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Figure 4. The tonotopic organization of the auditory cortex of four mammals. A: mouse; B: rat; C: ferret; D: cat. The brains are shown in the sagittal plane and show the primary auditory field (A1), secondary auditory field (A2), anterior auditory field (AAF), dorsoposterior field (DPF), ultrasonic field (UF), posterior auditory field (PAF), ventral auditory field (VAF), suprarhinal auditory field (SRAF), posterior pseudosylvian field (PPF), posterior suprasylvian field (PSF), and ventroposterior auditory field (VPAF).
that the upper hearing limit of cats exceeds 60 kHz and that there could be neurons present in the cortex special- ized for these ultrasonic frequencies.
Cortical Representation of Ultrasonic Frequencies
Mice, rats, ferrets, and cats are commonly used as animal models for acoustic research. The biological importance of ultrasound to these mammals is further reflected by the allotment of cortical space for ultrasonic sound perception in their respective auditory cortices.
As such, it is crucial to validate as well as expand our current understanding of their hearing abilities, espe- cially the neural correlates underlying the perception of ultrasonic frequencies.
Mice
In the mouse brain (Figure 4A), five auditory cortical fields can be delineated in both hemispheres: primary auditory field (A1), anterior auditory field (AAF), sec- ondary auditory field (A2), dorsoposterior field (DPF), and ultrasonic field (UF) (Stiebler et al., 1997). The A1 and AAF regions are both tonotopically organized but with reverse gradients. The properties of the neurons within these two fields are similar. For example, the fre- quency ranges for neurons found in both the A1 and AAF are between 2 and 45 kHz.
The mouse was the first animal where a specialized cortical region for processing ultrasonic frequencies was identified (Hofstetter and Ehret, 1992). Frequencies between 40 and 70 kHz are represented in the UF, with approximately 50% of neurons responding to frequencies between 50 and 60 kHz. However, unlike the A1 and AAF, the UF is not tono- topically organized (Stiebler et al., 1997), and it is still not clear whether the UF should be considered a part of the primary auditory fields alongside the A1 and AAF.
Tsukano and colleagues (2015) showed that the dorsome- dial field (DM), previously thought to be part of dorsal A1, is a separate area specialized for ultrasonic perception. This region contains neurons highly responsive to vocal- izations, with frequencies above 40 kHz, demonstrating how certain neurons in mouse cortex respond best to frequencies of behaviorally relevant sound features. This type of cortical organization can also be seen in other
rodents that rely on ultrasound for survival.
Rats
The central auditory system of rats is comparable to that of mice in both anatomical and functional organization. Five distinct cortical fields have been identified in the rat brain, and high-frequency neurons can be found in the following regions: A1, AAF, posterior auditory field (PAF), ventral auditory field (VAF), and suprarhinal auditory field (SRAF).
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