Synaptic Mechanisms and Network Architecture Underlying Spatiotemporal Properties of Simple Cell Receptive Fields in Primary Visual Cortex In Vivo

Neurons in sensory cortex represent information about the environment and relay this information to other cells in the network using electrical and chemical signals. The study of neuronal receptive fields—descriptions of the sensory stimuli that best drive a neuron to fire—can yield insights into how different groups of cells transform sensory information. In layer 4 (L4) of primary visual cortex (V1), neurons called simple cells are responsive to the specific orientation and spatial phase of an edge-like stimulus. Relatedly, simple cell receptive fields are characterized by elongated, non-overlapping, spatially restricted subregions in which visual stimuli can either increase or decrease the cell’s firing rate, depending on contrast. In this dissertation, we examine the synaptic and network mechanisms underlying the generation of simple cell receptive fields and their response characteristics to visual stimuli within their receptive fields. Our experimental data from in vivo, intracellular recordings provides a characterization of the spatiotemporal distribution of visually-evoked excitatory and inhibitory synaptic conductances. In contrast with a popular theory of functional connectivity in primary visual cortex, but consistent with anatomical studies of inhibitory neurons, we present evidence supporting unbiased connectivity arising from inhibitory simple cells in L4 of V1. In addition to our experimental findings, we provide two different network models of V1 L4 that combine anatomical and experimental data. Together, these models account for apparent discrepancies in experimental findings and offer mechanistic explanations for the characteristic delay between the onset of excitation and inhibition that has been observed across sensory cortex. The work here provides the most complete description to date of the spatial and temporal distribution of inhibition as it relates to simple cells in layer 4 of primary visual cortex