VIEWPOINT - THE CORE AND MATRIX OF THALAMIC ORGANIZATION

The integration of the whole cerebral cortex and thalamus during foreb rain activities that underlie different states of consciousness, requi res pathways for the dispersion of thalamic activity across many corti cal areas. Past theories have relied on the intralaminar nuclei as the sources of diffuse thalamocortical projections that could facilitate spread of activity across the cortex. A case is made for the presence of a matrix of superficially-projecting cells, not confined to the int ralaminar nuclei but extending throughout the whole thalamus. These ce lls are distinguished by immunoreactivity for the calcium-binding prot ein, D28K calbindin, are found in all thalamic nuclei of primates and have increased numbers in some nuclei. They project to superficial lay ers of the cerebral cortex over relatively wide areas, unconstrained b y architectonic boundaries. They generally receive subcortical inputs that lack the topographic order and physiological precision of the pri ncipal sensory pathways. Superimposed upon the matrix in certain nucle i only, is a core of cells distinguished by immunoreactivity for anoth er calcium-binding protein, parvalbumin, These project in highly order ed fashion to middle layers of the cortex in an area-specific manner. They are innervated by subcortical inputs that are topographically pre cise and have readily identifiable physiological properties. The parva lbumin cells form the basis for sensory and other inputs that are to b e used as a basis for perception. The calbindin cells, especially when recruited by corticothalamic connections, can form a basis for the en gagement of multiple cortical areas and thalamic nuclei that is essent ial for the binding of multiple aspects of sensory experience into a s ingle framework of consciousness. (C) 1998 IBRO. Published by Elsevier Science Ltd.