In Search of the Best Psychophysical Indicator of an M-Stream Deficit in Developmental Dyslexia

Robert F. Dougherty*, Deborah E. Giaschi*, Bruce H. Bjornson*, Stephanie L. Chamut*, Dorothy Edgell^ and Christopher J. Lyons*
*University of British Columbia, Canada; ^University of Victoria, Canada

Purpose. Our main goal was to determine which visual task of a battery of probable M-stream tasks, reveals the largest temporal processing deficit in older children with dyslexia. The next step will be to assess the value of this task as a screening test for use with preschool children.

Methods. We tested right-handed boys and girls between 10 and 12 years of age. Each was screened with an ophthalmic exam, a neurologic exam and a battery of neuropsychological tests aimed at detecting problems with phonological processing and reading. The dyslexia group consisted of subjects who tested 1.5 standard deviations below the norm on the reading tests but performed at least average on tests of general intellect. Control subjects were age- and IQ-matched to the dyslexic subjects. We selected tasks that have previously been reported to show deficits either in dyslexic subjects or in non-human primates with magnocellular LGN or MT lesions. We measured minimal speed thresholds for recognizing motion-defined shapes (MD shape), upper (Dmax) and lower (Dmin) displacement thresholds for direction discrimination, coherence thresholds for direction discrimination at three displacements, contrast sensitivity for an 8 Hz flickering blob, an 8 Hz drifting Gabor and a 15 Hz counter-phase flickering Gabor and critical flicker frequency (CFF) at high and low contrasts. We also measured grating acuity and motion coherence thresholds for isoluminant stimuli. For most tasks, threshold was measured with a 2-alternative forced-choice staircase algorithm. CFF was measured with the method of adjustment and the MD shape task employed a 10-alternative forced-choice method.

Results. Coherence thresholds showed the largest difference between the two groups. Dmin, MD shape and isoluminant grating acuity were similar in both groups. The average performance of the dyslexic group was poorer than that of the control group on the remaining tasks.

Conclusions. Visual deficits on tasks involving dynamic stimuli were clearly demonstrated in our group of dyslexic subjects. These results support the existence of M-stream involvement in reading disabilities and suggest that motion coherence tasks may be the best indicator of such involvement.

Supported by March of Dimes grant FY96-0479 & BC-MSF grant 95-36(95) to D.E.G.

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