What Occurs to a Tone as the Frequency Increases?
Curr Opin Neurobiol. Author manuscript; available in PMC 2013 February 22.
Published in concluding edited form as:
Curr Opin Neurobiol. 1992 Aug; ii(4): 449–456.
PMCID: PMC3579517
NIHMSID: NIHMS273160
Responses to sound of the basilar membrane of the mammalian cochlea
Mario A. Ruggero
Department of Otolaryngology, Academy of Minnesota, 2001 6th Street Southward.E., Minneapolis, Minnesota 55455, United states
Abstruse
Recent testify shows that the frequency-specific non-linear backdrop of auditory nerve and inner hair cell responses to sound, including their sharp frequency tuning, are fully established in the vibration of the basilar membrane. In turn, the sensitivity, frequency selectivity and non-linear properties of basilar membrane responses probably effect from an influence of the outer hair cells.
Introduction
Mechanical to electrical transduction in the cochlea, the hearing organ of mammals, is mediated by vibrations of the basilar membrane, on which rests the sensory epithelium—the organ of Corti—with its complement of inner and outer hair cells (for reviews, see [1•–three•]). In spite of the central part of the basilar membrane, its responses to sound have been measured in simply a few laboratories. Past far the most celebrated serial of investigations were carried out past Georg von Békésy, for which he was awarded the 1961 Nobel Prize for Physiology or Medicine [4]. Working principally in the temporal bones of human being cadavers, Békésy showed that the cochlea performs a spatial frequency analysis. According to Békésy's observations, each site of the basilar membrane responds to audio stimuli by vibrating linearly, i.e. in proportion to sound pressure. Every bit the vibration at each site of the basilar membrane stage lags the vibrations of more basal sites, a deportation traveling wave propagates on the basilar membrane from the cochlear base toward its apex. As it propagates, the traveling wave grows in amplitude, reaches a maximum and and then decays. The location of the peak is a function of stimulus frequency: vibrations in response to loftier-frequency sounds peak almost the cochlear base, while very low-frequency sounds travel all the fashion to the cochlear noon.
A turning point in the understanding of cochlear mechanics came in 1971, when Rhode [5] demonstrated that, at least in the cochlea of the live squirrel monkey, vibrations of the basilar membrane could be non-linear, growing at a charge per unit of less than 1 dB of vibration magnitude per 1 dB of stimulus increase. Further, he showed that this compressive non-linearity was frequency specific, existence demonstrable merely at stimulus frequencies close to that to which the basilar membrane site was most sensitive, i.e. its feature frequency (CF). Unfortunately, Rhode'southward discovery of a cochlear not-linearity in the squirrel monkey could not exist confirmed in other species, in spite of repeated attempts, for about a decade (for a review, see [six]). Therefore, in order to harmonize the plain linear and poorly frequency-tuned mechanical vibrations of the basilar membrane with the non-linear and sharply frequency-tuned responses of auditory nerve fibers, it was proposed that a '2d filter' exists in the organ of Corti [7]. The present review will outline contempo evidence demonstrating that the second filter is unnecessary and that, in fact, probably all frequency specific non-linear properties of auditory nerve and inner hair cell responses have mechanical counterparts in the vibration of the basilar membrane. Show is also presented which indicates that the sensitivity and frequency selectivity of basilar membrane responses result from an influence of the outer pilus cells.
New methodology
Both Rhode's discovery of a CF-specific basilar membrane non-linearity, and subsequent confirmation and extensions of his findings were obtained largely using a method based on the Mössbauer effect [5,8–12]. Because this method is extremely not-linear, authentic measurements are possible merely over a narrow range of response magnitudes, making it difficult to record vibration waveforms with large peaks (such as responses to clicks and other transients), and confounding the distinction betwixt cochlear non-linearities and those introduced by the method itself. Recently, Doppler-shift light amplification by stimulated emission of radiation velocimetry systems take been applied to the measurement of basilar membrane vibration. These instruments are highly linear and at least one guild of magnitude more sensitive than the Mössbauer technique [13 xv,sixteen•]. Many of the recent findings presented in this review were obtained using light amplification by stimulated emission of radiation velocimeters.
