The influence of TMS over MT on perceptual memory in structure-from-motion rivalry

Background. Ambiguous images confront the visual system with inconclusive or conflicting information. In response perception wavers seemingly unpredictably between alternative interpretations. Several factors influence the system's choice between conflicting percepts, thereby revealing mechanisms of perceptual organization. One such factor is prior perception: once an ambiguous image has been encountered, perception on subsequent encounters depends on what was previously perceived. The neural basis of this memory is uncertain. Here we investigate whether the memory traces can be altered by transcranial magnetic stimulation (TMS) applied during memory retention. Methods. An ambiguous structure-from-motion stimulus was presented intermittently and observers reported perception on every presentation. The blank interval between presentations was varied in a blocked fashion. At long blank durations the interaction between consecutive percepts is known to result in priming: the same percept tends to repeat over presentations. At short blank durations one finds mostly suppressive interactions: perception alternates on consecutive presentations. We applied triple pulse TMS at 25 Hz over the human middle temporal area and associated areas (hMT+) during the blanks that separate presentations. Results. For four observers we found that TMS over hMT+ increased the probability for perception to alternate from one stimulus presentation to the next, relative to no TMS and to TMS over the vertex. This suggests that perceptual memory traces in structure-from-motion rivalry may reside in hMT+. Current models of perceptual memory treat these traces as a form of neural adaptation. Within the context of these models our results can be interpreted as an elevation of adaptation due to TMS