Neural encoding of walking direction in biological motion: Evidence from direction-specific adaptation and functional neuroimaging

Abstract

Two questions were addressed in the current study: are there neurons tuned to specific directions of point-light biological motion, and if so, in which areas of the brain are these neurons located? We used a visual selective adaptation method to investigate the first question. After visual adaptation to point-light walkers oriented 45 degrees to either the left or right of the front direction, subsequently presented walkers oriented near the front direction were perceived to walk in a direction pushed away from that of the adapted direction. This aftereffect can be obtained with spatially nonoverlapping adapting and test stimuli. These results support the existence of biological motion selective neurons tuned to specific walking directions in the human visual system. Next, we measured cortical responses using event-related fMRI while observers viewed the same biological motion sequences, oriented 45 degrees to the left, 45 degrees to the right, or in the front direction. Regions of Interests (ROIs) potentially involved in processing biological motion (pSTS, EBA, hMT+, and LO) were identified with independent localizer scans. Although fMRI response amplitudes could not differentiate the different walking directions in any of the ROIs, spatial patterns of activation across voxels revealed that the pSTS and the hMT+ contained information about the walking direction in biological motion. In contrast, activation patterns in the EBA could differentiate different global forms of the body, i.e., front view vs. profile views. Evidence from an additional control experiment as well as from published studies reveals that directional encoding in the hMT+ is not unique for biological motion. We conclude that there are neurons tuned to different walking directions in point-light biological motion, and that the pSTS is the likely site of these neurons in the human visual system.