A new view of specific and nonspecific thalamocortical connections

Past theories about the circuitry that promotes integration of the whole cerebral cortex and thalamus during forebrain activities that underlie different states of consciousness have relied on the intralaminar nuclei as the sources of diffuse thalamocortical projections that could facilitate spread of activity across many cortical areas. Evidence is presented to show the presence of a matrix of superficially projecting cells extending throughout the whole thalamus that could form a substrate for diffusion of activity across the cortex. The superficially projecting cells in monkeys are distinguished by immunoreactivity for the calcium-binding protein calbindin. They are found in all thalamic nuclei and are increased in some nuclei. They not only project to superficial layers of the cortex but do so over wide areas, unconstrained by boundaries between areas. They are innervated by subcortical inputs that lack the topographic order and physiological precision of the principal sensory pathways. Superimposed on the matrix, but only in certain nuclei, is a core of cells characterized by immunoreactivity for another calcium-binding protein, parvalbumin. These project to middle layers of the cortex in an area-specific and topographically ordered manner. They are innervated by subcortical inputs that are typically precise in having a high degree of topographic order and readily identifiable physiological properties. The parvalbumin cells provide the sensory and other inputs to the cortex that are to be used as a basis for perception. The diffusely projecting calbindin cells can form a basis for the engagement of multiple cortical areas and thalamic nuclei, especially when recruited by corticothalamic connections. Diffusion of activity across multiple areas and thalamic nuclei is essential for the binding of all aspects of sensory experience into a single framework of consciousness.