Cortical reinstatement: A direct approach to testing hippocampal indexing theory

Episodic memories are crucial elements that determine our sense of identity. Episodic memory formation is mostly dependent on the proper function of the hippocampus. However, mechanisms of episodic memory formation, maintenance and retrieval are only beginning to be elucidated. One of the leading scientific theories about the hippocampal role in memory is indexing theory. This concept builds on the premise that hippocampal neurons are multi-synaptically bi-directionally linked with sensory and associative cortical regions. Nevertheless, it is not clear if cortical reinstating occurs on a neuron-to-neuron basis. We created an artificial pattern of cortical activity by optogenetically stimulating a random population of neurons in the cortical layers of the retrosplenial cortex (RSC) and observed increased c-Fos activity in cells of several structures in the hippocampus. Based on the results, we replayed the hippocampal index by optogenetic activation of Fos-positive neurons in the hippocampus and attempted the in vivo observations of GCaMP signal activity in the retrosplenial cortex. Our ultimate goal is to compare the degree of overlap between the initially activated neurons and the reactivated population in RSC.