The subcortical auditory structures in the Mongolian gerbil: II. Frequency-related topography of the connections with cortical field AI.

We investigated the frequency-related topography of connections of the primary auditory cortical field (AI) in the Mongolian gerbil with subcortical structures of the auditory system by means of the axonal transport of two bidirectional tracers, which were simultaneously injected into regions of AI with different best frequencies (BFs). We found topographic, most likely frequency-matched (tonotopic) connections as well as non-topographic (non-tonotopic) connections. AI projects in a tonotopic way to the ipsilateral ventral (MGv) and dorsal divisions (MGd) of the medial geniculate body (MGB), the reticular thalamic nucleus and dorsal nucleus of the lateral lemniscus, and the ipsi- and contralateral dorsal cortex of the inferior colliculus (IC) and central nucleus of the IC. AI receives tonotopic inputs from MGv and MGd. Projections from different BF regions of AI terminate in a non-tonotopic way in the ipsilateral medial division of the MGB (MGm), the suprageniculate thalamic nucleus (SG) and brachium of the IC (bic), and the ipsi- and contralateral external cortex and pericollicular areas of the IC. The anterograde labeling in the intermediate and ventral nucleus of the lateral lemniscus, parts of the superior olivary complex, and divisions of the cochlear nucleus was generally sparse; thus a clear topographic arrangement of the labeled axons could not be ruled out. AI receives non-tonotopic inputs from the ipsilateral MGm, SG, and bic. In conclusion, the tonotopic and non-tonotopic corticofugal connections of AI can potentially serve for both conservation and integration of frequency-specific information in the respective target structures.