Hemispheric asymmetries and pathway differences in the visual masking of letters



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Research in visual masking has been primarily concerned with specifying the shape of the Masking function, assessing the effects of various stimulus parameters, and the development of theories of visual masking. There has been correspondingly little interest in possible hemispheric asymmetries and visual pathway effects in masking. The present Investigation attempted to examine the effects of these latter variables by assessing the amount of masking of letter stimuli under monoptic and dichoptic viewing conditions. A completely within subjects design with 8 subjects (896 trials per subject) and 4 factors was utilized. The factors were: Viewing Condition (monoptic and dichoptic); hemisphere (left and right); Pathway (ipsilateral or contralateral); and, Interstimulus interval (ISI), with ISI values of 0, 20, 40, 60, 80, 100, 120 milliseconds. Four letter stimuli (T, E, A, X) were used as targets and a surrounding non-overlapping ring was utilized as a mask. The significant effects of greatest interest were: Pathway (less masking with ipsilateral presentations); Viewing Condition by ISI (more masking at the low ISI values for dichoptic viewing); and Hemisphere by Pathway (less masking to the left hemisphere for ipsilateral presentations). The main effect of Hemisphere approached significance. There was less masking for the left hemisphere at all ISI values with the exceptions of 100 and 120 msec. The left hemisphere function was displaced approximately 14 milliseconds to the left (less masking) than the right hemisphere function. The results indicated a superiority for ipsilateral processing and a trend toward less masking for the left hemisphere. The overall superiority of ipsilateral processing was attributed to the greater absolute number of sustained channels (channels maximally sensitive to high spatial frequency stimuli such as letters) in the ipsilateral pathways and to a higher likelihood of inhibition of sustained channel activity by transient channel activity(interchannel inhibition) in the contralateral pathways. It was proposed that the left hemisphere-right hemisphere difference did not reflect complex hemispheric visual processing asymmetries. Rather, it was suggested that since vocal responses were utilized, information loss during interhemispheric transfer (right to left) could produce an apparent superiority for left-hemisphere processing since with right hemisphere presentations interhemispheric transfer would have to occur. Two additional findings were as follows. First, there was more masking at low ISI values for dichoptic viewing. This finding was not readily explainable and more investigation is called for. Second, there was a very strong tendency for less masking with presentations to the left eye, independently of hemisphere or pathway. This finding suggested that differential eye sensitivity may be an important factor in visual masking. It was proposed that this differential eye sensitivity might be due to quantitative and/or qualitative differences in sustained and transient channels for the two eyes.



Neurological function, Reading, Visual ability