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Grapheme to phoneme conversion is a critical step in reading processing, as notably evidenced by its role in literacy acquisition (
According to the dual route cascade model (
Studies identifying the neural correlates of the two routes by contrasting word versus pseudoword (PW) reading yielded diverging results. In reading tasks, greater activation for PWs than words was found in both, left occipito-temporal and inferior frontal regions (
More recent studies confirmed these results by linking the lexical processing of words to bilateral posterior cingular, inferior-middle temporal and temporo-parietal regions (
In addition, lesion-based studies of acquired dyslexia found that surface dyslexia, where due to impaired lexical pathways patients are able to read pronounceable PWs and unable to read (irregular) words, goes in line with deficits in inferior temporal (
The inconsistent findings related to the anatomical underpinnings of reading routes may be due to differences in tasks applied, ranging from reading paradigms (
In addition to the degree of the lexicality or familiarity of the words being read, the orthographic depth hypothesis (
Of note, the engagement of a given pathway is not exclusive, i.e., reading processing generally involves both routes, but one route may be predominantly activated compared to the other depending on the orthographic depth index of the language (
Only few studies have brought evidence for a modulation of brain activity during reading by orthographic depth of the used language. In a PET study contrasting English and Italian monolinguals,
Of note, relative early latencies have also been found to be critically engaged in graphemic/phonologic conversion.
Collectively, the results indicate that a modulation of orthographic depth may impact reading routes around 300 ms after stimulus onset. Grapheme to phoneme mapping in languages with shallow orthographies seems to rely on regions involved in grapho-phonological processing (superior temporal, supramarginal and opercular inferior frontal regions), indicating an activation of non-lexical pathways. In contrast, grapheme to phoneme mapping in languages with deep orthographies seem to rely on regions involved in lexico-semantic processing (inferior and middle temporal and triangular inferior frontal regions), indicating an activation of lexical pathways. Thus, orthographic depth may indeed impact reading route selection.
However, because previous studies used between-subject or cross-language designs, the conclusions about the effect of orthographic depth that can be drawn from current literature are limited. In between-subject designs (
The aim of the present study was to investigate the impact of orthographic depth on reading route selection using an experimental design excluding possible effects related to differences in stimuli or readers. We presented the same PWs to highly proficient bilinguals and manipulated the orthographic depth of PW reading by embedding them among two separated language contexts respectively implicating shallow or deep orthography. The use of PWs as target stimuli will probably strengthen non-lexical processing independent of language contexts. In contrast, the reading route predominantly engaged during a language context will depend on its orthographic depth: the deep language context may strengthen lexical and the shallow language context non-lexical pathways. Consequently, if orthographic depth modulates reading routes, reading in the shallow context will support the non-lexical pathways routinely recruited to process PWs. In contrast, non-lexical pathways routinely recruited to process PWs may be less engaged when reading in the deep compared to the shallow context. Together, we predict a differential engagement of non-lexical pathways between pre-lexical and semantic processing stages (~300 ms) reflected in a stronger activation of grapho-phonological (superior temporal, supramarginal and inferior frontal) areas in the shallow versus the deep language context when reading identical PWs across language context. The study of early/high proficient bilinguals enabled controlling for socio-cultural effects and the use of identical stimuli across conditions for effects due to linguistic and/or physical differences, thus isolating the effect of orthographic depth in the 1 versus 1 within-subject design.
