Perceived global motion-dependent activity in human early visual cortex under two attentional conditions

H Boyaci1, B Akin2, K Doerschner1, S Eroglu1, F Fang3, D Kersten4, C Ozdem5, D Taslak6

1Department of Psychology & National MR Research Center, Bilkent University, Turkey
2Department of Radiology, Medical Physics, University Hospital Freiburg, Germany
3Department of Psychology, Peking University, China
4Department of Psychology, University of Minnesota, MN, United States
5Department of Psychology, Vrije Universiteit, Brussel, Belgium
6Faculty of Medical Research and Education Hospital, Bozok University, Turkey
Contact: hboyaci@bilkent.edu.tr

When a two dimensional silhouette of a "Pac-Man" oscillates about its center, the local motion signals are solely determined by the Pac-Man's "mouth". Nevertheless the surface is perceived to oscillate as a whole. Here we measured the cortical activity using functional magnetic resonance imaging (fMRI) while observers were fixating at the oscillating Pac-Man's center, under two attentional conditions. In one condition participants performed a demanding fixation task, in the other condition they viewed the stimulus passively while maintaining fixation at its center. The amplitude of oscillations were adjusted such that the local motion signals were restricted to the right visual field, therefore in the null hypothesis we expected to find no activity in the right hemisphere visual areas. In the passive view condition we found that the fMRI signal in most of the early visual areas in the right hemisphere was correlated with the perceived global motion, and rejected the null hypothesis. In the fixation task condition the perceived global motion-dependent activity was limited to areas V3A/B, LO1 and MT+. These results provide evidence of inter-hemispherical modulation of early cortical activity due to the perception of global form and motion.

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