Stop looking at my feet!

There’s something about feet that allows us to detect the movement of other people or animals even in the most difficult viewing conditions.

That’s the implication of a study by Nikolaus Troje and Cord Westhoff, who were investigating the remarkable human sensitivity to biological movement. Previous research has shown that in the pitch dark, all it takes for us to be able to recognise the shape of a walking human, is for someone to wear a spot of light on each of the main joints of their body.

However, if the image is turned upside down, we’re useless at recognising human movement in this way, leading researchers to suggest it’s the signature configuration of the joints (disrupted when inverted) that allows us to recognise biological motion.

However, Troje and Westhoff’s experiments suggest this can’t be the sole explanation. They presented participants with a display that looked like either a person, cat or pigeon, walking in the dark, with a light on each of their main joints (see here), but they distorted the positioning of the lights, thus removing the configural information previously thought to be so vital. Crucially, they found that even with the configural information removed, the participants were still better at recognising which direction the person/animal was walking in when they viewed the image the right way up, compared with upside down. That is, inverting the images must have disrupted some other source of information the participants were using besides the spatial arrangement of the joints.

So they tried inverting some parts of the image but not others, and found the secret lay in the feet. Only inverting the feet disrupted performance, while contrarily, even with the positioning of the lights distorted, and with the rest of the body upside down, so long as the joints of the feet were shown moving the right way up, the participants were able to tell which direction the person or animal was walking in.

The researchers said the movement of feet may serve as a kind of ‘life detector’, providing “…a reliable cue for the presence and the location of an animal in the visual environment”. They added: “The observation that it is relatively easy to get close to wild animals in a car, a canoe, or similar vehicle might be due to the absence of the typical movement of feet”.
Troje, N.F. & Westhoff, C. (2006). The inversion effect in biological motion perception: Evidence for a ‘life detector’? Current Biology, 16, 821-824.

Link to host lab.
Link to interactive demo of the stimuli.
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