Elaboration of a conditioned reflex in a single experiment with simultaneous recording of neural activity

Abstract

In 18 of 20 experiments with click CS, and in 5 of 10 experiments with flash CS, we have elaborated a conditioned EMG response in a single session (less than 60 pairings). The US was direct electrical stimulation of the rabbit's motor cortex that evoked a movement of the forelimb. Electrical stimulation of the lateral hypothalamus was used as reinforcement. Parameters for this reinforcement were chosen to evoke a feeding reaction or self-stimulation in the freely behaving animal. The elaborated EMG response satisfied most of the required characteristics of a conditioned reflex. These included spontaneous recovery after extinction, savings of long duration, specificity to stimulus pairing, and weak efferent and afferent generalization. In most experiments with click CS, the elaborated phasic response had an amplitude of 0.1–6 mV with a latency of 12–16 msec. In form and latency the conditioned response was similar to the unconditioned startle reaction of the same animal under chloralose anesthesia, or to its unanesthetized response to a loud sound. However, the conditioned response differed from the startle reaction in that it was localized. Extracellular recordings of 2–5 neurons were simultaneously made from sensory motor cortex near the point at which the US was applied. In 7 experiments 17 neurons were followed through the entire cycle of elaboration and extinction of the conditioned response. Seven neurons showed a statistically significant ( P < 0.05 increase of the response to CS during conditioning. Latencies were 20–140 msec. Interactions between neurons were studied by computing cross-correlograms and joint PST scatter diagrams. These measures were less informative than we had hoped because of the low level of spontaneous and evoked activity, and because of the small numbers of stimulus presentations that were needed for elaboration and extinction of the conditioned reflex. In isolated presentations of the CS after elaboration of the CR, we sometimes observed neural responses to click with a latency less than 6 msec. We propose that at least one of the pathways involved in the localized conditioned startle reflex reported here goes through the sensory motor cortex.