Neural Substrates for Pattern Separation and Completion in the Dorsal Pallium of a Weakly Electric Fish

Abstract

The dorsodorsal division (DD) of the teleost telencephalon has been implicated in memory processes similar to those associated with the mammalian hippocampus. The network connectivity and neural activity underlying this involvement have remained unclear. This thesis attempts to elucidate both. Attempts have been made to record the neural activity of DD neurons, but none have succeeded in correlating the recorded firing with any meaningful stimuli. In this thesis, I present single-unit electrophysiological recordings of DD neurons that reveal persistent activity in the form of up-states which are evoked by two modalities of naturalistic sensory stimuli – visual and electrosensory. The anatomy of DD was a little better understood than the neural activity. Recent anatomical work has shown that DD is strongly inter-connected with the cortical-like dorsolateral division (DL) of the pallium, re-inforcing its similarity to mammalian hippocampal structures. This same work has also revealed much of DD’s extrinsic connectivity. It was not, however, of a resolution fine enough to disambiguate the connections of the various DD subregions, nor to clarify the existence and structure of intrinsic DD connectivity. In this thesis, I further elucidate the connectivity of DD, by isolating its subregions. This was done by means of very small and precise neurotracer injections. These injections revealed strong recurrent connectivity within individual DD subregions, multiple pathways between DD and DL, and striking similarities between the connectivities of DL and DD and those of the mammalian dentate gyrus and CA3 respectively. From the results of these investigations I propose a model of homology between the teleost DD-DL loop and the putative pattern separation and completion networks contained in the mammalian cortico-hippocampal circuitry, as well as a role for the observed persistent activity in DD within this model. I further propose the dorsal teleost telencephalon as an excellent model system for the further study of the network mechanisms of pattern separation and completion.