This chapter focuses on a subset of cortical neurons—the simple cells of layer 4 in the visual cortex of the cat. Simple cells are the primary recipient of the major visual input from the thalamus—the relay cells of the lateral geniculate nucleus (LGN). Simple cells are highly sensitive to the orientation, direction and speed of motion, retinal disparity (depth), and spatial frequency (size) of a visual stimulus, while geniculate relay cells are largely insensitive to these stimulus features. The synapse between geniculocortical axons and the simple cells of layer 4 is, therefore, the point at which many of the interesting features of cortical neurons are generated for the first time. As a result, simple cells have been the focus of many models of cortical organization and function, some of which can be tested with intracellular methods. Intracellular recording has been employed in two types of experiments: with electrical stimulation of the visual pathways or with visual stimulation.
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