Development of Parallel Auditory Thalamocortical Pathways for Two Different Behaviors
The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.