To study local– global relationships in interhemispheric interactions, tachistoscopically presented pairs of lines (1.15°) were compared for their relative orientation by 48 neurotypical adults. orientations of line stimuli (local aspect of the task) were vertical, horizontal, forward slash or backslash, as were those of the interstimulus axes. the latter created a global context that could influence line discrimination. stimulus pairs were presented within a field (not requiring callosal participation for line orientation comparison) or one on each side of the visual field meridian (requiring callosal participation). the primary purpose of the design was to determine whether local or global violations of stimulus “homotopy” across the meridian would impose costs of interhemispheric integration. the rationale for this expectation is that the fiber projection of the corpus callosum is highly symmetric across the midsagittal plane (i.e., homotopic). the expected “callosal homotopy” effect was significantly upheld as a whole but broke down or became extravagant in certain specific conditions, with specific costs of interhemispheric integration varying from null to a highly significant 20- ms as a function of interactions of interstimulus and stimulus orientations. the corpus callosum seems to be particularly sensitive to local stimulus orientation in interaction with long- range stimulus context orientation.

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