Figure 7.

Dual inhibitory and excitatory model of callosal function in spatial attention. A, Inhibitory model. cTBS applied over the left IPS in participants with a high FA/HMOA index triggers a leftward attentional shift because the inhibitory stimulation is applied to regions already inhibited by the right dominant hemisphere (left panel). In participants with a low FA/HMOA index, there is a weak reciprocal inhibition of the two hemispheres across the respective attentional networks. cTBS over the left IPS thus triggers compensatory activation of other cortical areas within the left hemisphere (red cross), with the result that spatial attention is directed to the contralateral (right) visual field (right panel). B, Excitatory model. In participants with a large corpus callosum (large volume), cTBS applied over the right IPS results in smaller rightward attentional shifts. This is due to the fact that, whereas this stimulation triggers some inhibition of the right hemisphere, there is also high interhemispheric connectivity, causing rebalancing activation across both hemispheres (left panel). In participants with a smaller corpus callosum (small volume), cTBS over the right IPS results in larger rightward attentional shifts due to stimulation triggering inhibition within the dominant right hemisphere and the smaller corpus callosum being less efficient in rebalancing activation across both hemispheres (right panel). C, Dual model. The “typical” responses to cTBS can in turn be better accounted for not by a purely inhibitory model (as originally proposed by Kinsbourne, 1977, 1987, 1993), but rather by a dual inhibitory and excitatory model of callosal function, differentially maintaining right hemispheric dominance in spatial attention.