Motion illusion yields a neural surprise. (Brain's Moving Experience).

The primary motor cortex has a well-earned reputation as the brain's action center. This strip of tissue initiates limb movements ranging from walking up steps to wielding dining utensils.

However, the primary motor cortex also harbors an introspective side, according to a new study. This brain structure revs up during an experimentally induced illusion in which people mistakenly perceive that a stationary part of their body is moving, say neuroscientist Eiichi Naito of the Karolinska Institute in Stockholm and his colleagues.

In addition to its better-known duties, the primary motor cortex belongs to a brain network that monitors limb positions and plans upcoming actions, the investigators propose in the Dec. 5 Neuron. Scientists have traditionally attributed the sense of one's bodily position to so-called somatosensory tissue, located adjacent to the primary motor cortex.

The "extraordinary" new findings add to preliminary indications that the primary motor cortex not only issues motor commands but "also participates in the analysis of sensory information coming from the muscles," say Victor de Lafuente and Ranulfo Romo, both of the National Autonomous University of Mexico in Mexico City, in a commentary published in the journal.

Naito's group studied brain activity in eight adults as they experienced an illusory perception of hand movement. This trickery stems from mild vibrations applied by a small device to a tendon of a wrist muscle. After a few seconds, the vibrating tendon stimulates its connected muscle and creates a false sense of the wrist flexing back and forth.

Moreover, when volunteers hold their hands palm to palm, the vibrating device on either the right or left wrist tendon makes them perceive both hands as moving back and forth. Primary motor cortex on the right side of the brain regulates left-side movements, and that on the left side controls right-side movements. Thus, in this experiment, the hemisphere controlling the unstimulated hand received no sensory information related to the wrist-muscle activity.

Using functional magnetic resonance imaging scans, the researchers determined that the illusion of movement in an unstimulated hand was accompanied by a characteristic elevation of blood flow in primary motor cortex in the hemisphere that controls movement in that hand. A surge in blood flow serves as a marker of heightened brain-cell activity.

The same illusory movement yielded increased blood flow in several other parts of the cortex and the inner brain.

In a second set of eight volunteers experiencing the motion trick, the scientists used a magnetic device (SN: 9/23/00, p. 204) to spark neural activity in the primary motor cortex and thus magnify the illusion. Participants perceived more pronounced illusory movement in an unstimulated hand when their brains were prodded in this way. During the illusion, the heightened brain activity also coincided with increased electrical activity in the wrist muscle of the unstimulated hand--a sign of primary motor cortex activation.