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Jan Jastorff, Guy A. Orban; fMRI reveals distinct processing of form and motion features in biological motion displays. Journal of Vision 2008;8(6):676. doi: 10.1167/8.6.676.
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Functional imaging experiments have implicated several visual cortical areas in the recognition of biological motion stimuli (review by Giese and Poggio 2003). In order to identify the contribution of these areas to the perception of point-light displays, we conducted two human fMRI experiments using each a 2×2 factorial design. The point-light stimuli consisted of twelve white dots presented on a black background, displaying leftward and rightward walking human figures. In the first experiment, one factor manipulated the global form of the stimuli by spatially scrambling the starting position of each dot. The second one modified the kinematics of the stimuli by changing the trajectory of each individual dot to simple translation. In the second experiment, the first factor degraded local opponent motion by presenting the points belonging to one side of the human body twice, moving in phase. The second factor kept the local opponent motion intact but degraded coordinated limb movement by spatially scrambling the starting position of the dots in y-direction. Random effects analysis over 17 subjects showed that within cortex activated by biological motion, activity increasingly differentiated between the conditions starting from the Lateral Occipital Sulcus via the Middle Temporal Gyrus being most specific for point-light walker in the Fusiform Gyrus. Moreover, we found a clear dorsal / ventral segregation of form and motion processing. While more ventral areas showed a main effect of form and to a lesser extend of opponency, more dorsal areas exhibited an interaction between form and motion or a main effect of motion. These results indicate a prominent role of the fusiform areas in processing of biological motion. Furthermore, they are in close correspondence with monkey electrophysiology, defining two classes of neurons (“snapshot” - and “motion” neurons) in the lower and upper bank of the STS respectively (see VSS abstract Vangeneugden et al, 2008).
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