CONTRAST SENSITIVITY IN DYSLEXIA

Contrast sensitivity was determined for dyslexic and normal readers. W hen testing with temporally ramped (i.e. stimuli with gradual temporal onsets and offsets) gratings of 0.6, 4.0, and 12.0 cycles/deg, we fou nd no difference in contrast sensitivity between dyslexic readers and controls. Using 12.0 cycles/deg gratings with transient (i.e. abrupt) onsets and offsets, we found that dyslexic individuals had, compared t o controls, markedly inferior contrast sensitivity at the shortest sti mulus durations (i.e. 17, 34, and 102 ms). This deficit may reflect mo re sluggish temporal summation. There was no difference in sensitivity to 0.6 cycles/deg gratings with transient onsets and offsets. Under t hese conditions, the two groups showed a consistent and equal increase in sensitivity relative to the ramped baseline condition at 0.6 cycle s/deg at the longer stimulus durations. This demonstrates that dyslexi c readers have no deficit in their ability to detect stimulus transien ts, a finding which appears to be inconsistent with a transient system deficit. That detection of the low-frequency stimuli was mediated by the transient system is further indicated by the fact that these stimu li were more susceptible to forward masking than were the high-frequen cy stimuli. The effects of masking of both high and low spatial-freque ncy stimuli were about equal for dyslexic readers and controls. This i s not in agreement with the transient system deficit theory, according to which one would expect there to be less masking of high spatial-fr equency stimuli in the case of dyslexic readers.