Shape-From-Motion is intact even when motion perception is impaired: a TMS study

摘要

Shape from motion (SFM) is a phenomenon that a spatial region is defined by 2D motion (typically moving random dots) and segregated from background as a shape. Because the shape in SFM is defined by motion, SFM perception supposedly coincides with MT+ activity as well as perception of motion. However, the temporal relationships between motion processing and SFM perception, and causal relationships between them are yet to be clarified. The main objective of this study was to investigate the temporal and causal relationships between motion perception and SFM perception. For this purpose, we examined the performance of SFM task while impairing subjectsa?? motion perception by applying Transcranial Magnetic Stimulation (TMS) to MT+ (Beckers & Zeki, 1995). Prior to main experiment, 14 subjects underwent a MRI session, where the anatomical structure of the brain was obtained and the area MT+ was functionally localized for each subject. In the main experiment, single-pulse online TMS was delivered to the functionally localized MT+ at various timings from the stimulus onset (-120 ms to 320 ms). The stimuli were T or reversed T shape defined by moving random dots against static random-dot background. Subjects were asked to judge whether the shape moved either to the left or right (motion task), or whether the shape was either T or reversed T (shape task). The results revealed a clear dissociation between the performance of motion and shape tasks. The performance for motion task was impaired by TMS to the area MT+ at a particular timing. However, the performance for shape task was intact even when TMS was applied with the timing where motion task was severely impaired. These results indicated that perception of motion is not required for the perception of SFM, and suggest that a visual pathway bypassing MT+ contributes to SFM perception.