Reconsidering Tonotopic Maps in the Auditory Cortex and Lemniscal Auditory Thalamus in Mice

Abstract

The auditory thalamus and auditory cortex (AC) are pivotal structures in the central auditory system. However, the thalamocortical mechanisms of processing sounds are largely unknown. Investigation of this process benefits greatly from the use of mice because the mouse is a powerful animal model in which various experimental techniques, especially genetic tools, can be applied. However, the use of mice has been limited in auditory research, and thus even basic anatomical knowledge of the mouse central auditory system has not been sufficiently collected. Recently, optical imaging combined with morphological analyses has enabled the elucidation of detailed anatomical properties of the mouse auditory system. These techniques have uncovered fine AC maps with multiple frequency-organized regions, each of which receives point-to-point thalamocortical projections from different origins inside the lemniscal auditory thalamus, the ventral division of the medial geniculate body (MGv). This precise anatomy now provides a platform for physiological research. In this mini review article, we summarize these recent achievements that will facilitate physiological investigations in the mouse auditory system.

Reciprocal connectivity between secondary auditory cortical field and amygdala in mice

“…This could be addressed via systematic projection mapping between the auditory cortex and the amygdala. In the current study, we visualized comprehensive connectivity between the auditory cortex and the amygdala by injecting cholera toxin subunit b (CTB), a bidirectional tracer, into four tonotopic regions of the auditory cortex, according to a map that was functionally determined using flavoprotein fluorescence imaging (Tsukano et al, 2017). We demonstrated that the A2 is specifically connected with the amygdala, which is consistent with the idea that A2 is a higher-order region in the mouse auditory cortex.…”

“…Overall, our findings endorse the notion that A2 is a higher-order auditory region in the mouse auditory cortex. The auditory subregions of DM, AAF, A1, and A2 have been found to be tonotopic (Tsukano et al, 2017), and all of these fields receive substantial projections from the lemniscal auditory thalamus of the ventral division of the medial geniculate body (MGv) (Tsukano et al, 2017;Ohga et al, 2018). Because the general concept explains that cortical areas that receive afferents from MGv are primary auditory areas, a question arises regarding whether these fields, including A2, are all primary areas in the mouse cortex.…”

“…The mouse auditory cortex includes at least four tonotopic regions (Issa et al, 2014;Tsukano et al, 2017), which we refer to here as DM, AAF, A1, and A2 ( Fig. 1A,B).…”

“…The auditory cortex is a center of auditory processing, and it has been found to have a functional map (Stiebler et al, 1997;Guo et al, 2011). Recent optical imaging studies have indicated that the auditory cortex of the mouse brain is composed of at least four tonotopic subfields (Issa et al, 2014) that were tentatively annotated as the dorsomedial field (DM), anterior auditory field (AAF), primary auditory cortex (A1), and secondary auditory field (A2) in a previous report (Tsukano et al, 2017). Subregions in the mouse auditory cortex may be functionally specialized (Honma et al, 2013;Baba et al, 2016;Shepard et al, 2016), and so connective patterns with other brain regions may vary according to subregion.…”

Laminar specific gene expression reveals differences in postnatal laminar maturation in mouse auditory, visual, and somatosensory cortex

“…These will include the relatively low density laminar area of vGlut2 + fibers above the high intensity Cux1 + band (presumptive layers 2/3) in ACx, which can be distinguished from the dorsal area with a higher density of vGlut2 + fibers, and a relatively thick vGlut2 + band in the lower layers. The dorsal‐to‐ventral length of the Cux1 + patch in ACx was much longer than what would be expected for A1 (Llano & Sherman, ; Tsukano et al, ) and therefore will include other auditory cortical regions. More detailed analysis will be necessary to confirm this.…”