Co-activation, cell assemblies and learning

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Donald O. Hebb's Organization of Behavior (1949) 1 set an agenda that continues to guide research in neuroscience. In it, Hebb sought to provide a neuropsychological framework to account for the complex phenomena of perception, ideation, attention, and, in particular, the formation of learned associations. Hebb drew on an extensive body of anatomical, physiological and psychological evidence, much of it generated by his own contributions, to argue that the substrate of associations lay in activity-dependent changes at synapses. He postulated that repeated co-activity of synaptically connected cells leads to a lasting cellular change that strengthens thier connection. This process was envisioned to lead the slow development of diffuse ‘cell assemblies’ in the cortex, thalamus and basal ganglia that encoded associations and were thus able to modify behavior 1. This provocative hypothesis presented in 1949 continues to stimulate research in 1985: a recent investigation by Carew, Hawkins, Abrams and Kandel 2 examines the role of co-activity of pre-and post-synaptic neurons in classical conditioning in the simple nervous system of the opisthobranch mollusc, Aplysia californica.