Neuronal Regulation vs Synaptic Unlearning in Memory Maintenance Mechanisms

Hebbian learning, the paradigm of memory formation, needs further mechanisms to guarantee creation and maintenance of a viable memory system. One such additional mechanism is Hebbian unlearning, a process hypothesized to occur during sleep. It can remove spurious states and eliminate global correlations in the memory system. The problem of spurious states is unimportant in the biologically interesting case of memories that are sparsely coded 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 leads to dynamical maintenance 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 eld. On longer time scales it acts to maintain memories by letting the stronger synapses grow to their upper bounds. In aging, NR further increases the synaptic values to overcome the loss of synaptic degredation.