Extrathalamic modulation of cortical function.

THE DEMONSTRATION OF EXTRATHALAMIC CORTICAL AFFERENTS As recently as 25 years ago, it was commonly assumed that essentially all afferents to the neocortex arose from the thalamus, With the development of methods for visualizing noradrenergic, serotonergic, and dopaminergic neuronal processes, it gradually became clear that axons arising from subthalamic cell groups that contain these putative transmitters travel rostrally through the brainstem and monosynaptically innervate the neocortex. With the more recent demonstration that acetylcholinergic (ACh) neurons in the basal forebrain project monosynaptically into the neocortex, it is now clear that there are at least four substantial extrathalamic projections to the neocortex. Recently, these afferents have been demonstrated to be much more dense and more highly organized than was initially thought. The existence and nature of these cortical afferents raises major questions about cortical organization and function. Thalamocortical and corticocortical systems appear to be organized to implement both serial and parallel processing of information through modules that are interconnected with precise topography, thus permitting functional segregation.