Elisabeth Beyersmann
email: lisi.beyersmann@mq.edu.au

Petroula Mousikou

Sascha Schroeder

Ludivine Javourey-Drevet

Johannes C Ziegler

Jonathan Grainger

The dynamics of morphological processing in developing readers: A cross-linguistic masked priming study

Keywords: morphology, cross-linguistic, reading development, embedded words

Empirical evidence from masked priming research shows that skilled readers can rapidly identify morphological structure in written language. However, comparatively little is known about how and when this skill is acquired in children. The present work investigates the developmental trajectory of morphological processing in a two-year longitudinal study that involves two large cohorts of German and French primary school children. The masked priming paradigm was used within an experimental design that allowed us to dissociate effects of (i) non-morphological embedded word activation, (ii) morpho-orthographic decomposition, and (iii) morpho-semantics.

Four priming conditions were used: affixed word (farmer-FARM), affixed nonword (farmity-FARM), non-affixed nonword (farmald-FARM), and unrelated control (workald-FARM). The results revealed robust embedded word priming effects across both languages. However, morphoorthographic and morpho-semantic effects were only evident in the French sample. These findings are discussed in the context of a theoretical framework that specifies the distinct roles played by embedded words and affixes, their distinct developmental trajectories, and how the intrinsic linguistic properties of a given language may impact on morphological processing.

Research on morphological processing in reading has shown that skilled readers can rapidly decompose letter strings into morphological units (e.g., [START_REF] Taft | Morphological representation as a correlation between form and meaning[END_REF][START_REF] Taft | Lexical storage and retrieval of prefixed words[END_REF].

However, comparatively little research has examined how and when children acquire this important skill [START_REF] Rastle | The place of morphology in learning to read in English[END_REF]. Here, we report data from a two-year longitudinal study involving two large cohorts of German and French primary school children. The aim of the present study was to explore the developmental trajectory of children's morphological processing skills in visual word recognition, and to provide a more fine-tuned theoretical perspective of the mechanisms involved in identifying morphological sub-structures during reading. A masked primed lexical decision design was used that allowed for the dissociation of prefix, suffix, and embedded word priming effects across the two different languages.

Over the past 20 years, the masked priming paradigm has emerged as the gold standard for research investigating morphological processing in skilled readers (e.g., [START_REF] Forster | Form priming with masked primes: The best match hypothesis[END_REF][START_REF] Giraudo | Words matter more than morphemes: An investigation of masked priming effects with complex words and non-words[END_REF][START_REF] Grainger | Masked morphological priming in visual word recognition[END_REF][START_REF] Longtin | Morphological decomposition in early visual word processing[END_REF][START_REF] Longtin | Morphological priming without morphological relationship[END_REF][START_REF] Rastle | The broth in my brother's brothel: Morphoorthographic segmentation in visual word recognition[END_REF]. One of the key findings from this area of research is that target word recognition is significantly facilitated by the prior presentation of a semantically transparent complex prime (e.g., farmer-farm) and a semantically opaque pseudo-complex prime (e.g., corner-corn), but not by the prior presentation of a mono-morphemic control prime (e.g., cashew-cash), suggesting that skilled readers initially decompose complex and pseudo-complex words into morphemic sub-units. Morphological segmentation effects in skilled readers have been widely replicated across several languages (for reviews, see [START_REF] Amenta | Morphological processing as we know it: an analytical review of morphological effects in visual word identification[END_REF][START_REF] Rastle | Morphological decomposition based on the analysis of orthography[END_REF], and have also, in more recent years, found support from studies combining masked priming with the recording of event-related potentials (e.g., [START_REF] Morris | Semantic transparency and masked morphological priming: an ERP investigation[END_REF]; for a review see [START_REF] Beyersmann | Morphological processing without semantics: An ERP study with spoken words[END_REF].

As opposed to the widely replicated and robust pattern of morphological priming effects in adults, studies investigating such effects in children are much rarer and the obtained results tend to be more ambiguous (e.g., [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Lázaro | Morphological effects in word identification: tracking the developmental trajectory of derivational suffixes in Spanish[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF][START_REF] Schiff | The Development of the Hebrew Mental Lexicon: When Morphological Representations become Devoid of Their Meaning[END_REF]. What generally seems to be the case is that the kind of automatic morphological segmentation effects that are typically seen in adults, emerge at a relatively late stage in children's reading development, at a time when children have already mastered most other basic reading skills [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Dawson | Morphological effects in visual word recognition: Children, adolecents, and adults[END_REF][START_REF] Schiff | The Development of the Hebrew Mental Lexicon: When Morphological Representations become Devoid of Their Meaning[END_REF]. Importantly, the time of acquisition seems to vary across languages. For instance, a study with English-speaking third and fifth graders [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF] found significant priming effects with semantically transparent complex words (e.g., farmer-farm), yet no priming with semantically opaque complex words (e.g., corner-corn) was observed. These results indicate that English-speaking primary school children are sensitive to the "morpho-semantic" but not the "morpho-orthographic" relationship between the prime and the target, suggesting that they have not yet reached a level of automatization of form-based morphological processing during reading. Similarly, a study with Hebrew-speaking children provided evidence for morpho-semantic but not morpho-orthographic priming in fourth graders, whereas both morpho-semantic and morpho-orthographic priming effects were found in a group of seventh graders, suggesting that morpho-orthographic decomposition mechanisms are not acquired until high school [START_REF] Schiff | The Development of the Hebrew Mental Lexicon: When Morphological Representations become Devoid of Their Meaning[END_REF]. In line with the findings from English and Hebrew, masked priming results from Spanish fourth, fifth, and sixth graders also show that morphological suffix priming effects (e.g., lechero -journalero) are only evidenced in the most experienced developing readers, but not in fourth and fifth grade [START_REF] Lázaro | Morphological effects in word identification: tracking the developmental trajectory of derivational suffixes in Spanish[END_REF]. In contrast, an earlier onset for morpho-orthographic priming in children's reading development has been reported by a study with French-speaking third, fifth, and seventh graders [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF]. All three groups of children revealed significant morphoorthographic and morpho-semantic priming effects, which were comparable to those reported for adults. Hence, the results from this study suggest that French-speaking children might acquire morpho-orthographic decomposition mechanisms earlier in reading development than English, Hebrew, and Spanish speaking children.

Besides masked priming, the role of morphological processing in children's reading acquisition has also been explored using a range of other tasks, including for example simple lexical decision and naming tasks. Although these kind of tasks are open to strategic processing and therefore less able to shed light on the automatic processing that is known to be crucial for morpho-orthographic decomposition, there are a range of visual lexical decision studies that provide evidence for sub-lexical morphological processing in primary school children, which we will briefly summarize here (e.g., [START_REF] Casalis | How language affects children's use of derivational morphology in visual word and pseudoword processing: evidence from a cross-language study[END_REF][START_REF] Dawson | Morphological effects in visual word recognition: Children, adolecents, and adults[END_REF][START_REF] Lázaro | Exploring the derivative suffix frequency effect in Spanish speaking children[END_REF][START_REF] Lázaro | How orthographic transparency affects morphological processing in young readers with and without reading disability[END_REF][START_REF] Lázaro | Morphological effects in word identification: tracking the developmental trajectory of derivational suffixes in Spanish[END_REF][START_REF] Quémart | Exploring the role of bases and suffixes when reading familiar and unfamiliar words: Evidence from French young readers[END_REF]. For instance, [START_REF] Lázaro | Exploring the derivative suffix frequency effect in Spanish speaking children[END_REF] showed that Spanish children responded faster to words with frequent suffixes than infrequent suffixes as early as Grade 2, suggesting that Spanish-speaking children are sensitive to affix representations from an early age. However, we note that the above mentioned Spanish masked priming results indicate that morpho-orthographic processing is not automatized until sixth grade [START_REF] Lázaro | Morphological effects in word identification: tracking the developmental trajectory of derivational suffixes in Spanish[END_REF], hence suggesting that sensitivity to affixes is not a sufficient condition for morpho-orthographic decomposition. Moreover, [START_REF] Quémart | Exploring the role of bases and suffixes when reading familiar and unfamiliar words: Evidence from French young readers[END_REF] reported evidence for a morpheme interference effect in French third and fifth graders (i.e., children found it harder to reject morphologically complex nonwords than morphologically simple nonwords), whereas [START_REF] Casalis | How language affects children's use of derivational morphology in visual word and pseudoword processing: evidence from a cross-language study[END_REF] showed that this effect is more prominent in French than in English fourth graders. These findings further showcase the importance of cross-linguistic differences in the developmental trajectory of morphological form processes.

