The psychophysical limits of agency: Biological motion percepts are dramatically weakened in slow-moving displays.

Abstract

The most important stimuli we perceive may be other agents, given their effects on our fitness. Accordingly, perception may be specialized for processing agents, and one of the best-studied examples is biological motion: displays of surprisingly few moving dots ("point-light walkers"; PLWs) nevertheless give rise to rich percepts of locomoting agents. Is this a complex instance of general motion perception, such that PLWs simply inherit the lower thresholds of general motion perception? Or might biological motion have a distinct lower "speed limit"? Observers viewed PLWs moving in place, always embedded in noise (with extra irrelevant moving dots). PLWs moved at typical speeds or at speeds that were (a) considerably slower than has been previously tested, but (b) still far above motion perception thresholds - always testing displays that were both duration-matched (with less overall motion) and cycle-matched (simply lasting longer). Across eight experiments, observers reported spontaneous percepts, or had to discriminate various properties - such as facing direction, apparent gender, or even whether a PLW was present in the first place. We always obtained the same results (which were also apparent as powerful phenomenological demonstrations): spontaneous impressions of biological motion were markedly diminished in slow displays, and discrimination of each of these aspects of biological motion in noise was greatly impaired in slower displays, although the motion was still readily visible. These explorations of "slow visual cognition" support the characterization of biological motion perception as psychophysically distinct from general motion perception.