The modular organization of brain systems. Basal forebrain: the last frontier.

Computational anatomical studies suggest that specific clusters of projection neurons in the basal forebrain together with specific prefrontal and posterior cortical associational regions constitute distributed parts of functional parallel circuits. The predictable sequence of cell clusters consisting of various types of noncholinergic cell populations in the basal forebrain suggests further subdivisions within these circuits. It is possible that similar to the parallel basal ganglia circuits (Alexander and Crutcher, 1990), large number of specialized channels and sub-channels exist within this triangular circuitry that permit parallel, multilevel processing concurrently. The location and size of the active modules may temporarily vary according to the prevalence of state-related diffuse ascending brain stem and specific telencephalic inputs. From this latter group of afferents, the prefrontal input may function as an external threshold control which allocates attentional resources via the basal forebrain to distributed cortical processes in a selective, self-regulatory fashion.