Summer Research Description: Cortical circuits are comprised of structurally and functionally diverse interneurons. The classification of these interneuron populations can be achieved by examining immunocytochemical expression. Previously, two classes of calcium binding proteins, parvalbumin (PV) and calretinin (CR), have been found to comprise non-overlapping interneuron populations in the cortex of various mammals. In the primary visual cortex of the macaque monkey, PV makes up the largest portion of calcium binding proteins, however, in frontal areas, CR makes up the largest population. This significant change in proportion suggests that the number of each cell type is important to quantify for different cortical areas. In this study, we stereologically counted cells expressing PV or CR in two visual areas, V4 and MT, of two macaque monkeys. These two areas were chosen because they provide a sample of cell numbers for mid-level areas in the dorsal (MT) and ventral (V4) visual pathways. Overall, quantifying these populations across different cortical areas may provide insight to their underlying function.