A theoretical and experimental case for a visual deficit in specific reading disability

Abstract

We present three lines of evidence indicating a low-level visual deficit in a large percentage of specifically-disabled readers. This research is presented within the theoretical framework of spatial frequency analysis, in particular, the framework of transient and sustained subsystems. a. Measures of visible-persistence duration as a function of spatial frequency in normal and specifically-disabled readers show that disabled readers have a different pattern of temporal processing across spatial frequencies. These differences disappear when transient system activity is reduced. b. On measures of pattern-contrast sensitivity specifically-disabled readers are less sensitive than controls at low spatial frequencies, but equal or more sensitive at high spatial frequencies. c. On measures of temporal-contrast sensitivity disabled readers are less sensitive than controls at all flicker rates with the greatest differences being at the higher temporal frequencies. These three sources of evidence indicate that disabled readers have a deficient transient system. On the other hand, a fourth series of experiments investigated sustained system functioning in the two groups and failed to show any differences between them. It is argued that the existing confusion over visual deficits in disabled readers may be related to our findings of a transient but not a sustained system deficit. Further analyses indicate that these differences do not result from failing at reading, are present before reading instruction commences and occur in approximately 75% of subjects tested. An attempt is made to relate these findings to existing classifications of sub-types of specific reading disability. Some discussion of how a transient system deficit may influence the reading process is offered.

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