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Are the visual disturbances experienced by dyslexics during reading due to a failure in saccadic re-mapping mechanisms?

Stephen R. Jackson, Yu-Ju Chou, Elizabeth Liddle

School of Psychology, The University of Nottingham stephen.jackson@nottingham.ac.uk

Abstract

Electrophysiological recordings in non-human primates indicate that regions of parietal cortex may be involved in re-mapping visual space immediately prior to the execution of a saccadic eye movement, and that these mechanisms may serve to maintain space constancy across saccades. Dyslexic individuals frequently experience perceptual disturbances during reading: often reporting that words and letters appear to 'float about' on the printed page. We hypothesised that these perceptual disturbances might arise as a consequence of a failure in the re-mapping mechanisms responsible for maintaining space constancy across saccades. We investigated this by examining saccade-dependent spatial mislocalisation in a group of adult dyslexic individuals and a group of controls. We demonstrate for the first time that the saccade-dependent spatial mislocalisation bias observed in non-dyslexics is significantly reduced in the dyslexic group. That is, unlike control subjects who show a large saccade-dependent spatial bias in their reporting of the location of a briefly presented (20ms) visual probe stimulus, the reports of dyslexic adults remain spatially accurate even when the stimulus is presented very close to the onset of a saccadic eye movement. These data suggest that the physiological mechanisms that permit spatial representations to be rapidly and dynamically re-mapped in advance of a saccadic eye movement may not be operating normally in dyslexics.

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