Revised estimates of human cochlear tuning from otoacoustic and behavioral measurements

Abstract

We develop an objective, noninvasive method for determining the frequency selectivity of cochlear tuning at low and moderate sound levels. Applicable in humans at frequencies of 1 kHz and above, the method is based on the measurement of stimulus-frequency otoacoustic emissions and, unlike previous noninvasive physiological methods, does not depend on the frequency selectivity of masking or suppression. The otoacoustic measurements indicate that at low sound levels human cochlear tuning is more than twice as sharp as implied by standard behavioral studies and has a different dependence on frequency. New behavioral measurements designed to minimize the influence of nonlinear effects such as suppression agree with the emission-based values. A comparison of cochlear tuning in cat, guinea pig, and human indicates that, contrary to common belief, tuning in the human cochlea is considerably sharper than that found in the other mammals. The sharper tuning may facilitate human speech communication.T he mammalian cochlea acts as an acoustic prism, mechanically separating the frequency components of sound so that they stimulate different populations of sensory cells. As a consequence of this frequency separation, or filtering, each sensory cell within the cochlea responds preferentially to sound energy within a limited frequency range. In its role as a frequency analyzer, the cochlea has been likened to a bank of overlapping bandpass filters, often referred to as ''cochlear filters.'' The frequency tuning of these filters plays a critical role in our ability to distinguish and perceptually segregate different sounds. For instance, hearing loss is often accompanied by a degradation in cochlear tuning, or a broadening of the cochlear filters. Although quiet sounds can be restored to audibility with appropriate hearing-aid amplification, the loss of cochlear tuning leads to pronounced, and as yet largely uncorrectable, deficits in the ability of hearing-impaired listeners to extract meaningful sounds from background noise (1).The bandwidths of cochlear filters have been measured directly in anesthetized, non-human mammals by recording from the auditory-nerve fibers that contact the sensory cells (2). Filter bandwidths in humans, however, must be determined indirectly from noninvasive measurements. Traditionally, such studies have relied on psychophysical (i.e., behavioral) measures of filter bandwidth based on the phenomenon of masking; that is, the ability of one sound to interfere with, or ''mask,'' the perception of another. Strong masking is interpreted as indicating that frequency components of the masker fall within the passband of the cochlear filter whose output is used to detect the signal. Interference then occurs because both signal and masker stimulate an overlapping group of sensory cells. Since the pioneering work of Harvey Fletcher (3), filter bandwidths have been obtained by measuring listeners' thresholds for detection of a pure tone in background noises with particular spectral characteristics. These tone th...

Development, validation and application of a generator for distortion product otoacoustic emissions

“…Allowing for the higher Q-factor is reasonable, since Liu et al also stated that further improvements of their model is needed in order to obtain sharper tuning properties. This is in agreement with findings measured on humans by Glasberg and Moore [22] and Shera et al [23].…”

Representations of the pitch of complex tones in the auditory nerve

“…This result is consistent with the progressive sharpening of peripheral tuning with increasing CF when expressed as a Q factor. The exponent for Q would be 0.37, which closely matches the 0.37 exponent found by Shera et al (2002), based on pure-tone tuning curves from AN fibers in the cat.…”

“…The bandwidths of the band-pass filters are constrained to be a power function of the CF (Shera et al, 2002). The population model has no free parameters; rather, parameters of the stimulus (F0 and SPL) are selected to fit the measured "rate-place profiles" expressing the normalized driven discharge rate as a function of CF (Sachs & Young, 1979).…”

“…However, these studies have largely neglected possible rateplace cues to pitch available when individual harmonics are resolved by the peripheral auditory system. Stimuli used in these studies had fundamental frequencies in the range of human voice (100-300 Hz), which may produce few, if any, resolved harmonics in typical experimental animals, which have a poorer cochlear frequency selectivity compared to humans (Shera, Guinan and Oxenham, 2002). Human psychophysical studies suggest that the low pitch produced by stimuli with resolved harmonics is stronger and less dependent on phase relationships among the partials than the pitch based on unresolved harmonics (Shackleton and Carlyon, 1994).…”

Perception of across-frequency asynchrony and the role of cochlear delays

“…The BM delays have been estimated from measurements of the compound action potential (CAP) (Elberling, 1974;Eggermont, 1979;Schoonhoven et al, 2001), derived-bands and tone-burst auditory brainstem responses (Eggermont and Don, 1980;Neely et al, 1988;Don et al, 1993;Donaldson and Ruth, 1993;Don et al, 1998;Harte et al, 2009), distortion-product, transient-evoked, and stimulus-frequency otoacoustic emissions (Neely et al, 1988;Bowman et al, 1997;Ramotowski and Kimberley, 1998;Schoonhoven et al, 2001;Shera et al, 2002;Sisto and Moleti, 2007;Harte et al, 2009), and by using latency-frequency responses obtained postmortem (Von Békésy, 1949) and then assuming a compensation term for the effects of death on the cochlear function (Ruggero and Temchin, 2007). The latency (s) of peak BM responses has often been described by a power law, s ¼ af -b , where f is the tone frequency a and b are constants whose values differ across studies.…”