Fourteen healthy female French/German bilinguals participated in the study (all right-handed,
French-German bilingualism characteristics of participants (
Variable | French |
German |
|||
---|---|---|---|---|---|
mean | SD | mean | SD | ||
Age of acquisition (years) | 1.61 | 2.21 | 0.93 | 1.87 | 0.43 |
Lived in region speaking (years) | 16.65 | 6.33 | 11.92 | 9.34 | 0.19 |
First language mother | 14 | 35 | 79 | 41 | 0.01* |
Language spoken with mother | 50 | 50 | 57 | 49 | 0.78 |
First language father | 50 | 50 | 50 | 50 | 1.00 |
Language spoken with father | 58 | 49 | 46 | 50 | 0.55 |
Language taught in school | 48 | 43 | 52 | 43 | 0.88 |
Language spoken with peers at school | 52 | 44 | 41 | 43 | 0.65 |
Language spoken with family | 45 | 32 | 45 | 32 | 1.00 |
Spoken at work | 52 | 26 | 48 | 26 | 0.81 |
Watching TV/listening radio | 30 | 22 | 66 | 24 | 0.01* |
Speaking with friends | 59 | 22 | 38 | 21 | 0.09 |
Reading books | 43 | 24 | 54 | 26 | 0.45 |
Mental arithmetic | 64 | 35 | 27 | 31 | 0.04* |
Learned in school only | 7 | 26 | 0 | 0 | - |
Learned “on road” only | 0 | 0 | 0 | 0 | - |
Learned at workplace only | 0 | 0 | 0 | 0 | - |
Speaking | 94 | 6 | 92 | 11 | 0.57 |
Comprehension | 97 | 4 | 96 | 5 | 0.82 |
Reading | 90 | 11 | 90 | 12 | 0.91 |
Writing | 77 | 22 | 80 | 17 | 0.64 |
DIALANG score | 852 | 124 | 864 | 59 | 0.77 |
All participants filled out a questionnaire evaluating French and German language skills consisting of three parts (
Target stimuli of the study were orthotactic (i.e., orthographically legal) PWs composed of 4 to 6 letters (to avoid eye movements). One hundred and twenty PWs were generated using WordGen software (
French and German words were presented in addition to the PWs to strengthen language context (see “Procedure and Task”). Four hundred and eighty French Words were selected from Lexique database (
One hundred and twenty symbol strings (symbols) were created by changing the font of the PWs to “symbols” in MS Word (Microsoft Corporation, 2010). symbols were intended to be part of future research and were not analyzed in the present study. Examples of symbols are: Nατε, Δαυδ, Mελλε, Aπασε, Γαυτελ, Γρυττε.
The task in this study was to read aloud French and German words, PWs and symbols displayed on a computer screen.
Participants were seated in an electrically shielded and sound attenuated booth 90 cm in front of a 21-inch LCD screen. Stimulus delivery and response recording were controlled using E-Prime 2.0 (Psychology Tools, Inc., Pittsburgh, PA, USA). Stimuli were presented in the center of the screen and displayed in black font color on white background. Each trial started with the presentation of a fixation cross of 400 ms duration, followed by a pseudo-randomly determined stimulus (66% word, 17% PW or 17% symbol, see next paragraph) displayed for 472 ms to allow comfortable reading (Courier New, pt. 24). A response window displaying a fixation cross with a random duration between 1200 and 1700 ms was presented after the stimuli (inter trial interval;
Since the aim of the study was to investigate the effect of orthographic transparency on reading identical stimuli, orthographic depth of PW reading was manipulated by creating two separated language context sessions (experimental phase;
At the beginning of each language context session, a short text written in the corresponding language was presented in order to activate the given language. Next, a 2 min training block with words (not included in experimental phase) in the language of the selected context session was started to familiarize the procedure and verify the apparatus, before initiating the experimental phase. To reduce fatigue, stimuli presentation of one language context session was divided into four blocks separated by 1–2 min breaks. One block comprised of randomly selected 30 PWs, 30 symbols, and 120 words and lasted around 6 min. The order of blocks was randomized across participants. Both language context sessions were separated by a pause of at least 10 min. The order of language context sessions was randomized across participants.
Continuous EEG was acquired at 1024 Hz through a 128-channel Biosemi ActiveTwo system (Biosemi, Amsterdam, Netherlands) referenced online to the CMS-DRL ground, which functions as a feedback loop driving the average potential across the montage as close as possible to the amplifier zero. Electrode impedances were kept below 20 kOhm. EEG data preprocessing and analyses were conducted offline using Cartool (
Response accuracy of PWs and words was assessed by auditory inspection of the audio files generated with E-Prime to determine whether different language contexts were created successfully. Expected pronunciations were a priori defined by a native German and a native French speaker. Five types of errors were defined: language intrusion (complete or partial German pronunciation in French context or complete or partial French pronunciation in German context), orthography (adding, exchanging or omitting letters), phonology (unusual phonological coding of correct orthographic form), intonation (wrong lexical stressing), and other errors leading to an incorrect response (e.g., abortion, correction, no response, pronunciation in a third language).