Questions thus still remain concerning the generalizability of morphological priming effects across different languages. Intrinsic linguistic properties, such as orthographic transparency and morphological productivity, influence the way morphemes are processed within specific languages (e.g., [START_REF] Beyersmann | Morphological processing across modalities and languages[END_REF][START_REF] Mousikou | Orthographic consistency influences morphological processing in reading aloud: Evidence from a cross-linguistic study[END_REF]. German and French differ in two important aspects, morphological productivity and orthographic complexity, which leads to two key predictions, outlined below.

First, as shown by methods developed to quantify morphological productivity across different languages (e.g., [START_REF] Juola | Measuring linguistic complexity: The morphological tier[END_REF][START_REF] Juola | Assessing linguistic complexity[END_REF][START_REF] Kettunen | Can Type-Token Ratio be Used to Show Morphological Complexity of Languages[END_REF], German is morphologically more complex than French. [START_REF] Sadeniemi | Complexity of European Union Languages: A comparative approach[END_REF] argue that the productivity of German morphology is likely due to the abundant presence of compound words (e.g., Creutz & Lagus, 2005;[START_REF] Creutz | Morfessor and hutmegs: Unsupervised morpheme segmentation for highly-inflecting and compounding languages[END_REF][START_REF] Fleischer | Wortbildung der deutschen Gegenwartssprache[END_REF][START_REF] Meyer | Compound comprehension in isolation and in context: The contribution of conceptual and discourse knowledge to the comprehension of German novel noun-noun compounds[END_REF]. In line with these quantitative differences, it has been shown that German skilled readers are more sensitive than French skilled readers to the processing of embedded stems, presumably because of the highly productive German compounding system and the frequent exposure to stem + stem concatenations in the German language [START_REF] Beyersmann | Morphological processing across modalities and languages[END_REF].

These intriguing findings from adults predict that the activation of embedded stems would play a particularly important role in German reading acquisition compared to reading acquisition in a language with a sparser compounding system like French. In contrast, although compounding also exists, even if to a lesser extent, in French [START_REF] Nicoladis | Word family size and French-speaking children's segmentation of existing compounds[END_REF] Morphological segmentation into stems and affixes therefore represents the key to morphological processing in the French language and predicts that French children may benefit from morphological structure sooner in their reading development compared to German children. The goal of the present study was to put these predictions to test by directly comparing the magnitude of embedded stem and morphological priming effects across two large cohorts of German and French speaking third and fourth graders1 .

Second, although German and French are both characterized by a rich system of inflectional and derivational affixes (e.g., [START_REF] Rey-Debove | Le domaine de la morphologie lexicale [The domain of lexical morphology[END_REF][START_REF] Roelcke | Sprachtypologie des Deutschen: historische, regionale und funktionale Variation [Language typology of German: historical, regional, and functional variation[END_REF], the two languages differ with respect to their "orthographic complexity" [START_REF] Schmalz | Getting to the bottom of orthographic depth[END_REF], which is known to affect children's reading acquisition (e.g., for a comparison of 7 different orthographies, see [START_REF] Aro | Learning to read: English in comparison to six more regular orthographies[END_REF]. French is orthographically more complex than German [START_REF] De Simone | Order among chaos: Cross-linguistic differences and developmental trajectories in pseudoword reading aloud using pronunciation Entropy[END_REF][START_REF] Schmalz | Unpredictability and complexity of print-to-speech correspondences increase reliance on lexical processes: more evidence for the orthographic depth hypothesis[END_REF], due to its large number of complex letter-sound correspondences (e.g., [START_REF] Schmalz | Unpredictability and complexity of print-to-speech correspondences increase reliance on lexical processes: more evidence for the orthographic depth hypothesis[END_REF][START_REF] Schmalz | Getting to the bottom of orthographic depth[END_REF][START_REF] Van Den Bosch | Measuring the complexity of writing systems[END_REF]. The different degrees of complexity of the French and German orthographies are also reflected in the number of grapheme-to-phoneme correspondence (GPC) rules implemented in the Dual Route Cascaded (DRC) models of German [START_REF] Ziegler | The DRC model of visual word recognition and reading aloud: An extension to German[END_REF] and French [START_REF] Ziegler | Speed of lexical and nonlexical processing in French: The case of the regularity effect[END_REF]. The number of rules, particularly the number of complex rules, and the number of context-sensitive rules is higher in French than in German (see also Table 1 in [START_REF] Schmalz | Getting to the bottom of orthographic depth[END_REF]. French contains many multiletter rules, where several letters are required to denote a single phoneme (e.g., aient → /ε/ in French), as well as many context-sensitive regularities, where surrounding letters affect a grapheme's pronunciation (e.g., in French, the letter g is pronounced as /ʒ/ when followed by an i or e, as in gélatine). Therefore, learning to read within a complex orthography may be more likely to shift children's reliance from single letter decoding onto multi-letter decoding early on in reading development [START_REF] Mousikou | Orthographic consistency influences morphological processing in reading aloud: Evidence from a cross-linguistic study[END_REF]. This thus makes a second important prediction that French readers should be more proficient at processing affixes compared to German readers, and would also explain why French children already show evidence for morpho-orthographic processing as early as Grade 3 [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF]. Although these initial findings logically imply that the developmental trajectories of morphological processing skills should vary across languages like German and French, this hypothesis has never been put to test in a tightly controlled cross-linguistic examination.

The goal of the present masked priming study was to investigate the development of embedded word and morphological processing in German and French primary school children by building on the "word and affix" model of complex word reading (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF], a theoretical framework that clearly dissociates the different contributions of embedded word activation and morphological segmentation in children's reading development. The model is not only amongst the most recent theoretical frameworks having emerged from decades of masked priming research in children and adults, but is also unique in the sense that it specifies the multiple stages of reading development that are required to reach the same level of morphological processing skills that are typically seen in adults. The latest version of the model (Beyersmann & Grainger, in press) implements three key mechanisms that are at play during the early stages of complex word recognition: edge-aligned embedded word activation, morpho-orthographic full decomposition, and feedback from semantics (see Figure 1). The model predicts that embedded words and whole words are activated via a whole-word pathway, by simply mapping letters onto whole-word representations in the orthographic lexicon, whereas morpho-orthographic full decomposition occurs via a separate pathway by which letter strings are decomposed into their component morphemes.

Active units in the orthographic lexicon are then mapped onto a third layer of semantic representations. Based on bidirectional links between the orthographic lexicon and semantics, the model accounts for semantic influences on lexical processing via feedback connections from semantics.

One of the key predictions of this model, which is of particular relevance to the current investigation, is that beginning readers can draw on their whole-word knowledge when discovering form-meaning relationships between morphologically complex words and their embedded stems. Embedded stems represent a critical starting point in children's development of morphological processes [START_REF] Beyersmann | Embedded stems as a bootstrapping mechanism for morphological parsing during reading development[END_REF], based on which a specialized morphological parsing system can develop once children gain more reading experience (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF]. Grainger and Beyersmann argue that the development of the morphological parsing system goes through three consecutive stages, by which children first acquire the ability to identify embedded words, then the ability to process morpho-semantics (by discovering form-meaning regularities), and finally the ability to decompose words into morpho-orthographic subunits (see also, [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF]. How quickly children move through these three developmental stages might differ between different languages, as is already shown in the above-mentioned studies with English-, Hebrew-, and French-speaking children [START_REF] Beyersmann | Morphological processing during visual word recognition in developing readers: Evidence from masked priming[END_REF][START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF][START_REF] Schiff | The Development of the Hebrew Mental Lexicon: When Morphological Representations become Devoid of Their Meaning[END_REF].

