Ohga S1, Tsukano H1, Horie M2, Terashima H3, Nishio N1, Kubota Y4, Takahashi K4, Hishida R1, Takebayashi H5, Shibuki K1.
1Department of Neurophysiology, Brain Research Institute, Niigata University
2Department of Morphological Sciences, Graduate School of Medical and Dental Sciences, Kagoshima University
3NTT Communication Science Laboratories, NTT Corporation,
4Department of Otolaryngology, Graduate School of Medicine and Dental Sciences, Niigata University
5Division of Neurobiology and Anatomy, Graduate School of Medicine and Dental Sciences, Niigata University
Tonotopy is an essential functional organization in the mammalian auditory cortex, and its source in the primary auditory cortex (A1) is the incoming frequency-related topographical projections from the ventral division of the medial geniculate body (MGv). However, circuits that relay this functional organization to higher-order regions such as the secondary auditory field (A2) have yet to be identified. Here, we discovered a new pathway that projects directly from MGv to A2 in mice. Tonotopy was established in A2 even when primary fields including A1 were removed, which indicates that tonotopy in A2 can be established solely by thalamic input. Moreover, the structural nature of differing thalamocortical connections was consistent with the functional organization of the target regions in the auditory cortex. Retrograde tracing revealed that the region of MGv input to a local area in A2 was broader than the region of MGv input to A1. Consistent with this anatomy, two-photon calcium imaging revealed that neuronal responses in the thalamocortical recipient layer of A2 showed wider bandwidth and greater heterogeneity of the best frequency distribution than those of A1. The current study demonstrates a new thalamocortical pathway that relays frequency information to A2 on the basis of the MGv compartmentalization.