In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the “base” of the cochlea (near the stapes) and low-frequency waves approaching the “apex” of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the “cochlear amplifier.” This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

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Mechanics of the Mammalian Cochlea

Phantom auditory perception (tinnitus): mechanisms of generation and perception

J.R. Pierce, The Nature of Musical Sound. M.R. Schroeder, Concert Halls: From Magic to Number Theory. N.M. Weinberger, Music and the Auditory System. R. Rasch and R. Plomp, The Perception of Musical Tones. J-C. Risset and D.L. Wessel, Exploration of Timbre by Analysis and Synthesis. J. Sundberg, The Perception of Singing. E.M. Burns, Intervals, Scales, and Tuning. W.D. Ward, Absolute Pitch. D. Deutsch, Grouping Mechanisms in Music. D. Deutsch, The Processing of Pitch Combinations. J.J. Bharucha, Neural Nets, Temporal Composites, and Tonality. E. Narmour, Hierarchical Expectation and Musical Style. E.F. Clarke, Rhythm and Timing in Music. A. Gabrielsson, The Performance of Music. W.J. Dowling, The Development of Music Perception and Cognition. R. Shuter-Dyson, Musical Ability. O.S.M. Marin and D.W. Perry, Neurological Aspects of Music Perception and Performance. E.C. Carterette and R.A. Kendall, Comparative Music Perception and Cognition. Index.

The psychology of music

A litter of four cats, born and raised in a soundproofed chamber, was studied in an attempt to determine which, if any, features of the auditory‐nerve response from routinely available cats might be due to the chronic effects of noise exposure. Two features of routine‐normal response were especially suspect in this regard: (1) a ’’notch’’ in the distribution of single‐unit thresholds centered at characteristic frequencies (CF’s) near 3 kHz and (2) a compression of the distribution of rates of spontaneous discharge for units with CF above 10 kHz. A third feature of response in routine animals was the presence of a small number (roughly 10%) of units with virtually no spontaneous discharge and very high thresholds, sometimes 80 dB less sensitive than high‐spontaneous units of similar CF. In the data from chamber‐raised animals, the high‐spontaneous units showed exceptionally low thresholds at all CF regions, however, there were signs of the midfrequency notch in the threshold distribution of at least two of these animals. The compression of the spontaneous rate distribution was not seen in any of the three most sensitive animals. The data suggest that there is a significant amount of ’’normal pathology’’ in the high‐CF units from routine animals. Low‐spontaneous, high‐threshold units were present in all four chamber‐raised ears with the same characteristics as in routine animals (exceptionally narrow tuning curves and exceptionally low maximum discharge rates) and at roughly the same percentage of the unit sample. A class of units with medium spontaneous rates and intermediate thresholds could also be identified. The possible significance of a classification of auditory‐nerve units according to spontaneous rate is discussed.

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Auditory-nerve response from cats raised in a low-noise chamber.

Hearing sensitivity in mammals is enhanced by more than 40 dB (that is, 100-fold) by mechanical amplification thought to be generated by one class of cochlear sensory cells, the outer hair cells. In addition to the mechano-electrical transduction required for auditory sensation, mammalian outer hair cells also perform electromechanical transduction, whereby transmembrane voltage drives cellular length changes at audio frequencies in vitro. This electromotility is thought to arise through voltage-gated conformational changes in a membrane protein, and prestin has been proposed as this molecular motor. Here we show that targeted deletion of prestin in mice results in loss of outer hair cell electromotility in vitro and a 40-60 dB loss of cochlear sensitivity in vivo, without disruption of mechano-electrical transduction in outer hair cells. In heterozygotes, electromotility is halved and there is a twofold (about 6 dB) increase in cochlear thresholds. These results suggest that prestin is indeed the motor protein, that there is a simple and direct coupling between electromotility and cochlear amplification, and that there is no need to invoke additional active processes to explain cochlear sensitivity in the mammalian ear.

Prestin is required for electromotility of the outer hair cell and for the cochlear amplifier.

