The Cortical Control of
Movement
Electrical microstimulation was used to study primary
motor and premotor cortex in monkeys. Each stimulation
train was 500 ms in duration, approximating the time scale
of normal reaching and grasping movements and the time
scale of the neuronal activity that normally accompanies
movement. This stimulation on a behaviorally relevant
time scale evoked coordinated, complex postures that involved
many joints. For example, stimulation of one site caused
the mouth to open and also caused the hand to shape into
a grip posture and move to the mouth. Stimulation of this
site always drove the joints toward this final posture,
regardless of the direction of movement required to reach
the posture. Stimulation of another site caused the left
eye to close, the head to turn to the right, the left
hand to move to the space on the left side of the head,
and the hand to turn such that the palm faced outward,
as if the monkey were protecting itself from a potential
threat to the side of the head. Stimulation of other cortical
sites evoked different postures. Postures that involved
the arm were arranged across cortex to form a map of hand
positions around the body. This stimulation-evoked map
encompassed primary motor and the adjacent premotor cortex.
Primary motor cortex appeared to represent 7 mainly postures
of the arm that brought the hand into central space, and
postures of the fingers that were consistent with manipulation
of objects. These findings suggest that motor cortex does
not contain a map of the muscles as was previously thought,
but instead a map of locations in space to which movements
are directed. They also suggest that the differences between
primary motor and premotor cortex may be related to the
control of different types of actions in different regions
of space.
