Hippocampal CA1 Subregion as a Context Decoder

Example and proposed mechanism about CA1 neuron representations in Allen et al.'s study. The animal learned to associate a certain outcome with a sequence of four odors presented in the following order: A, B, C, and D (Context 1). The example shows how the information of the third item of a sequence flows within the hippocampal circuit in two different scenarios. Top, C is presented as the third item. Sensory information about item C is delivered from EC to CA1 by two parallel paths. EC layer II efferents provide input to the CA3–DG circuit, where recurrent synapses of CA3 allow the matching of incoming information about item C with the third item of the previously stored sequence. The resulting activity from this match in CA3 is sent to CA1, converging with the direct input from EC layer III, which triggers an abstract representation of context 1. Bottom, D is presented as the third item. In this case, the CA3–DG circuit does not recognize it as the third item of the sequence, resulting in the disambiguation of the overlapping sequences (ABC vs ABD), generating a distinct output pattern to CA1 that, in conjunction with the direct input from EC, triggers the representation of another context.