Examples of language intrusions demonstrated on the PWs “nate” (correct response German = [’na:tə]; correct response French = [nat]), “melle” (correct response German = [’mεlə]; correct response French = [mεl]) and “apsase” (correct response German = [a’pa:zə]; correct response French = [’apa:z]) are: [’na:tə] resp. (’mεlə] (both complete) or [‘apa:zə] (partial) in French context and [nat] resp. [mεl] (both complete) or [a’pa:z] (partial) in German context. Examples for orthographic errors demonstrated on the PW “grutte” (correct response German = [’grυtə]; correct response French = [g
To investigate whether response accuracy rates differentiate or interact across conditions, a 2 × 2 repeated-measures analysis of variance (ANOVA) with factors language context (French vs. German) and Stimulus Type (Words vs. PW) was performed. In addition, a paired
To investigate whether error types in PW reading differentiate across language contexts, a one-way repeated-measures multivariate analysis of variance (MANOVA) was performed. language context (French, German) was included into the analysis as independent and Language Intrusion Errors, Orthographic Errors and Phonological Errors in PW reading were included as dependent variables. Due to low incidences (see first paragraph of “Behavioral Results”), intonation errors and errors labeled as “other” were excluded from the analysis to increase statistical power. A series of one-way univariate analyses were performed as
Production latencies [reaction times (RT)] were assessed with a speech analysis software (Praat;
To investigate whether production latencies differentiate or interact across conditions, a 2 × 2 repeated-measures ANOVA with factors language context (French vs. German) and Stimulus Type (Words vs. PW) was performed. In addition, a paired
Unless otherwise stated, significance threshold was set at
Waveform analyses were performed to determine time periods where ERP amplitude differences occurred between the conditions PWs in French context versus PWs in German context.
Time-frame wise paired
A topographic pattern analyses was applied to the ERP to determine whether and when distinct configurations of brain network were engaged in response to the PWs when read in the French vs. German context. This approach is based on evidence that the ERP map topography does not vary randomly across time, but remains quasi-stable over 20–100 ms functional microstates before rapidly switching to other period of stable topography (
The most dominant topographic maps appearing in the visual evoked potentials (VEPs) of the group-averaged ERPs from each condition over time were identified with a modified hierarchical cluster analysis, the topographical atomize and agglomerative hierarchical clustering (T-AAHC;
The present multivariate topographic analyses have the advantage of being reference-independent (
Electrical source estimations were calculated using a distributed linear inverse solution and the local autoregressive average (LAURA) regularization approach
Mean accuracy (SD) on the whole group of 12 subjects were for Words in French context 98% (7%), words in German context 98.5% (8%), PWs in French context 93% (4%) and PWs in German context 93% (3%). For words in French context, 0% intrusion, 0.23% (2.43%) orthographic, 01.06% (6.47%) phonological, 0.38% (2.34%) intonation and 0.16 % (2.49%) other errors were observed. For PWs in French context, 3.8% (4.09%) intrusion, 1.88% (2.13%) orthographic, 1.03% (1.23%) phonological, 0.48% (0.76%) intonation and 0 % other errors were observed. For words in German context, 0% intrusion, 0.38% (3.24%) orthographic, 0.35% (3.30%) phonological, 0.59% (5.16%) intonation and 0.16 % (2.49%) other errors were observed. For PWs in German context, 3.4% (3.15%) intrusion, 2.8% (1.75%) orthographic, 0.13% (0.37%) phonological, 0.3% (0.47%) intonation, and 0.13 % (0.37%) other errors were observed.
Repeated-measures ANOVA with factors language context (French vs. German) and Stimulus Type (Words vs. PW) was performed to investigate whether accuracy rates differentiate or interact across conditions. This analysis revealed no main effect of language context [
Repeated-measures MANOVA with language context (French, German) as independent and Language Intrusion Errors, Orthographic Errors and Phonological Errors as dependent variables was performed to investigate whether error types in PW reading differentiate across language contexts. This analysis revealed a significant multivariate effect for language context [
Mean RTs (SD) on the whole group of 12 subjects were for words in French context 720 ms (168 ms), Words in German context 706 ms (165 ms), PWs in French context 730 ms (172 ms) and PWs in German context 718 ms (164 ms).
