Action Segmentation in the Brain: The Role of Object–Action Associations

Motion information has been argued to be central to the subjective segmentation of observed actions. Concerning object-directed actions, object-associated action information might as well inform efficient action segmentation and prediction. The present study compared the segmentation and neural processing of object manipulations and equivalent dough ball manipulations to elucidate the effect of object–action associations. Behavioral data corroborated that objective relational changes in the form of (un-)touchings of objects, hand, and ground represent meaningful anchor points in subjective action segmentation rendering them objective marks of meaningful event boundaries. As expected, segmentation behavior became even more systematic for the weakly informative dough. fMRI data were modeled by critical subjective, and computer-vision-derived objective event boundaries. Whole-brain as well as planned ROI analyses showed that object information had significant effects on how the brain processes these boundaries. This was especially pronounced at untouchings, that is, events that announced the beginning of the upcoming action and might be the point where competing predictions are aligned with perceptual input to update the current action model. As expected, weak object–action associations at untouching events were accompanied by increased biological motion processing, whereas strong object–action associations came with an increased contextual associative information processing, as indicated by increased parahippocampal activity. Interestingly, anterior inferior parietal lobule activity increased for weak object–action associations at untouching events, presumably because of an unrestricted number of candidate actions for dough manipulation. Our findings offer new insights into the significance of objects for the segmentation of action.