Sound Frequency Representation in the Auditory Cortex of the Common Marmoset Visualized Using Optical Intrinsic Signal Imaging

Abstract

Natural sound is composed of various frequencies. Although the core region of the primate auditory cortex has functionally defined sound frequency preference maps, how the map is organized in the auditory areas of the belt and parabelt regions is not well known. In this study, we investigated the functional organizations of the core, belt, and parabelt regions encompassed by the lateral sulcus and the superior temporal sulcus in the common marmoset (Callithrix jacchus). Using optical imaging of intrinsic signals, we obtained evoked responses to band-pass noise stimuli in a range of sound frequencies (0.5-16 kHz) in anesthetized adult animals and visualized the preferred sound frequency map on the cortical surface. We characterized the functionally defined organization using histologically defined brain areas in the same animals. We found tonotopic representation of a set of sound frequencies (low to high) within the primary (A1), rostral, and rostrotemporal areas of the core region. In the belt region, the tonotopic representation existed only in the mediolateral (ML) area. This representation was symmetric with that found in A1 along the border between areas A1 and ML. The functional structure was not very clear in the anterolateral area. Low frequencies were mainly preferred in the rostrotemplatal area, while high frequencies were preferred in the caudolateral area. There was a portion of the parabelt region that strongly responded to higher sound frequencies (>5.8 kHz) along the border between the rostral and caudal parabelt regions.

Significance Statement In this study, we examined functional organizations for sound frequency representation in the core, belt, and parabelt regions in the marmoset using optical intrinsic signal imaging. In addition to the auditory areas in the core region, the ML in the belt region continuously represented a range of frequencies from low to high, with tonotopic organization, which was symmetric to that in the A1 in the core region about the border between the A1 and ML. The posterior and anterior belt regions represented mainly low and high frequencies, respectively. The parabelt region contained a distinct region with preference for high frequencies. These results suggest that the functional organization unique to each auditory region would process sound information specific to this species.