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.
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.
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.
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.