Subdivisions and Connections of Auditory Cortex in Marmoset Monkeys

Abstract

The current working model of auditory cortex is comprised of three levels of processing: a core region of three primary areas, surrounded by a belt region of secondary areas, and a parabelt region lateral to the belt. The anatomical and physiological properties of the belt fields bordering the core medially are the least understood, however recent studies have begun to examine response properties in some of these areas. The focus of the present thesis is to test and refine the current model by comparing the corticocortical and thalamocortical connections of caudal and rostral divisions of the medial belt (CM, RM) and the core (A1, R). In addition the connections of the lateral belt and the parabelt regions were examined. Multiple injections of retrograde and bidirectional tracers were made into rostral and caudal divisions of the medial belt, core, lateral belt and parabelt regions of auditory cortex in marmoset monkeys (Callithrix jacchus jacchus). Architectonic analyses of the marmoset auditory cortex was performed in order to provide a basis for identifying individual cortical areas and facilitate reconstruction of the connectional data. Rostral and caudal medial belt injections revealed distinct connections of the core, belt and parabelt respectively. The caudal division of the medial belt was further distinguished by thalamic inputs from multisensory nuclei in the thalamus, consistent with observations of auditory and somatosensory convergence in this area (Schroeder, C. E. et al. 2001, Fu, K. G. et al. 2003). Connections of the rostral (R) and caudal (A1) core included core and belt areas, with strong interconnections between areas R and A1 and distinct connections with the belt. The lateral belt and parabelt both exhibited rostrocaudal topography and the parabelt had strong connections with the belt, but not the core, consistent with serial processing in auditory cortex. These results revealed minor revisions to the model however, overall, the connection patterns of the marmoset are consistent with the current working model of primate auditory cortex.