Parvalbumin immunoreactivity reveals layer IV of monkey cerebral cortex as a mosaic of microzones of thalamic afferent terminations. 
Correlative light and electron microscopic immunocytochemical methods were used to study the pattern of staining for the calcium-binding protein parvalbumin (PV) in the primary visual area (area 17) and area 3b of the first somatic sensory area of the monkey cerebral cortex.  A conspicuous feature of the light microscopic staining pattern is the presence of focal aggregations of immunoreactive terminal-like puncta within the major thalamic recipient layers (IV and VI).  At the electron microscopic level these aggregations of puncta are found to be immunoreactive terminals most of which form asymmetric synapses, principally on dendritic spines and, to a lesser extent, on dendritic shafts.  Outside the aggregations, most PV-immunoreactive terminals form symmetric synapses.  Correlative observations in the present and other studies indicate that the aggregations of PV-immunoreactive terminals forming asymmetric synapses arise from thalamic afferent fibers while those forming symmetric synapses arise from intrinsic gamma-aminobutyric acid neurons.  The aggregations of PV immunoreactivity in layers IV and VI form microzones of preferred thalamic afferent terminations which may contribute to the formation of functional columns based upon focussed thalamic inputs.