The auditory cortical projections onto the medial geniculate body in the cat. An experimental anatomical study with silver and autoradiographic methods.

The auditory cortical projections to the medial geniculate body (MGB) were studied in the cat. Lesions carefully restricted to each one of the subdivisions of the auditory and peri-auditory cortex were made and degenerating fibres were mapped in the MGB. In other experiments L-[4-5-3-H]leucine was injected into the cortex of AI, AII and SF and its transport to the MGB was studied by autoradiography. The results show that fibres arise in the deep layers of the cortex of AI, pass through the deep subdivision of the dorsal nucleus of the MGB as well as through the magnocellular MGB. and end in the pars lateralis of the ventral nucleus of the MGB, Arising from AII, axons pass through the magnocellular MGB and end in the superficial and deep subdivisions of the dorsal nucleus extending to the most caudal part of the MGB. The ectosylvian posterior auditory cortex projects diffusely to all subdivisions of the MGB as well as to its magnocellular part. Of the peri-auditory areas, only the suprasylvian fringe projects to the parvicellular MGB, and it sends axons to the dorsal nucleus of the MGB. The SF and the ectosylvian anterior periauditory area send fibres to the magnocellular MGB. The insular cortex does not project to the MGB but sends heavy projections to lower nuclei of the auditory pathway. The complementary use of degeneration and autoradiographic methods shows that each method may be used to eliminate the drawbacks of the other. This strategy appears as being most adequate for the study of reciprocal projections such as those found between the MGB and the three auditory areas AI, AII, and the ectosylvian posterior auditory area.