Changes in unit postsynaptic responses at sensorimotor cortex with conditioning in rabbits.

A localized, conditioned startle reaction (LCSR) to a click (as conditional stimulus) could be established and recorded electromyographically in rabbits if complex unconditional stimulus was employed. The complex consisted of localized, electrical stimulation applied concurrently to sensorimotor cortex and hypothalamus. The latency of this LCSR was only 12-14 ms. Postsynaptic responses were recorded intracellularly or quasi-intracellularly within the sensorimotor cortex near the placement of the electrodes for the cortical stimulation. The latency distributions of the averaged postsynaptic responses were compared for the naive, conditioned and extinguished dates. Latencies in naive animals were similar to those in the extinguished state, but were significantly (P less than 0.011) different from the Latencies of click-responses in the conditioned state. An increase in the number of responses with 8-17 ms latencies was found after conditioning. Of 30 neurons recorded in conditioned animals six responded with an extremely short latency of 4 to 7 ms. Similar latencies were found in response to much more intense clicks capable of evoking an unconditioned startle reaction in naive animals. Changes in postsynaptic potentials thus occur at the cortex independent of proprioceptive feedback from the conditioned movement. The short latency neuronal responses suggest that a pathway for the LCSR may pass through the sensorimotor cortex. The appearance of responses at less than 7 ms to a previously neutral click in conditioned animals supports the idea that an increase in synaptic effectiveness underlies the neuronal mechanism of conditioning.