Cortical convergence from different frequency domains in the cat primary auditory cortex.

Primary auditory cortex (AI) has a tonotopic map consisting of orderly isofrequency (IF) bands, and cortical connections are commonly supposed to link domains preferring similar characteristic frequencies (CFs) within AI and in auditory association cortex. The interaction of different frequency channels, however, has not fully been understood in terms of anatomical substrates. Here, by injecting two anterograde tracers in different frequency domains of cat AI, without overlap of the injection cores, we attempted to relate the anatomical mapping of cortical outputs to physiologically defined fields in the auditory cortex. Consistent with previous studies, patches of labeled axon terminals were oriented largely along the IF axis. In regions distant from the injection sites, however, terminal patches were divergent in distribution. This divergence resulted in a complex geometry of partial overlap of projections originating from the two injection sites. The relative extent of the overlap tended to vary depending on the distance between the two injection sites. Physiological mapping for tonotopy across auditory fields revealed that projectional overlap was characteristic of dorsal AI and the dorsoposterior field and, to a lesser extent, in the secondary auditory field. Considering the differences in frequency representation in different AI IF bands, the anatomical convergence of projections tuned to different CFs could contribute to the spectral integration of sound components. Furthermore, the different extent of convergence in the functionally distinct fields might reflect field-specific processing of acoustic signals.