Involvement of cortical area MT in the perception of depth from motion explored with Transcranial Magnetic Stimulation

The unambiguous perception of depth from motion requires the integration of visual retinal motion with a signal indicating pursuit eye movement direction. In previous work (Nawrot & Johnson, 2023, ECVP), we used Transcranial Magnetic Stimulation (TMS) to show that the Frontal Eye Field (FEF) may be the source of the necessary pursuit signal. In the current study, we used TMS to investigate the role of cortical area MT in these computations. Right hemisphere MT was localized as the region approximately 5 cm lateral and 3 cm superior to the inion that, when stimulated, produced visible phosphenes in the contralateral visual field. Following localization, triple-pulse (33 Hz) TMS was applied to the right MT of 35 participants either 30 msec before (early stimulation) or 90 msec after visual stimulus onset (late stimulation) during the performance of three different psychophysical tasks: i) pursuit, ii) translational motion perception, and iii) MP depth perception. The pursuit task was a step-ramp. Psychophysical tasks required the observer to report perceived depth or perceived motion direction (2AFC) of a computer-generated random-dot stimulus making a single translation (leftward or rightward) of duration (t). Between trials, t varied in two interleaved staircases, one for each direction of translation. TMS of right hemisphere MT produced a small decrease in pursuit latency in the contraversive direction (7 msec early and 5 msec late), but no change in ipsiversive pursuit. For depth perception, TMS produced 14 msec (early) and 20 msec (late) of speeding for leftward stimulus translation. For rightward stimulus translation, TMS produce 13 msec of speeding for late, but no change for early stimulation. There was no change in motion perception latency for either direction of stimulus translation, for either early or late TMS. The concordance of pursuit and depth effects suggests MT may have a role in their integration.