Critical duration, contrast sensitivity, and specific reading disability

Critical durations and contrast sensitivities for sine-wave gratings of four different spatial frequencies - were measured in normal and disabled readers. Two groups, each of ten subjects, with an average age of 14 years, and matched as to seZ age; IQ, and socio‚ÄövÑvÆeconomic status, were used. The results showed that while critical duration for controls increases significantly with spatial frequency, this is not so for disabled readers, suggesting that the two groups may differ in terms of the temporal properties of their spatial frequency channels. It Was also found, for stimulus durations approximately equal to fixation durations, that disabled readers were relatively less sensitive than were controls at low to medium spatial frequencies. At all duration's there was a marked contrast sensitivity loss at 4 c/deg. Controls, in respect of both critical duration and contrast sensitivity function, produced results similar to those found in other studies on normal adult subjects. The suggestion of spatial frequency-selective differences in critical duration and the clear finding of such differences in contrast sensitivity may indicate a fundamental abnormality in the visual-temporal integration of spatial Stimuli. The existence of such qualitative differences in children at this comparatively mature age renders a developmental explanation unlikely. An explanation discounting visual perceptual differences seems even more untenable. The present study does not preclude a multi-factorial etiology for SRD; it does; however, indicate that abnormality .in spatial - frequency-specific channels of visual information processing is one factor which can now confidently be included.