Our goal was therefore to closely monitor the developmental stages of morphological processing in two large cohorts of German-and French-speaking third and fourth graders, using masked morphological priming.

The experimental design was chosen such that it allowed us to determine the unique contribution of non-morphological embedded word priming, morpho-orthographic priming and morpho-semantic priming. Four priming conditions were created for each target word. These included an affixed word prime (e.g., farmer-FARM), an affixed nonword prime (e.g., farmity-FARM), a non-affixed nonword prime (e.g., farmald-FARM), and an unrelated prime (e.g., workald-FARM). To examine the influence of embedded word processing, the non-affixed condition was compared against the unrelated control condition2 . To investigate the influence of morpho-orthographic processing, the affixed nonword condition was compared against the nonaffixed condition. Finally, to explore the influence of morpho-semantic processing, the affixed word condition was compared against the affixed nonword condition. We note that the latter comparison raises a complex issue of prime lexicality. Under masked priming conditions, orthographically related nonword primes tend to have a greater facilitatory impact than word primes, due to lexical competition between word prime and the word target (also referred to as the "prime-lexicality effect"; [START_REF] Forster | The prime lexicality effect: Form-priming as a function of prime awareness, lexical status, and discrimination difficulty[END_REF]. However, [START_REF] Grainger | Effects of lexicality and pseudo-morphological complexity on embedded word priming[END_REF] showed that the effect of prime lexicality extends to non-affixed primes including the target word as an embedded letter sequence (e.g., inhibitory priming from dragon-DRAG but not dragip-DRAG), but that in complex words, the effect of prime lexicality is counterbalanced by morpho-orthographic complexity (i.e., equal priming for corner-CORN and cornry-CORN). We therefore reasoned that any additional priming in the affixed word condition (farmer-FARM) could only be due to the shared morpho-semantic relationship between the prime and the target.

Cross-linguistically, we hypothesized that German speaking children should rely primarily on the activation of embedded words rather than on morpho-orthographic and morpho-semantic processing [START_REF] Beyersmann | Morphological processing across modalities and languages[END_REF], and that evidence for embedded word priming should emerge as early as Grade 3 [START_REF] Hasenäcker | Masked morphological priming in German-speaking adults and children: Evidence from response time distributions[END_REF], 2020). French speaking individuals were also expected to show evidence for significant embedded word priming (Beyersmann, Grainger, Casalis, & Ziegler, 2015), but additionally demonstrate greater reliance on morpho-orthographic and morpho-semantic processing mechanisms, compared to their German peers . Based on prior evidence, we predicted that morpho-orthographic segmentation effects in French would already emerge as early as Grade 3 [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF].

Finally, our research aimed at comparing the processing of prefixed and suffixed nonwords in French and German. Accordingly, four additional priming conditions were created using prefixed items (e.g., affixed word: reload-LOAD; affixed nonword: exload-LOAD; non-affixed nonword: erload-LOAD; unrelated control: erwork-LOAD). Based on the idea that both edges of the letter string act as anchor points for embedded word activation (Beyersmann & Grainger, in press;[START_REF] Fischer-Baum | Both-edges representation of letter position in reading[END_REF][START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF], we hypothesized that comparable embedded word priming effects should be observed in both prefixed items (with final embedded words) and suffixed items (with initial embedded words), under the assumption of parallel letter processing. In addition, the study's aim was to test differences in morpho-orthographic and morpho-semantic priming effects between prefixed and suffixed items. Prior work from skilled readers has shown that suffixed words tend to be subject to early, morpho-orthographic segmentation, whereas prefixed words appear to have a quasilexical status, and are therefore more likely to be subject to post-lexical, morpho-semantic processing (e.g., Beyersmann, Ziegler, & Grainger, 2015;[START_REF] Kim | Morphological decomposition in the recognition of prefixed and suffixed words: Evidence from Korean[END_REF]. Other studies, however, have failed to find processing differences between prefixed and suffixed words [START_REF] Beyersmann | Embedded Stem Priming Effects in Prefixed and Suffixed Pseudowords[END_REF][START_REF] Heathcote | Do 'blacheap' and 'subcheap' both prime 'cheap'? An investigation of morphemic status and position in early visual word processing[END_REF][START_REF] Mousikou | Morphological processing in single-word and sentence reading[END_REF]. Whether or not prefixes and suffixes have a different status in the reading system therefore continues to be a matter of debate. Here, our aim was to examine differences between prefixes and suffixes within the context of reading development and to test, for the first time, if children are more efficient at segmenting items containing prefixes than items containing suffixes, or vice versa. Developing readers initially learn to decode the letters of a word serially, from left to right, which may promote the acquisition of a left-to-right scanning mechanisms in visual word recognition (e.g., [START_REF] Acha | The effects of length and transposed-letter similarity in lexical decision: evidence with beginning, intermediate, and adult readers[END_REF][START_REF] Ziegler | Developmental dyslexia in different languages: Language-specific or universal?[END_REF]. As a result, prefix processing may emerge earlier during reading development than suffix processing. However, it is also possible that children acquire the ability to extract suffixes sooner than prefixes, because suffixes are less likely to modify the meaning of the stem morpheme.

Method Participants

A total of 304 children, 171 French and 133 German participated in the study. Children were assessed twice with a one-year interval: once in Grade 3 and once in Grade 4 (5-6 months into the school term in both grades). Three German children had to be excluded, because they were foreign language speakers who had spent less than two years living in Germany, and another three children were excluded because they had reported learning disabilities. Moreover, several children who participated in the first testing session were no longer available in the following year, at the time of the second testing session, and therefore were also removed from the sample. This reduced the total number of participants to 139 French and 113 German children (see Table 1). Participants were native speakers of their respective language, had normal or corrected-to-normal vision, and reported no hearing, reading, or language difficulties. Prior to participating in the study, the children's parents provided written, informed consent.

Materials

Two different item sets were selected for each language: a set of prefixed items and a set of suffixed items (see Appendix A for a full list of materials). The prefixed materials included 48

French target words and 48 German target words, which were matched as closely as possibly on number of letters, syntactic word class, and word frequency. Word frequency was extracted from Manulex [START_REF] Lété | MANULEX: A grade-level lexical database from French elementary school readers[END_REF] for French, and childLex (Schroeder, Würzner, Heister, Geyken, & Kliegl, 2015) for German. 3 All target words in the prefixed set corresponded to infinitive forms of verbs. The psycholinguistic properties of the French and German words are displayed in Table 2. Each target was preceded by an affixed word prime

CHARGER [feleave-CHARGE]; karhasten-DENKEN [carpaint-THINK]

). The French prefixed words included the prefixes dé-, en-, pré-, and re-, and the German prefixed words included the prefixes auf-, mit-, ab-, and an-, which were each repeated 12 times. The French and German prefixed words were matched on number of letters.

Prefixed nonword primes were created using the target word as a stem (e.g., charger) and a prefix (e.g., ex-), which in combination with the stem resulted in a nonword (excharger). To ensure that the selected stem-affix combinations did not result in another existing word, while controlling for stimulus length and affix repetition, the prefixes in this condition were different from the prefixes in the affixed word condition. The French prefixed nonwords included the prefixes in-, bi-, dis-, and ex-, and the German prefixed nonwords included the prefixes hin-, vor-, um-, and zu-. The French prefixes were each repeated 12 times. In German, it was impossible to use all prefixes an equal number of times, because German prefixes are highly productive and often result in real words when combined with a stem. Therefore, the German prefixes hin-and um-were repeated 16 times, and the prefixes vor-and zu-were repeated 8 times. Non-suffixed nonword primes were created using the target word as a stem (e.g., charger) and a nonmorphemic letter-sequence (e.g., fe-), which in combination with the stem resulted in a nonword (fecharger). Unrelated primes were non-affixed nonwords that were orthographically unrelated to the target. All nonwords contained orthographically legal letter combinations and were pronounceable. Within each language, the items in the four prime conditions were matched on length.

