Disruptions to human speed perception induced by motion adaptation and transcranial magnetic stimulation

To investigate the underlying nature of the effects of transcranial magnetic stimulation (TMS) on speed perception, we applied repetitive TMS (rTMS) to human V5/MT+ following adaptation to either fast- (20 deg/s) or slow (4 deg/s)-moving grating stimuli. The adapting stimuli induced changes in the perceived speed of a standard reference stimulus moving at 10 deg/s. In the absence of rTMS, adaptation to the slower stimulus led to an increase in perceived speed of the reference, whilst adaptation to the faster stimulus produced a reduction in perceived speed. These induced changes in speed perception can be modelled by a ratio-taking operation of the outputs of two temporally tuned mechanisms that decay exponentially over time. When rTMS was applied to V5/MT+ following adaptation, the perceived speed of the reference stimulus was reduced, irrespective of whether adaptation had been to the faster- or slower-moving stimulus. The fact that rTMS after adaptation always reduces perceived speed, independent of which temporal mechanism has undergone adaptation, suggests that rTMS does not selectively facilitate activity of adapted neurons but instead leads to suppression of neural function. The results highlight the fact that potentially different effects are generated by TMS on adapted neuronal populations depending upon whether or not they are responding to visual stimuli.