Discharge patterns of single fibers in cat auditory nerve in response to controlled acoustic stimuli

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Discharge Patterns of Single Fibers in the Cat's Auditory Nerve

Spike discharges from single fibers in the auditory nerve of anesthetized cats were recorded with micro‐electrodes. Rates of discharge were measured as functions of the frequencies and levels of either single tones or two tones presented simultaneously. We found that the presence of a second tone diminishes the responses to the first tone if appropriate stimulus parameters are chosen. All fibers tested showed this two‐tone inhibition. Response areas and inhibitory areas were defined from isorate contours. The general characteristics of the inhibitory areas are found to be similar for a population of over 300 fibers.

Two-tone inhibition in auditory-nerve fibers.

The activity of single auditory‐nerve fibers was examined with and without electric stimulation of the crossed olivocochlear bundle (COCB). In the presence of tones at the characteristic frequencies (CF) of the fibers, the effect of COCB stimulation is, in general, to reduce the spike‐discharge rate. The time characteristics of the effect were studied for several parameters of the electric shocks, acoustic stimuli, and fiber characteristics. Some effects of COCB stimulation on “spontaneous” and acoustically generated activity are discussed with reference to cochlear mechanisms.

Effects of Electric Stimulation of the Crossed Olivocochlear Bundle on Single Auditory‐Nerve Fibers in the Cat

Bekesy's interest in improving telephone communication led him eventually to make observations on the mechanical properties of the basilar membrane For apical parts of the membrane he obtained resonance curves with steep high‐frequency slopes and gradual low‐frequency slopes Tuning curves for auditory‐nerve fibers in anesthetized cats also show significant low‐frequency tails Even fibers with high characteristic frequencies (CF) respond to tones throughout a wide range of low and middle frequencies For low‐frequency tones at moderate and high stimulus levels the responses of high‐CF fibers tend to be time‐locked either to the same phase of the stimulus or to a phase that differs by a half cycle For complex stimuli such as speech, which contains many low‐frequency components, these fibers behave as broadly tuned elements, discharging with time patterns that are principally determined by the temporal characteristics of the acoustic waveform Results of experiments using low‐ and high‐frequency masking stimuli reinforce the view that responses of high‐CF fibers to low‐frequency stimuli should not be ignored in theories that seek to describe the role of the auditory nerve in speech communication

Tails of tuning curves of auditory-nerve fibers.

Discharge patterns of auditory‐nerve fibers in anesthetized cats were recorded in response to a set of nine steady‐state, two‐formant vowels presented at 60 and 75 dB SPL. The largest components in the discrete Fourier transforms of period histograms were almost always harmonics of the vowel fundamental frequency that were close to one of the formant frequencies, the fundamental frequency or the fiber characteristic frequency (CF). For any fiber, the position of its CF relative to the formant frequencies (F1 and F2) appears to determine which of these components dominates the response. Specifically, the response characteristics of the tonotopically arranged array of fibers can be described in terms of five CF regions: (1) a low‐CF region below F1 in which the largest response components are the harmonics of the fundamental frequency closest to CF; (2) a region centered around CF = F1 in which the first formant and its harmonics are the largest components; (3) an intermediate region between F1 and F2 with prominent components at both the fiber CF and the fundamental frequency; (4) a region centered around CF = F2 in which harmonics close to the second formant are the largest for frequencies above the fundamental; and (5) a high‐CF region in which response spectra tend to show broad, multiple peaks at the formant and fundamental frequencies. These CF regions are related to the phonetic descriptions of vowels. For example, the extent of the low‐CF region is largest for ‘‘open’’ vowels (which have a high F1), and the intermediate region is distinct only for ‘‘spread’’ vowels for which F1 and F2 are more than 1.5–2 octaves apart. For all vowels, response activity for the majority of fibers is concentrated near the formant frequencies, in contrast to responses to broadband noise for which components near CF are dominant.

Nelson Y. S. Kiang

Papers

Discharge Patterns of Single Fibers in the Cat's Auditory Nerve

Two-tone inhibition in auditory-nerve fibers.

Effects of Electric Stimulation of the Crossed Olivocochlear Bundle on Single Auditory‐Nerve Fibers in the Cat

Tails of tuning curves of auditory-nerve fibers.

Speech coding in the auditory nerve: I. Vowel-like sounds