Neuronal regulation versus synaptic unlearning in memory maintenance mechanisms

Hebbian learning, the paradigm of memory formation, needs further mechanism s to guarantee creation and maintenance of a viable memory system. One such proposed mechanism is Hebbian unlearning, a process hypothesized to occur during sleep. It can remove spurious states and eliminate global correlatio ns in the memory system. However, the problem of spurious states is unimpor tant in the biologically interesting case of memories that are sparsely cod ed on excitatory neurons. Moreover, if some memories are anomalously strong and have to be weakened to guarantee proper functioning of the network, we show that it is advantageous to do that by neuronal regulation (NR) rather than synaptic unlearning. Neuronal regulation can account for dynamical ma intenance of memory systems that undergo continuous synaptic turnover. This neuronal-based mechanism, regulating all excitatory synapses according to neuronal average activity, has recently gained strong experimental support. NR achieves synaptic maintenance over short time,scales by preserving the average neuronal input field. On longer time scales it acts to maintain mem ories by letting the stronger synapses grow to their upper bounds. In agein g, these bounds are increased to allow stronger values of remaining synapse s to overcome the loss of synapses that have perished.