Abstract
It is generally assumed that frequency selectivity varies along the cochlea. For example, at the base of the cochlea, which is a region sensitive to high-frequency sounds, the best frequency of a cochlear location increases toward the most basal end, that is, near the stapes. Response phases also vary along cochlear locations. At any given frequency, there is a decrease in phase lag toward the stapes. This tonotopic arrangement in the cochlea was originally described by Georg von Békésy in a seminal series of experiments on human cadavers and has been confirmed in more recent works on live laboratory animals. Nonetheless, our knowledge of tonotopy at the apex of the cochlea remains incomplete in animals with low-frequency hearing, which is relevant to human speech. The results of our experiments on guinea pig, gerbil, and chinchilla cochleas, regardless of the sex of the animal, show that responses to sound differ at locations across the apex in a pattern consistent with previous studies of the base of the cochlea.
SIGNIFICANCE STATEMENT Tonotopy is an important property of the auditory system that has been shown to exist in many auditory centers. In fact, most auditory implants work on the assumption of its existence by assigning different frequencies to different stimulating electrodes based on their location. At the level of the basilar membrane in the cochlea, a tonotopic arrangement implies that high-frequency stimuli evoke largest displacements at the base, near the ossicles, and low-frequency sounds have their greatest effects at the apex. Although tonotopy has been confirmed at the base of the cochlea on live animals at the apex of the cochlea, however, it has been less studied. Here, we show that a tonotopic arrangement does exist at the apex of the cochlea.
Introduction
The ear is a versatile organ that transduces sounds over a wide range of levels and frequencies. One of the fundamental properties of the auditory system, from the cochlea through the brainstem and all the way to the cortex, is tonotopy, whereby different spatial regions respond best to different frequencies of sound, termed their best frequency (BF). Thus, different regions of each structure code for different frequencies of sound. Although still a controversial subject, this type of frequency-to-place mapping is considered necessary to extract pitch information from sounds (Oxenham et al., 2004).
Within the cochlea, the existence of a tonotopic arrangement was first demonstrated by von Békésy (1947, 1949) in his experiments on human cadavers in which he showed that apical cochlear regions had lower BFs than basal ones. In live mammals, the cochlea has been shown to have exquisite frequency selectivity and remarkable sensitivity to quiet sounds (Rhode, 1971). To a large extent, this performance is achieved through active signal processing within the cochlea, which involves cochlear amplification by outer hair cells (Brownell et al., 1985; Liberman et al., 2002). Tonotopy has also been found within the cochlear basal high-frequency region in chinchilla (Rhode and Recio, 2000: Recio and Rhode, 2000) and gerbil (Ren, 2002).
Although a tonotopic arrangement in the mechanical vibrations of the organ of Corti (OoC) has been shown within the base of the cochlea, tonotopy has not been explicitly demonstrated within the apex. The application of single-neuron labeling techniques to the auditory nerve in several species (cat, Liberman, 1982; gerbil, Müller, 1996; chinchilla, Müller et al., 2010) has yielded frequency-position data that suggest a tonotopic organization throughout the cochlea. In addition, the mechanical tuning recorded at the level of basilar membrane was found to match the auditory nerve tuning measured from the same chinchilla cochlea (Narayan et al., 1998). Given these similarities, it has long been expected that the tonotopic arrangement inferred from auditory nerve fiber labeling experiments would also exist at the apex of the cochlea.
Until relatively recently, recordings of mechanical cochlear responses at the apex of live animals have been marred by technical difficulties because of the need of opening the otic capsule. The application of optical coherence tomography (OCT), however, has surmounted such difficulties (Recio-Spinoso and Oghalai, 2017; Dong et al., 2018). A detailed demonstration of tonotopy within the apex of the mouse cochlea exists (Nankali et al., 2022). The higher-frequency range of mouse hearing, however, calls into doubt whether recordings from the mouse cochlear apex are relevant to what happens in the apex of humans and other animals that hear at lower frequencies.
A recent study (Burwood et al., 2022) showed that amplitudes and phases measured at several locations at the apex of the guinea pig cochlea have similar values across sites, which would indicate an absence of a tonotopic arrangement at the apex as well as revealing an inconsistency with traveling wave theory. Based on our previous work (Recio-Spinoso and Oghalai, 2017; Dong et al., 2018), we questioned these findings. Here, we present an analysis of sound-evoked vibrations measured using OCT at two apical locations of the same cochlea in guinea pigs, gerbils, and one chinchilla. Our data argue that animals with low-frequency hearing have mechanical tonotopy within the apical turn of their cochlea.
