Cosyne 2008 Workshops
March 4, 2008
Snow Bird, Utah
Daniel Ben Dayan Rubin (with Stefano Fusi)
Long Memory Lifetimes Require Complex Synapses and Limited Sparseness
Theoretical studies have shown that memories last longer if the neural representations are sparse, i.e. when each neuron is selective for a small fraction of the events creating the memories. Sparseness reduces both the interference between stored memories and the number of synaptic modifications which are necessary for memory storage. Paradoxically, in cortical areas like the infero-temporal cortex, where presumably memory lifetimes are longer than in the medial temporal lobe, the neural representations are less sparse. We resolve this paradox by analyzing the effects of sparseness on complex models of synaptic dynamics in which there are metaplastic states with different degrees of plasticity. For these models, memory retention in a large number of synapses across multiple neurons is significantly more efficient in case of many metaplastic states, i.e. for an elevated degree of complexity. In other words, larger brain regions allow to retain memories for longer only if the synaptic complexity increases with the total number of synapses. However the initial memory trace, the one experienced immediately after memory storage, becomes weaker both when the number of metaplastic states increases and when the neural representations become sparser. Such a memory trace must be above a given threshold in order to permit every single neuron to retrieve the information stored in its synapses. As a consequence, if the initial memory trace is reduced because of the increased synaptic complexity, then the neural representations must be less sparse. We conclude that long memory lifetimes allowed by a larger number of synapses require more complex synapses, and hence, less sparse representations, which is what is observed in the brain.