The suffixed materials included 48 French target words and 48 German target words, which were matched as closely as possible on number of letters, syntactic word class, and word frequency. As opposed to the prefixed item set which consisted of a selection of verb targets, all target words in the suffixed set were either nouns or adjectives. This choice of materials was constrained by the German language: Due to the high productivity of most German prefixes, which result in real words when combined with a stem, it was practically impossible to use the same affixes in the affixed word and affixed nonword conditions. To ensure that the items in the two languages were comparable, we opted for creating the German and the French items using the same procedure. Suffixed nonword primes were created using the target word as a stem (e.g., fille) and a suffix (e.g., -ible), which in combination with the stem resulted in a nonword (fillible). Again, to maintain nonword status in this condition, the suffixes used to create suffixed nonwords were different from the suffixes used in the suffixed word condition (see Appendix A). The French suffixed nonwords included the suffixes -able, -ible, -oir, and -ure, and the German suffixed nonwords included the suffixes -keit, -isch, -ling, and -lein. The French suffixes were each repeated 12 times. The German suffixes -keit and -lein were repeated 12 times, the suffix -isch 14 times, and the suffix -ling 10 times. Non-suffixed nonword primes were created using the target word as a stem (e.g., fille) and a non-morphemic letter-sequence (e.g., -ache), which in combination with the stem resulted in a nonword (fillache). Unrelated primes were non-affixed nonwords that were orthographically unrelated to the target. All nonwords contained orthographically legal letter combinations and were pronounceable. Within each language, the items in the four prime conditions were matched on length.

For the purpose of the lexical decision task, 98 French nonword targets (48 for the prefixed materials and 48 for the suffixed materials) and 98 German nonword targets (48 for the prefixed materials and 48 for the suffixed materials) were created from words by replacing one or two letters (e.g., brader [to flog something] -> *brager). In both the prefixed and suffixed materials, nonword targets were matched to real word targets on length. To mimic the structure of the primes preceding real word targets, primes preceding nonword targets were constructed in a similar fashion. Each nonword target was preceded by four different types of primes: primes that consisted of the nonword targets and the affix used in the affixed word condition (e.g., rebrager-BRAGER), primes that consisted of the nonword targets and the affix used in the affixed nonword condition (e.g., exbrager-BRAGER), primes that consisted of the nonword targets and the non-morphemic letter sequence used in the non-affixed nonword condition (e.g., febrager-BRAGER), and primes that consisted of a nonword the non-morphemic letter sequence used in the unrelated condition (e.g., feminder-BRAGER).

Procedure

On each trial, a 500-ms forward mask consisting of hash keys was presented first, followed by a 50-ms prime in lowercase letters, followed by the target in uppercase letters.

Participants were instructed to decide if the presented item was a real word or a nonword. The target remained present until a response was made or until 60 seconds elapsed. Participants were instructed to respond as quickly and as accurately as possible. Each component of the experiment (prefixed and suffixed) took about 10 minutes to complete (i.e., approx. 20 minutes per participant). The experiment formed part of a longer longitudinal study and a number of other tasks were administered between the two testing components. To ensure that every participant saw every target only once, we created four counterbalanced experimental lists. An equal number of participants were assigned to each list. The word and nonword items were presented in randomized order. Moreover, the order of trial presentation within each list was randomized for each participant. Six practice items consisting of both words and nonwords were presented prior to the experimental trials.

In addition, children's reading fluency was assessed using the 1-min-reading test [START_REF] Gentaz | Differences in the predictors of reading comprehension in first graders from low socio-economic status families with either good or poor decoding skills[END_REF] in French, and the SLRT II reading test [START_REF] Moll | SLRT-II: Lese-und Rechtschreibtest: Weiterentwicklung der Salzburger Lese-und Rechtschreibtests (SLRT): Manual[END_REF] in German. The 1-min-reading and SLRT reading tests involved reading aloud a list of words and a list of nonwords. Based on each sample, we calculated a z-score for correctly read words and a z-score for correctly read nonwords. The average of the two scores was used as an index of reading ability in the analyses. Each child was assessed twice: once in Grade 3 and once in Grade 4.

Results

Analyses were performed using (generalized) linear mixed-effects models [START_REF] Baayen | Mixed-effects modeling with crossed random effects for subjects and items[END_REF] as implemented in the lme4 package (Version 1.1-21; [START_REF] Bates | Fitting Linear Mixed-Effects Models Using lme4[END_REF] in the statistical software R (Version 3.6.0, 2019-07-05, "Planting of a Tree", RCoreTeam, 2019). RTs were log transformed to normalize residuals and were then analyzed using a linear mixed-effects (LME) model. For the error analysis, a generalized linear mixed-effects (GLME) model was created using logit transformation and a binomial link function. The significance of the fixed effects was determined with type III model comparisons using the Anova function in the car package (Version 3.0-3; [START_REF] Fox | An R companion to applied regression[END_REF]. Post hoc comparisons were carried out using cell means coding and single df contrasts with the glht function of the multcomp package (Version 1.4-10; [START_REF] Hothorn | Simultaneous inference in general parametric models[END_REF] using the normal distribution to evaluate significance. Factor Prime Type was decomposed into three individual contrasts to determine the contribution of (i) embedded word processing, (ii) morpho-orthographic processing, and (iii) morpho-semantic processing. The first contrast was used to examine the influence of embedded word processing on the masked priming data, by comparing the non-affixed condition against the unrelated control condition. The second contrast was used to investigate the influence of morpho-orthographic processing on masked priming, by comparing the affixed nonword condition against the non-affixed nonword condition. The third contrast was used to explore the influence of morpho-semantic processing on masked priming, by comparing the affixed word against the affixed nonword condition.

Reaction Times

The French target word lever was mistakenly repeated and therefore removed. 4 Outlier trimming was performed separately within each grade. First, incorrect responses to nonwords (12.2% of the Grade 3 data; 9.7% of the Grade 4 data) were removed. Then, latencies below 300 or above 8000 ms were considered as extreme values and were also removed (1.1% of the Grade 3 data; 0.9% of the Grade 4 data). A base model, including only participants and items as random intercepts, was fitted to the data and data points with residuals exceeding 2.5 SDs were removed, following the procedure outlined by [START_REF] Baayen | Analyzing reaction times[END_REF] 2.2% of the Grade 3 data; 2.2% of the Grade 4 data). The LME model included the effect-coded fixed effects of Affix Type (Prefix, Suffix), Prime Type (Affixed Word, Affixed Nonword, Non-affixed Nonword, Unrelated), Language (French, German), Grade (Grade 3 vs. Grade 4) and Reading Fluency (standardized score in each grade), and their interactions. Trial Order (standardized) was included as a covariate to control for task effects such as fatigue or habituation. Random intercepts and random slopes for the effect of Prime Type were used for both participants and items. Mean model RTs are displayed in Figures 2 and3. The full model output, including all main effects and interactions, is provided in Appendix B.

There was a significant main effect of Grade (χ 2 (1) = 3880.50, p < .001), showing that fourth graders responded overall faster than third graders. There was also a significant main effect of Affix Type (χ 2 (1) = 41.20, p < .001), showing that children responded overall faster to suffixed than prefixed trials. The interaction between Affix Type and Grade was significant (χ 2 (1) = 6.45, p = .011), indicating that the Grade effect was larger for suffixes (z = 46.51, p < .001) than for prefixes (z = 41.80, p < .001). The interaction between Affix Type and Language was significant (χ 2 (1) = 18.58, p < .001), because the effect of Affix Type was significant in French (z = 7.56, p < .001), but not in German (z = 1.50, p = .135). The interaction between Grade and Language was significant (χ 2 (1) = 88.98, p < .001), because the effect of Grade was larger in German (z = 48.50, p < .001) than in French (z = 39.27, p < .001).

