Experiment Module: Activity Pattern of Neurons in the Motor Cortex of Monkeys

One very instructive way of studying how the brain controls movement is by observing the activity patterns of neurons while movements are actually in progress.

This is what Weinrich and Wise did in 1982. They conditioned a group of monkeys to perform a simple task that involved touching an illuminated target. First a light came on to tell the monkey which of several targets it was going to have to touch. But the monkeys were supposed to wait for the target itself to light up before actually reaching out and touching it. After a time interval of varying length, the target light came on, telling the monkey to go ahead and touch it. If the monkey performed this task correctly (waited for the second signal, then touched the target), it was rewarded with fruit juice.

Using electrodes implanted in the monkeys' premotor cortexes, the researchers then made extracellular recordings of activity from individual premotor neurons while the monkeys performed this conditioned task. The figure here shows the activity recorded from one premotor neuron during each of the three steps of this task. 1) Baseline activity of the neuron while the monkey waits for the first signal. 2) The first instruction stimulus is given (the first light comes on) at the point identified by the arrow in the recording, triggering the discharge of the neuron. 3) Very shortly after the second light comes on and the monkey begins to move its arm, the neuron stops discharging.

The observation that the neurons of the premotor area are selectively activated before a movement begins is consistent with their posited role in the activation sequence for voluntary movements.

In other studies, the electrical activity of individual neurons in the primary motor cortex has been recorded in monkeys who have been conditioned to perform a simple motor task. Once again, the experimental protocol involved two types of signals: an informative signal, given first, to provide the monkey with advance information about the movement that it was going to have to execute, and an imperative signal, given one second later, telling the monkey to go ahead and execute the movement toward the target.

The researchers discovered three types of neurons with three distinct activity patterns in the primary cortexes of monkeys who carried out this protocol: 1. preparation neurons that became active as soon as the informative signal was given and that ceased activity as soon as the imperative signal was given; 2. execution neurons that became active after the imperative signal was given and that were involved only in the execution of the movement; and 3. preparation and execution neurons that became active after the informative signal and continued being active after the imperative signal, thus connecting the one process to the other.

These experiments partially explain how the nervous system carries out a movement following a stimulus: it selects a behavioural response, programs the appropriate movement, and then executes it.

Source: Neuroscience: Exploring the Brain, by
Bear, Connors, and Paradiso



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