Paying attention at many levels.

Paying attention at many levels

An animal is continuously bombarded with sensory input--all the sights, sounds, smells and skin sensations delivered by the environment. Somehow the brain selects from this barrage the relatively few stimuli important for the animal's immediate behavior. This essential screening occurs at many levels within the brain. But surprisingly, scientists now report, the screening process begins before the signals reach the brain's complex processing centers, perhaps even before they reach the brain.

"The screening occurs right when information comes into the central nervous system, not as some higher function of the cortex,' Mary C. Bushnell of the University of Montreal reported last week in Dallas at the meeting of the Society for Neuroscience. The new data stem from scientists' increased ability to study awake animals trained in particular tasks.

In their recent experiments, Bushnell and Ronald Dubner of the National Institute of Dental Research in Bethesda, Md., trained monkeys to press a button to begin a trial, to wait for a cue and then to release the button to get a juice reward. Each monkey learned to recognize two cues--a light signal and small increase in heat from a heating element on its face.

The scientists recorded the electrical activity of nerve cells that receive input from the face's pain receptors. These cells are in a lower brain area called the dorsal horn of the medulla, but are comparable to cells found in the spinal cord that respond to pain elsewhere on the body.

In each test, whether the monkey was to respond to the heat or the light cue, the scientists applied the same amount of heat to the monkey's face. But the response of the dorsal horn cells differed according to the cue relevant to the monkey's current task. When the monkey was instructed to respond to the heat cue, its dorsal horn cells showed increased activity after the heat increment. But when the monkey was instructed to respond to the visual cue, the dorsal horn cells responded to the heat increment with half this activity or less.

The dorsal horn cells receive the message instructing whether or not to pay attention to the pain input from higher brain regions, but the scientists do not yet know what brain areas are involved. The magnitude of the dorsal horn response reflects how well the animal can detect a small change in stimulus, Bushnell reports. The animal is better able to analyze an expected stimulus than an unexpected one. The scientists expect to find the same sort of early screening in cells of the spinal cord. They have recently trained monkeys to respond to heat on their hands, but have not yet recorded the activity of the spinal cord cells.