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Shannon Fitzhugh, Thomas Shipley, Peter Marshall; The relation between motor cortex activity and perception of form coherence for biological motion stimuli. Journal of Vision 2007;7(9):491. doi: https://doi.org/10.1167/7.9.491.
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In humans, biological motion is easily perceived from minimal displays where 13 lights mark the 12 major joints of the human body and the head. Viewing point light displays of human action activates the motor system (Saygin et al. 2004). Here we use electroencephalography (EEG) to study the associations between motor cortex activation and biological motion perception. Specifically, we use desynchronization of the mu rhythm over central electrode sites as an index of covert motor system activation during action observation (Pineda, 2005). Desynchronization occurs not only with action execution, but also with imagined action, and observation of human actions (Muthukumaraswamy et al., 2004). The current study directly explores the relation between observers' perception of form in biological motion stimuli and desyncronization of the mu rhythm.
Participants viewed point light displays of eight familiar actions (walking, running, cartwheel, jumping jacks, throwing a baseball, hitting a tennis ball, kicking a soccer ball, and hitting a baseball). The form coherence of the point light displays was continuously varied by adding random location offsets to each point. The resulting display perceptually oscillated from a coherent, easily perceived, form to incoherent, independent element motions. Participants observed the point light displays while EEG was collected using a 21-electrode EEG cap. Participants then watched the same displays while rating the displays for clarity of form, using a rating dial.
Preliminary analyses indicate that the power in the mu band (7–13Hz) at central electrode sites was negatively correlated with reported form coherence. As the form became more coherent, the mu rhythm was desynchronized. Similar relations were not observed for form coherence of a non-biological stimulus (a point-light rotating circle). These results suggest a tight temporal link between ongoing perception of action and motor cortex activity during observation of biological motion.
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