Thalamocortical interactions

Glutamatergic pathways dominate information processing in the brain, but these are not homogeneous. They include two distinct types: Class 1, which carries the main information for processing, and Class 2, which serves a modulatory role. Identifying the Class 1 inputs in a circuit can lead to a better understanding of its function. Also, identifying Class 1 inputs to a thalamic nucleus tells us its main function (e.g. the lateral geniculate nucleus, or LGN, is the relay of retinal Class 1 input), and such identification leads to a division of thalamic relays into first and higher order: the former receives Class 1 inputs from subcortical sources; the latter, from layer 5 of cortex, which it then relays to another cortical area. When a cortical area directly connects with another, it often has a parallel, transthalamic connection through these higher order relays. This leads to a novel appreciation of cortical functioning and raises many new questions.

► Glutamatergic inputs in thalamic and cortical circuitry are either Class 1 or Class 2.
► We suggest that Class 1 is information bearing and Class 2 is modulatory.
► Class 1 inputs represent what a thalamic nucleus relays to cortex.
► A transthalamic information route exists between cortical areas.
► Many or all cortical areas directly connected have a parallel transthalamic route.