Materials and Methods
Experiments were performed on cohorts of guinea pigs, gerbils, and one chinchilla. Data recorded from the guinea pigs and gerbils were previously published (Recio-Spinoso and Oghalai, 2017; Dong et al., 2018) but reanalyzed for the purpose of this article. These experiments were performed at Oghalai's laboratory, initially at Stanford University and later at the University of Southern California. The chinchilla experiments were performed in Recio-Spinoso's laboratory. All procedures were approved by Stanford University, University of Southern California, and Universidad de Castilla-La Mancha Institutional Animal Care and Use Committees.
Animal preparation
Guinea pig
Data for this work originates from six guinea pigs. Methods for the surgical preparation of guinea pigs were previously described (Recio-Spinoso and Oghalai, 2017), and only a brief explanation will be provided here. Adult albino guinea pigs of either sex were used (Charles Rivers Laboratory). Guinea pigs were initially anesthetized with ketamine hydrochloride (40 mg/kg, s.c.) and xylazine (5 mg/kg, s.c.). Deep anesthesia was maintained with supplemental doses of both anesthetics to eliminate withdrawal reflexes. The head of the guinea pig was fixed in a head holder. The left middle ear bulla was opened to allow imaging of the cochlear apex. The tensor tympani was sectioned. We did not open the otic capsule to perform imaging or vibrometry.
Gerbil
Data for this work originates from one representative gerbil, but the results were verified to be similar in 12 additional gerbils as published in Dong et al. (2018). Methods for the surgical preparation of gerbils were previously described (Dong et al., 2018). Briefly, adult gerbils of either sex weighing 40–60 g were used (Charles Rivers Laboratories). Gerbils were initially anesthetized with ketamine hydrochloride (100 mg/kg, s.c.) and xylazine (10 mg/kg, s.c.). Deep anesthesia was maintained with supplemental doses of both anesthetics to eliminate withdrawal reflexes. The head was fixed using a head holder, and the left pinna was removed. The left bulla was then opened to expose the apex of the cochlea.
Chinchilla
Data for this work originates from one male chinchilla. Adult chinchillas weighing 400–500 g were used (Granja Vergé). Chinchillas were initially anesthetized with ketamine hydrochloride (40 mg/kg, i.m.) and xylazine (8 mg/kg, i.p.). Deep anesthesia was maintained with supplemental doses of both anesthetics to eliminate withdrawal reflexes. The body temperature of the animal was maintained at 37.5°C using a heating pad and a rectal probe with a servo-controlled heating system. The chinchilla head was fixed in a head holder and the right ear was removed. Surgery was performed to open the right bulla. The tensor tympani tendon was sectioned, and the stapedius muscle was detached from its anchoring. We did not open the otic capsule to perform imaging or vibrometry.
OCT vibrometry and data analysis
We imaged the left ear of guinea pigs and gerbils and the right ear in the chinchilla. Digitally generated sounds were delivered through a speaker coupled to the ear. They were calibrated using a calibrated probe-tip microphone (one-fourth inch, Brüel & Kjær). For guinea pigs and gerbils, the same custom-built OCT device was used. It used a broadband swept source with a wavelength of 1300 nm and a 200 kHz sweep rate (Lee et al., 2015). Data were collected using custom software written in Python and the CUDA (Compute Unified Device Architecture) platform. For chinchilla, vibrometry was performed using a Telesto Spectral Domain OCT system (Thorlabs) and ThorImage OCT software, version 5.4.2 (Thorlabs). For all animals, OoC vibrations were obtained from multiple points within the apical turns of the intact cochlea in vivo. We also collected middle ear (ME) vibration data from the malleus-incus complex.
We analyzed the data by performing fast Fourier transform (FFT) analysis on the vibratory responses. After finding the peak vibratory magnitude at the stimulus frequency, we determined the mean and SD of the noise floor by averaging the magnitude over the FFT bins contained within a 100 Hz band adjacent to the stimulus frequency. If the peak magnitude at the stimulus frequency was less than the noise floor threshold (mean + 3 SD), the measurement was not kept. All data were analyzed using custom software written using MATLAB R2020b (MathWorks).
