We report 2 experiments designed to demonstrate that unilateral tachistoscopic stimulation would yield a response time (RT) advantage over bilateral stimulation in a simple experiment, whereas the opposite pattern would occur in a complex version of the same task, as predicted by the intrahemispheric resource limitation model of Banich and colleagues. Experiment 1 was a go/no-go task in which participants had to press a key when two shapes (circles or squares) were identical on the computer screen. A unilateral field advantage was obtained that was accentuated in several task conditions that yielded overall longer RTs, mostly in the bilateral condition. Experiment 2 was similar but required a more complex judgment: The go trials were to 2 stimuli identical on 1 dimension (shape or color) but not both or neither. The RTs were significantly and substantially longer than in Experiment 1 and exhibited a nonsignificant bilateral field advantage, which differed significantly from the unilateral field advantage obtained in Experiment 1. These results support the intrahemispheric resource limitation model of Banich and colleagues. However, several within-experiment effects are in direct opposition to this model and are best explained as limitations of commissural relay of perceptual information.

No abstract

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.

Our assay of gender-specific risk for severe brain disease around the world has not been able to support the idea according to which any one gender-prevalent gonadal steroid hormone dominates as a neuroprotective agent at natural concentrations.