Abstract

Locomotion speed provides important social information about an individual’s fitness, mood and
intent. Visual estimation of locomotion speed is a complex task for the visual system because viewing
distance must be taken into account, and the estimate has to be calibrated by recent experience of
typical speeds. Little is known about how locomotion speed judgements are made. Previous research
indicates that the human visual system possesses neurons that respond specifically to moving human
forms. 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 three
adaptation experiments show that these judgements involve both a low-level sensory component and
a high-level decision component. A simple theoretical scheme is proposed, in which neurons sensitive
to image flicker rate (temporal frequency) provide a sensory speed code, and a benchmark ‘norm’ value
of the speed code, based on prevailing locomotion speeds, is used to make decisions about objective
speed. The output of a simple computational model of the scheme successfully captured variations in
locomotion speed in the stimuli used in the experiments. The theory offers a biologically-motivated
account of how locomotion speed can be visually estimated.