Visual processing and specific reading disability

Specific reading disability is a pervasive disorder which affects the academic, social and emotional lives of a large proportion of people living in a world where literacy is taken for granted. While many hypotheses have been raised as to the cause of this disorder, the possibility that the disorder is related to visual processing anomalies has not as yet been fully investigated. Based on the proposal that the visual system contains two separate mechanisms for the processing of spatial (sustained mechanism) and flickering (transient mechanism) stimuli, the operation of these two systems in groups of normal and specifically disabled readers was investigated. Initially, parameters of the contrast sensitivity function were investigated in specifically disabled and normal readers. It was found that at luminance levels less than 30 cd/m2 specifically disabled readers were generally less sensitive than normal readers at lower spatial frequencies and equally or more sensitive at the higher spatial frequencies. This suggested a deficit in some part of the transient system functioning in specifically disabled readers. The operation of the transient and sustained systems was then directly investigated through the use of flickering stimuli. In two experiments - one manipulating spatial frequency and the other temporal frequency - it was found that as spatial frequency and temporal frequency increase so did the differences between specifically disabled and normal readers with specifically disabled readers being less sensitive than controls to all the stimuli. Four experiments were then carried out investigating the effect of masking by uniform field flicker. In the first of these experiments using a 6 Hz uniform field flicker and a stationary test grating, the presence of uniform field flicker decreased sensitivity in normal readers at low spatial frequencies but had little effect on sensitivity across that range in specifically disabled readers. At higher spatial frequencies while no effect was seen in the results of normal readers uniform field flicker decreased sensitivity in specifically disabled readers. Overall the effect of the uniform field flicker was to make the contrast sensitivity function of specifically disabled readers more like that of normal readers. In the final three experiments it was generally found that low rates of uniform field flicker had a depressing effect on the contrast sensitivity to a moving or flickering grating of both specifically disabled and normal readers. However, when higher rates of uniform field flicker were used it was found that the contrast sensitivity of specifically disabled readers was not further affected while that of normal readers showed a further decrease in sensitivity. These findings suggest that disabled readers have a deficiency in some part of their transient system which would lead to disordered interactions between the transient and sustained systems. This deficiency would lead to information reaching the cognitive areas of their brains in a degraded form thus making reading for these children extremely difficult.