Responses to tones
The basilar membrane responds to single tones with a transverse Air-conditioning sinusoidal vibration. Responses to tones with a frequency well removed from the CF grow linearly as a function of stimulus intensity. Responses to tones with frequencies near the CF, on the other hand, usually grow linearly for stimulus levels close to neural threshold, simply at slower rates—slopes as low as 0.2 dB/dB at the CF and even lower immediately to a higher place the CF—at moderate stimulus intensities (Fig.1a) [eight–x,12,16•]: Information technology is not still clear whether this compressive non-linearity persists at intense stimulus levels, as indicated by the earlier Mössbauer studies. Some recent reports bear witness CF input—output curves whose slopes tend to become linear at fourscore–100 dB sound pressure level (SPL) [17••,18••]. If the responses to tones are plotted as families of iso-intensity contours, they strongly resemble the equivalent plots for auditory nerve fibers (Fig.1b). Responses plotted as iso-velocity or iso-deportation curves (i.e. sound intensities required to elicit a constant response magnitude) are very similar to the frequency-threshold tuning curves of auditory nerve fibers with a comparable CF (Fig.1d) [8,12,xix]. It is not sure whether neural thresholds are more closely matched to deportation or velocity of basilar membrane motion, Measurements in the 17 kHz region of the guinea pig basilar membrane suggest that neural threshold at the CF corresponds to a basilar membrane velocity of 40 µm s−i or a displacement of 0.35 nm [8]. For the chinchilla 9 kHz site, corresponding values are 100 µm s−i or 2 nm [12]. Theoretical analyses of frequency tuning in the basilar membranes of healthy republic of guinea pigs and chinchillas suggest that such tuning cannot be achieved by passive mechanisms, powered solely by the free energy of the acoustic stimulus. Rather, information technology seems, basilar membrane responses to audio describe boosted energy from an active process, the 'cochlear amplifier' [20–25].
Not-linearity and frequency tuning in the basilar membrane. The panels depict iv alternative representations of the same set of data: mechanical responses to tones in an exceptionally sensitive chinchilla cochlea. (a) Input-output functions: peak velocity of basilar membrane responses to tones equally a role of tone intensity. The parameter is tone frequency in kHz. The thin solid line represents a hypothetical linear input-output office (i.e. velocity proportional to sound pressure). Responses at frequencies near the feature frequency (CF) (9 kHz) grow at increasingly not-linear, rates (i.due east. gradient condign progressively smaller than 1 dB/dB) with increasing stimulus intensity. Responses at v kHz, a frequency well beneath the CF, grow linearly at all intensities. (b) Iso-intensity functions: height velocity of basilar membrane responses to tones as a part of tone frequency. The individual curves connect responses obtained at the same acoustic level (the parameter, expressed in decibels referenced to 20 µPa). Vertical slices through this plot produce input–output functions such as those in panel (a), while horizontal slices produce iso-velocity or frequency-tuning curves such equally those in console (d). (c) The velocity data of panel (b) are normalized to sound pressure level (left ordinate), yielding a family of iso-gain functions. In addition, gains are indicated relative to stapes motion (right ordinate) [57]. Due to the non-linear compressive growth of responses almost the CF, the gains are largest and most sharply frequency tuned at the lowest sound pressure levels. Note that, at low stimulus levels, basilar membrane vibrations at the CF are more than 10,000 times larger than those of the stapes. (d) Frequency tuning in the basilar membrane and in the auditory nerve. Iii iso-velocity and i iso-displacement functions (solid and dashed lines, respectively) for basilar membrane responses are compared with an average frequency-threshold tuning curve (dotted line) from auditory nerve fibers. The basilar membrane iso-velocity curves (0.1, 0.two and 0.iv mm southward−ane) are separated from each other by 6 dB at low frequencies, indicating a linear response growth. In dissimilarity, at the CF (9 kHz) the curves are separated by 17 dB and 25 dB, indicating compressive non-linear growth. Panels (b) and (d) are adapted from [xvi•]; panel (c) is adjusted from [33•].
