The fastest (and simplest), the earliest: The locus of processing of rapid forms of motion aftereffect

Adaptation to directional motion has been shown to bias the perceived direction of a subsequently presented stationary or flickering test stimulus toward the opposite direction with respect to that of adaptation. This phenomenon, called motion aftereffect, is usually generated with adaptation periods of tens of seconds or minutes and has been shown to depend upon the functional integrity of visual area V5/MT. Rapid forms of MAE, arising and decaying within half a second (rMAE), can also be generated with sub-second adaptation durations. In order to investigate the neural substrate underlying the rMAE, repetitive transcranial magnetic stimulation (rTMS) has been used just after the adaptation stimulus over areas V1/V2, V5/MT, or over the vertex. Results showed that, besides some reduction in strength of the rMAE when rTMS was delivered over V5/MT, it was maximally disrupted when stimulation was delivered over early visual areas V1/V2. This is the first study where a causal role of early visual cortices in MAE is demonstrated. Moreover, this finding supports the existence of multiple loci along the visual stream in which gain control takes place and generates the MAE as a byproduct. The specific locus is likely to depend on the specific stimulus used.