Auditory Cortical Areas Activated by Slow Frequency-Modulated Sounds in Mice

Abstract

Species-specific vocalizations in mice have frequency-modulated (FM) components slower than the lower limit of FM direction selectivity in the core region of the mouse auditory cortex. To identify cortical areas selective to slow frequency modulation, we investigated tonal responses in the mouse auditory cortex using transcranial flavoprotein fluorescence imaging. For differentiating responses to frequency modulation from those to stimuli at constant frequencies, we focused on transient fluorescence changes after direction reversal of temporally repeated and superimposed FM sweeps. We found that the ultrasonic field (UF) in the belt cortical region selectively responded to the direction reversal. The dorsoposterior field (DP) also responded weakly to the reversal. Regarding the responses in UF, no apparent tonotopic map was found, and the right UF responses were significantly larger in amplitude than the left UF responses. The half-max latency in responses to FM sweeps was shorter in UF compared with that in the primary auditory cortex (A1) or anterior auditory field (AAF). Tracer injection experiments in the functionally identified UF and DP confirmed that these two areas receive afferent inputs from the dorsal part of the medial geniculate nucleus (MG). Calcium imaging of UF neurons stained with fura-2 were performed using a two-photon microscope, and the presence of UF neurons that were selective to both direction and direction reversal of slow frequency modulation was demonstrated. These results strongly suggest a role for UF, and possibly DP, as cortical areas specialized for processing slow frequency modulation in mice.

Multiscale Optical Ca2+ Imaging of Tonal Organization in Mouse Auditory Cortex

“…Accordingly, for purposes of robust fiducial orientation, we favored calculation of the ventrally directed L→H gradient in AAF (Figures 2E and 2F), which tracks local maxima of response strength. Reassuringly, this ventrally directed gradient concurs with results from intrinsic imaging methods (Honma et al, 2013). Finally, for AII, the robust activity we monitor under transcranial Ca 2+ imaging enabled observation of a dorsoventral gradient (Figures 2E and 2F), a feature not previously discerned in electrode studies using anesthesia (Guo et al, 2012).…”

“…Alternatively, a complementary view has come from wide field optical imaging that simultaneously surveys expansive cortical regions. For instance, to gauge neural tissue activity, these approaches monitor local changes in blood flow or altered flavoprotein oxidation (Honma et al, 2013; Takahashi et al, 2006); alternatively, regions of depolarization may be directly detected via voltage-sensitive dyes bulk-loaded into neuropil (Grinvald and Hildesheim, 2004). While these spatially expansive approaches provide holistic global maps, they are often limited by low signal fidelity and spatial resolution.…”

Multiscale mapping of frequency sweep rate in mouse auditory cortex

“…In other species, certain regions prefer faster FM sweeps, such as a ventral region at the border of the primary auditory cortex (AI) and the secondary auditory cortex (AII) in cats (Mendelson et al, 1993), various lateral belt areas in rhesus monkeys (Tian and Rauschecker, 2004), and the anterior auditory field (AAF) in mice (Trujillo et al, 2011). In addition, regions of the ultrasonic field (UF) have been noted to prefer frequency modulations (Stiebler et al, 1997) and are selective for reversals in FM direction (Honma et al, 2013; Tsukano et al, 2016, 2015). While these studies highlight the potential specialization of cortical regions for FM sweep processing, a more comprehensive mapping of FM selectivity, which would facilitate the search for dedicated FM areas, is lacking.…”

Functional imaging of neuronal activity of auditory cortex by using Cal-520 in anesthetized and awake mice

“…So far, various Ca 2+ indicators including rhod-2 [6], fura-2 [7,8], OGB-1 [9][10][11][12][13], fluo-2 [14], fluo-4 [9,15,16], Alexa Fluor 594 together with fluo-5F [17,18], and genetically encoded Ca 2+ indicators (GCaMP3 [1] and GCaMP6s [3]) [14,19,20] have been widely used in studies of the Au1 with two-photon Ca 2+ imaging in both in vitro and in vivo conditions. The in vivo experiments, particularly the functional mapping experiments, have provided important insights into the topographic organization of the Au1, but the degree of precision in tonotopic mapping has been controversial.…”