Touched by an illusion

Why would multiple real stimuli register as a single stimulus in the brain? A paper published September 18 in the online edition of Science explains this quirk of perception, known as the tactile funneling illusion.

Yale neurobiologist Anna W. Roe, Ph.D., and her colleagues studied the illusion in a portion of the primary somatosensory cortex (SI) of squirrel monkeys. They found that whenever they administered a mild electrical stimulus simultaneously to two nonadjacent fingers of the animal’s hand, the SI showed two separate activation spots that corresponded to the two sites of stimulation. By contrast, when they delivered simultaneous stimuli to two adjacent fingers, the SI showed a single activation spot located midway between the two sites. The study indicates that, contrary to previous thought, a finger’s “receptive field” for sensory stimuli can sometimes extend beyond the finger itself—a notion that could someday find clinical application, for example in rehabilitation after injury or stroke.