VISUAL BACKWARD-MASKING IN CENTRAL AND PERIPHERAL-VISION IN LATE-ADOLESCENT DYSLEXICS

1. Visual backward masking in central and peripheral vision was measur ed in groups of late-adolescent dyslexic and matched normal readers. 2 . For each subject a target duration threshold was measured by randoml y presenting target letter stimuli at a number of retinal locations in central vision and in the right and left visual field. 3. Using the t arget duration estimated for each subject at each retinal location, ba ckward masking was measured as a function of seven inter-stimulus inte rvals (ISIs). Recognition error was the dependent measure and a comple tely randomized design was used across all conditions of the experimen t. 4. There were no differences between late-adolescent dyslexics and matched controls in target duration thresholds for isolated target sti muli in central and peripheral vision. 5. In central vision late-adole scent dyslexic subjects experienced significantly more backward maskin g at brief ISIs, a finding similar to that found in preadolescent dysl exic subjects by Williams, LeCluyse and Bologna [(1990) Clin. Vision S ci. 5, 335-343]. 6. With masking in the right visual field late-adoles cent dyslexic subjects showed some evidence for disinhibition at an IS I of 0 ms and increased masking at intermediate ISIs. No differences i n backward masking were found between the groups in the left visual fi eld. 7. The findings are interpreted in terms of a sustained and trans ient channel model of visual processing. Differences in visual process ing in central vision in the dyslexic group are explained in terms of longer durations of visible persistence and increased visual integrati on between stimuli that are presented in rapid temporal succession. Di fferences in the right visual field in the dyslexic group are explaine d in terms of a delayed transient channel response which shifts its in hibitory activity from brief to intermediate ISIs. 8. The present find ings provide evidence for the continuation of a visual processing defi cit evident in preadolescent dyslexics into late-adolescence and indic ate that the visual processing deficit in dyslexia may not be due to a temporary developmental delay. 9. The present study also outlines a s et of criteria for the selection of adult dyslexic subjects and provid es some evidence that their use selected subjects with a test profile similar to preadolescent dyslexics.