Fickle Finger’s Funny Feel: Digit illusion modifies touch perception

If your left index finger suddenly grew about an inch longer, you’d have good reason to freak out. Your brain, however, within seconds would calmly adapt to this bizarre circumstance by altering its map of the body’s surface, a new study suggests. The elongated finger’s sense of touch would change, according to an experiment led by neuroscientist Patrick Haggard of University College London.

The researchers’ conclusion rests on observations of people who underwent arm manipulations that create illusions of finger growth or shrinkage.

Even faced with abrupt increases in body size, the brain adjusts its representation of relevant body parts that it uses to interpret touch on the skin, Haggard and his team conclude in the July 26 Current Biology. Parts of the brain’s outer layer, or cortex, primarily handle tactile information.

The brain modifies its body map for the gradual expansions of skin surface as a child grows, Haggard’s team proposes. However, the brain doesn’t remap after reductions of skin surface, which would be uncommon, as in amputations. This explains, at least in part, why the brain continues to cause sensations where a limb has been removed, according to the investigators.

More typically, though, “the brain’s map of the body changes minute by minute and constructs our conscious experience of the external world,” Haggard says.

The researchers studied 10 volunteers who were blindfolded and had their right arms put in slings. They were then instructed to hold the tip of their left index finger with their right index finger and thumb. At the same time, a device vibrated tendons of the biceps muscles, the triceps muscles, or a tendonfree part of the right forearm.

Volunteers reported that biceps vibrations elicited an illusion of the right arm extending outward slightly and, therefore, of an elongation of the left index finger. Triceps vibrations induced an illusion of the right arm flexing and shrinkage of the left index finger. Forearm vibrations yielded no bodily illusions.

While undergoing vibrations, each participant was touched successively on the left index finger and the forehead with two small metal rods and asked to judge whether the distance between rods on the finger felt bigger or smaller than it did on the forehead. For each condition, finger and forehead rods were the same distance apart in 10 trials and different by 15 millimeters in another 10 trials.

When the left index finger felt longer than it really was, participants consistently overestimated the distance between rods placed on that finger. Estimates were usually accurate when the finger felt shorter than it really was and when no illusion occurred.

Vilayanur S. Ramachandran, a neuroscientist at the University of California, San Diego, says that the results raise intriguing questions about the malleability of touch perception. For instance, he wonders whether, if two rods were placed on a finger as researchers somehow induced the illusion that the finger was expanding, the perceived distance between the rods would expand proportionately.