Ipsilateral sensitivity to visual motion is restricted to V5/MT+ in the right cerebral hemisphere

Abstract

Previous experiments have demonstrated that transcranial magnetic stimulation (TMS) of human V5/MT+ in the right cerebral hemisphere can induce deficits in visual motion perception in both the contra- and ipsi-lateral visual hemi-fields. However, when TMS is applied to V5/MT+ in the left hemisphere, motion deficits are restricted to the contra-lateral hemi-field (Thakral and Slotnick, 2011). An explanation for this result might lie in differential stimulation of sub-divisions within V5/MT+ across the two hemispheres. V5/MT+ has two major sub-divisions; MT/TO-1 and MST/TO-2. MST/TO-2 contains neurons with large receptive fields (RFs) that extend up to 15° into the ipsi-lateral hemi-field. RFs of MT/TO-1 neurons are smaller and do not extend significantly into the ipsi-lateral field. We wanted to re-examine this functional asymmetry between V5/MT+ in the right and left hemispheres and ascertain whether the pattern of motion deficits is dependent upon the extent to which either MT/TO-1 or MST/TO-2 are disrupted by TMS. MT/TO-1 and MST/TO-2 were identified in six subjects using fMRI localisers that directed target points for TMS. Subjects identified the translational direction (up/down) of a threshold level of coherently moving dots presented in either the left or right visual field whilst TMS pulses were applied synchronously with stimulus onset. Application of TMS to MT/TO-1 and MST/TO-2 in the right hemisphere disrupted direction discrimination in both the contra- and ipsi-lateral visual fields, whereas deficits following application of TMS to MT/TO-1 and MST/TO-2 in the left hemisphere were restricted to the contra-lateral visual field. This result suggests an enhanced role for the right hemisphere in processing full-field translational motion, but contrary to our hypothesis, effects differ across hemispheres rather than within sub-divisions of V5/MT+. This corresponds to literature investigating timing differences across the left and right hemispheres (Ffytche et al., 2000), however the reasons for this asymmetry are still unclear.

Meeting abstract presented at VSS 2017