target presentation, participants had to click on a button in the centre of the screen for launching the scene display. Thus, the position of the mouse at the beginning of the search was identical for all tasks. Three levels of task difficulty were defined, based mainly on the target visual complexity and the number of photographs in the scene that might be mistaken for it because of their visual similarity to it [START_REF] Chelazzi | Serial attention mechanisms in visual search: a critical look at the evidence[END_REF]. Levels of task difficulty were evenly distributed among the four structures (i.e., 10 scenes by level and structure). A gender-balanced group of 24 experienced computer users with ages between 24 and 29 and normal eyesight (assessed using the Bioptor test kit) participated in the experiment. Thus, all participants were expert mouse users with alike quick motor reactions; they were also experienced in visual search activities on computer displays. Therefore, target selection time and spotting accuracy were likely to reflect visual search performance reliably, and task learning effects were prevented.

RESULTS

Averaged selection times, error numbers and percentages were computed over all subjects (24) by condition, scene structure and task difficulty. First, we applied a n-factors ANOVA procedure on the data, then, paired t-tests whenever possible.

ANOVA Procedure

Table 1 shows that scene target presentation mode, scene structure and task difficulty are significant factors. Considering selection times, results are highly significant for both target presentation mode and task difficulty; considering error numbers, they are highly significant for the target presentation mode only. These results suggest that scene structure has less influence on results than target presentation mode and task difficulty. These quantitative and qualitative results confirm those presented in [START_REF] Kalyuga | Managing split attention and redundancy in multimedia instruction[END_REF][START_REF] Pelz | The coordination of eye, head and hand movements in a natural task[END_REF] for more complex tasks and other combinations of modalities: speech+graphics versus text+graphics. They partly validate hypothesis A: additional oral information on the location of a visually familiar target on the display significantly improves visual search efficiency effectively. Such messages also increase visual search comfort, and will get wide user acceptance.

Effects of Scene Structure

In the reference condition (VP), the four spatial structures can be ordered as follows according to increasing averaged selection times: Radial (5626 ms), Random, Matrix, Elliptic (6250 ms Concerning accuracy (see table 5), "actual errors" only are considered in this subsection and the next one. They include mouse clicks on non targets and clicks on the background (i.e., targets not found); clicks near the target (22) are considered as hits. In the VP condition, rates of actual errors range from 21\% (Random structure) to 30\% (Matrix structure) of the total number of actual errors (141). Differences between structures are then moderate. Contrastingly, there is a sharp difference between the Radial structure (14\% over a total of 66 actual errors) and the three other structures (from 24\% to 36\%) in the MP condition. A likely interpretation of this unexpected result is that the zones defined on the screen by the chosen spatial phrases match the Radial structure best and the Elliptic one worst. This interpretation is supported by some spontaneous comments collected during the debriefing interviews.Participants' subjective judgments are at variance with their performances. In the VP condition, more than half of the participants expressed a marked preference for Elliptic layouts compared to the other structures, and two thirds of them judged either the Matrix or the Radial structure the most inefficient layout. In the MP condition, judgments were more varied: the Radial and Elliptic structures were preferred by 11 and 8 participants respectively, while the Matrix and Elliptic structures were viewed as most inefficient by 7 and 6 participants respectively. Participants' performances and subjective judgments concerning the Matrix structure in the VP condition are in accordance with the results presented in [START_REF] Perkins | The interchange online network: simplifying information access[END_REF]. These quantitative and qualitative results suggest two main conclusions. First, messages including information on target location facilitate visual search for familiar pictures or graphical objects whatever the display layout. However their efficiency may be reduced in cases when spatial phrases and scene spatial structure imply different partitions of the display. This result contributes to validating hypothesis B. Therefore, display layout should be taken into account when designing verbal messages meant to help users to spot familiar pictures or graphical objects on crowded displays. Second, participants' performances and judgments relative to the VP condition suggest that 2D arrays may prove to be less appropriate than some other symmetrical layouts for displaying small collections of pictures or graphical objects. Further experimental research is needed to ascertain this conclusion which, if proved to be valid, might induce designers of graphical user interfaces and picture browsers to question the current prevailing use of 2D arrays for designing display layouts.

Effects of Task Difficulty

Participants' selection times validate our classification of scenes into three levels of difficulty (40 scenes per condition and level).

In both conditions, averaged selection times increased noticeably from level 1 (Easy) to level 3 (Very Difficult). For the VP condition, the difference between any pair of levels is statistically significant, the difference between levels 1 and 3 being highly significant (t=-6.40; p<.0001). For the MP condition, differences between level 1 and 3, and 2 and 3, are highly significant (t=-5.29; p<.0001 and t=-5.33; p<.0001 respectively), while the difference between levels 1 and 2 did not reach significance. Error rates also increased from level 1 to 3 in both conditions. A careful analysis of participants' performances shows that average selection times increase from level 1 to level 3 less rapidly in the MP condition (25%) than in the VP condition (35%). This observation suggests that spatial information messages are particularly useful for performing difficult visual search tasks. These results contribute to validating the second part of hypothesis A. Therefore, it seems worth while to assist users in difficult visual search activities through spatial information messages. As such short oral messages will be well accepted by potential users, or so it seems according to participants' subjective judgments, their use for helping users to carry out easy visual search tasks may also be considered.

