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Abstract:
Visual perception of moving faces activates parts of the human superior temporal sulcus (STS) whereas static facial information is mainly processed in areas of ventral temporal and lateral occipital cortex. However, recent findings show that the latter regions also respond more to moving faces than to static faces (Schultz and Pilz, 2009). This study investigated the origin of this activation increase. We considered the following causes: (1) facial motion per se (2) increased static information due to the higher number of frames constituting the movie stimuli, and/or (3) increased attention towards moving faces, which would increase the response in face-sensitive areas through top-down modulation. We presented non-rigidly moving faces to participants lying in an fMRI scanner. We manipulated static face information and fluidity of the motion in the stimuli by presenting ordered and scrambled sequences of frames at temporal resolutions of 25 frames per second (fps) which were perceived as fluid motion when ordered, but as very non-fluid when scrambled, 12.5 fps (still perceived as fluid when ordered) and 5 fps (significantly less fluid when ordered, with smallest effect of scrambling). To control the influence of attention, subjects were asked to perform a target detection task that was unrelated to the face stimuli (one-back matching task on a stream of letters presented at fixation). Results confirm the increased activation induced by facial motion in the face-sensitive fusiform and superior temporal regions. A purely attention-based effect can be ruled out given that the task performance was far from ceiling performance and equal across conditions. While activation generally increased with the number of frames, a significant reduction of activation was observed due to frame-scrambling the stimuli. These results indicate that the activation increase induced by moving faces is due to the motion of the stimulus (i.e. temporal order of the frames) and not only to increased static information or attentional modulation.
