Ipsilateral corticocortical projections related to binaural columns in cat primary auditory cortex

Projections from the high-frequency representations of auditory cortical fields A and P were autoradiographically labeled using anterograde transport of a mixture of tritiated proline and leucine. A single injection of isotope into either cortical field resulted in multiple regions of dense labeling within ipsilateral AI. As the result of an injection of radioactive label into field A in one experiment, densely labeled regions were found to be systematically related to the binaural map throughout the high-frequency representation of AI. Contralateral dominant suppression responses were located in regions of dense labeling while summation responses were located in regions of less dense labeling. In contrast, it was previously found that callosal axon terminations were more densely concentrated in summation columns than in contralateral dominant suppression columns (Imig and Brugge, '78). Thus, the two classes of binaural columns differ with respect to the density of innervation they receive from these ipsilateral and contralateral populations of neurons. In other experiments in which isotope was injected into field A, a systematic relationship between density of ipsilateral labeling and binaural response class was only seen in a portion of AI; in other regions no relationship was evident. A simple interpretation of these data follows. Within a contiguous territory in field A is a population of neurons whose axons provide more dense innervation to contralateral dominant suppression columns than to summation columns in ipsilateral AI. Injections of radioactivity confined to this territory result in a systematic relationship between the density of labeling and the binaural map throughout AI. Outside this territory is a population of neurons whose axon terminations are not systematically related to the binaural map in AI. Isotope injections which engage both territories may result in a systematic relation between density of labeling and the binaural map in one portion of AI, while in another, no relationship may be evident. There is some indication that projections from field P may also be related to binaural columns in AI in the same manner as are the projections from field A.