The Elephant and the Tweezers

Vintage artwork of two elephants standing in a landscape.Buffon, 1838, from vintageprintable

When we were very young we had a joke: how do you get an elephant into a matchbox?

The answer went: look at the elephant through the wrong end of a telescope. It will then be very tiny, so you can pick it up with a pair of tweezers and pop it in the matchbox. Don’t forget to shut the drawer quickly!

I was reminded of this by reading about an experiment by Salvatore Aglioti.

It’s described in V.S. Ramachandran’s fascinating book “Phantoms in the brain”. Aglioti was looking for evidence that, as many scientists have suggested, alongside the “mammalian” visual pathway there is a relic of an older one, which we share with birds and fish. This is the pathway that probably accounts for the remarkable ability called “blindsight”.

There is a very well know visual illusion (Titchener circles or Ebbinghaus illusion), which you’ve probably seem many times, and which relies on size contrast. In the picture below the orange disk is exactly the same size in both cases, but at a conscious level it’s impossible to convince oneself that this is the case. The image below is from Wikimedia.

At left an orange disk is surrounded by six larger grey ones. At right, and orange disk is surroubded by eight smaller ones. Both orange disks are of identical size but size contrast makes the one surrounded by bigger disks perceptually much littler.

Aglioti set up an experiment, using physical objects rather than pictures, and asked experimental subjects to reach out and pick up the central object between finger and thumb (think of the tweezers!). LEDs were attached to a subject’s thumb, index finger and wrist, and their movements tracked by two cameras.

What this revealed is that the subjects opened up their “tweezers” according to the actual size of the objects, not the size perceived at a conscious level. In fact when the illusion was arranged so that disk of different sizes appeared the same, the subjects moved their thumb and wider further apart for the larger disks.

So at an unconscious level the brain is not deceived by the illusion. What Ramachandran calls the “how” pathway accurately guides the movements of our tweezers, despite the misleading signals from the parallel “what” pathway.

There’s no camera to track your hand movements, but try it anyway. Reach out simultaneously with the thumb and forefinger of each hand to grasp the two orange circles below. Don’t think about it, just do it intuitively. I think you will find you can do this fairly accurately and the optical illusion doesn’t make you over- or underestimate how far apart your thumb and forefinger should be.  You don’t have to suddenly compensate at the last moment.

At left an orange disk is surrounded by six larger grey ones. At right, and orange disk is surroubded by eight smaller ones. Both orange disks are of identical size but size contrast makes the one surrounded by bigger disks perceptually much littler.

The existence of the parallel visual pathway isn’t by any means accepted by all researchers, but Aglioti’s findings are very intriguing and as Ramachandran has pointed out they may have implications for many physical activities, particularly sports.

For an excellent summary of the current state of play, see the erudite article by Melvin Goodale.

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About partialinsight

One evening I had a stroke. Half my sight vanished overnight. Adapting made me grasp how amazing the visual system and brain are. It also taught me to understand disability completely differently and I'm grateful for the lesson.
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