Changes in excitability to weak-intensity extracellular electrical stimulation of units of pericruciate cortex in cats.

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Abstract
The excitability to weak (nA) extracellular electrical stimulation was measured among single neurons of the pericruciate cortex of awake cats as a function of behavioral state. Levels of neuronal excitability were compared after classical conditioning of a facial movement, during extinction of the conditioned response, and during unpaired presentations of conditioned and unconditioned stimuli (CS and US). Studies were made among neurons that were ultimately projective to musculature of the face and among those responsive to auditory stimuli. Neurons projective to facial muscles via polysynaptic corticofugal pathways showed decreased levels of excitability to weak extracellular stimulation following conditioning with forward pairing of the CS and US, extinction with backward pairing of the stimuli and presentations of the US alone. These changes in excitability were attributable solely to the effects of US presentation and were not relatable to either conditioning or extinction of the behavioral response. Small decreases in rates of spontaneous firing were found to accompany the decreases in neural excitability. Data from cats that received exposure to the US prior to but not during the recording sessions indicated that the effects of US presentation persisted for at least 28 days. Units classified according to auditory responsiveness did not show any significant changes in their levels of excitability to extracellular nA stimulation that could be related to experimental state. The changes in excitability described above were specific for cells related to motor rather than to auditory functions. Significant nonassociative changes in neural excitability appear to occur during conditioning and extinction due to presentations of the unconditioned stimulus and that the mechanism of these changes is different from that of changes in postsynaptic excitability found, after conditioning, on intracellular stimulation of similar cortical neurons. Behaviorally, the decreases in excitability may support a phenomenon that causes a reduction in the rate of conditioned response performance during initial training periods, is acquired nonassociatively following exposure to the US and is overshadowed by the more obvious effects of conditioning.

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