Ipsilateral sensitivity to visual motion Is eestricted to V5/MT+ in the right cerebral hemisphere

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.