Modifying body representation through vision and vice versa

There’s a few interesting multi-modal illusions involving vision and another sense. Proprioception, the ability to sense the position of one’s body parts, is one sense that gets a bad rep; it’s not even included in the classic 5 senses. Yet it’s certainly quite important, and people that lose proprioception have difficulty functioning at first, a condition that is mitigated over time through vision, as related by Oliver Sacks.

Can you modify vision through proprioception, and vice versa? There’s a neat illusion related in Carlston et al. 2010 that shows just this. A subject sits in the dark for several minutes. A blinding, 1 ms flash then occurs; after a certain number of seconds, the person perceives a positive afterimage of the scene. If the scene includes one’s own limbs, then interesting things happen: if you move the limb away from its original position, it seems to fade in the afterimage. If you move it back to the original spot, it seems to fade back in.

The fading appears to extend not only to one’s own limbs, but also to tools. You can hold a ball in your hand, for example, then drop it, and it seems to be removed from the afterimage. Admittedly this is hard to believe, so I tried to replicate the experiment for myself and see what it’s all about. I don’t have the 3,000$ flash system used in the original experiment, but I did buy a 20$ flash in a pawn shop. It took a little while to get the effect going, but basically the key is to hold the flash very close to the limb whose image is to be manipulated. Then a very clear blue-black afterimage appears after a few seconds, clear enough to try some of the effects described in the paper.

Here’s a video showing how to get it running:

It’s perhaps not as mindblowing as it appears in the paper; actually I don’t really like how the paper is written because there’s few actual experiments and the style is bombastic (it’s not very sciency). But the effect is real nonetheless, and I’m sure it would be more impressive with a 2000W lighting system handy.

Carlson, T., Alvarez, G., Wu, D., & Verstraten, F. (2010). Rapid Assimilation of External Objects Into the Body Schema Psychological Science, 21 (7), 1000-1005 DOI: 10.1177/0956797610371962

One response to “Modifying body representation through vision and vice versa”

  1. This reminds me of a demo Ramachandran et al had at Demo Night at the Vision Sciences Conference last year. In that case they did have a very powerful flash. The various illusions they describe (legs on the ceiling, etc) were very powerful.

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