Priming Effects

There was a significant main effect of Prime Type (χ 2 (3) = 66.18, p < .001) 5 . Contrast coding revealed a significant effect of embedded word priming (Δ = 49 ms, z = 2.82, p = .005), and a significant effect of morpho-orthographic priming (Δ = 17 ms, z = 2.15, p = .032). The effect of morpho-semantic priming was not significant (Δ = 4 ms, z = 1.48, p = .139). Crucially, 5 The analyses focus on the description of our key priming effects, which were 1) embedded word priming, 2) morpho-orthographic priming, and 3) morpho-semantic priming. Therefore, the simple contrasts between the affixed conditions and the unrelated condition are not reported below. We note however, that these simple priming effects were highly significant in both French (affixed word vs. unrelated: z = 5.39, p < .001; affixed nonword vs. unrelated: z = 5.21, p < .001) and German (affixed word vs. unrelated: z = 4.69, p < .001; affixed nonword vs. unrelated: z = 4.48, p < .001).

the 3-way interaction between Prime Type, Affix Type and Language was significant (χ 2 (3) = 9.02, p = .029), suggesting that priming effects were modulated by differences between participant cohorts and item sets.

To follow up the 3-way interaction between Prime Type, Affix Type, and Language, contrast coding was used to analyse priming effects separately for prefixes and suffixes, and for each language group. A summary of the individual priming effects and effect sizes is presented in Table 3. Robust embedded word priming effects were observed in both languages, independently of affix type. In the German cohort, morpho-orthographic and morpho-semantic priming effects were entirely absent (see Table 3). In the French cohort, an interesting dissociation was observed between affix types. For prefixes, a significant effect of morphoorthographic priming was observed (Δ = 43 ms, z = 3.19, p = .001), while the effect of morphosemantic priming was not significant (Δ = -24 ms, z = 1.44, p = .151). For suffixes, there was a significant effect of morpho-semantic priming (Δ = 27 ms, z = 2.07, p = .038), but the effect of morpho-orthographic priming was not significant (Δ = 2 ms, z = 0.89, p = .373).

Individual Differences in Reading Fluency

There was a significant main effect of Reading Fluency (χ 2 (1) = 188.02, p < .001), indicating that children with high reading proficiency responded faster than children with low reading proficiency. The interaction between Affix Type and Reading Fluency was significant (χ 2 (1) = 24.35, p < .001), showing that the effect of Affix Type decreased with increasing reading proficiency. There was also a significant interaction between Grade and Reading Fluency (χ 2 (1) = 181.96, p < .001), indicating that the effect of reading proficiency was larger in Grade 3 than in Grade 4. The interaction between Language and Reading Fluency was also significant (χ 2 (1) = 58.56, p < .001), indicating that in the low reading proficiency range, French children responded faster than German children, but in the high reading proficiency range, German children responded faster than French children. In addition, the 3-way interaction between Affix Type, Language and Reading Fluency was significant (χ 2 (1) = 38.11, p < .001), because the Affix Type by Reading Fluency was significant in French (z = 8.41, p < .001), but not in German (z = 0.83, p = .205). The 3-way interaction between Grade, Language and Reading Fluency was also significant (χ 2 (1) = 18.79, p < .001), because the Grade by Reading Fluency interaction was larger in German (z = 12.01, p < .001) than in French (z = 6.83, p < .001).

Most importantly, the factor Prime Type was modulated by individual differences in reading fluency (see Figure 4). There was a significant three-way interaction between Prime Type, Language and Reading Fluency (χ 2 (3) = 9.03, p = .029), because individual differences in reading fluency differently affected priming in French children compared to German children. In French children, morpho-orthographic priming effects were modulated by reading fluency (z = 3.33, p < .001), with larger priming effects being observed in children with lower levels of reading fluency (see left panel of Figure 4). Morpho-semantic priming effects were also modulated by reading fluency (z = 2.16, p = .031), but here French children with lower levels of reading fluency showed a larger inhibitory morpho-semantic priming effect. Embedded word priming effects were not modulated by individual differences in reading fluency in French children (z = 0.70, p = .486). In contrast, in the German data, none of the priming effects were modulated by reading fluency (all p >.1; see right panel of Figure 4).

Accuracy

The GLME model included the same fixed effects, random effects and interactions as the LME model, except that we had to exclude the random intercepts and random slopes for the effect of Prime Type for participants and items, because the model did not converge. The mean model error rates are displayed in Figures 5 and6. The detailed model output is provided in Appendix B.

The accuracy data showed a similar numerical trend as the RTs, suggesting that there was no speed-accuracy trade-off. There was a significant main effect of Grade (χ 2 (1) = 78.09, p < .001), showing that fourth graders responded overall more accurately than third graders. There was also a significant main effect of Language (χ 2 (1) = 4.19, p = .041), such that French children responded overall more accurately than German children. The interaction between Affix Type and Grade was significant (χ 2 (1) = 5.11, p = .024), reflecting the fact that the Grade effect was larger for prefixes (z = 8.23, p < .001) than suffixes (z = 4.46, p < .001). The interaction between Language and Grade was also significant (χ 2 (1) = 24.36, p < .001), because the Grade effect was larger in French (z = 9.64, p < .001) than in German (z = 2.79, p = .005).

Priming Effects

There was a significant main effect of Prime Type (χ 2 (3) = 10.68, p = .014), showing that the global difference between conditions was significant. However, none of the three contrasts were significant (embedded word priming: z = 1.14, p = .253; morpho-orthographic priming: z = 0.36, p = .718; morpho-semantic priming: z = 1.74, p = .082).

Individual Differences in Reading Fluency

There was a significant main effect of Reading Fluency (χ 2 (1) = 122.73, p < .001), indicating that children with high reading proficiency responded more accurately than children with low reading proficiency. The interaction between Language and Reading Fluency was significant (χ 2 (1) = 12.41, p < .001), suggesting that the effect of reading fluency was larger in German than in French.

Discussion

The aim of the present study was to shed light on the use of morphological knowledge during reading development, and to compare the developmental trajectories in two different languages. To this end, we carried out a two-year longitudinal masked priming study in two large cohorts of French and German speaking third-and fourth-graders. The study was designed in a way that it allowed for the dissociation of embedded word activation, morpho-orthographic and morpho-semantic processing, using four priming conditions: affixed word (farmer-FARM), affixed nonword (farmity-FARM), non-affixed nonword (farmald-FARM), and unrelated control primes (workald-FARM). In addition, to examine differences between prefix and suffix processing in children's reading development, the materials included half prefixed and half suffixed word and nonword trials. The study revealed several key findings, which are summarised below.

Embedded Word Priming

One of the first points to note is that the results showed robust embedded word priming effects that were stable across languages, grades, and affix types. These priming effects suggest that third-and fourth-graders are already able to activate words embedded in initial and final string positions, independently of whether they are accompanied by a real affix or a non-affix.

These findings converge with prior evidence from skilled readers (e.g., [START_REF] Beyersmann | Embedded Stem Priming Effects in Prefixed and Suffixed Pseudowords[END_REF]Beyersmann et al., 2018;[START_REF] Crepaldi | Seeing Stems Everywhere: Position-Independent Identification of Stem Morphemes[END_REF][START_REF] Heathcote | Do 'blacheap' and 'subcheap' both prime 'cheap'? An investigation of morphemic status and position in early visual word processing[END_REF]. One mechanism by which readers identify embedded words in initial and final string position is edgealigned encoding of letter position, using the spaces at each end of the letter string as anchor points for positional, orthographic encoding (Beyersmann & Grainger, in press;[START_REF] Fischer-Baum | Both-edges representation of letter position in reading[END_REF][START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF]. Significant embedded word priming has been previously reported for words embedded in edge-aligned string position (e.g., bookpime-BOOK), but not for words embedded in mid (e.g., pibookme-BOOK) or outer string position (e.g., bopimeok-BOOK), supporting the idea that words embedded in edge-aligned position receive the strongest form of lexical activation (Beyersmann et al., 2018). As such, the activation of embedded words can be achieved on the basis of an entirely non-morphological process of mapping letters onto existing whole-word representations in the orthographic lexicon, which is consistent with Grainger and Beyersmann's "word and affix" model (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF].