There is some evidence that in vivo basilar membrane responses to tones may incorporate, in addition to sinusoidal vibrations, CF-specific DC and/or very low-frequency components [15,26,27]. The being of such displacement responses remains in doubt, every bit they were obtained principally in severely damaged cochleae, using very intense stimuli [26,27]. DC responses may have also been recorded in healthier cochleae [15]. A preliminary report of another study, using a specially suitable displacement- (rather than velocity-) sensitive laser interferometer, emphatically states that DC components are absent in responses of the basilar membrane at the base of the cat cochlea (NP Cooper and WS Rhode: Association for Enquiry in Otolaryngology, Midwinter Meeting Abstracts 1992, xix).
Responses to clicks
Basilar membrane responses to clicks take been recorded in alive animals using the Mössbauer technique [11], a capacitive probe [28], and laser velocimetry [16•,29••]. While the earlier studies were hampered past not-linear methodology [11] and/or the severely deteriorated state of the experimental cochleae [28], they revealed responses that grew not-linearly with click intensity in a mode qualitatively consistent with responses to tonal stimuli. Responses to low-level clicks, recorded with laser velocimetry in the basal region (3.5 mm from the oval window) of sensitive chinchilla cochleae, consist of transient oscillations with periodicity appropriate to the CF (Fig.2a). These responses have about symmetrical spindle-shape envelopes with delays to the maxima (average group delays) of roughly 0.seven ms, measured relative to the onset of stapes move (Fig.2a, top). As click level is raised, the envelopes become skewed, with earlier cycles of oscillation growing faster than later cycles, which grow at non-linear (compressive) rates. At high stimulus intensities, the initial bike of oscillation emerges from the baseline noise, growing linearly. This linear bicycle has an irreducible latency of 0.one ms, presumably corresponding to travel fourth dimension from the oval window to the recording site, three.v mm away. Fourier transformations of the fourth dimension-domain responses match excellently the non-linear frequency and level dependencies evident in responses to tones (Fig.ane) [29••]. Thus, the total frequency selectivity of basilar membrane responses at the base of the chinchilla cochlea appears to be developed within 0.6 ms of the arrival of the traveling wave. Feasibly, this filibuster could result from the operation of the 'cochlear amplifier'.
Basilar membrane responses to clicks in normal cochleae and in those transiently poisoned by furosemide. (a) The tracings draw normal fourth dimension-domain responses to clicks presented at several elevation intensities (dB SPL indicated next to each tracing). The abscissa indicates time (ms) elapsed since the arrival of the acoustic click at the tympanic membrane. (b) The solid lines depict the frequency spectra obtained past Fourier transformation of three of the waveforms (indicated past arrows) in (a). Dashed lines stand for the frequency spectra for responses obtained immediately following an intravenous furosemide injection. Note that response sensitivity is drastically reduced at and near the characteristic frequency (CF), but not altered at low frequencies. Notation also that the effect of furosemide is potent at a moderate stimulus level (48 dB), but pocket-size for intense clicks (88 dB). Furosemide reduced the responses to 48 dB clicks (meridian panel) to such an extent that they became cached in the baseline noise. Responses were fully recovered 100 min after furosemide injection (dotted lines). Panels in (b) adapted from [17••].
2-tone suppression
Two-tone charge per unit suppression is a non-linear auditory nerve miracle consisting of a reduction in the response to one tone due to the presence of a second tone (for a review, run into [30]). Rate suppression is frequency specific in that only responses to probe tones with a frequency nearly the CF can be suppressed. A mechanical counterpart of ii-tone charge per unit suppression was kickoff shown past Rhode in the squirrel monkey cochlea [31] using intense probe and suppressor tones. More recently, studies involving much healthier cochleae of chinchilla and guinea hog have demonstrated that most characteristics of neural rate suppression are shared past mechanical suppression in the basilar membrane [18••,32,33•,34]. Thus, the mechanical suppression effect can be elicited past moderate-level suppressor tones with frequencies both higher and lower than the CF, which, when presented alone, evoke responses smaller than the response to the probe tone alone. The consequence is CF-specific and physiologically vulnerable, is largest at low-probe tone levels, and grows with suppressor level at faster rates for lower-than-CF suppressors than for higher-than-CF suppressors [xviii••]. In the example of low-frequency suppressors, the suppression effect waxes and wanes with a periodicity respective to the suppressor frequency [18••,34].
