Contrast sensitivity in dyslexia

Abstract

Contrast sensitivity was determined for dyslexic and normal readers. When testing with temporally ramped (i.e. stimuli with gradual temporal onsets and offsets) gratings of 0.6, 4.0, and 12.0 cycles/deg, we found no difference in contrast sensitivity between dyslexic readers and controls. Using 12.0 cycles/deg gratings with transient (i.e. abrupt) onsets and offsets, we found that dyslexic individuals had, compared to controls, markedly inferior contrast sensitivity at the shortest stimulus durations (i.e. 17, 34, and 102 ms). This deficit may reflect more sluggish temporal summation. There was no difference in sensitivity to 0.6 cycles/deg gratings with transient onsets and offsets. Under these conditions, the two groups showed a consistent and equal increase in sensitivity relative to the ramped baseline condition at 0.6 cycles/deg at the longer stimulus durations. This demonstrates that dyslexic readers have no deficit in their ability to detect stimulus transients, a finding which appears to be inconsistent with a transient system deficit. That detection of the low-frequency stimuli was mediated by the transient system is further indicated by the fact that these stimuli were more susceptible to forward masking than were the high-frequency stimuli. The effects of masking of both high and low spatial-frequency stimuli were about equal for dyslexic readers and controls. This is not in agreement with the transient system deficit theory, according to which one would expect there to be less masking of high spatial-frequency stimuli in the case of dyslexic readers.

Visual motion sensitivity in dyslexia: evidence for temporal and energy integration deficits

“…Either or both of these factors would decrease their ability to detect global coherent motion and would be consistent with anatomical evidence of structural impairment in both the retino-cortical M-pathway and within the extrastriate dorsal stream of dyslexics [18,28,44]. However, other studies have suggested that sensory de®cits in dyslexia are associated more with a problem detecting short duration stimuli whether they are dynamic or not ( [19,20] for review see [17]). Such a de®cit would be dicult to reconcile with the hypothesis of a visual de®cit speci®c to the M-stream.…”

“…Sensitivity for these stimulus parameters are usually unimpaired in dyslexics [10,28,31,33,34]. Some psychophysical studies have failed to con®rm that such visual de®cits are associated with dyslexia ( [20,21,61], see also, [17] for review). Nevertheless most experimental evidence is consistent with the hypothesis that the M (or transient) pathway is a locus of their subtle visual impairments (for reviews see [29,30,56]).…”

On the relationship between dynamic visual and auditory processing and literacy skills; results from a large primary‐school study

“…Negative findings of overall group differences in sensory processing have also been reported (e.g. Gross-Glenn et al, 1995;Walther-M . u uller, 1995;Hill et al, 1999), suggesting that differences in sample selection might mediate some of the different patterns of result found across studies.…”

Assessing the role of different spatial frequencies in word perception by good and poor readers

“…Numerous researchers have found that dyslexic and nondyslexic controls exhibit different patterns of sensitivity to spatial frequencies (e.g., Borsting et al, 1996;Cornelissen, 1993;Demb, Boynton, Best, & Heeger, 1998;Evans, Drasdo, & Richards, 1993, 1996Gross-Glenn et al, 1995;Lovegrove, Bowling, Badcock, & Blackwood, 1980;Lovegrove et al, 1982;Martin, Cornelissen, Fowler, & Stein, 1993;Martin & Lovegrove, 1984. In particular, studies indicate that dyslexic individuals have reduced sensitivity to certain spatial frequencies and that this reduction tends to be greatest in the low-to midfrequency range (i.e., between 2 and 8 cpd; Borsting et al, 1996;Cornelissen, 1993;Demb et al, 1998;Evans et al, 1993Evans et al, , 1996Lovegrove et al, 1980;Lovegrove et al, 1982, Experiment 2;Martin & Lovegrove, 1984see Skottun, 2000, for a review).…”