Cross-Linguistic Differences in Morphological Priming

Clearly distinct patterns of morpho-orthographic and morpho-semantic priming were observed across the two languages. In the German cohort, morpho-orthographic and morphosemantic priming effects were entirely absent. In contrast, the French data revealed significant effects of morpho-orthographic and morpho-semantic priming that were modulated by individual differences in reading fluency and affix type. The interaction between Prime Type and Affix Type showed that, although the activation of embedded words played an equally important role in both prefixed and suffixed trials, it was only in the prefixed trials that French participants additionally benefitted from decomposition into morpho-orthographic subunits, whereas in the suffixed trials, the only additional benefit was from morpho-semantic overlap between prime and target. One explanation for the application of a more form-based segmentation mechanism for prefixes than for suffixes is that participants relied on a left-to-right decoding mechanism. Leftto-right reading techniques are not uncommon in children who are still in the earlier stages of learning to read (e.g., [START_REF] Acha | The effects of length and transposed-letter similarity in lexical decision: evidence with beginning, intermediate, and adult readers[END_REF][START_REF] Ziegler | Developmental dyslexia in different languages: Language-specific or universal?[END_REF]. When primes consisted of a prefix, children likely decoded the left aligned prefix first, and subsequently the right-aligned stem, which must have helped them gain faster access to the morpho-orthographic representations of prefixed words. When primes consisted of a suffix, children likely decoded the left aligned stem first, and then the right-aligned suffix, thus resulting in a slower, more semantically driven segmentation process.

One may ask why the application of a left-to-right scanning mechanism was not also reflected in larger priming effects for stems embedded in initial string position compared to stems embedded in final string position. In line with the principles of the "word and affix" model (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF], we hypothesize that the activation of embedded words and suffixes are based on two distinct processes. Embedded words can be activated via the embedded whole-word pathway, whereas affixes are subject to affix activation and morpho-orthographic full decomposition. The earlier acquisition of the embedded word activation mechanism leads to more efficient mappings between the orthographic input level and the orthographic lexicon, such that the activation of embedded words can rapidly proceed for words embedded in both initial and final string position. As a result, the activation of embedded words is less affected by a left-to-right scanning strategy. In contrast, the careful sequential scanning of letters would be of greater importance for the activation of comparatively weaker affix representations in children's orthographic lexicon. These findings suggest that morphological processing in French primary school children must be driven by different component processes than in German primary school children and shows that the development of morphological processing mechanisms is influenced by the intrinsic linguistic properties of the language that children are exposed to.

Morphological Priming as a Function of Individual Differences in Reading Fluency

In the German cohort, the priming effects were not modulated by individual differences in reading fluency (but see [START_REF] Hasenäcker | Morphological priming in children: Disentangling the effects of school-grade and reading skill[END_REF], who reported an effect of reading proficiency on masked priming latencies, with greater embedded word priming effects in good vs. poor German 2-4 graders). In the French cohort, two interesting interactions were observed between reading fluency and morphological priming. First, morpho-orthographic priming effects were modulated by reading fluency, with larger facilitatory priming effects in children with lower levels of reading fluency (see black dotted line in left panel of Figure 4). Second, morphosemantic priming effects were modulated by reading fluency, with larger inhibitory priming effects in children with lower levels of reading fluency (see grey dotted line in left panel of Figure 4). A potential explanation for the modulation of morphological priming by children's individual differences in reading fluency is that French children at the lower end of the reading fluency spectrum must rely more heavily on morpho-orthographic segmentation processes (for a similar pattern of findings, see Beyersmann, Casalis, Ziegler, & Grainger, 2015;Beyersmann, Grainger, et al., 2015). French children with higher reading proficiency, on the other hand, were more expert at decomposing letter strings into morpho-semantic reading units. As has been previously argued by Beyersmann, Casalis, et al. (2015), participants with higher levels of language proficiency tend to map sub-lexical orthography onto whole-word orthographic representations, and therefore rely to a lesser extent on form-based morphological segmentation processes. In other words, good readers are able to rapidly access the lexical representations that are associated with the input string, thus gaining faster access to the semantics of the embedded morphemes, which would explain why increased morpho-semantic priming was observed in this participant group. This also provides an explanation for the inhibitory pattern of morphoorthographic priming pattern in highly fluent readers, given the semantic incompatibility of the morphemic sub-units in this condition. Poorer readers, on the other hand, would be more likely to compensate for their less efficient whole-word activation by relying to a greater extent on morpho-orthographic segmentation.

Another point to note is that morpho-semantic priming (defined by the difference in priming between farmer-FARM and farmity-FARM), but not embedded word priming (defined by the difference in priming between farmald-FARM and walkald-FARM), was modulated by individual differences in reading fluency in the French cohort. This indicates that proficient French readers were able to access whole-word representations (as evidenced by increased priming in the farmer-FARM compared to the farmity-FARM condition). However, the lack of an interaction between embedded word priming and individual differences in reading fluency suggests that they did not stand out in their ability to identify embedded word units. The combination of these two findings shows that proficient developing readers of French are better at rapidly activating complex whole-word representations and their associated higher-level semantic representations, whereas less proficient readers make more use of lower-level formbased decoding stages.

Developmental Aspects of Complex Word Recognition

While German children primarily relied on one single mechanism (embedded word activation), French children used three clearly distinct mechanisms to process morphologically complex words (embedded word activation, morpho-orthographic processing, and morphosemantic processing). Here we discuss how the "word and affix" model of complex word reading (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF] can account for these findings. The dissociation between embedded word activation and morphological segmentation effects provides support for the notion that embedded stem activations are handled via an independent pathway. Embedded stems, which also typically act as free-standing words in children's reading (with the exception of bound stems), do not require setting up any specialized morphological representations. Children therefore quickly acquire the ability to match the orthographic input with existing whole-word orthographic representations, which includes the activation of embedded words that only form a subset of the letters of the input string. The activation of embedded words then acts as a bootstrapping mechanism for morphological parsing in children's reading development (Beyersmann & Grainger, in press;Beyersmann, Grainger, et al., 2019;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF]. To become proficient at applying more sophisticated morpho-orthographic and morpho-semantic processes, children need to first acquire abstract affix representations. The model therefore predicts a number of consecutive developmental stages, which children have to go through in order to read complex words fluently. The developmental milestones in this process include the acquisition of an embedded word activation mechanism, followed by the acquisition of a morpho-semantic feedback mechanism, followed by the development of a morpho-orthographic full decomposition mechanism (Beyersmann & Grainger, in press;[START_REF] Grainger | Edge-aligned embedded word activation initiates morpho-orthographic segmentation[END_REF].

