Abstract
Various types of visual impairment have been documented in developmental dyslexia but their relation to the word recognition deficit remains unclear. The present study examined the spatiotemporal deployment of processing resources for visual word recognition in developmental dyslexics and normal reading adults. Participants performed a visual word recognition task using 200 ms stimuli (5-letter words) with randomly oscillating signal-to-noise ratio (SNR; noise component made of white noise) applied independently on each letter position. The total amount of signal over stimulus duration was normalized across letter positions and trials. SNR profiles were generated for each trial by integrating 5–60 Hz sinewaves (5 Hz steps) of random amplitudes and phases. Individual classification images of encoding effectiveness for each letter through time were constructed by the weighted subtraction of SNRs leading to errors from those associated with correct target identification. They were transformed into Z-scores by bootstrapping (1000 iterations) and normalized linearly to a 0–1 range. The same procedure was applied on time-frequency analyses of the SNRs for each trial to obtain a classification image of encoding effectiveness in the time-frequency domain. The mean temporal and time-frequency classification images of adult dyslexics were compared to those of normal readers matched on age and IQ. Temporal domain classification images did not differ between groups. However, a complex pattern of differences between groups was observed in the time-frequency domain, as shown by a significant interaction of group × letter position × time × frequency. The time-frequency classification images were strikingly similar across normal readers (average of pairwise correlations between participants and letters, r = .83) and across dyslexics (r = .86). The present results suggest that the spatiotemporal deployment of processing resources during word recognition, as revealed by the letter-position specific time-frequency profiles, is markedly different between impaired and normal readers.
Acknowledgement: This research was supported by a grant to Martin Arguin and a graduate scholarship to Simon Fortier-St-Pierre from the Fonds Québécois de Recherche en Nature et Technologies (FQRNT)