Topographic organization of the auditory thalamocortical system in the albino rat

The organization of the auditory thalamocortical connections was studied in rats. Retrograde transport of horseradish peroxidase conjugated to wheat germ agglutinin following injections into parietal, occipital and temporal cortex was used. The medial geniculate body, the suprageniculate, the lateral part of the nucleus posterior thalami, the posterior part of the nucleus lateralis thalami, and the nucleus ventroposterior project to the investigated part of the neocortex. Corresponding to different patterns of labeling, five areas of auditory neocortex were distinguished: 1. The rostral area is innervated by neurons of the nucleus ventroposterior, the lateral part of the nucleus posterior thalami, and the medial division of the medial geniculate body. 2. The dorsal area is innervated by neurons of the suprageniculate, the posterior part of the nucleus lateralis thalami and the rostral region of the dorsal division of the medial geniculate body. 3. The caudal area is innervated by neurons of the posterior part of the nucleus lateralis thalami, the suprageniculate, the medial division, the caudal region of the dorsal division and the ventrolateral nucleus of the medial geniculate body. 4. The ventral area is innervated by neurons of the suprageniculate, the medial division, the caudal region of the dorsal division, and the ventrolateral nucleus of the medial geniculate body. 5. The core area of the temporal cortex is exclusively connected to the caudal region of the medial division and the ventral division of the medial geniculate body. The findings of the present study indicate topographic organizations of the ventral division of the medial geniculate body and of the corea area. Four segments (a-d) of the ventral division each show a different set of topographic axes. They correspond to sets of topographic axes in the core area of the auditory cortex. These topographies characterize the segments which are each exclusively connected to one of the four fields of the core area.