Iacaruso, M; (2014) Population coding of natural scenes in mouse visual cortex. Doctoral thesis , UCL (University College London).

Abstract

A fundamental aim in systems neuroscience is to understand how sensory information is represented by neural circuits under natural conditions. Natural scenes present complex combinations of features such as orientations and spatial frequencies that change over time. The complex spatiotemporal dynamics of natural stimuli is represented by sensory networks operating in a sparse regime where only a small fraction of neurons is active. But, how neurons with different feature selectivity interact to encode the spatiotemporal properties of complex natural scenes, while maintaining a sparse population code, remains unclear. Neuronal activity was measured in layer 2/3 of the primary visual cortex with two‐photon calcium imaging during natural scenes in anaesthetized mice between postnatal days 29 and 40. The mean population firing rate during natural movies was similar to the spontaneous level of activity measured during prolonged darkness. High and low-pass spatially filtered versions of the movie increased and decreased the mean population firing rate, respectively, and revealed neurons responding to spatial frequency components of the raw movie. Analysis of the information content of a small population of cells revealed that the mean mutual information increased with the spatial frequency along with mean network activity. The activation probability was significantly lower during low-pass movies than in the absence of visual input. This was due to a reduction in activity of neurons preferring high-pass movies, suggesting that spontaneous background network activity was reduced. During whole-cell recordings membrane potential (Vm) was consistently more depolarized during high pass movies regardless of the selectivity of the neurons. Differential modulation of the mean Vm and the Vm variance determined the firing rate of the high pass and low pass preferring groups during spatially filtered movies. These results suggest that the bidirectional modulation of ensembles of neurons with spatial frequency tuning might arise from differential modulation of voltage fluctuations and steady state depolarization.

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