Glutamatergic pathways in thalamus and cortex are divided into two distinct classes: Driver, which carries the main information between cells, and Modulator, which modifies how driver inputs function. Identifying driver inputs helps to reveal functional computational circuits, and one set of such circuits identified by this approach are cortico-thalamo-cortical (or transthalamic corticocortical) circuits. This, in turn, leads to the conclusion that there are two types of thalamic relay: first order nuclei (such as the LGN) that relay driver input from a subcortical source (i.e., retina), and higher order nuclei (such as the pulvinar) which are involved in these transthalamic pathways by relaying driver input from layer 5 of one cortical area to another. This thalamic division is also seen in other sensory pathways: for the somatosensory system, first order is and higher order is POm; and for the auditory system, first order is MGBv and higher order is MGBd. Furthermore, this first and higher order classification extends to most of thalamus. For sensory systems, LGN (vision), VPM/L (somatosensation), and MGNv (audition) are first order, and their respective higher order mates, respectively, are pulvinar, POm, and MGNd, and this division extends beyond sensory pathways so that most of thalamus by volume consists of higher order relays. Many, and perhaps all, direct driver connections between cortical areas are paralleled by an indirect cortico-thalamo-cortical (transthalamic) driver route involving higher order thalamic relays. Such thalamic relays 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 this transthalamic circuit as well as speculations as to why these two parallel routes exist will be offered.