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.
Highlights
Predators are known to use the movement speed of other animals as a basis for prey selection during chases, preferring slower targets[1,2,3]
After viewing a video depicting slowed-down or speeded-up human locomotion for a short time, experimental participants made judgements about the locomotion speed depicted in brief test videos of the same scene
The results showed that apparently ‘natural’ or ‘normal’ locomotion speed shifted towards the speed of the previously viewed stimulus; it was slower after adapting to slowed-down locomotion, and faster after adapting to speeded-up locomotion
Summary
Predators are known to use the movement speed of other animals as a basis for prey selection during chases, preferring slower targets[1,2,3]. The results showed that apparently ‘natural’ or ‘normal’ locomotion speed shifted towards the speed of the previously viewed stimulus; it was slower after adapting to slowed-down locomotion, and faster after adapting to speeded-up locomotion This shift in perceived locomotion speed is a form of normalisation to recently experienced stimuli, similar to other normalisation effects in the perception of faces, blur and colour[11,12,13], indicating that judgements of an object’s speed are calibrated on the basis of prevailing speeds in recent experience. A set of three psychophysical experiments used PLW displays to study the processes that mediate calibration of locomotion speed judgements. The first experiment tested whether PLW displays are effective as a stimulus for driving adaptation to locomotion speed, as predicted if previously identified neural systems responsive to human locomotion are involved. Any adaptation effects would reflect neural processes that do not respond to retinal speed
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