Concurrent tonotopic processing streams in auditory cortex

The basis for multiple representations of equivalent frequency ranges in auditory cortex was studied with physiological and anatomical methods. Our goal was to trace the convergence of thalamic, commissural, and corticocortical information upon two tonotopic fields in the cat, the primary auditory cortex (AI) and the anterior auditory field (AAF). Both fields are among the first cortical levels of processing. After neurophysiological mapping of characteristic frequency, we injected different retrograde tracers at separate, frequency-matched loci in AI and AAF. We found differences in their projections that support the notion of largely segregated parallel processing streams in the auditory thalamus and cerebral cortex. In each field, ipsilateral cortical input amounts to similar to70% of the number of cells projecting to an isofrequency domain, while commissural and thalamic sources are each similar to15%. Labeled thalamic and cortical neurons were concentrated in tonotopically predicted regions and in smaller loci far from their spectrally predicted positions. The few double-labeled thalamic neurons (<2%) are consistent with the hypothesis that information to AI and AAF travels along independent processing streams despite widespread regional overlap of thalamic input sources. Double labeling is also sparse in both the corticocortical and commissural systems (similar to1%), confirming their independence. The segregation of frequency-specific channels within thalamic and cortical systems is consistent with a model of parallel processing in auditory cortex. The global convergence of cells outside the targeted frequency domain in AI and AAF could contribute to context-dependent processing and to intracortical plasticity and reorganization.