Repeated-measures ANOVA with factors language context (French vs. German) and Stimulus Type (Words vs. PW) was performed to investigate whether production latencies differentiate or interact across conditions. This analysis revealed no main effect of language context [
Evoked potential waveforms to the PWs presented in the two language context are depicted in
Paired
Agglomerative hierarchical clustering was applied on the ERPs to identify the pattern of predominating topographic maps of the electric field at the scalp in the cumulative group-averaged data. The output of the topographic pattern analysis is displayed in
In order to localize the effect in the brain space, paired
We investigated the spatio-temporal impact of orthographic depth on reading. Identical PWs were presented to highly proficient bilinguals embedded either in a deep orthographic (French) or in a shallow orthographic (German) language context. The lexical context in which the stimuli were presented (80% words and 20% PWs) has been designed to force initial automatic word reading in the pre-activated context and to force PW reading in the corresponding orthographic depth. Our results show that orthographic depth induced by language context indeed impacts brain response to reading physically identical stimuli. The topography of the ERPs to identical PWs differed 300–360 ms post-stimulus onset when the PWs were read in different orthographic depth context, indicating distinct brain networks engaged in reading during this time window. Analysis of electrical source estimation over the period of topographic modulation showed a differential engagement in left inferior-frontal, left superior parietal and left anterior cingular areas in the deep versus shallow condition.
The topography of the ERPs to identical PWs differed around 330 ms post-stimulus onset when the PWs were read in different orthographic depth context. Because distinct topographies necessarily follow from distinct configuration of the underlying brain network (e.g.,
The 330 ms latency of the topographic modulation has been associated to processing stages involved in grapheme to phoneme conversion in previous studies (
The dual route cascade model posits a lexical and a non-lexical route among which graphemes and phonemes are being mapped (
Alternatively, one might consider that the modulation of reading route selection by orthographic depth was not restricted to word reading, but directly impacted PW processing. Thus, PW reading might have recruited non-lexical pathways in the shallow and lexical pathways in the deep context. However, the use of low-lexical target stimuli (PWs) and the absence of a differential engagement of lexico-semantic networks across conditions in the results of electrical source estimation over the period of topographic modulation (see “Location of the effect of orthographic depth”) speak against a direct and in favor of an indirect modulation of reading route by orthographic depth. Nevertheless, further research is needed to unravel the precise mechanism underlying the orthographic-related reading route modulation.
Given the fact that the 300–350 time window has also been associated to (early) semantic processing in reading (
Our results thus suggest that distinct brain networks support PW reading 300–360 ms post-stimulus onset when they were read in different orthographic depth context. We propose that these distinct brain networks reflect a modulation of the non-lexical grapheme to phoneme conversion routinely engaged in PW reading by the activation of different reading routes in word reading across language contexts. More precisely, reading (German) words in a shallow context may preferentially activate non-lexical pathways, which strengthen the engagement of the non-lexical pathways routinely recruited in PW reading in the shallow versus the deep context. In contrast, reading (French) words in a deep orthographic context may preferentially recruit lexical pathways, which reduce the reliance on routinely recruited non-lexical pathways in PW reading in the deep versus the shallow context. Thus, the topographic modulation in PW reading might indirectly reflect the engagement of different reading routes across the orthographic depth of language contexts.
Statistical analyses of electrical source estimations over the period of topographic modulation support the hypothesis of orthographic-related reading route modulation by showing differential engagement in the deep versus shallow conditions in left inferior-frontal, left superior parietal and left anterior cingular areas.
The left inferior frontal region (part of Broca’s area complex) was activated stronger when reading in the German than French context. The inferior frontal activation might indicate enhanced engagement of phonological processing when reading PWs in the shallow orthography in contrast to reading in the deep orthography. Previous findings showed that inferior frontal regions are involved in grapheme to phoneme conversion (
However, our findings contrast with the results by
Additionally, inferior frontal regions have been associated to the motor control of speech articulators (
A more pronounced engagement of non-lexical networks could have been expected, as numerous reading studies have demonstrated broad networks to critically underlie grapho-phonological processing covering temporal, parietal and frontal brain regions (
The differential engagement of parietal–cingular areas may follow from a modulation of attentional demands across language context.
