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Abstract:
Identifying living moving things in the environment is a priority for animals, as these could be prey, enemies or mates. When the shape of the moving object is hard to see (fog, twilight, great distance, small animal), motion becomes an important cue to detect it. The neural correlates of the detection of an isolated living entity on the basis of its motion are largely unknown.
To study this phenomenon, we developed a single-dot stimulus, thus eliminating all possible sources of information about form, spatial arrangement, shape or structure of the object. The dot moved such that it appeared self-propelled, or moved by an external force, or something intermediary according to a small set of parameters. Self-propelled stimuli were perceived as more animate (= more likely to be alive) than the externally-moved stimuli, with a gradual transition occurring in the intermediary morphs following a classic psychometric function (cumulative gaussian).
In an fMRI experiment, 20 subjects had to categorize these stimuli into alive and non-alive. A region of the left medial posterior parietal cortex (mPPC) showed BOLD signal correlating with the probability of animacy judgments about the moving dot. While activation in parts of the early visual cortex showed the same response, the mPPC was the only region in which changes in percept had a stronger effect on activation than physical changes in the stimuli. In addition, only the mPPC showed BOLD signal increases when a stimulus was judged to be animate, irrespective of its physical characteristics.
This study shows that parts of the early visual cortex but particularly the medial posterior parietal cortex (mPPC) are involved in judging the animacy of an isolated translating visual stimulus, without information about its form.
