The apparent late onset of left cortical specialization for print is quite surprising, because preschool children typically present some letter knowledge, this knowledge being one of the most important predictors of subsequent reading proficiency ( 17, 18). This process could occur either through phonological mapping ( 16) or audiovisual integration ( 7). That is, a relatively high level of automatization in letter-speech sound associations appears necessary for left hemispheric posterior lateralization to print, taking time, cognitive resources, and explicit teaching. Collectively, these findings, as well as the relatively small amount of available data in preschool children, suggest that the left hemispheric specialization for letter strings emerges only after formal instruction or intensive training. Moreover, whereas N1 tuning for letter strings can be induced in preschool children with relatively short grapho-phoneme training, this tuning is neither left-lateralized nor long-lasting, disappearing within a few weeks after training interruption ( 15). Typically, sensitivity to print has been shown after 1 y ( 13), or 1.5 y of schooling ( 14), thus, after formal reading education.
11 and 12) is absent in preschool children, even if they can already categorize or name letters.
In developmental studies with direct measures of brain activity (electro/magneto-encephalography, EEG/MEG), the typical reading adult occipito-temporal response evoked by letter strings at approximately 200 ms after stimulus onset (i.e., N1 or N170 refs. 9 for a metaanalysis) and in natural settings, significant left lateralization appears only when children have already acquired formal reading ability, approximately at 7 y of age ( 10). Collectively, these findings indicate that limited knowledge of print in young children, before formal education, is sufficient to develop specialized left lateralized neuronal circuits, thereby pointing to an early onset and rapid impact of left hemispheric reentrant sound mapping on posterior cortical development.įunctional magnetic resonance imaging (fMRI) reveals the typical left lateralized adult-like brain pattern for letter strings only in proficient readers (ref. These findings were replicated in a second experiment ( n = 26 preschool children), where familiar symbols and line drawings of objects evoked right-lateralized and bilaterally specific responses, respectively, showing the specificity of the early left hemispheric dominance for letter strings. These results point to the high potential of this rapidly collected behavior-free measure to assess reading ability in developmental populations. Interindividual differences in the amplitude of this electrophysiological response are significantly related to letter knowledge, a preschool predictor of later reading ability.
Within 2 min of recording only, letter strings evoked a robust specific response over the left occipito-temporal cortex at the predefined frequency of 1.2 Hz (i.e., 6 Hz/5). Here, we recorded scalp electrophysiological (EEG) brain responses in 5-y-old ( n = 40) prereaders presented with letter strings appearing every five items in rapid streams of pseudofonts (6 items per second). In children, this left hemispheric cortical specialization for letter strings is typically detected only after approximately 1 y of formal schooling and reading acquisition. Reading, one of the most important cultural inventions of human society, critically depends on posterior brain areas of the left hemisphere in proficient adult readers.
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