Selective inhibition in CA3: A mechanism for stable pattern completion through heterosynaptic plasticity
Engrams compete for successful retrieval in the CA3 of the hippocampus, but the detailed mechanisms of their formation remain elusive. Recent research reveals that hippocampal inhibitory neurons respond selectively to stimuli and exhibit diverse plasticity, implying their significance in engram formation. Conventional attractor network models for CA3 commonly employ global inhibition where inhibitory neurons uniformly reduce the activation of excitatory neurons. However, these models may not fully capture the diverse competition conditions arising from sparse distributed coding and not accurately reflect the specific roles of inhibitory neurons in engram competition. We propose an engram formation mechanism in CA3, highlighting the critical role of the association between excitatory and inhibitory neurons through heterosynaptic plasticity. In the proposed model, an inhibitory neuron is associated with specific neural assemblies and it selectively inhibits the excitatory neurons for retrieval that belong to competing assemblies. With a simplified dentate gyrus (DG) in a feed-forward structure, this proposed mechanism results in sparsely distributed engrams in CA3. The sparse distributed coding implemented in the model allows us to investigate the effects of selective inhibition on pattern completion under various configurations, such as partially overlapping competing engrams. Our results show that selective inhibition leads to more stable and improved retrieval performance than global inhibition alone. We observed the neural activities of the hippocampal subregions in the model for pattern separation and pattern completion, aligning with experimental findings on their respective roles.