Are dyslexics??? visual deficits limited to measures of dorsal stream function?

Abstract

We tested the hypothesis that the differences in performance between developmental dyslexics and controls on visual tasks are specific for the detection of dynamic stimuli. We found that dyslexics were less sensitive than controls to coherent motion in dynamic random dot displays. However, their sensitivity to control measures of static visual form coherence was not significantly different from that of controls. This dissociation of dyslexics' performance on measures that are suggested to tap the sensitivity of different extrastriate visual areas provides evidence for an impairment specific to the detection of dynamic properties of global stimuli, perhaps resulting from selective deficits in dorsal stream functions.

Motion and Form Coherence Detection in Autistic Spectrum Disorder: Relationship to Motor Control and 2:4 Digit Ratio

“…Consequently impairment of the dorsal stream or the magnocellular system has been implicated following reports of high motion coherence thresholds (Atkinson et al, 1997;Hansen et al, 2001;Milne et al, 2002;Spencer et al, 2000;Stein & Walsh, 1997). However, others have challenged the inference of a purely magnocellular impairment from high motion coherence thresholds (Bertone et al, 2003;Skottun, 2000), and, as was noted in the Introduction, there is no direct physiological evidence of magnocellular abnormality in autism.…”

“…We followed the procedure outlined by Hansen et al to determine psychophysical thresholds for coherent motion and form detection (Hansen et al, 2001).…”

“…Additionally, it is not the case that abnormal motion detection occurs only in the developmental disorder of autism. Deficits in motion perception have also been reported in dyslexia (Eden et al, 1996;Hansen, Stein, Orde, Winter, & Talcott, 2001;Talcott et al, 1998) and in William's syndrome (Atkinson et al, 1997(Atkinson et al, , 2003. It is now apparent that the deficit does not characterize all children with these disorders, but occurs in clusters of individuals within the disorders.…”

Developmental dyslexia: specific phonological deficit or general sensorimotor dysfunction?

“…A number of studies do provide evidence that dyslexics have elevated detection thresholds or abnormal visual evoked potentials for stimuli in the spatial and temporal ranges of the magnocellular system [24, [49][50][51][52][53], although it has been disputed whether some of the stimuli used uniquely tap the magnocellular system [54,55]. However, a growing number of studies report findings inconsistent with a visual deficit specific to the magnocellular system [20,22,31,56-59], often finding that visual deficits, when present, cover the whole range of spatial and temporal frequencies.…”

“…Questions have also been raised as to whether group differences could be explained by attention or memory rather than sensory deficits [57,60]. Moreover, visual deficits seem to be restricted to a subset of dyslexics: looking at 7 recent-studies displaying individual data, one finds 37/128 (29%) dyslexics with elevated visual thresholds in the target conditions [22,24,31,34,49,51,53]. Finally, no demonstration has been provided that magnocellular dysfunction, when present, engenders visual problems that are more proximal to reading, like visual instability, crowding or stress.…”