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In terms of tonotopic organization, the A1 shows a progres- sion of characteristic frequencies from low (~1 kHz) to high (~60 kHz) along a posterior-to-anterior gradient (Polley et al., 2007). The tonotopic gradient then reverses in a mirror- like fashion at the posterior and anterior borders of the A1 to form the boundaries of the PAF and AAF, respectively (Figure 4B) (Rutkowski et al., 2003; Polley et al., 2007).
Unlike mice, an ultrasonic field has not been identified in rats, although, because the tonotopic organization in the rat is comparable to that of the mouse, Kalatsky and colleagues (2005) hypothesized that a distinct region representing ultrasonic frequencies might likely also be present in rats. Overall, despite the similarities between the suggested cor- tical maps, further investigation is needed to improve our understanding of ultrasonic representations in the auditory cortex of rodents. This could potentially lead to discoveries that could, in turn, be extended to other mammals.
Like the auditory cortex of other mammals, the ferret auditory cortex is divided into multiple subregions. These include the two primary areas, the A1 and AAF
(Bajo et al., 2006) and the secondary areas: anterior dorsal field, posterior pseudosylvian field (PPF), and posterior suprasylvian field (PSF). The PPF and the PSF are found immediately ventral to the A1 (Figure 4C). Bizley and colleagues (2005) described the functional organization of the different regions within the ferret auditory cortex and subsequently mapped the tonotopic organization of these areas.
As discussed for mice and rats and also for most other mammals, the frequencies in these fields are organized from high to low in a rostrocaudal manner, with fre- quency reversals taking place at the borders between adjacent fields (Bizley et al., 2005). However, this reversal pattern is not present in ferrets. Instead, the frequencies are organized where the gradients of the A1 and AAF meet dorsally and decrease ventrally (Figure 4C) (Kaas, 2011). Therefore, the A1 and AAF are orga- nized tonotopically, with higher frequencies represented toward the dorsal tip. The physiological properties of the ferret A1 (such as tonotopic organization and neu- ronal properties) are similar to those seen in the cat A1 (Kaas, 2011), but when comparing audiograms of ferrets and cats, the ferret’s audiogram is shifted toward lower frequencies (Figure 2).
Similar to ferrets, the cat auditory cortex can be divided into one or more primary areas and several secondary areas (Bizley et al., 2005). To help describe the func- tional and tonotopic organization of the cat auditory cortex, Reale and Imig (1980) analyzed how clusters of neurons (and sometimes single neurons) respond to vari- ous frequencies. In addition to describing the tonotopic organization of the core auditory region, the A1 and AAF, Reale and Imig (1980) also described the presence and tonotopic organization of the PAF and the ventroposte- rior auditory field (VPAF). Furthermore, they delineated the belt auditory region into the A2, temporal area (T), dorsoposterior area (DP), and ventral area (V).
More recently, Hall and Lomber (2015) confirmed the four functionally distinct tonotopic areas within the cat auditory cortices (A1, AAF, PAF, and VPAF) and reported a reversal in tonotopic gradients between neigh- boring regions (Figure 4D). In the cat, the A1 increases in its tonotopic gradient as it extends from the anterior division of the posterior ectosylvian sulcus (PES) to the posterior portion of the anterior ectosylvian sulcus (AES). At the posterior edge of the PES, the A1 reaches the minimal values of its tonotopic gradient, forming a low-frequency reversal border as it nears the PAF (Hall and Lomber, 2015). High-frequency reversal borders also exist at the A1-AAF and PAF-VPAF borders and are likely a location where ultrasonic selective neurons may be found.
Ultrasonic-Selective Neurons in the Cat Auditory Cortex
Following the principles of tonotopic organization, it
seems that neurons with the highest characteristic fre- quencies could potentially be located at the periphery of each auditory region. Neurons can be classified as being either broadly or narrowly tuned, responding maximally to a large range or a narrow range of frequencies, respec- tively. This classification provides insight into the type of sensory input the neurons convey and their roles within a specific cortical field. High-frequency selective neu- rons have been found to be narrowly tuned (Phillips and Irvine, 1982), supporting the claim that high-frequency reversal borders (e.g., between the A1 and AAF) contain mostly such neurons. For example, Carrasco and Lomber (2010) identified neurons selective for frequencies reach- ing 60 kHz around the border between the A1 and AAF.
22 Acoustics Today • Spring 2021

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