Results
Vibratory responses to single tones in the guinea pig
We demonstrate the existence of tonotopy by measuring sound-evoked responses at four apical locations in the guinea pig (cochlear turns 3.5, 3.25, 2.5, and 2.25). The locations were 95, 92, 80, and 75% from the base, respectively. There were little differences between the responses at the 95 and 92% sites, as well as between the 80 and 75% sites. We therefore show only the results at the 95 and 75% sites. (The separation between both sites is 400 µm for a 20 mm cochlea.) In all the locations, the measuring point is near the reticular lamina at the lateral edge of the outer hair cell region or the medial edge of the Hensen cells (Recio-Spinoso and Oghalai, 2017).
Amplitude responses were obtained using Fourier analysis; an example of the results of this type of analysis for one cochlea is in Figure 1. The waterfall plot of displacements in Figure 1A shows a low-pass profile (i.e., amplitude values change slightly below a corner frequency and then decrease in value after that frequency) at the 95% site. Displacements measured in the second turn, or 75% site, (Fig. 1B) display a spectral peak at 600 Hz, particularly at mid- to high-intensity levels.
Differences in tuning at two apical locations in the guinea pig cochlea. A, B, Waterfall plots of displacement versus frequency curves at two apical locations (A, 95%; B, 75%). The 95% site corresponds to the most apical location recorded and is located approximately in the 3.5 cochlear turn. The 75% site is located in turn 2.25. C, D, Displacement versus level functions were obtained from responses to 200 and 600 Hz stimuli, respectively, at two locations, 95% (solid black lines) and 75% (red lines). Dashed lines indicate linear growth. E, F, Velocity versus frequency curves were computed from data in A and B, respectively.
Displacement amplitudes as a function of stimulus levels for two stimulus frequencies are shown in Figure 1, C and D. For 200 Hz signals, vibration amplitudes at the 95 and 75% sites are virtually identical (Fig. 1C). At 600 Hz, however, amplitudes at the 95% site are smaller than at the 75% site (Fig. 1D). Not surprisingly, the results in Figure 1, A and B, expressed in velocity units reveal bandpass characteristics with peaks at 300 (Fig. 1E) and 600 Hz (Fig. 1F), respectively.
The tonotopic arrangements of mechanical responses were further demonstrated using vibration amplitudes as a function of frequency (Fig. 2A–D) at two stimulus levels for two cochleas [guinea pig (GP)08 and GP09]. These responses were used to confirm that mechanical responses at more apical locations, that is, the 95% site, are different from the responses at more basal locations, that is, the 75% place, in particular, the shape of the amplitude curve and the high-frequency roll-off. At the lowest stimulus levels, 30 dB SPL in Figure 2A and 20 dB SPL in Figure 2C, the amplitude versus frequency curves at the most apical site (95%, black lines with symbols) display a low-pass shape, whereas responses at the 75% site show a bandpass profile. Similar results were obtained from the responses to 70 dB SPL tones (Fig. 2B,D).
Low-pass and bandpass selectivity of apical responses in the guinea pig. A, B, Plots represent data obtained from responses to 30 and 70 dB SPL tones, respectively, as a function of frequency shown in Figure 1, A and B. C, D, Similar data obtained from a different preparation at the stimulus levels indicated. E, Data from A and B expressed as velocities. F, Velocities obtained from the data in C and D.
Figure 2E displays velocity versus frequency curves obtained from the results in Figure 2, A and B. A tenuous spectral peak at 200 Hz is shown in the results of the 95% site at 30 dB SPL (Fig. 2E, black line with open symbols). More apparent bandpass characteristics were found in the responses to 70 dB SPL at that location (Fig. 2E, black lines with filled symbols) and in the velocity data recorded at the 75% site, regardless of the stimulus level (Fig. 2E, red lines with symbols). Results in Figure 2F also show similarities to those in Fig. 2E. There are differences in BFs among the sound-evoked velocities as a function of stimulus frequency at the 95 and 75% sites. Moreover, stimulus frequencies that evoke the largest velocities in Figure 2, E and F, increase slightly with stimulus level. For example, in Figure 2E (red lines), BF increases from 600 Hz at 30 dB SPL to 700 Hz at 70 dB SPL.
Based on a cochlear frequency map for the guinea pig (Tsuji and Liberman, 1997), the characteristic frequencies (CFs) at the 95 and 75% sites are 178 and 595 Hz, respectively. Such CFs are similar to the BFs obtained from low-level stimuli at the 95% (∼200 Hz) and 75% (600 Hz) shown in Figure 2.
