Motor theory: Graziano

MOTOR THEORY - Motor primitives for arm movement and speech

Text extracts and stimulation graphic from Graziano Michael SA, Taylor Charlotte SR, Moore Trin. Complex movements evoked by microstimulation of precentral cortex. Neuron Vol. 34, 841-851, May 30, 2002. Speech element table from The Physical Foundation of Language 2001.
[See also the further account in Graziano, Michael SA, Taylor Charlotte SR, Moore Trin and Cooke Dylan F. The cortical control of movement revisited. Neuron Vol. 36, 349-362, October 24, 2002. "One possibility is that the mechanisms for speech were built on a preexisting mechanism for motor control" (p. 355)
Systematic arm postures representation in motor cortex

The primate brain is thought to contain a map of the body that is used to control movement. This map is stretched across the cortex in front of the central sulcus, with the feet at the top of the brain and the face near the bottom.

Stimulation on a behaviorally relevant time scale evoked coordinated, complex postures that involved many joints. Stimulation always drove the joints towards the final posture, regardless of the direction of movement required to reach the posture.

Within the large arm and hand representation, the stimulation-evoked postures were organized across the cortex. Postures that involved the arm were arranged across the cortex to form a map of hand positions around the body. The map of hand locations was embedded in a larger, rough map of the monkey's body.Primary motor cortex corresponded mainly to the representation of the central space directly in front of the monkey's chest.

Postures Illustrating the Topographic Map Found in Precentral Cortex of Monkey.
The circle on the brain shows the area that could be reached with the electrode. The magnified view at the bottom shows the locations of the stimulation sites. The area to the left of the lip of the central sulcus represents the anterior bank of the sulcus. Stimulation on the right side of the brain caused movements mainly of the left side of the body. For all sites, stimulation trains were presented for 500 ms at 200 Hz.