Does The "Shrunken Finger Illusion" Fool Your Brain?

When you fall for an optical illusion it’s often said your "mind is playing tricks on you.”

In this sense, it suggests that these blips are caused by some failure of your cognitive processing. However, according to a study published in Current Biology, this optical illusion could be more to do with our visual system than our imagination.

Take a look at the illusion that they used in their study above. It involves just a ping-pong ball sliced in half and your own hand. If you place the semi-spherical ball on top of the tip of your middle finger and give a glance downwards, it will appear that your finger has shrunk. Even though we are aware that the ball is cut in half, however, our visual system “fills in” the obscured bottom half of the ball, creating the impression that your finger is shorter than it is.

According to experimental psychologist Vebjørn Ekroll from KU Leuven and colleagues, this is all down to the visual system and not the cognitive function of the brain.

They arrived at this conclusion by conducting an experiment whereby they asked participants to identify where they thought their fingertip was whilst keeping the ping pong half on their finger. The researchers then introduced halved ping pong balls with progressively increasing radiuses. Oddly enough, the impact of the illusion was heightened as the ball got wider and the amount of finger exposed was decreased.

“Rationally, we know that our finger is not actually shorter, but the illusion persists nonetheless,” the authors say in the study.

They added, “Our study shows that our visual system is behind the illusion, not our conscious mind.”

^{Image credit: Ekroll et al/Current Biology}