1School of Biological Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom; and 2Zoologisches Institut, Universität München, 80333 Munich, Germany
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ABSTRACT |
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Russell, I. J. and M. Kössl. Micromechanical Responses to Tones in the Auditory Fovea of the Greater Mustached Bat's Cochlea. J. Neurophysiol. 82: 676-686, 1999. An extended region of the greater mustached bat's cochlea, the sparsely innervated (SI) zone, is located just basally to the frequency place of the dominant 61-kHz component of the echolocation signal (CF2). Anatomic adaptations in the SI zone are thought to provide the basis for cochlear resonance to the CF2 echoes and for the extremely sharp tuning throughout the auditory system that allows these bats to detect Doppler shifts in the echoes caused by insect wing beat. We measured basilar membrane (BM) displacements in the SI zone with a laser interferometer and recorded acoustic distortion products at the ear drum at frequencies represented in the SI zone. The basilar membrane in the SI region was tuned both to its characteristic frequency (62-72 kHz) and to the resonance frequency (61-62 kHz). With increasing stimulus levels, the displacement growth functions are compressive curves with initial slopes close to unity, and their properties are consistent with the mammalian cochlear amplifier working at high sound frequencies. The sharp basilar membrane resonance is associated with a phase lag of 180° and with a shift of the peak resonance to lower frequencies for high stimulus levels. Within the range of the resonance, the distortion product otoacoustic emissions, which have been attributed to the resonance of the tectorial membrane in the SI region, are associated with an abrupt phase change of 360°. It is proposed that a standing wave resonance of the tectorial membrane drives the BM in the SI region and that the outer hair cells enhance, fine tune, and control the resonance. In the SI region, cochlear micromechanics appear to be able to work in two different modes: a conventional traveling wave leads to shear displacement between basilar and tectorial membrane and to neuronal excitation for 62-70 kHz. In addition, the SI region responds to 61-62 kHz with a resonance based on standing waves and thus preprocesses signals which are represented more apically in the CF2 region of the cochlea.
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INTRODUCTION |
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The greater mustached bat, Pteronotus
parnellii parnellii, is the only New World bat to emit constant
frequency (CF) echolocation calls, a specialization it exploits to hunt
insects within the forest canopy and thereby occupy an ecological niche
not available to its close relatives (Schnitzler and Kalko 1998). The
typical echolocation call of P. parnellii consists of a 15- to 25-ms-long CF component followed by a brief, downward sweeping FM.
The fundamental frequency of the call is close to 30 kHz (CF1), and
there are three harmonics, the loudest at ~60 kHz (CF2). By
comparison with closely related species, P. parnellii has an
extended cochlear duct that is used to analyze the Doppler-shifted
echoes of the dominant 61 kHz (CF2) component of the CF calls. The
echoes are Doppler-shifted to a few hundred hertz above the call
frequency due to changes in the relative velocity between the bat and
its target. The incredibly sharp frequency tuning of the expanded, echo-response region of the cochlea enables P. parnellii to
resolve fine FMs in the echoes to the CF calls caused by insect wing
beats and to localize and distinguish the insects against the
scattering of acoustic signals by the foliage of the bat's hunting
grounds. In order that the frequency of the return echo to the CF call falls precisely within the very narrow bandwidth of this "sweet spot" in the echo-response region of the cochlea, the bat
continually adjusts the frequency of its call (Doppler compensation
behavior). The combination of extremely narrow frequency filtering in
the echo-response region and the setting of the echo-response frequency to a few hundred hertz above the call frequency enables the cochlea to
detect minute changes in the echo while strongly rejecting direct
stimulation from the intense CF call (for review, Neuweiler 1990
).
The sharply tuned neuronal responses to the echoes is due to mechanical
processing within the echo response region of the cochlea. There are
adaptations in cochlear anatomy that may serve to enhance frequency
tuning to the CF2 echoes. On the basis of an inner hair cell (IHC)
cochlear frequency map of the basilar membrane (BM), which was obtained
by characterizing and dye labeling auditory neurons and observing where
they terminated on the IHCs along the length of the BM
(Kössl and Vater 1985a), the frequency representation of the 60-kHz range is expanded along the BM in the
sparsely innervated zone (SI) and in the CF2 echo frequency response
region (CF2) (Fig. 1A). The
thickness of the BM in the SI region is increased (Fig. 1C),
largely due to an extension of the pars pectinata on the upper surface.
