| The mammalian auditory cortex is responsible for the higher order processing of acoustic information, mediated through connectional interactions among numerous thalamic nuclei and cortical areas. Most studies of the auditory forebrain have focused on primary areas that contain tonotopic frequency representations. However, the limits of acoustically responsive cortex extend to areas far beyond these regions---including the frontal lobe---though their role remains obscure and open to further investigation. In an effort to better understand and constrain the possible interrelations among the auditory cortical areas in the cat, the thalamocortical, commissural, and corticocortical systems were investigated using retrograde tract tracing.; The studies in this dissertation demonstrate new physiological and anatomical features of the auditory forebrain. The first group of studies focuses on two tonotopic areas, the primary auditory cortex (AI) and the anterior auditory field (AAF). The first study demonstrates distinct physiological and anatomical differences between these two fields, suggesting dual streams for the processing of tonotopically-organized information. In the subsequent study, the separation of frequency channels in both areas is shown to be imprecise, with the discovery of heterotopic projections from frequency-mismatched loci in the thalamus and cortex. These heterotopic sources are postulated to support plastic reorganization in the auditory cortex, and a dynamic theory for such plasticity is explored in the chapter that follows.; The final group of studies explores the entire domain of the thirteen areas recognized as auditory, focusing individually on the thalamocortical, commissural, and corticocortical systems. Among these, the commissural system is shown to be the most stereotyped in its connections, with the major projections originating from homotopic loci in the contralateral hemisphere, but which also exhibit variability of laminar origins from dorsal and ventral fields. Each area also receives a numerically equivalent input from a unique profile of thalamic nuclei grouping functionally related areas. The corticocortical projections provide the major source of extrinsic input to each area, with unique and complex patterns of areal and laminar origin. The final study demonstrates a global topographic metric for all auditory thalamic and cortical projections, and provides an isotropic principle guiding the organization of the auditory forebrain. |
