Why our tongues and fingers "see" the world differently

The oral size illusion is a strange phenomenon in which we get wildly different estimates of size depending on what we used to do the measuring: our fingers or... our tongues. Here's how this odd little phenomenon works.

It's only certain objects that activate the oral size illusion, with very small objects posing the biggest challenge. Researchers recently put the illusion to the test by asking experimental subjects to match pegs and holes of similar size using only their fingers and tongues:

"Individuals overestimate the diameter of 1-mm-deep stimulus holes presented to the tongue when they use their fingers to select a hole of matching diameter. The aim here was to determine whether the oral size illusion evident for 1-mm-deep holes would also occur with 1-mm-high pegs of similar diameters.

The illusion was studied in 24 individuals who were blindfolded during the trials. The two sets of test stimuli were (a) 5 cylindrical pegs (1 mm high) and (b) five circular holes (1 mm deep), each of 2.5, 5.0, 7.5, 10.0, 12.5 mm dia. The stimuli were held to the mouth with one hand, while the fingers of the free hand were used to select a matching object from a comparator series of either 1-mm-deep holes or 1-mm-high pegs ranging from 2.0 to 18 mm dia. Each stimulus was presented four times in a random order."

And what did the researchers find? It turns out that most objects didn't create a mismatch - it was only when the subjects were asked to compare their stimulus to comparison holes that they ran into trouble. I'll warn you now - this next passage contains frankly shocking amounts of the words "pegs" and "holes":

"The participants overestimated the diameter of the 1-mm-deep stimulus holes when these were matched with 1-mm-deep comparator holes. However, there was no illusion with four out of five 1-mm-high stimulus pegs when matched with 1-mm-high comparator pegs. In the ‘reverse' experiment, there was no illusion with four out of five stimulus holes when these were matched with 1-mm-high comparator pegs. However, an illusion was evident for all the 1-mm-high stimulus pegs when these were matched with the 1-mm-deep comparator holes. Regardless of the nature of the stimulus (‘hole' or ‘peg'), a mismatch between the stimulus and comparator was consistently seen only when the fingers probed comparator holes."

So after all this, what's going on with the oral size illusion? The researchers explain:

"The oral size illusion is not due to any intrinsic differences in the sensitivities of the tongue or fingers. Rather, the illusion is due to the inability of the fingertip to access small comparator holes."

So...it's just that our fingertips are too big to estimate properly? You know, a lot of sensory illusions are the byproduct of something that actually helps us understand the world better, the result of little sensory shortcuts that we evolved because they're very helpful most of the times and only occasionally make us see things incorrectly. In this case, though? Yeah, it just sounds like evolution couldn't be bothered to get this right.

[via NCBI ROFL]