Interhemispheric transfer time differs between fast and slow healthy adult readers

Abstract

The visual system forms the basis of visual word decoding processes. Reading is a left-lateralized function. The interaction between the two hemispheres via the corpus callosum is required for successful reading. It is known that callosal function and morphology are affected in reading disorders. This study investigated the differences in callosal transfer speed of verbal and nonverbal stimuli in healthy university students. We hypothesized that if the callosal transfer has a role in slow reading, transfer speed would differ between slow and fast readers. Moreover, if the difference was affected by the type of stimulus, this will provide information about the level of neural processing at which the difference is based/aroused. Fifty-one participants were grouped as slow (n = 15, 8 female) and fast (n = 36, 22 female) readers. Three types of stimuli (word, legal pseudoword, and non-verbal grating) were presented from the right or left visual field. Latencies of the evoked potentials (N1) were used to measure interhemispheric transfer time. We found that slow readers have a slower right-to-left transfer speed at the parietal site, which is related to the visual word decoding process. The finding was similar to previous studies examining individuals with dyslexia. This difference was not seen with grating stimuli; we suggest that the difference originates at the orthographic visual lexical level rather than at earlier basic visual processing. We did not observe any effect of lexical and sublexical routes on the callosal transfer time because of evaluated time windows.