Timing the impact of literacy on visual processing

Abstract

Learning to read requires the acquisition of an efficient visual procedure for quickly recognizing fine print. Thus, reading practice could induce a perceptual learning effect in early vision. Using functional magnetic resonance imaging (fMRI) in literate and illiterate adults, we previously demonstrated an impact of reading acquisition on both high- and low-level occipitotemporal visual areas, but could not resolve the time course of these effects. To clarify whether literacy affects early vs. late stages of visual processing, we measured event-related potentials to various categories of visual stimuli in healthy adults with variable levels of literacy, including completely illiterate subjects, early-schooled literate subjects, and subjects who learned to read in adulthood (ex-illiterates). The stimuli included written letter strings forming pseudowords, on which literacy is expected to have a major impact, as well as faces, houses, tools, checkerboards, and false fonts. To evaluate the precision with which these stimuli were encoded, we studied repetition effects by presenting the stimuli in pairs composed of repeated, mirrored, or unrelated pictures from the same category. The results indicate that reading ability is correlated with a broad enhancement of early visual processing, including increased repetition suppression, suggesting better exemplar discrimination, and increased mirror discrimination, as early as ∼ 100-150 ms in the left occipitotemporal region. These effects were found with letter strings and false fonts, but also were partially generalized to other visual categories. Thus, learning to read affects the magnitude, precision, and invariance of early visual processing.

Keywords: brain plasticity; education; reading.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Stimuli and procedure. (A) Examples of visual categories used in the experiment. (B) Schematic representation of the experimental design. (Top) After a fixation cross, two successive stimuli within the same category were displayed with a 400-ms stimulus-onset asynchrony (SOA). The pairs could be exactly the same, a mirror version of each other or different exemplars (as above). (Middle) ERPs averaged across subjects and conditions. The GFP time course is plotted in green in the lower part of the figure. (Bottom) Scalp maps showing the topographic distribution of P1s and N1s evoked by the first and second stimuli, respectively.

Fig. 2.

Early and late effects of reading ability on electrophysiological responses to visual stimuli. (Left) Early effects of literacy in the post-P1 time window (140–180 ms). The topographic map shows the beta weights of the correlation between scalp voltages and the reading scores of the participants (number of words and pseudowords read per minute), across all categories (in mV/number of additional stimuli read per minute). For each category, voltages from the two occipital clusters collapsed (green lines) are plotted against the subject’s reading scores. (Right) Late effects of literacy in the post-N1 time window (200–240 ms). (Insets) Posterior views of the brain showing the cortical sources of these early and late literacy effects obtained by correlating reading scores with the reconstructed activation at each vertex, at a time point corresponding to the center of the time window (160 and 220 ms, respectively).

Fig. 3.

Impact of reading ability on the lateralization of N1. (A) Scalp map of the N1 topography at 176 ms after the stimulus in the grand average (all subjects and conditions collapsed). Arrows indicate the symmetric occipitotemporal clusters selected (10 eletrodes for each hemisphere; in red). The boxplots represent the voltages from these two clusters on the left hemisphere minus those from the right hemisphere, calculated for each subject from the activation evoked by letter strings, across a 40-ms window centered on the N1 peak (i.e., left lateralization index of N1). Scalp maps on the N1 peak are plotted for each of the six subgroups of participants with increasing levels of reading ability, from illiterates (ILB) on the left to ex-illiterates (EXP and EXB) in the middle to literates (LB2, LP, and LB1) on the right. (B) Correlation of the N1 lateralization index with the participant’s reading scores for each category.

Fig. 4.

Effect of reading ability on the lateralization of occipitotemporal responses at the N1 stage (source analysis). Time course of reconstructed source activity associated with literacy (beta weights of the correlation between source activity and reading scores) for four ROIs of equivalent size (230 vertices) based on individual source reconstruction: (1) left occipital (blue), (2) right occipital (orange), (3) left ventral occipitotemporal (green), and (4) right ventral occipitotemporal (red), separately for strings and faces. (Insets) Estimated cortical activity at 168 ms. For letter strings, a strong peak of literacy-related activity is found at 168ms (N1 stage) in the left ventral occipitotemporal cortex (in green). For faces, a smaller peak of activity occurs in the right occipitotemporal cortex (in red), in parallel with a reduction of activity on the left side, followed by a later enhancement (∼250 ms).

Fig. 5.

Effects of repetition priming as a function of reading ability. For each subject and category, repetition suppression (i.e., different pairs minus identical pairs trials) (A) and mirror repetition priming (i.e., mirror pairs minus identical pairs trials) (B) effects were calculated on the average voltages of a left occipitotemporal cluster of 10 electrodes (green dashed line) in the 100- to 148-ms interval after the second stimuli. Then a regression against reading scores was performed for each category. In each case, the scalp map shows the beta weights of the correlation between repetition suppression and reading scores for all categories collapsed. Below each category, source reconstructions show the correlation with reading score calculated for each vertex at the peak of this interval (i.e., 124 ms). r, Pearson’s correlation coefficient. A positive correlation indicates that as reading fluency increases, the capacity to discriminate between two unrelated items improves (A), as does the capacity to discriminate an item from its mirror image (B). Improvements are seen for letter strings, as well as for faces and, to a lesser degree, false fonts and houses.