The activity within superior parietal areas (BA 7) was stronger when reading in the French than German context. In reading, superior parietal areas have been advanced to contribute to visual attention, which could be involved in modifying the reading strategy (
However, alternative explanations could account for the effect found, as parietal areas have been put forward to be involved in a variety of cognitive tasks, including eye movements (especially IPS;
The anterior cingular activity (parts of BA 24, BA 32, and BA 33) was stronger for reading in the German than French context. The cingulate cortex has been linked to inhibitory control and error detection (
Cingular activities have been found in proficiency-related control processes (
The temporal dynamic of the responses to PWs in a German versus French context also support our hypothesis for a modulation of reading route by orthographic depth. Given the timing of the effect between early (letter identification) and later (semantic) processing and our design enabling isolating the effect of orthographic depth, the topographic difference are likely to reflect different networks engaged in grapheme to phoneme conversion across language context 330 ms post-stimulus onset. We propose that the engagement of the non-lexical reading route routinely involved in PW reading is modulated by the activation of distinct reading routes in word reading across language context. Reading (German) words in a shallow context may activate non-lexical processing, which reinforces the involvement of the non-lexical pathways routinely recruited in PW reading, reflected in a stronger engagement of frontal phonological areas in the shallow versus the deep orthographic context. In contrast, reading (French) words in a deep orthographic context may weaken the non-lexical pathways routinely recruited in PW reading. The recruitment of less routine non-lexical pathways in PW reading might be reflected in a stronger engagement of visuo-attentional parietal areas in the deep versus shallow orthographic context.
Since in the present paradigm, many (real) words were used to create language context, the additional inclusion of words into the analyses may have helped to disentangle the nature of the effects found. However, we think that the joint analysis of words and PWs would unlikely help clarifying the interpretation of the present results because of the following reason: To compare words and PWs (e.g., in terms of pathways engaged), an interaction would be needed between the factors stimulus type (words, PWs) and orthographic depth (shallow, deep;
Several limitations of the current study constrain the interpretability of our results. First, investigating the language system in bilinguals might be less straightforward compared to investigating monolinguals, due to the two languages cohabiting in the brain. This complexity is majorly linked to switching between languages and inhibition of one language (
Second, language skills are probably never perfectly matched across languages, even if statistically comparable in our group. Marginal effects can be argued with regard to the language used to perform mental arithmetic’s, the first language spoken by the mother and the language preferred to watch TV. However, we consider these differences to have unlikely impacted reading performance because of the following reasons: the variables showing differences across languages are related to oral language production, in contrast, variables directly linked to the task, i.e., written language skills (reading books, school, computer-based reading evaluation) showed no differences across languages. In addition, behavioral results (equal RTs/accuracy across languages) speak in favor of balanced proficiency and potential effects of proficiency are minimized by the two separated language context session (
Third, the use of PWs as target stimuli might have enhanced attentional demands and the differences found might reflect controlled instead of automatic processing. Indeed, response accuracy was lower during PW than word reading, indicating that PW reading might have enhanced cognitive control. However, equal RTs across words and PWs suggest that the inaccurate responses occurred pre-attentively during “automatic” reading. Additionally, equal RTs across stimulus type reflect that the “word-likeliness” of the PWs (unlike letter strings or non-words) and the strong language context (generated by adding four times more words than PWs) possibly facilitated the task. More importantly, the task was the same across conditions, thus control strategies should unlikely explain the results. Further, equal RTs and accuracy of PW reading across conditions support a comparable engagement of cognitive control processes, which should thus be cancelled out in the analysis. Finally, the effects found around 300 ms are unlikely to reflect higher processing mechanisms such as cognitive control strategies (
Fourth, the low spatial resolution of EEG inverse solution limits the interpretability of the spatial aspects of our data. However, the high-density EEG montage (128 channels) enables that the localization accuracy with LAURA is in the order of the grid size, i.e., about 0.6 cm3 (
Fifth, a further limitation of the study is the small sample size (
Sixth, due to differences in brain representations of language processing between bi- and monolinguals (
Finally, the dual route cascade model, in its original form, may be too rigid as a template to project our results. Instead, our results should be discussed in terms of a parallel engagement of both routes, but one may be predominantly activated compared to the other depending on the orthographic regularity of the language.
The present study reveals insights into the neural underpinnings of orthographic regularity processing. Our findings complement current literature on reading processing and support the orthographic depth hypothesis (
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work was supported by a grant from the Swiss National Science Foundation to Jean-Marie Annoni (No. 325130_138497). We would like to thank Michaël Mouthon for technical assistance with EEG recordings. Cartool software (
LAURA inverse solution is a weighted minimum norm method together with the LAURA regularization approach. The LAURA method calculates a current density value at each solution point. The local auto-regressive average regularization approach describes the spatial gradient across neighboring solution points (