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Thursday stream 1 Session 14.00 - 15.40 Length 25 minutes
M. F. Bradshaw, P. B. Hibbard and J. Everatt.
Psychology Department, University of Surrey m.bradshaw@surrey.ac.uk
Abstract
At least two anatomically distinct pathways project from the retina to the cortex termed the parvocellular and magnocellular. It is thought that dyslexics may have a deficit in the M-pathway (Stein and Walsh, 1997) which may account for their impaired performance on certain psychophysical tasks (Everatt et al, Perception, 1999). To test this we compared the performance of normals and dyslexics on three visual tasks in photopic and scotopic lighting conditions. The latter diminishes the activity of cells in the P-pathway (Purpura et al, 1988 Proc. Nat. Acad, 85 4534-4537). Thresholds for dyslexics (N=15) and non-dyslexics (N=15) were determined using a TUD staircase procedure in three tasks (i) coherent motion (SNR), (ii) disparity thresholds (corrugation phase detection) and (iii) biological motion (SNR). The stimuli comprised bright dots (13 cd m-2) or (0.05 cd m-2) in the photopic and scotopic conditions respectively, on a dark background. Before commencing the scotopic trials, subjects dark-adapted for 30 mins. Disparity and biological motion thresholds did not differ significantly between the groups although both groups performed worse under scotopic conditions (p < 0.01). Thresholds for coherent motion detection were 20% and 32% for non-dyslexics and dyslexics respectively in photopic conditions whereas in scotoptic conditions the difference disappeared (~ 32%). These results suggest that when interpreting the performance of dyslexic and non-dyslexic observers in psychophysical tasks the interaction between the M and P pathways must be considered.
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