Visual-vestibular and visuovisual cortical interaction: new insights from fMRI and pet.

PET and fMRI studies have revealed that excitation of the vestibular system by caloric or galvanic stimulation not only activates the parietoinsular vestibular cortex but also bilaterally deactivates the occipital visual cortex. Likewise, visual motion stimulation not only activates the visual cortex but also deactivates the parietoinsular vestibular cortex. These findings are functionally consistent with the hypothesis of an inhibitory reciprocal visual-vestibular interaction for spatial orientation and motion perception. Transcallosal visuovisual interaction between the two hemispheres was found by using half-field visual motion stimulation: activation of motion-sensitive areas hMT/V5 and deactivations of the primary visual cortex contralateral to the stimulated hemisphere. The functional significance of these inter- and intra-sensory interactions could be that they (A) allow a shift of the sensorial weight between two incongruent sensory inputs and (B) ensure a correspondence of the two hemispheres during evaluation of contradictory motion stimulation of the right and left hemifields. In terms of mathematical modeling, these findings may reflect the concepts of a sensory conflict mechanism or a mismatch between expected and actual sensory input.