Auditory cortico-cortical connections in the owl monkey.

Two tonotopically organized cortical fields, the primary (A1) and rostral (R) fields, comprise a core of auditory cortex in the owl monkey. Injections of tritiated proline were made into each of these fields to determine their projections to the auditory fields in the ipsilateral and contralateral hemispheres using autoradiographic methods. Neurons in R project to the rostromedial (RM) and primary fields in both hemispheres, and to the posterolateral (PL) and anterolateral (AL) fields in the ipsilateral hemisphere. In addition, the rostral fields in the two hemispheres are connected. Neurons in the primary field project to RM and R in both hemispheres and to AL, Pl, and the caudomedial (CM) field in the ipsilateral hemisphere. The primary fields in the two hemispheres are connected. Single injections into A1 and R often result in labeling of two or more columns of tissue in the ipsilateral and contralateral target fields. Cortico-cortical axon terminations are concentrated in layer IV of fields AL and RM and in upper layer III and layer IV of R and CM. In A1, axon terminals of neurons whose cell bodies lie in A1 in the opposite hemisphere are concentrated in upper layer III and layer IV; axon terminals of neurons located in field R of the same hemispheres are concentrated in layers I and II. Layer IV of Pl contains the greatest concentration of cortico-cortical axon terminals; the supragranular layers contain a somewhat lower concentration. Neurons in R project contralaterally in the anterior commissure while A1 neurons send their axons contralaterally in the corpus callosum.