Cochlear gains in the guinea pig
Cochlear gain was defined as the ratio of the vibration amplitude measured at the organ of Corti relative to ME motion. Examples of gains relative to ME motion as a function of frequency are shown in Figure 3, A–D, for four preparations. Cochlear gains at the 95% site (black lines with symbols) had an approximate low-pass shape. By contrast, gains at the 75% site show bandpass tuning at 20 and 70 dB SPL (red lines with open and filled symbols, respectively). Amplitude gains at the 95 and 75% sites are not constant as a function of stimulus level, a consequence of the nonlinear behavior of the cochlea.
Gain functions reveal differences in tuning and phase responses at two apical locations in the guinea pig. A–D, Cochlear gains relative to ME motion computed from responses at the 95% and 75% sites reveal low-pass selectivity (black lines with symbols) and bandpass tuning (red lines with symbols), respectively. E, Results indicate that phases at the 95% site (black lines with symbols) lag phases at the 75% site (red lines). F, Curvature versus frequency curves obtained from 70 dB SPL phases. Positive values (i.e., above the dashed gray lines) indicate phases are convex; negative values indicate phase functions are concave. The y-axis is correct only for GP05; the other curvature values were displaced by multiples of 0.01 radians/Hz2.
OoC phases relative to ME motion for the recordings in Figure 3, A–D, are shown in Figure 3E. Only the phases from the responses to 70 dB SPL tones are shown. Phase accumulated with frequencies and showed a steeper slope at the 95% site compared with the 75% site. Such results are consistent with the traveling wave theory proposed by von Békésy (1947), where the wave takes more time to travel to more apical cochlear locations.
In addition to the overall phase differences between the 95 and 75% sites shown in Figure 3E, the overall shapes of the two sets of phases in that figure are different. Phases at the 95% site display a convex shape for frequencies up to ∼1000 Hz, which are different from the phases at the 75% site. The shape of the phase curves can be quantified by computing their second derivative with respect to frequency, that is, the curvature (Oxenham and Dau, 2001). Positive (negative) curvature values in Figure 3F indicate that the phase is convex (concave). For animal GP05, for example, phases at the 95% site (black lines) are positive (i.e., convex) up to 800 Hz, and they become concave after that. At the 75% site, however, the opposite behavior happens (red lines). Phases are concave up to a certain frequency (∼800–900 Hz), and then their shape becomes convex. Similar results occur in the other three cochleas in the same figure.
Vibratory responses to single tones in the gerbil
A representative set of OoC vibratory responses to sound from one gerbil are shown in Figure 4. Results in Figure 4 were obtained from recordings at the second (red lines) and third turns (black lines) in the same cochlea (Dong et al., 2018). Velocity versus frequency curves obtained from responses to 10 dB SPL tones (Fig. 4A, open circles) exhibit bandpass tuning with a BF that increases from 450 Hz at the third turn (Fig. 4A, black lines with open symbols) to 2200 Hz at the second turn (red lines with open symbols). Using a gerbil cochlear map (Müller, 1996), the normalized distances from the recording sites at the third and second turns are 89 and 64%, respectively. Velocity versus frequency curves in Figure 4A show that BF changes with level at the 89 and 64% sites but in different ways. Whereas at the third turn BF increases with level from 450 Hz (10 dB SPL) to 650 Hz (60 dB SPL), and at the second turn BF decreases with level from 2200 Hz to 1300 Hz. That is, there is an approximate one-half octave increase or decrease in BF with stimulus level in the results in Figure 4A.
Differences in tuning and response phase at two apical cochlear locations in the gerbil cochlea. A, Velocity versus frequency curves obtained from tone-evoked responses at the 89% (black lines with symbols) and 64% sites (red lines with symbols) for two stimulus levels. B, Response phases relative to ME motion indicate that responses at the 89% site lag responses at the 64% site.
The response phases relative to ME motion from the same gerbil also reveal differences as a function of the position of the recording site along the cochlea. Phases in Figure 4B obtained from recordings at the 89% site (black lines) lag response phases at the 64% (red lines) at all frequencies and stimulus levels studied. There are also differences in the shapes of the phase versus frequency curves in Figure 4B, which are like the differences in shapes shown in Figure 3E.
Responses to acoustic clicks in the chinchilla
Vibratory responses to clicks at two apical locations were measured in one chinchilla. Figure 5, A and B, show photographs indicating the location of the two recording sites (red arrows). Visualizations were possible by creating a cross-sectional image (or B-scan) at each of the two sites. B-scans in Figure 5, C and D, were obtained at the regions indicated by the red arrows in Figure 5, A and B, respectively.
