Associative learning in invertebrates
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Updated: Apr 14, 2026

Measuring Associative Learning in Chemotaxis of the Nematode Caenorhabditis elegans
Published on: June 17, 2025
Robert D Hawkins1, John H Byrne2
1Department of Neuroscience, Columbia University, New York, New York 10032 New York State Psychiatric Institute, New York, New York 10032.
Abstract:
This work reviews research on neural mechanisms of two types of associative learning in the marine mollusk Aplysia, classical conditioning of the gill- and siphon-withdrawal reflex and operant conditioning of feeding behavior. Basic classical conditioning is caused in part by activity-dependent facilitation at sensory neuron-motor neuron (SN-MN) synapses and involves a hybrid combination of activity-dependent presynaptic facilitation and Hebbian potentiation, which are coordinated by trans-synaptic signaling. Classical conditioning also shows several higher-order features, which might be explained by the known circuit connections in Aplysia. Operant conditioning is caused in part by a different type of mechanism, an intrinsic increase in excitability of an identified neuron in the central pattern generator (CPG) for feeding. However, for both classical and operant conditioning, adenylyl cyclase is a molecular site of convergence of the two signals that are associated. Learning in other invertebrate preparations also involves many of the same mechanisms, which may contribute to learning in vertebrates as well.
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