A View of Cortex from the Thalamus

First order thalamic nuclei (e.g., the LGN) relay driver input from a subcortical source (e.g., retina), and higher order nuclei (e.g., the pulvinar) relay driver input from layer 5 of one cortical area to another and are involved in cortico-thalamo-cortical (or transthalamic corticocortical) circuits. Most of thalamus can be divided into first and higher order components. Many, and perhaps all, direct driver connections between cortical areas are paralleled by indirect transthalamic ones. Such transthalamic circuits represent a heretofore unappreciated role in cortical functioning, and this assessment challenges and extends conventional views regarding both the role of thalamus and mechanisms of corticocortical communication. Evidence for these transthalamic circuit as well as speculations as to why these two parallel routes exist will be offered.