Half a century of Hebb

history Hebb's idea of reverberatory activity in a cell assembly. Today the validity of Hebb's theory is still uncertain. Granted, the existence of the Hebbian synapse is not in doubt, but it is still unclear whether delay activity is truly reverberatory activity. If the neur-al network models are correct, then feedback loops are indeed the mechanism by which delay activity is maintained. This idea has been challenged, however, on the grounds that the persistence of delay activity is due to mechanisms that are intrinsic to single neurons, such as plateau potentials. Moreover, there is still no evidence that LTP stabilizes reverber-atory activity, as Hebb postulated. In addition, Hebb's belief in just two memory traces may be incorrect. We know now that synaptic plasticity is not a uni-tary process, but rather many processes on different time scales. Besides LTP, there are shorter-lasting forms of plasticity like post-tetanic potentiation, augmentation , facilitation and depression. If these forms of synaptic plasticity are involved in memory, the number of memory traces will have to be increased. Looking to the future, what strategy might definitively prove or disprove Hebb's theory? Here are some recommendations. First, study the simplest examples of neur-al activity that could possibly be 'rever-beratory,' and focus on whether synaptic feedback loops are really involved. Choosing the simplest examples is essential, because it will be technically demanding to settle the mechanistic question of whether reverberatory activity actually exists. Second, investigate the role of LTP in these examples. Third, attempt to manipulate Hebbian synaptic plasticity in vitro so as to store reverberatory activity patterns as long-term memories in culture or brain slice preparations—a synthetic rather than analytic strategy. Hebb's synapse is not his only legacy. Even after half a century, his theory as a whole is still inspiring, because it is a general framework for relating behavior to synaptic organization through the dynamics of neural networks. That is why further exploration of Hebb's ideas is a promising route for translating our accumulating knowledge about the molecular and cellular events underlying synaptic plasticity into a real understanding of learning and memory. For a more detailed history of these ideas, see Orbach, J. In 1949, Donald Hebb predicted a form of synaptic plasticity driven by temporal contiguity of pre-and postsynaptic activity. This prediction was verified decades later with the discovery of long-term potentiation, securing Hebb's place in the scientific pantheon. But the Hebbian synapse was just …