OoC click responses reveal differences at two apical sites in the chinchilla cochlea. A, B, Photographs of a right cochlea with 1 mm arrows indicating the location and width of the B-scan. C, D, B-scans obtained at the locations indicated by the arrows in A and B, respectively. E, Traces represent click-evoked velocities measured at the umbo of the middle ear (gray line), the most basal (red line), and the most apical sites (black line). Vertical dashed lines in E indicate vibration onsets at the 73% and 99.6% sites. The amplitude of the click stimulus was attenuated 20 dB relative to the maximum value in our system. F, G, Fourier transform amplitudes and phases, respectively, of the OoC waveforms in E. Animal, OCT12.
OoC measurements were obtained at the midpoint of each B-scan [field of view (FOV) = 0.5 mm]. Traces in Figure 5E represent middle-ear (gray line) and OoC click responses (red and black lines); the traces correspond to the maximum values recorded along the optical axis (red dashed lines in the B-scans). Maximum values were measured approximately at depths of 1.75 mm (Fig. 5C) and 2 mm (Fig. 5D). OoC click responses occur after a delay of >1 ms relative to the onset of ME motion and lack the fast response previously shown by Cooper and Rhode (1996). OoC responses at the two sites do not occur at the same time either. In fact, the response at the most apical site (Fig. 5E, black line) starts ∼700 µs after the beginning of the vibrations at the most basal site (Fig. 5E, red line). There are also clear differences between the OoC responses in Figure 5E. For example, the initial oscillations in both responses have different periods.
The frequency selectivity and timing of the OoC responses in Figure 5E were further analyzed using the Fourier transform. Figure 5F displays response amplitudes obtained from the responses at the most apical site (black line), with BF = 123 Hz. According to a chinchilla cochlear map (Müller et al., 2010), the distance from this site to the base is 20.17 mm (normalized distance = 99.6%). The red line in Figure 5F shows amplitudes from a site with BF = 527 Hz, which is located at 14.74 mm (73%) from the base. Both response amplitudes in Figure 5F display a bandpass shape. The response amplitude at the 73% site (Fig. 5F, red line) exhibits two spectral peaks. Short-time Fourier analysis shows that the largest spectral peak originates from the initial response oscillations, and the smaller peak arises from the latter part of the response. This pattern has been seen in other click responses at this location in other chinchillas.
Raw phases computed from the responses measured at the 99.6% site (Fig. 5G, black lines) lag response phases at the 73% place (Fig. 5E, red line) at all the frequencies shown. This behavior is similar to the one shown in Figures 3E and 4B. Both phase versus frequency curves in Figure 5G, however, display a convex shape.
Discussion
Tonotopy, also called frequency-to-place mapping, is an important feature of the processing of sounds by the mammalian auditory system. The tonotopic arrangement described by von Békésy (1947, 1949) in his experiments in human cadavers shows that the apex of the cochlea is more sensitive to low-frequency sounds than other sites closer to the stapes. That is, the BF increases as the recording site approaches the stapes. Experiments in live laboratory animals have confirmed this finding within the cochlear base. Here, we show that tonotopy also exists within the low-frequency cochlear apex of three different mammals that all have low-frequency hearing approximating that of humans.
Using OCT systems, we have recorded the responses to sound at several locations of the apex of live and intact guinea pig, gerbil, and chinchilla cochleas. In the guinea pig, OoC responses measured at 95 and 75% sites peaked at 200 and 600 Hz, respectively (Figs. 1, 3; Recio-Spinoso and Oghalai, 2017). Measurements in the gerbil originate from the 89 and 64% sites with respective BFs at 450 and 2200 Hz (Fig. 4). Velocity-frequency curves in the gerbil exhibit a bandpass shape thus displaying tuning to their respective BFs. In the chinchilla, OoC click responses were recorded at the 99.6 and 73% sites. Amplitude versus frequency curves computed from the velocity responses at both sites exhibit bandpass tuning with different shapes and BFs (123 and 527 Hz at the 99.6 and 73% sites, respectively).
Gain functions also show important differences between the processing of sounds at the 95 and 75% sites in the guinea pigs (Fig. 3). Whereas gain versus frequency curves at the most apical site reveal a low-pass shape; similar curves at the 75% site show bandpass tuning. This indicates that the most extreme apical end of the cochlea is more sensitive to frequencies below 200–300 Hz. In contrast, the 75% site is more sensitive to frequencies in the 500–600 Hz range. Similar observations were obtained in gerbils (Fig. 4; Dong et al., 2018).