Two-tone distortion
When listening to pairs of tones, humans tin hear additional tones that are non present in the acoustic stimulus. These two-tone distortion products are also known as combination tones because their pitches match those of combinations of the primary frequencies (fane and f2, ftwo > fane), such equally f2−f1 and 2f1−fii. Abundant psychoacoustical and neurophysiological evidence (for a review, see [xxx]) long pointed to a cochlear origin of combination tones, but basilar membrane studies failed to demonstrate them convincingly [31,35]. Cubic (2f1-f2) and other higher-club distortion products take recently been detected in basilar membrane responses to tone pairs [33•,36–38], Although detailed studies have yet to exist published, it is clear that the magnitude of these mechanical distortion products (effective levels as large as 17 dB below primary levels [37,38]) tin can account well for the presence of their counterparts in the auditory nerve. A mechanical ftwo−f1 baloney product has also been recorded from the basilar membrane of the guinea pig cochlea [39•].
Lability of responses to audio
The CF-specific non-linearity that Rhode discovered in the basilar membrane of the squirrel monkey disappeared later on death [40]. With hindsight, it now appears obvious that failure to demonstrate sensitive and non-linear basilar membrane responses in other studies in live animals (for a review, see [vi]) was due to surgical damage inflicted during experimental manipulations of the cochlea. This became evident when an contained, sensitive and frequency-specific measure out of cochlear function, namely the threshold of tone-pip-evoked compound action potentials, was shown to correlate well with mechanical sensitivity [8,12]. A causal relation between loss of mechanical sensitivity and non-linearity, on the one paw, and height of compound action potential thresholds, on the other, was suggested by a parallel deterioration of both measures in cochleae that produced initially sensitive responses to tones [viii,12]. More than recently, the force of mechanical two-tone suppression and the sharpness of tuning displayed past basilar membrane responses to clicks have too been linked to the sensitivity of mechanical responses to CF tones [16•,29••,33•]. In add-on, stimulation with intense sounds seems to have been responsible for eliminating the CF-specific not-linearity in two guinea squealer cochleae [8,41].
While the foregoing studies clearly indicated that basilar membrane non-linearities and sensitivity were physiologically vulnerable, they could not place the cells afflicted by surgical or acoustic trauma. A clue to their identity was the finding of outer hair jail cell damage in cochleae where basilar membrane recordings had been performed [42]. The clearest link to appointment betwixt organ of Corti function and basilar membrane vibration has been established past measuring the effects of systemic furosemide injection on basilar membrane responses to sound [17••]. Furosemide, a diuretic, drastically but reversibly alters cochlear part primarily by abolishing the endocochlear potential and reducing the receptor potentials of inner and outer pilus cells. Upon intravenous injection in chinchilla, furosemide causes a big CF-specific reduction and linearization of basilar membrane responses to tones and clicks (Fig.2b). These results nigh inescapably imply that the receptor potential of outer hair cells controls the vibration of the basilar membrane (presumably via a motile response; reviewed in [one•,3•]). Although furosemide must likewise bear upon inner hair cells, it is the outer hair cells that are implicated here considering of the demonstration of a differential effect on inner hair cells of DC currents applied extracellularly or intracellularly [43]. Injecting negative DC currents into Scala media (a procedure analogous to decreasing the endocochlear potential by means of furosemide) causes CF-specific reductions in auditory nerve and inner pilus jail cell sensitivity [43,44]. In contrast, alterations in inner hair cell responses induced past intracellular current injection are not frequency specific [44]. Thus, it appears that the cochlear amplifier resides in the outer hair cells, which draw energy from the large positive endocochlear potential.
