Abstract
The primary auditory cortex (AI) is the representative recipient of information from the ears in the mammalian cortex. However, the delineation of the AI is still controversial in a mouse. Recently, it was reported, using optical imaging, that two distinct areas of the AI, located ventrally and dorsally, are activated by high-frequency tones, whereas only one area is activated by low-frequency tones. Here, we show that the dorsal high-frequency area is an independent region that is separated from the rest of the AI. We could visualize the two distinct high-frequency areas using flavoprotein fluorescence imaging, as reported previously. SMI-32 immunolabeling revealed that the dorsal region had a different cytoarchitectural pattern from the rest of the AI. Specifically, the ratio of SMI-32-positive pyramidal neurons to nonpyramidal neurons was larger in the dorsal high-frequency area than the rest of the AI. We named this new region the dorsomedial field (DM). Retrograde tracing showed that neurons projecting to the DM were localized in the rostral part of the ventral division of the medial geniculate body with a distinct frequency organization, where few neurons projected to the AI. Furthermore, the responses of the DM to ultrasonic courtship songs presented by males were significantly greater in females than in males; in contrast, there was no sex difference in response to artificial pure tones. Our findings offer a basic outline on the processing of ultrasonic vocal information on the basis of the precisely subdivided, multiple frequency-organized auditory cortex map in mice.
Highlights
THE PRIMARY SENSORY CORTEX of the mammalian brain is widely known to receive the first thalamic inputs that convey peripheral sensory information, such as hearing and vision
The regions overlapped to the places where Stiebler et al (1997) drew the ultrasonic field (UF) and dorsoposterior field (DP) in their auditory cortical map, hereby referred to as Stiebler’s UF and DP in this report, were confirmed by several studies (Honma et al 2013; Joachimsthaler et al 2014; Stiebler et al 1997), and both areas are known to be activated by directional changes of slow frequency modulation (FM) sounds (Honma et al 2013; Tsukano et al 2013b)
The dorsal area of the auditory cortex (AI) mapped between Stiebler’s UF and DP (Fig. 2D). To evaluate these data quantitatively, we positioned regions of interest (ROIs) across Stiebler’s UF and DP, and the same ROIs were placed onto the image of the responses to a 35-kHz tone obtained from the same mouse (Fig. 2F)
Summary
THE PRIMARY SENSORY CORTEX of the mammalian brain is widely known to receive the first thalamic inputs that convey peripheral sensory information, such as hearing and vision. The larger dorsal branch of the fork-shaped frequency gradients travels toward the high-frequency area of the AI, referred to as the UF, as reported by Guo et al (2012), whereas the smaller ventral division has an axis of the frequency organization toward the AII (Fig. 1C) (Issa et al 2014). This new map was obtained using optical imaging. Existence of multiple frequency organizations in the mouse auditory cortex may unify the auditory cortical maps of mice and other mammals
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