Adaptation reveals sensory and decision components in the visual estimation of locomotion speed

<p>Locomotion speed provides important social information about an individual’s fitness, mood andintent. Visual estimation of locomotion speed is a complex task for the visual system because viewingdistance must be taken into account, and the estimate has to be calibrated by recent experience oftypical speeds. Little is known about how locomotion speed judgements are made. Previous researchindicates that the human visual system possesses neurons that respond specifically to moving humanforms. This research used point-light walker (PLW) displays that are known to activate these cells,in order to investigate the process mediating locomotion speed judgements. The results of threeadaptation experiments show that these judgements involve both a low-level sensory component anda high-level decision component. A simple theoretical scheme is proposed, in which neurons sensitiveto image flicker rate (temporal frequency) provide a sensory speed code, and a benchmark ‘norm’ valueof the speed code, based on prevailing locomotion speeds, is used to make decisions about objectivespeed. The output of a simple computational model of the scheme successfully captured variations inlocomotion speed in the stimuli used in the experiments. The theory offers a biologically-motivatedaccount of how locomotion speed can be visually estimated.</p>