Our results provide insights into the developmental trajectory of morphological parsing mechanisms across two different languages. In German, the prominence of embedded word priming effects suggests that children in this cohort had not yet moved past the initial developmental stage of embedded word activation. This finding is not entirely surprising, given that the German language is characterized by a productive compounding system (e.g., Creutz & Lagus, 2005;Creutz et al., 2005;[START_REF] Fleischer | Wortbildung der deutschen Gegenwartssprache[END_REF][START_REF] Meyer | Compound comprehension in isolation and in context: The contribution of conceptual and discourse knowledge to the comprehension of German novel noun-noun compounds[END_REF]. Due to the frequency of exposure to stem-stem concatenations, German speakers are likely to develop a high level of proficiency in processing embedded words [START_REF] Beyersmann | Morphological processing across modalities and languages[END_REF]. The absence of morphoorthographic or morpho-semantic priming effects in German children confirms the hypothesis that embedded word activation represents the primary word identification mechanism in the developing German reading system. Our data do not answer the question of whether German children ever move past the initial stage of embedded word activation. They may continue to use embedded word activation as a primary tool to process complex words once they have become skilled readers. We know, however, from previous work, that in contrast to children, German speaking adults do show evidence for morpho-orthographic decomposition, in addition to embedded word processing [START_REF] Hasenäcker | Masked morphological priming in German-speaking adults and children: Evidence from response time distributions[END_REF]. This implies that German children, on their path to becoming skilled readers, do indeed develop morphological segmentation mechanisms that go beyond the early stage of embedded word activation, but that this does not happen until the later stages of primary school.

The masked priming effects in the French data show that children in this cohort, although matched to their German peers in age and grade-level, have already acquired the ability to rapidly decompose letter strings into morpho-orthographic and morpho-semantic sub-units by Grade 3. These findings converge with the results of [START_REF] Quémart | The role of form and meaning in the processing of written morphology: A priming study in French developing readers[END_REF], suggesting that morpheme segmentation mechanisms are acquired relatively early in French children's reading development. Grainger and Beyersmann's model predicts that in order to reach a level of automaticity in morpho-orthographic processing, children must sequentially move through the earlier developmental stages of embedded word activation and morpho-semantic processing. The model thus implies that French children move through the developmental stages faster and acquire the skill to rapidly and automatically decompose letter strings into their morphemes sooner in reading development than German children. There are two explanations for why this may be the case. One explanation for why morphological processing may be of greater importance in a language like French is that the primary word formation process is one that includes the concatenation of stems and affixes. The segmentation into stem + affix provides an avenue for rapid access to the meaning of the embedded constituents and can also be used as a tool to derive meaning from novel words with morphologically complex structures. An alternative explanation for why French children acquire morphological segmentation mechanisms early in their reading development is that the nature of French orthography induces processing mechanisms that are more likely to pick up sub-lexical reading units. French orthography is high in complexity, due to its large number of complex letter-sound correspondences (e.g., [START_REF] Schmalz | Unpredictability and complexity of print-to-speech correspondences increase reliance on lexical processes: more evidence for the orthographic depth hypothesis[END_REF][START_REF] Schmalz | Getting to the bottom of orthographic depth[END_REF][START_REF] Van Den Bosch | Measuring the complexity of writing systems[END_REF], including many multi-letter rules and context-sensitive regularities. Importantly, French is orthographically more complex than German [START_REF] De Simone | Order among chaos: Cross-linguistic differences and developmental trajectories in pseudoword reading aloud using pronunciation Entropy[END_REF][START_REF] Schmalz | Unpredictability and complexity of print-to-speech correspondences increase reliance on lexical processes: more evidence for the orthographic depth hypothesis[END_REF]. When words contain complex correspondences, the identification of sub-lexical reading units is necessary to access full information about the word's phonology and semantics.

Therefore, learning to read within a complex orthography may shift children's reliance from single letter decoding onto multi-letter decoding early on in reading development. The complexity of the French orthography thus provides another possible explanation for the earlier acquisition of morphological segmentation mechanisms in French compared to German children.

Limitations and Future Directions

A point that potentially raises concern is that the observed embedded word priming effects may be attributed to the orthographic overlap between prime and target, and to principles of letter position encoding, rather than the activation of the embedded word as a lexical unit.

However, what speaks against the argument of lower-level orthographic processing is that the activation of embedded words is known to be modulated by morphological family size (Beyersmann & Grainger, 2018) and conditional suffix probability of the embedded word unit (Grainger & Beyersmann, 2020), suggesting that embedded words are indeed activated to the level of the lexicon. Moreover, the here reported evidence for morphological processing in the French sample demonstrates that the observed priming effects, at least in the French sample, were not just a result of the orthographic similarity between primes and targets, but rather due to the complex interplay between embedded word and morpheme activation mechanisms. An interesting follow up of the present study would be the cross-linguistic examination of conditional suffix probability effects in German and French speaking children, as a direct test of the lexical mechanisms underlying embedded word processing. The here observed differences in morphological processing between German and French make the prediction that effects of conditional suffix probability should develop sooner in French compared to German speaking children. From a broader developmental perspective, it is important to be reminded that the present study's methodological approach, that is, masked priming, provides insights into the rapid, more form-based aspects of morphological processing and does not necessary reflect the entire chain of processes that are activated during visual word recognition and -more generally -during reading acquisition. To understand the influence of morphological knowledge on children's reading skills, a wider range of methods should be used, perhaps with longer prime durations, to investigate the later stages of the reading process, as well as paradigms that would allow us to examine morphological processing across modalities. Related to this, a recent lexical decision study with adults showed that morphological processing may be more prominent in the visual than in the auditory modality [START_REF] Beyersmann | Morphological processing across modalities and languages[END_REF], but this has yet to be confirmed with children.

Due to the tightly controlled experimental design, the German and

A further limitation concerns the fact that the current cross-linguistic investigation was based on the comparison of two languages that not only differ with respect to their productivity of morphological word formation processes (i.e., more productive compounding in German than in French), but also in terms of orthographic complexity (i.e., more complex letter-sound correspondences in French than in German). It is therefore impossible to clearly tease apart the linguistic pre-requisites that may affect how children learn to read complex words within each of these languages. To gain a deeper understanding of how language-specific differences in orthography and morphology might influence reading acquisition, we need to build on the present study to investigate morphological processing within a broader, universal context. Effects of visual word recognition need to be studied by considering the "full linguistic environment" of a particular language [START_REF] Frost | Towards a universal model of reading[END_REF], including phonology, orthography, morphology, and semantics, rather than just taking into account one or the other. For instance, in Hebrew, the letters of stem morphemes are not represented consecutively, whereas in most Indo-European languages they are. This is just another example of how the complex interplay between orthography, morphology and semantics imposes processing constraints that vastly differ across languages. Hence, cross-linguistic investigations are in a unique position to shed light onto how statistical properties of different writing systems might influence the development of complex word reading mechanisms in young children, and therefore represent a powerful empirical tool for future research.

Appendix

  , French compounds are often non-concatenated (e.g., chef de police [engl. chief of police]) or hyphenated (e.g., grand-père [engl. grandfather]). As a result, French developing readers are primarily exposed to stem + affix concatenations (i.e., prefixed and suffixed words).

(

  recharger-CHARGER [recharge-CHARGE]; mitdenken-DENKEN [think along-THINK]), an affixed nonword prime (excharger-CHARGER [excharge-CHARGE]; hindenken-DENKEN [think towards-THINK]), a non-affixed nonword prime (fecharger-CHARGER [fecharge-CHARGE]; kardenken-DENKEN [karthink-THINK]), and an unrelated prime (felaisser-

  Each target was preceded by an affixed word prime (fillette-FILLE [little girl -GIRL]; steinchen-STEIN [little stone -STONE]), an affixed nonword prime (fillible-FILLE [girlable-GIRL]; steinkeit-STEIN [stonity-STONE]), a non-affixed nonword prime (fillache-FILLE [girlach-GIRL]; steinucht-STEIN [stonel-STONE]), and an unrelated prime (gommache-FILLE [gummach-GIRL]; piratucht-STEIN [pirate-STONE]). The French suffixed words included the suffixes -esse, -ette, -eur, and -ier, and the German suffixes words included the suffixes chen, -haft, -heit, and -lich, which were each repeated 12 times.

  French materials were restricted to verb targets in the prefixed set and noun/adjective targets in the suffixed set. It is possible that the differences in syntactic word class contributed to the observed differences between prefixed and suffixed priming in the French sample. Therefore, a broader investigation of morphological priming across a wider range of syntactic word classes in future research would be desirable.

