Intracellularly recorded responses of neurons of the motor cortex of awake cats to presentations of Pavlovian conditioned and unconditioned stimuli

Abstract

The electrical responses to a conditioned stimulus (click CS) and an unconditioned stimulus (glabella tap US) were studied in single units of the pericruciate cortex of awake cats. There has been no previous direct, in vivo characterization of the intracellular effects in cortical neurons of a CS and US used for associative Pavlovian conditioning. Earlier studies showed that ablation of the pericruciate cortex prevented the development of short latency blink CRs produced by associative pairing of these stimuli. Both the CS and the US produced depolarizing EPSPs and increases in spike discharge in intracellularly recorded units. The magnitude of the EPSPs was greater in response to the tap US than to the click CS, and the degree of unit discharge in response to the tap US was greater than that to the click CS. The onset latencies of the spike responses were distributed in three ranges. For click CS the ranges were 6–15, 20–35 and 40–85 ms. For tap US the ranges were 6–25, 30–45 and 50–85 ms. An additional response was observed in the period 0.5–6.0 ms after tap onset. It was thought to be elicited mechanically by vibration of the electrode tip following tap delivery. Pentobarbital anesthesia abolished the unconditioned motor response to tap, but failed to abolish the neuronal responses in any of the latency ranges, suggesting that the responses were not produced by feedback from the movement. Both the CS and the US are needed for the development of blink conditioning, and are distinguished behaviorally by the US producing an unconditioned motor response whose form resembles that of the conditioned response which develops after the stimuli are paired. Both the click CS and the tap US also produced hyperpolarizing IPSPs. Injection of steady, hyperpolarizing current disclosed two types of IPSPs in response to the tap US. One reversed with hyperpolarization; the other increased in magnitude with hyperpolarization. The magnitude of inhibition, measured by reduced discharge, was greater in response to the click CS than to the tap US. We conclude that the major differences between effects of this CS and US on cells of the motor cortex of cats are that the CS elicits less activity than the US and has a larger inhibiting effect. The US elicits greater depolarization than does the CS, as well as an initial discharge component of greater magnitude and longer duration.