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Electrical stimulation of the reticular formation, pars oralis of the spinal trigeminal, abducens, and accessory abducens nuclei was used to assess the role of these sites in the elicitation, reflex modification, and classical conditioning of the rabbit's nictitating membrane response (NMR). Although electrical brain stimulation of the targeted sites revealed comparable levels of unconditioned responses, the spinal trigeminal nucleus was the only site at which reflex modification and conditioned response acquisition occurred reliably. These findings suggest that a locus of conditioned stimulus and unconditioned stimulus interaction, mediating either or both reflex modification and NMR conditioning, is on the sensory side of the reflex arc, at the pars oralis of the spinal trigeminal nucleus.

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Elicitation of responses by electrical brain stimulation (EBS) was related to the synaptic distance of the target nucleus from the accessory abducens. Specifically, responses to EBS in the spinal trigeminal nucleus (TRIG) and red nucleus (RN) increased as a positive function of stimulation parameters. Responding to EBS in the interpositus nucleus (IP) was lower, and responding to EBS in the inferior olive (IO) was negligible. EBS in the TRIG, IP, and RN nuclei was then paired with a tone conditioned stimulus (CS). The CS modified responses for EBS in RN and TRIG but not IP. CS-EBS pairings yielded conditioned response (CR) acquisition, in which Groups TRIG, IP, and RN reached asymptotes of 90%, 70%, and 43% CRs, respectively. Thus, contrary to previous findings, EBS in the efferent pathway can support CR acquisition. The results are discussed with respect to the role of projections from the RN to the cerebellar cortex and the TRIG nucleus.

No abstract