Acquisition of conditioned facial reflexes in the cat: cortical control of different facial movements.

The motor cortex plays a role in determining which of three different facial movements is acquired in Pavlovian conditioning experiments. Three separate facial reflexes can be distinguished by recording electromyographic activity from the orbicularis oculi (eye blink) and levator orii (nose twitch) muscles. One in a pure eye blink; a second is a nose twitch; the third is a compound eye blink and nose twitch. Which of these movements is elicited by a click (conditioned stimulus) following associative conditioning is reflected by the pattern of unit activity elicited by the click at the motor cortex. Activity is enhanced, after conditioning, in those units that project polysynaptically to the specific muscles performing the learned movement. This enhancement of activity is, in turn, relatable to an enhanced electrical excitability of the involved neurons. Analogous changes in the excitability of neurons of the motor cortex to applied currents can be produced by local application of cholinergic agents. Iontophoresis of acetylcholine, aceclidine (a cholinomimetic drug), or intracellularly applied cyclic GMP produces changes in single neuron membrane resistance that increase neuronal excitability. The units of the motor cortex that respond preferentially to these agents and to the click conditioned stimuli with short latencies have been identified as pyramidal cells of layer V. The axons of these neurons form the pyramidal tract, a pathway characterized as serving voluntary movement. It appears that this system supports rapid transmission and processing of auditory-motor information used to perform learned movements adaptively, selectively, and discriminatively.