A dyslexic and a normal subject were screened for the presence and extent of "Scotopic Sensitivity Syndrome," and examined using visual evoked response (VER) testing.
Both were given two lrlen acetate overlays by an lrlen screener: one being the most beneficial and one being non-beneficial. A flicker masking technique was used to record the integrity of the magnocellular pathway in both subjects. VERs were recorded with the acetates under flicker- and non-flicker mask conditions.
A magnocellular defect was found in the dyslexic subject but not in the normal subject. When tested with the beneficial overlay, the magnocellular defective subject had increased N1-P1 and P1-N2 amplitudes, increased N1-P1 latency differences, and decreased P1-N2 latency differences as compared to the no overlay condition. The nonbeneficial overlay also produced these effects, but to a lesser extent. No beneficial effect of the overlays was found for . the normal subject. Decreased luminance produced by doubling overlay density was found to decrease N1-P1 times, increase P1-N2 times, and decrease N1-P1 and P1- N2 amplitudes for the normal reader's VER. Decreased luminance, caused by doubling overlay density, increased N1-P1 times, decreased P1-N2 times, and increased N1-P1 and P1-N2 amplitudes in the VER of the dyslexic subject.
"Scotopic Sensitivity Syndrome" may be objectively diagnosed by using the VER to asses the magnocellular defects that seem to accompany it. Potentiation of the magnocellular pathway is possible with colored filters, as indicated by the fact that the beneficial acetate produced a more "normal" VER in the dyslexic subject. A proposed mechanism of action for the overlay in magnocellular defective individuals could involve modification of the magnocellular input to the VER.
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