Comparison of intrinsic connectivity in different areas of macaque monkey cerebral cortex.

We have used small injections of biocytin to label and compare patterns of intraareal, laterally spreading projections of pyramidal neurons in a number of areas of macaque monkey cerebral cortex. In visual areas (V1, V2, and V4), somatosensory areas (3b, 1, and 2), and motor area 4, a punctate discontinuous pattern of connections is made from 200-microns-diameter biocytin injections in the superficial layers. In prefrontal cortex (areas 9 and 46), stripe-like connectivity patterns are observed. In all areas of cortex examined, the width of the terminal-free gaps is closely scaled to the average diameter of terminal patches, or width of terminal stripes. In addition, both patch and gap dimensions match the average lateral spread of the dendritic field of single pyramidal neurons in the superficial layers of the same cortical region. These architectural features of the connectional mosaics are constant despite a twofold difference in scale across cortical areas and different species. They therefore appear to be fundamental features of cortical organization. A model is offered in which local circuit inhibitory "basket" interneurons, activated at the same time as excitatory pyramidal neurons, could veto pyramidal neuron connections within either circular or stripe-like domains; this could lead to the formation of the pattern of lateral connections observed in this study, and provides a framework for further theoretical studies of cerebral cortex function.