The projection of the auditory cortex upon the diencephalon and brain stem in the cat.

Summary The site of termination in the diencephalon and brain stem of fibers arising in the auditory cortex of the cat has been studied with the method of Nauta and Gygax. Such corticofugal fibers end in the corpus striatum, the reticular nucleus of the ventral thalamus, the medial geniculate and the posterior group of the dorsal thalamus, in the interstitial nucleus of the brachium of the inferior colliculus, the parabrachial region and adjoining midbrain tegmentum, the inferior colliculus (of both sides) and the pontine nuclei. All fields of the auditory cortex contribute to this projection, but there are different patterns of connections to the subdivisions of the medial geniculate body and to the posterior group of the thalamus. A I, A II and EP send fibers to the ventral and deep dorsal nuclei of the medial geniculate body, but in varying proportions to each of these nuclei, A I sending the greatest number and EP the fewest number of fibers to the ventral nucleus. The insulo-temporal region is related to the dorsal and deep dorsal nuclei. This cortico-geniculate projection is well organized and probably reciprocates the geniculo-cortical projection. There appears to be a correlation between the field of origin in the auditory cortex and the site of termination of corticothalamic fibers in the medial geniculate body and in the posterior group respectively: those fields (A I and A II) which send a considerable number of fibers to the ventral nucleus of the medial geniculate body are related predominantly to the lateral division of the posterior group, whereas the insulo-temporal region and EP, which send most of their fibers to the dorsal division of the medial geniculate nucleus, project to more medial parts of the posterior group. The 3 parts of the inferior colliculus—the dorsomedial cortex, the lateral nucleus and the central nucleus—all receive fibers from the auditory cortex. In the ventral nucleus of the medial geniculate body and in the central nucleus of the inferior colliculus the degenerating corticofugal fibers have a parallel arrangement between the cell columns present in these nuclei, are of smaller diameter than the afferent fibers to these nuclei from lower levels of the auditory pathway, and enter the nuclei at the end of the column opposite to that of the ascending afferent fiber; these observations have suggested certain principles of the columnar organization in subcortical sensory relay nuclei.