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Shan Zhang, Ayse P. Saygin; Form and Motion in Biological Motion Perception: An Event-related Potential Paradigm. Journal of Vision 2020;20(11):950. doi: https://doi.org/10.1167/jov.20.11.950.
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Point-light biological motion (PL-BM) is a complex stimulus that comprises interrelated form and motion cues. Despite the inherently dynamic nature of PL-BM, much remains to be understood about the temporal aspects of BM perception. The event-related potential (ERP) technique provides excellent temporal resolution, but typically involves time-locking to overall stimulus onset, which can make it challenging to explore subtler and ongoing aspects of processing, especially for dynamic stimuli. Here, we developed a novel variant of the ERP method, which features applying sparse visual events onto continuously presented, dynamic PL-BM stimuli. Subjects viewed PL-BM stimuli depicting locomotion with black dots corresponding to the joints of a moving body. A brief contrast reversal (i.e., change to white dots) is then applied to individual frames of the stimulus at an average rate of 3/s, with the goal of inducing a feed-forward wave of visual processing without disturbing the continuity of the ongoing motion. Each trial featured either an intact or a spatially scrambled PL-BM animation matched for local motion and motion energy. Evoked responses to the contrast-reversals showed the expected visual ERP componentry and distribution, indicating the feasibility of the approach. Furthermore, the occipital P1 (90-110ms) and parietooccipital N1 (150-170ms) components were enhanced for intact vs. scrambled PLWs. Frame-level analyses showed that while the response to the stimulus onset could dominate evoked potentials to dynamic stimuli, our ERP paradigm provides a promising approach to study the temporal aspects of BM processing by acting as a probe to “catch the visual system in action.” Follow-up experiments using this paradigm will aim to inform how form and motion cues are processed and integrated during biological motion perception.
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