Patterns of cortico-cortical connections related to tonotopic maps in cat auditory cortex.

Topographic distributions of cortico-cortical projections from the primary (AI), anterior (A), posterior (P), ventroposterior (VP), and second (AII) auditory fields were studied in relation to tonotopic maps in combined anatomical and electrophysiological experiments. Distributions of axon terminals were determined by autoradiographic labeling with tritiated proline and leucine. Each of fields A, AI, P, and VP is connected with the other three in the same hemisphere as well as with a number of other auditory cortical areas. Additionally, neurons in each of the fields studied were found to project to the lateral bank of the collateral fissure. In general, regions near the injection site receive more densely labeled projections than do more distant targets. Neurons in each field were found to project to one or more areas in th opposite hemisphere. Only similar portions of the best-frequency representations in fields A, AI, P, and VP are interconnected. A single isotope injection generally produced multiple patches of labeling within each of several cortical fields. Within AI, projections from contralateral fields A and AI and from ipsilateral fields A and P terminate in patches which are often elongated in a direction parallel to the low-to-high best-frequency gradient. A divergence in the projections from one field upon another is apparent in many experiments. Within fields A, AI, P, and VP, patches of label are distributed along a band of cortex oriented parallel to isofrequency lines.