AN INVESTIGATION OF SOME SENSORY AND REFRACTIVE VISUAL FACTORS IN DYSLEXIA

The role of visual factors in dyslexia has been a long-standing source of controversy. Recent research has suggested that there may be a def icit of the transient visual subsystem in dyslexia. The evidence for t his hypothesis comes principally from investigations of spatial and te mporal contrast sensitivity and visual persistence. This evidence is r eviewed and it is noted that previous work has never applied two of th ese purported ''tests of transient function'' to the same subject grou p. The hypothesised transient system deficit in dyslexia was investiga ted in a study comparing 43 control with 39 dyslexic children who mere matched for age, sex, and intelligence. Comprehensive psychometric an d optometric data were obtained, including visual acuities and refract ive errors. The spatial contrast sensitivity function was determined i n such a way as to investigate further the findings of Lovegrove, Mart in, Bowling, Blackwood, Badcock and Paxton [(1982) Neuropsychology, 20 , 309-315] and Martin and Lovegrove [(1984) Neuropsychologia, 22, 73-7 7]. It might be expected, from the work of Merigan and Maunsell [(1990 ) Neuroscience, 5 347-352], that a better test of magno-cellular funct ion would be to investigate the modulation threshold for a virtually u niform field that was flickering sinusoidally at 10 Hz. This temporal contrast sensitivity was studied in a similar way to Brannan and Willi ams [(1988) Clinical Vision Sciences, 3, 137-142]. A non-verbal simula ted reading visual search task was used to investigate the effect of a ny visual deficits on a test that was, in its low-level visual require ments, similar to reading. The following factors were found to be sign ificantly associated with dyslexia: reduced visual acuity, impaired fl icker detection at 10 Hz, reduced low spatial frequency contrast sensi tivity, and slightly slower performance at a simulated reading visual search task. The two alleged ''tests of transient function'' were only weakly correlated with one another (r = 0.183), suggesting that these variables do not measure the same function. Much of the dyslexic grou p's slightly slower performance at the simulated reading task could be accounted for by the psychometric variable of visual sequential memor y. Like reading, the simulated reading task requires the accurate perc eption of sequential characters. Hence, it seems unlikely that the low -level visual deficits in the dyslexic group were major causes of thei r poor reading performance. Alternative explanations for the results a re discussed.