A pilot project is proposed to study the laminar terminations of the projection from the intralaminar nuclei of the thalamus to the primary visual cortex in the cat. Recent evidence in the rat and tree shrew suggests that the visual cortex receives projections from two adjacent yet distinctly separate populations of neurons within the intralaminar zone of the thalamus. In the rat the definitive intralaminar nuclei project to layers V-VI, whereas a paralaminar system, that includes portions of the ventral medial (VM) and ventral anterior nuclei (VA), projects to layer I of visual cortex. This proposed segregation of intralaminar and paralaminar projections to the cortex will be tested in the visual system of the cat. This investigation will have three major subdivisions. First, it will be determined if the intralaminar thalamus of the cat is segregated into two systems that terminate within either layer I or layers V-VI in the visual cortex. Second, this investigation will determine if the VA nucleus in the cat is part of a paralaminar system that contributes to layer I of visual cortex. Finally, this investigation will determine if the same neurons in VA project to both the visual cortex and to the motor cortex through axonal collateralization. The data for this investigation will be generated using the following procedures: 1) retrograde transport of horseradish peroxidase, 2) anterograde transport of tritiated amino acids, and 3) double labeling techniques using either fluorescent dyes or a combined wheat germ agglutinin-horseradish peroxidase/tritiated wheat germ agglutinin technique. The data obtained in this study will allow for an understanding of the organization of the projections of the intralaminar thalamic zones as well as speculation as to their potential roles in visual processing in the cat.