Uniformity and specificity of long-range corticocortical connections in the visual cortex of the cat

Progress in the understanding of brain function depends on comprehensive knowledge of neural structure and organization, based on reliable quantitative data. We quantified and analysed data about long-range connections between different visual cortical areas of the cat. The analyses were based on a numeric evaluation of the retrograde distribution of anatomical label in 16 visual cortical areas after injections of the middle suprasylvian cortical region. Although connection profiles and connection densities varied substantially between injections in different individual animals, characteristic features of corticocortical connectivity emerged. Our results suggest that long-range connections in the cortex obey global constraints, such as a linear relationship between connection density and area size of connection origin, or a sequential organization of laminar connectivity between different visual areas. Within the limits set by these global constraints, individual areas also possess specific connectivity characteristics, which hint on specialized functional interactions between cortical regions.