Figure 1 .

 1 Figure1. The "word and affix" model of complex word reading (adapted fromBeyersmann & Grainger, in press). Orthographic input is mapped onto the orthographic lexicon based on two mechanisms that operate in parallel: (embedded) word activation and affix activation. The principle of "morpho-orthographic full decomposition" operates in the links between the orthographic input (a string of letters) and the entities activated in the orthographic lexicon, by comparing the sum of the letters in the embedded word and the affix with the letters of the input. Representations within the orthographic lexicon are mapped onto a third layer of semantic representations. Connections between layers are bidirectional, thus allowing for bottom-up as well as top-down transfer of information between the three layers.

Figure 6 .

 6 Figure 6. Error Rates and Standard Errors for French and German fourth graders.

Table 1 .

 1 A. Items used in the study. Analysis of variance table for children's response times and accuracy Demographics of child participants.

	French prefixed stimuli
	affixed word	affixed	non-affixed	unrelated
	prime	nonword prime	nonword prime	prime	target
	1 déborder	inborder	daborder	dadurcir	BORDER
	2 décoller	incoller	dacoller	dabrûler	COLLER
	3 découper	incouper	dacouper	dacalmer	COUPER
	4 découvrir	incouvrir	dacouvrir	dadiviser	COUVRIR
	5 défaire	infaire	dafaire	damunir	FAIRE
	6 défiler	infiler	dafiler	dagâter	FILER
	7 démontrer	inmontrer	damontrer	dasecouer	MONTRER
	8 démouler	inmouler	damouler	dafumer	MOULER
	9 dépasser	inpasser	dapasser	dacocher	PASSER
	10 déplacer	inplacer	daplacer	daisoler	PLACER
	11 déménager	inménager	daménager	datraiter	MÉNAGER
	12 déranger	inranger	daranger	daobéir	RANGER
	13 endormir	bidormir	gidormir	gifilmer	DORMIR
	14 enfermer	bifermer	gifermer	gicoudre	FERMER
	15 enfoncer	bifoncer	gifoncer	gifendre	FONCER
	16 enfuir	bifuir	gifuir	giagir	FUIR
	17 enlever	bilever	gilever	gifinir	LEVER
	18 enneiger	bineiger	gineiger	givibrer	NEIGER
	19 entraîner	bitraîner	gitraîner	gidoucher	TRAÎNER
	20 envoler	bivoler	givoler	gigarer	VOLER
	21 encadrer	bicadrer	gicadrer	gicourir	CADRER
	22 encercler	bicercler	gicercler	gigonfler	CERCLER
	23 endurer	bidurer	gidurer	giskier	DURER
	24 enlacer	dislacer	gilacer	gisucer	LACER
	25 préchauffer	dischauffer	blochauffer	blohéberger	CHAUFFER
	26 prélasser	dislasser	blolasser	blofriser	LASSER
	27 prévenir	disvenir	blovenir	blologer	VENIR
	28 prédire	disdire	blodire	blofier	DIRE
	29 préjuger	disjuger	blojuger	blomêler	JUGER
	30 prélever	dislever	blolever	blonager	LEVER
	31 préoccuper	disoccuper	blooccuper	blogrogner	OCCUPER
	32 préposer	biposer	bloposer	blorugir	POSER
	33 prévoir	disvoir	blovoir	blorire	VOIR
	34 prédominer	disdominer	blodominer	blohabiter	DOMINER
	35 prédéterminer disdéterminer	blodéterminer	blosommeiller DÉTERMINER
	36 préfigurer	disfigurer	blofigurer	blohériter	FIGURER
	37 recharger	excharger	fecharger	felaisser	CHARGER
	38 recopier	excopier	fecopier	feglacer	COPIER
	39 redonner	exdonner	fedonner	fefâcher	DONNER
	40 redresser	exdresser	fedresser	femaigrir	DRESSER

Table 2 .

 2 Psycholinguistic properties of French and German words (SDs in Parentheses).

	Words	French	German
		Prefixed stimuli
	Number of letters of targets	6.02 (1.12)	6.02 (0.64)
	Frequency/million of targets	121.47 (296.36)	99.40 (132.41)
	Number of letters of affixed words	8.27 (1.22)	8.52 (0.82)
	Frequency/million of affixed words	15.63 (26.32)	4.50 (5.41)
		Suffixed stimuli
	Number of letters of targets	5.29 (0.82)	5.10 (0.93)
	Frequency/million of targets	132.60 (181.57)	108.53 (196.96)
	Number of letters of affixed words	8.02 (1.08)	9.10 (0.93)
	Frequency/million of affixed words	15.45 (19.14)	15.94 (32.58)

Table 3 .

 3 Summary of RT priming effects for each participant cohort and affix type.

			Morpho-orthographic	Morpho-semantic
	Affix Type Embedded stem priming	priming	priming
			French children
	Prefix	71ms, z =2.61, p=.009	43ms, z =3.19, p=.001	-24ms, z=1.44, p=.151
	Suffix	33ms, z=2.64, p=.008	2ms, z=0.89, p=.373	27ms, z=2.07, p=.038
			German children
	Prefix	39ms, z=2.20, p=.028	10ms, z=0.50, p=.619	3ms, z=0.19, p=.849
	Suffix	54ms, z=2.94, p=.003	18ms, z=1.00, p=.319	5ms, z=0.31, p=.758

We note that cross-linguistic differences in complex word recognition as a function of morphological productivity(Beyersmann et al.,

2020), do not necessarily generalize to the field of spoken word production[START_REF] Mousikou | Orthographic consistency influences morphological processing in reading aloud: Evidence from a cross-linguistic study[END_REF].[START_REF] Mousikou | Orthographic consistency influences morphological processing in reading aloud: Evidence from a cross-linguistic study[END_REF] used a reading aloud task to study children's ability to map letters onto sounds, showing that children's reading aloud skills varied not as a function of morphological productivity, but as a function of orthographic consistency. This finding is not inconsistent with the findings of the present study, because the consistency with which letters in a certain language map onto phonemes might be critical for reading aloud, but not necessarily for visual word recognition.

We interpret the difference between these two conditions in terms of embedded word processing, and not in terms of lower-level orthographic processing. Embedded word priming effects are thought to be modulated by the morphological family size(Beyersmann & Grainger, 2018) and conditional suffix probability of the embedded word unit(Grainger & Beyersmann, 2020), suggesting that embedded words are activated to the level of the lexicon. Moreover, although words produce priming when embedded in edge-aligned string positions (e.g., bookpime-BOOK), this is not the case for words embedded in mid (e.g., pibookme-BOOK) or outer string position (e.g., bopimeok-BOOK), supporting the idea that orthographic prime-target overlap alone is not sufficient to produce priming(Beyersmann et al., 2018).

It is worth noting that the comparison of word frequencies in the two languages is not straightforward, because of the different sizes of the corresponding corpora.

Six French items with orthographic illegalities(inborder, inpasser, inplacer, inmontrer, inmouler, inménager) were mistakenly included in the current set of materials. We therefore re-ran the analyses, which showed that the exclusion of these items did not change the direction or significance of the results.

Figure 2. Lexical Decision Latencies (in Milliseconds) and Standard Errors of French and German third graders.

Acknowledgements

This research was supported by grants from the French National Agency for Research (ANR) and the German Research Foundation (DFG), in particular the French-German ANR/DFG grant scheme in social sciences and humanities (ANR-15-FRAL-0003-01) and a DFG grant (SCHR-1268/2-1). The research also benefitted from support from grants ANR-16-CONV-0002, ANR-11-LABX-0036). EB was supported by a Discovery Early Career Researcher Award (DECRA) by the Australian Research Council (DE190100850), and JG by ERC grant 742141.

Figure 3. Lexical Decision Latencies (in Milliseconds) and Standard Errors of French and German fourth graders.