Memory in the Motor Cortex

The motor cortex--a strip of tissue on the top of the brain that tells our muscles what to do--now appears to do far more than simply orchestrate movements. In today's Science, neuroscientists report evidence suggesting that, at least as a prelude to movement, neurons in the motor cortex also do a kind of thinking: helping the brain recognize and remember the sequence of events in time.

More than 2 decades of experiments have indicated that in certain circumstances neurons in the motor cortex are active even when an animal isn't moving. In the 1980s and early 1990s, for example, a team led by Apostolos Georgopoulos of the University of Minnesota and the Veterans Affairs Medical Center in Minneapolis found that neuronal activity in the motor cortex can provide information about the direction of an upcoming movement and can also serve as a memory for the spatial locations of individual stimuli to which the animal was supposed to move.

Georgopoulos then wondered whether motor cortex cells also could keep track of several spatial locations when they are presented in a sequence. So in 1993, he and his colleagues began training two monkeys to perform a task that requires memorizing the order of events in time. The monkeys watched a series of yellow spots pop up on a screen in a random order around an invisible circle. Then, after three or four spots were present on the screen, one would turn blue. The monkeys' job was to move a cursor to the spot that had appeared right after the blue spot.

As the monkeys performed their task, many of the hundreds of neurons in the animals' motor cortices showed an increase in activity before any movement occurred. Surprisingly, hardly any of these responses was specifically related to a spot's location. Instead, many of the neurons seemed to be sensitive to the spot's order of appearance. The data suggest that the motor cortex can play an active role in processing abstract information.

Some neuroscientists are skeptical of the conclusions. For instance, some think it's possible that the neural responses seen in the study somehow correspond to a monkey's thoughts of moving in the direction of each new spot.

But if the findings hold up, the work may mean "that the information needed to perform complex cognitive tasks is distributed very widely" in the brain, says Steven Wise, a neuroscientist at the National Institute of Mental Health in Poolesville, Maryland. In that event, prospects for recovering from brain injuries may someday be brighter. If healthy areas share some functions of the damaged brain areas, Wise speculates, clinicians may be able to boost those functions and stimulate more complete recovery.

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