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Cats were maintained on Flaxedil after spinal transection at T-12 under ether anesthesia. Experimental animals were classically conditioned by electrical stimulation of the exposed superficial peroneal nerve (CS), paired with cutaneous shock to the ankle of the same limb (US). The CR was the gross efferent volley recorded from the exposed deep peroneal nerve. Control animals were divided into unpaired CS and US, CS-only, and US sensitization groups. Results showed that the experimental conditions produced increases in the amplitude of the gross efferent volley while unpaired CS and US, and CS-only control conditions produced no change or a decrease in amplitude. The US sensitization group showed that no sensitization was present at the intertrial intervals used in experimental conditions.Spinal "conditioning" has a long and controversial history, due in part to the earlier belief that learning is the special province of the cerebral cortex and in part to contradictory empirical findings. Shurrager and Culler (1940) first reported successful classical conditioning of a spinal flexion response in acute spinal waking dogs. Utilizing a weak tail-shock conditioned stimulus (CS) and a hindpaw-shock unconditioned stimulus (US), paired trials produced responding to CS while unpaired and CS-alone trials did not. Subsequently, several studies by Kellogg and associates (Kellogg, Deese, Pronko, & Feinberg, 1947;Kellogg, Pronko, & Deese, 1946) failed to verify the Shurrager and Culler (1940) results. However, the Kellogg studies also failed to replicate many, essential features of the Shurrager investigations. Kellogg and associates utilized chronic preparations and presented CS and

Efferent mechanisms controlling the nictitating membrane (NM) reflex response to air puff in the albino rabbit were analyzed using stimulation, lesions, and recording techniques. In brief, stimulation of the sixth nerve (abducens) yields short-latency NM extension. Stimulation of the fourth and seventh nerves and the superior cervical ganglion has essentially no effect on the NM. Stimulation of the third nerve causes short-latency retraction of the NM. Lesions and recording data are consistent with this result--the sole efferent neuronal control of NM extension is the sixth cranial nerve and of NM retraction is the third cranial nerve. The NM extension response appears to be mediated by mechanical actions via retraction of the eyeball by the retractor bulbi muscle, and NM retraction appears to result from direct activation of muscle fibers in the NM by the third nerve. The superior cervical ganglion appears to play no role in reflex NM retraction in the rabbit, in contrast to its action in the cat.We have recently adopted classical con-(1962), as an extremely useful system for ditioning of the rabbit nictitating membrane analysis of brain substrates of associative (NM) response, first developed by Gor-learning (Thompson, Cegavske, & Pattermezano, Schneiderman, Deaux, and Fuentes son, 1973). The preparation has many ad-"TheTata were collected primarily in the second vantages for electrophysiological recording author's laboratory at Irvine, with replication of of neuronal activity that develops during several experiments in the third author's labora-the course of learning (see discussion in tory at Kirksville. Portions of the research formed Thompson, Patterson, & Teyler, 1972). portions of the first author's PhD dissertation at Acquisition occurs in a single training ses-Irvme. The research was supported primarily by . ^ ,, . , .

Neuronal unit activity was recorded from the abducens (6th nerve) nucleus, the "final common path," during classical conditioning of the nictitating membrane (NM) response in the rabbit, with the use of a tone conditioned stimulus, an air puff unconditioned stimulus (UCS), 250-msec interstimulus interval, and 60-sec intertrial interval. Animals were given 2 days of conditioning training (104 trials in eight blocks per day) and 1 day of extinction. Control animals were given comparable periods of stimulus presentations, explicitly unpaired. Activity of small clusters of units-"multiple unit" recording-was compared with the amplitude-time course of the NM response. Between-blocks comparisons of neural and behavioral responses indicated an essentially perfect correlation during acquisition of the conditioned response (Day 1, r = .99; Day 2, r = .98) and a slightly lower correlation during extinction (r = .93) for the conditioning animals. Within-blocks comparisons indicated a close correspondence between the histograms of unit activity and the amplitude-time course of the NM response for the conditioning animals in all phase of training and for the control animals in the UCS trial blocks. *Analysis of the neural mechanisms of of processing, it is necessary to compare learning and memory has proved extraordi-putative learning-related changes in other narily difficult. The goal is localization and brain structures with the temporal course of analyses of the alterations in neuronal ac-altered activity in motoneurons. tivity that code learning-the "engram"We have adopted classical conditioning of

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