The M-pathway whose function has been reported to be impaired in dyslexia, contributes significantly to ventral object recognition as well as dorsal stream attentional processing. This experiment aimed to investigate whether transience of stimulus onset and offset is critical in explaining the impaired contrast sensitivity associated with dyslexia. 212 school children aged 5-15 years participated in the study. On the basis of Neale reading accuracy and Raven’s progressive matrices performance 20.3% of children were classified as learning disabled (LD), 17.9% as dyslexic (DD) and 61.8% as normal readers (NR). Contrast sensitivity was measured for flicker defined contrast reversing letters (58.5 Hz reversal) presented on a computer either with abrupt onset (duration 34 msec) or with ramped onset/offset stimulus profiles of duration from 34 to 85 msec. Thresholds were determined using a PEST procedure. Unpaired t-tests indicated that there was no significant difference in the contrast levels required for the discrimination of the E between reading groups for stimuli where the contrast was ramped on and off. However there was a significant difference (t(169) = 3.35, p=0.001) between the performance of the LD and NR groups when abrupt onset /offset was used, although the DD group did not perform significantly differently from the other two groups. Thus the processing of transients differentiates the performance of learning disabled and normal readers in flicker-defined letter recognition.

The research was supported by a grant #A000937 from the Australian Research Council.