Hack #104: Change the length of your arms!

Here’s a fantastic party-trick, if it works as reported in the Journal of Vision – make your arms feel like they are different lengths using a simple cut out piece of card.

Now, we talked about perception of depth in the book (Hack #22) and about how the senses interact (Chapter 5). One common theme was how visual information often tended influence our perception of information in the other modalities (at least for spatially organisation information, see Hack #53). What Nicola Bruno from the University of Trieste, and colleagues, seem to have found is an instance where a classic illusion of visual depth can distort your perception of your own body.

Ames’ trapaziodal window works by virtue of the assumption that things which appear larger are often just closer by. The Window (see a demo here) is a trapazoid, so that it gives the same appearence as a square with one side further away than the other. Like this:


Just like this the retinal-image is ambiguous between a trapazoid viewed flat on, and a square viewed with one side closer than the other. Normally you can use other information, like comparing the image between your two eyes, to deduce the correct perception of depth, but if you close one eye your brain has to fall back on just the ambiguous image information. And it seems your brain thinks squares are more likely and will deliver to your consciousness the perception of a slanted object, rather than a correct, flat-on, impression.

What Bruno et al did was have participants hold versions of the trapazoidal window illusion and judge the level of slant. Not only did they systemmatically mis-judge the slant of the object (despite getting clear information on how far away both sides were via the proprioception of their hands), but some participants reported ‘a stiking prioprioceptive distortion’ – namely that one hand appeared to be further away than the other, or one arm appeared longer than the other!

Unfortunately the research is only reported in abstract form (here) so I can’t get any more details of how exactly they built the illusionary trapazoid, but you can bet that I’ll be trying it out in the next few days. I suspect that, like the body schema illusions (Hack #64), this effect will work strongly on only a few people, so I’ll have to try it on a bunch of people before getting anything. I’ll let you know the results of my experiments, and I’d love to hear from anyone else who trys it.

One thought on “Hack #104: Change the length of your arms!”

  1. That trapezoid doesn’t work – the diagonals are longer than the large upright, so it can’t be a foreshortened square. It could be a foreshortened rectangle, but I don’t see that much more strongly than the actual trapezoid.
    If you add in the internal bars (like the rotating gif on the site you link to), the illusion’s much stronger, as it’s a closer resemblance to a barred window.

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