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ABSTRACT:
Due in part to studies of learning in Aplysia, a great deal is known about the cellular basis of classical conditioning. In comparison, relatively little is known about the mechanisms underlying operant conditioning. This deficit results, in part, from the lack of a suitably traceable model system that manifests operant conditioning and that is amenable to cellular and molecular studies. To address this issue, we developed an in vivo conditioning procedure using the feeding behaviour of Aplysia and virtual stimuli.
Virtual reality encompasses the replacement of physical stimuli with neural stimulation. In Aplysia, sensory receptors in the buccal cavity transmit stimuli to the buccal ganglion. In pilot studies, we have performed in vivo recordings from the anterior branch of the oesophageal nerve in feeding Aplysia. These studies revealed presumably afferent activity (30Hz/3s) that coincided with biting and swallowing. In a novel operant conditioning paradigm, we have substituted food reinforcement (unconditioned stimulus, US) with extracellular stimulation of the anterior branch of the oesophageal nerve via surgically implanted electrodes. The pattern of stimulation was modelled after the recordings from feeding Aplysia (30Hz/3s). In an experimental group, nerve stimulation was made contingent on spontaneous biting behaviour. In other words, the animals could stimulate themselves by biting. In a yoked control group, the animals received the same pattern and amount of stimulation as the animals in the experimental group, but independent of their behaviour. A third group did not receive any stimulation at all (no-US group). Training lasted for 10 minutes. The animals were then tested for spontaneous biting behaviour either immediately after training (Fig. a) or 24 hours later (Fig. b). Each test session lasted 5 minutes. At both time points, contingent reinforcement lead to a significantly higher rate of biting in the experimental group compared to both controls. These results show that contingent in vivo stimulation of the anterior branch of the oesophageal nerve in Aplysia is sufficient to induce an operant memory that lasts for at least 24 hours. Further studies will reveal whether the neuronal correlates found in the in vitro analogue can be found in the CNSs of animals trained operantly with in vivo stimulation of the oesophageal nerve.
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