Title

Thalamocortical Pathway Specialization for Primary and Ventral Auditory Cortices

Date of Completion

January 2011

Keywords

Psychology, Behavioral|Biology, Neurobiology

Degree

Ph.D.

Abstract

All mammalian species studied to date have multiple auditory cortical fields that are organized topographically like the cochlear epithelium of the ear, or cochleotopically. Physiological and behavioral studies have established that these neighboring fields are specialized to filter and represent sound. Hierarchical models suggest that cochleotopic cortices inherit thalamic input from the ventral division (MGBv) of the medial geniculate body (MGB), and functional differences are the result of different combinations of non-MGBv nuclei input. This dissertation explores the functional specialization of neighboring cochleotopic auditory cortices, while exploring ways in which these differences could be inherited via distinct thalamocortical pathways. ^ We propose that functional differences such as spectral bandwidth and response latency vary systematically across primary and ventral auditory cortices, and that these differences are in part due to different thalamic inputs. However, there is a dearth of studies examining thalamocortical connectivity patterns that also perform dense physiological recording, a method traditionally required to identify multiple cochleotopic cortices. We address this problem using Fourier intrinsic imaging to identify multiple cochleotopic cortices. Spike rate responses are examined in three fields to compare functional differences of spectral resolution and response latency, and we show that these receptive field properties vary systematically outside of the cochleotopic axis. Using anatomical tract tracing methods we demonstrate that thalamic afferents to primary and ventral cortices originate in separate rostral and caudal MGB, and that cochleotopic organization shifts along this anatomic axis. We employed in situ hybridization to demonstrate that caudal and rostral MGB have distinct glutamate transport systems, as the number of neurons expressing the gene for type 1 vesicular glutamate transporter (VGLUT1) increases along this anatomic axis. Finally, we demonstrate that thalamic expression patterns of VGLUT1 mRNA distinguish parallel thalamocortical pathways to two different cochleotopic auditory cortices. The results support a new model of functional heterogeneity along the caudal-to-rostral anatomic axis of MGB, and demonstrate a novel anatomic marker by which to distinguish thalamic afferents to different cochleotopic auditory cortices. ^