Sequence Selectivity in the Mouse Primary Visual Cortex: the Semantics and Syntax of the Canonical Cortical Microcircuit

Abstract

The structure and temporal workings of the canonical cortical microcircuit have been in the spotlight for decades as anatomists, physiologists and neuroscientists sought to uncover the mechanisms of the synaptic connections of the six layers composing the entire neocortex. Our lab has also been attempting to contribute to this investigation, focusing specifically on the sequence selectivity of layer 2 and 3 (L2/3) cells of the mouse primary visual cortex (V1), and looking into whether or not these neurons are also selective to sequence order and history. Our hypothesis was that they would be selective to order—they would respond differently to forward and backward sequences—and also to history—they would be able to recognize when the presentation of a known sequence is disrupted in the middle. Previous work in the lab had shown that L2/3 neurons in V1 either show an adaptation, or transient, response to repeated sequence stimuli, or a sustained response. The experiments conducted and summarized in this thesis show that out of the sustained cells, there is a subset of cells that are also order selective: they markedly respond differently to forward and backward sequences. We also found that these sequence-selective, sustained cells also recognize history: they fired in predictable manners to disruptions of the sequences at varying places. However, there were also results from our experiments that did not agree with our hypothesis. A key caveat was that our current analysis was limited to the averaging of all the cells’ responses to order and history instead of allowing for the categorization of various possible response types. All in all, our hypothesis that there exist L2/3 neurons in V1 that are sequence, order and history selective was supported. Future experiments would require the investigation of layer 4 cells’ responses to check our theory of the sequence selectivity hierarchy in V1.