8 - Integration of Thalamic Inputs to Cat Primary Visual Cortex

Abstract

This chapter discusses the functional organization of the geniculocortical pathway in the cat with respect to the response properties of layer 4 simple cells. It reviews the experiments that have examined the specificity and strength of geniculate inputs to layer 4 neurons. Geniculate cells provide synaptic input to simple cells only when their receptive fields spatially overlap a simple receptive field and match the sign (on or off) of the overlapping sub-region. This thalamic input is sufficient to account for the orientation selectivity (and perhaps direction selectivity) of simple cells. However, the thalamic input alone cannot account for all of the excitatory drive to a simple cell; approximately 50%–70% of the drive to a simple cell comes from intracortical sources. Intracellular investigations performed in vivo provide an increasingly mechanistic understanding of the three major influences on simple-cell function: thalamocortical excitation, intracortical inhibition, and intracortical excitation. Of the three, thalamocortical excitation is certainly the best understood. New evidence for the push–pull model underscores the central role played by inhibition in the organization of on and off sub-regions.