CONCLUSION AND FUTURE WORK

We have presented an experimental study that aims at assessing the actual contribution of voice system messages to visual search efficiency and comfort. Messages comprised one or two spatial phrases conveying coarse information on the target location on the display. 24 participants carried out 240 visual search tasks in two conditions differing from each other in initial target presentation only: visual presentation of the target \textit{versus} multimodal presentation, that is, visual presentation of the target simultaneously with oral indications on its location on the screen. Oral messages improved participants' selection times and accuracy noticeably. However, their influence varied according to display spatial layout: the benefits were smaller for 2D array layouts than for Radial layouts, although the use of 2D arrays for displaying pictures is currently prevailing. In addition, message usefulness increased with task difficulty. Most of these results are statistically significant. According to subjective judgments, oral messages were well accepted, and multimodal target presentations were preferred to visual presentations by a majority of participants. Therefore, designers of graphical user interfaces might consider resorting to short oral messages including coarse spatial information for drawing users' attention to some displayed object. As such messages are likely to be well accepted by users, they may provide designers of advanced conversational user interfaces with a useful alternative "pointing" technique which may appropriately replace visual enhancement in interaction contexts where gaze activity is intense and where there is a risk of visual attention overload and eyestrain. However, these results need to be consolidated and further refined before reliable recommendations inferred from them can be proposed to designers. Their actual scope has first to be assessed. In particular, is the influence of spatial layout on visual search efficiency and comfort independent of the number of items displayed simultaneously and of their type (e.g., textual labels, graphical icons, drawings, etc.)? To what extent are the efficiency and user acceptance of oral spatial information messages dependent on their length and complexity? We are considering addressing some of these issues in the near future.

Figure 1 :

 1 Figure 1: Matrix, Elliptic, Radial and Random structures.

Table 1 .

 1 ANOVA Procedure. Factors: target presentation mode, scene structure, task difficulty.

	Factors	Selection times	Error numbers
	Presentation t=1202.98; p<.0001	t=23.18; p<.0001
	Structure	t=6.26; p=0.0003	t=2.58; p=0.0005
	Difficulty	t=32.49; p<.0001	t=7.59; p=0.0005
	4.2

Multimodal vs Visual Target Presentation

	This result is highly significant (t=-34.07; p<.0001). Selection
	times and error numbers per condition are reported in tables 2 and
	3. Average selection times (Avg ST) and standard deviations (Std
	Dev) were computed over the total number of tasks per condition
	(Nb Obs).
	Moreover, participants expressed very positive judgments on
	multimodal target presentations, both in the questionnaires and
	during the debriefing interviews. For 75\% of them (18), target
	spotting had been easier (less hesitations) in the MP condition
	than in the VP reference condition. Most participants mentioned
	that they had experienced some strain and visual fatigue during
	the VP condition whereas they had felt perfectly comfortable
	during the MP condition. All participants considered that oral
	messages including coarse information on target location could
	provide efficient support to visual search activities, and two thirds
	(16) expressed a marked preference for the MP condition.

Spatial information messages improved participants' visual search performances significantl2. Actually, averaged target selection times computed over all participants are thrice longer in the VP condition than in the MP condition (5674 ms versus 1747 ms).

Table 2 . Participants' selection times per condition.

 2

	Condition	Avg ST ms	Std Dev ms	Nb Obs
	VP	5674	5985	2880
	MP	1747	1552	2880

Table 3 . Participants' errors per condition.

 3

	Condition	Nb Errors	% Errors	Nb Obs
	VP	150	5.2%	2880
	MP	79	2.7ù	2880

Table 4 . Participants' selection times per condition and structure.

 4 ). Selection time differences between the Radial and Elliptic structures, the Radial and Matrix structures, the Elliptic and Matrix structures are statistically significant; see table4where values (720 tasks per condition and structure) preceded by "-" or "+" are respectively inferior or superior to the corresponding average values per condition reported in table 1. These results are somewhat unexpected, since participants were experienced computer users, and the use of 2D arrays is currently prevailing for displaying pictures. For the MP condition, the ranking of the four structures is the same as for the VP condition (see table4) but only the difference between the Radial and Elliptic structures reaches statistical significance (t=2.75; p=.006).

	Structure	Condition	Avg ST ms	Std Dev ms
	Radials	VP MP	-5081 -1640	-5565 -1256
	Random	VP MP	-5626 -1737	-5819 -1437
	Matrix	VP MP	+5738 +1763	-5879 +1819
	Elliptic	VP MP	+6250 +1851	+6585 +1633

Table 5 . Participants' errors per condition and structure.

 5

	Structure	Condition	Nb Errors	% Errors
	Radials	VP MP	34 9	24% 14%
	Random	VP MP	29 17	21% 26%
	Matrix	VP MP	36 16	25% 24%
	Elliptic	VP MP	42 24	30% 36%