Responses to audio of in vitro cochleae and isolated hair cells
An of import step toward a more than complete analysis of cochlear mechanical processes was taken recently with the development of an in vitro method for studying apical regions of isolated guinea squealer cochleae, which are sufficiently viable to retain endocochlear potentials of 30–l mV and moderate-size microphonics [45,46,47•,48,49•,l•,51••] Application of this method, combining a confocal microscope with either a very sensitive laser velocimeter or video recordings, has yielded surprising and potentially very important results. According to the initial reports, outer pilus cells within the apical organ of Corti answer to intense sound with AC vibrations that are "several hundred times greater than the response of the basilar membrane," and are as sharply frequency-tuned every bit the responses of pilus cells or auditory nervus fibers in vivo [46]. Fascinatingly, super imposed on the AC vibrations is a steady low-velocity response component whose frequency tuning and displacement magnitude far exceed those of the AC response (Fig.3) [51••], This position shift of the organ of Corti appears to reflect a form of outer pilus cell motility. When stimulated acoustically with sinusoidal stimuli, isolated outer hair cells obviously undergo a steady (DC) length change, whose polarity depends on whether they are extracted from basal or apical cochlear sites [52]. The length change is vulnerable to metabolic inhibitors [53•] and is sharply frequency tuned, with the most effective frequency correlating well with outer hair cell length (which itself is highly correlated with longitudinal cochlear location) [54]. At confront value, these findings suggest that (in contradiction with long-held behavior [55]) sound pressure level is an acceptable stimulus for hair cells, that hair cells part intrinsically as extremely abrupt mechanical frequency filters, and that "the sharply tuned …responses measured in the basilar membrane … are induced by the vibrations of the outer pilus cells" [46]. The foregoing findings and implications, if confirmed, would be revolutionary. For the moment, these findings should be interpreted with circumspection, equally they may non apply to in vivo cochleae, particularly at their basal region, when using depression- or moderate-level stimuli.
Organ of Corti response to intense amplitude-modulated Otones measured at the 3rd turn of an isolated (in vitro) guinea pig cochlea. Cochlear vibrations were recorded with a laser velocimeter and later integrated to extract deportation waveforms. The carrier frequency is indicated next to each tracing. The pinnacle stimulus pressure at the tympanic membrane was 135 dB SPL (i.e. referenced to 20 µPa), just because of the attenuation due to fluid filling the middle ear, the effective pressure was 100 dB SPL. In response to earner frequencies between 844 Hz and 917 Hz the organ of Corti shifts its position (with a velocity of approximately 100 µm s−1) toward the scala vestibuli, presumably due to a lengthening of the outer pilus cells. Reproduced with permission from [51••].
Conclusions and prospects
It is now articulate that sharp frequency tuning at the base of the cochlea is fully established at the level of vibrations of the basilar membrane [eight–ten,12,16•,nineteen,29••]. Further, probably all the CF-specific non-linearities of auditory nerve responses as well take correlates in the basilar membrane [5,viii,11,12,16•,17••,18••,28,29••,31,32,33•,34,36–38,39•]. Thus, the concept of the 'second filter', at least as originally conceived (i.due east. interposed between the basilar membrane and the auditory nerve in a unidirectional path of cochlear signal flow, peripheral to fundamental), is unnecessary and probably invalid. Withal, mounting bear witness links the not-linear, labile, and frequency-selective properties of basilar membrane motility to the physiological state of the organ of Corti [56], in particular to the receptor potentials of outer hair cells [17••]. Thus, the original 'first filter,' the basilar membrane, must now be viewed as existence inextricably linked to the organ of Corti, forming a feedback loop. Important questions concerning cochlear mechanics remain unanswered. What mechanical signal transformations arbitrate between basilar membrane motion and deflection of inner hair cell stereocilia? Does the mechanical CF-specificity of the basilar membrane/organ of Corti complex arise dynamically out of an interaction among elements (outer hair cells, basilar membrane and tectorial membrane), which, individually, are non frequency tuned or only mildly so, or are some elements (the outer hair cells?) intrinsically and sharply frequency tuned? What is the machinery whereby outer hair cells influence the vibrations of the basilar membrane? And is the response of the basilar membrane at the apical region of the cochlea qualitatively similar to that at the basal region?
Supplementary Fabric
Correction
Acknowledgements
NC Rich collaborated in much of the research described herein, prepared well-nigh figures and helped to edit the manuscript. I thank W Dixon Ward for comments on a previous version of the paper. This work was supported by Grants DC-00110 and DC-00419 from the National Constitute on Deafness and Other Communication Disorders.
Abbreviations
| CF | feature frequency |
| SPL | sound pressure level |
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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3579517/