Tonotopy and response timing
Variations in response phases along different cochlear sites were originally described by von Békésy (1947) and have been confirmed by several contemporary studies, including the present one. Responses to a tone signal of a given frequency, for example, measured at different cochlear locations yield different phases, which are characteristic of a traveling wave mechanism (von Békésy, 1947).
In the guinea pig and gerbil, vibrational phases relative to ME motion reveal important differences in the processing of sounds between the two sites. Vibrations at the most apical site lag responses at more basal places in response to the same frequency of sound. This indicates that the wave arrived at the more basal location first. Similar results have been found within the middle (Meenderink and Dong, 2022, their Fig. 2) and the base of the cochlea (de la Rochefoucauld and Olson, 2007). Similar findings have also been shown in the chicken (Xia et al., 2016). The phase variation with recording locations represents the traveling wave, as described by von Békésy (1947). Moreover, there are differences in the curvature of the phase functions. In the guinea pig, below 700–800 Hz, phase responses at the 95% site are convex, which contrast with the curvature of the response phases at 75% (Fig. 3F).
OoC responses to clicks recorded at two apical locations also show differences between each site in the timing of their responses. Click-evoked vibrations at the 99.6 and 73% sites do not have the same onset latencies, as judged by the differences in times indicated by the dashed vertical lines (2 and 1.33 ms, respectively) in Figure 5E. Those values resemble the latencies of chinchilla auditory nerve fiber responses to rarefaction clicks with similar BFs (2.03 and 1.05 ms, respectively), once adjusted for a 1.244 ms neural delay (Temchin et al., 2005, their Fig. 10A). Response phases at the two apical locations in Figure 5 are also different. Phases at the 99.6% site lag the phases at the 73% site (Fig. 5G).
Comparison to previous work
Warren et al. (2016) provided evidence of tonotopy at the apex of the guinea pig cochlea by showing two clearly different apical tuning curves, albeit from two different preparations, with BFs at least one octave apart from each other (Warren et al., 2016, their Fig. 3C). Similarly, Hemmert et al. (2000) showed evidence of tonotopy and the traveling wave from recordings at two apical sites with BFs of 380 and 505 Hz in a temporal-bone preparation of the guinea pig cochlea (Hemmert et al., 2000, their Fig. 4). On the other hand, Burwood et al. (2022) showed that amplitudes and phases measured at several points at the apex of the guinea pig cochlea have similar values across sites, which disagrees with the previous study and the results presented here. For example, a visual comparison of the results in Figure 2B in Burwood et al. (2022) with our results in Figures 1 and 3 highlights important differences. That is, BFs in their Figure 2B do not change along the second and third turns.
The discrepancy cannot be attributed to species differences because most of the results for this report were also obtained in guinea pig. Nor can the discrepancy be attributed to differences in instrumentation because the chinchilla recordings in this work were done using the same OCT system as the one used by Burwood et al. (2022). The discrepancy might be that, in part, because of the use of pixel averaging by Burwood et al. (2022), OCT measurements have demonstrated a complex motion within the cochlear partition whereby different structures move with different amplitudes, tuning, and phases (Gao et al., 2014; Lee et al., 2016; Cooper et al., 2018; Dewey et al., 2021; Meenderink et al., 2022). The averaging of motions across the organ of Corti makes it challenging to interpret the results. Furthermore, as OHC electromotility drives these differences (Dewey et al., 2021), the differences found between normal and furosemide-treated animals detected by Burwood et al. (2022) are not surprising.
In conclusion, we have shown that responses to single tones and clicks measured in the apical region of guinea pig, gerbil, and chinchilla cochleas are not similar across the apex. In fact, there is a tonotopic arrangement, and response phases at different recording sites follow the pattern of a traveling wave mechanism.
Footnotes
This work was supported by Ministerio de Ciencias, Innovación y Universidades Grants EQC2018-0048200-P and PID2020-117266RB-C22 and the European Social Fund (A.R-S.) and National Institutes of Health–National Institute on Deafness and Other Communication Disorders Grants R21DC019998 (W.D.) and DC014450, DC017741, and DC019700 (J.S.O.).
The authors declare no competing financial interests.
- Correspondence should be addressed to Alberto Recio-Spinoso at reci0001{at}umn.edu