This thickening consists of extensive longitudinal fibers, indicative
of strong longitudinal coupling of the BM in the SI region (for review,
Kössl and Vater 1995
). The tectorial membrane (TM)
in the SI region is also highly specialized in that it is club-shaped
in cross-section with a reduced attachment to the spiral limbus
(Henson and Henson 1991
; Vater and Kössl
1996
).
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The frequency-dependent sensitivity of the inner ear can be assessed by
measuring acoustic distortion products in the external ear canal
(Gaskill and Brown 1990). It has been proposed that the
mechanoelectrical transducer function is a dominant nonlinearity that
could distort the cochlear response to sound in vivo (Frank and
Kössl 1997
; Lukashkin and Russell 1998
;
Patuzzi et al. 1989
; Santos-Sacchi 1993
)
and give rise to quadratic and cubic distortion products. There is
evidence that the emission of the distortion products from the cochlea
is filtered by the TM (Allen and Fahey 1993
;
Brown et al. 1992
). On the basis of distortion product
measurements in the mustached bat (Kössl and Vater
1996a
), it has been suggested that the TM in the SI region
resonates close to the CF2 echo frequency (~62 kHz), and according to
a model by Steele (1997)
, the motion of the TM would be
like that of a hammer on an anvil (the thickened BM) for frequencies
close to the CF2 echo. These anatomic features are thought to
contribute to the strong spontaneous (SOAEs) and stimulus-frequency-evoked otoacoustic emissions (SFOAEs) that are an
indirect indicator of mechanical resonance in this region of the
cochlea (Henson et al. 1985
; Kössl and
Vater 1985b
). The resonance frequency, as determined by
measuring SFOAEs, is a few hundred hertz above the CF2 call frequency
and close to the echo frequency (Fig. 1B). The strong
mechanical resonance in the cochlea of the mustached bat is also
evident in a threshold minimum and in long-lasting ringing of cochlear
microphonic potentials (Henson et al. 1985
;
Pollak et al. 1972
; Suga et al. 1975
). A
putative cochlear frequency map of the TM has been based on acoustic
distortion products. If this map is compared with the IHC-BM frequency
map (Fig. 1A), then it becomes apparent that the BM should
be tuned to the CF2 echo frequency (~62 kHz) at the transition
between the SI zone and the CF2 region. In the SI zone, the IHCs are
tuned to frequencies between 62 and 72 kHz. According to the putative TM map, the TM is tuned to 62 kHz over the whole length of the SI zone
(Fig. 1B).
In the present experiments, we measured distortion-product and
stimulus-frequency OAEs in the auditory meatus and compared these with
laser interferometer measurements of BM displacements in the SI region,
where frequencies between 62 and 72 kHz are represented. One aim was to
investigate the frequency and level-dependent properties of the BM in
the SI zone to see if they were similar to, or differed from, those
measured in the cochleae of nonecholocating mammals. These experiments
were, in part, initiated by Gale and Ashmore's (1997)
conclusion that the upper limit for outer hair cell (OHC) motility was
25 kHz and that bats, and presumably other mammals capable of detecting
ultrasound (Brown 1970
), must employ processes at
ultrasonic frequencies other than those involving OHC motility (for
review, Dallos 1992
) for amplifying mechanical signals
and for generating high-frequency resolution. Another aim was, indeed,
to investigate the role of the SI zone of the cochlea in the generation
of the cochlear resonance and the extraordinarily sharp frequency
tuning in the echo-response region of the BM.
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METHODS |
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Mustached bats (P. parnellii parnellii) were
anaesthetized with an initial dose of 2 mg of pentobarbital sodium per
100 g and either 1.2 mg of ketamine hydrochloride per 100 g
(Ketavet, Parke-Davis) or 0.13 mg of fentanyl dihidrogencitrate per
100 g (Hypnorm, Janssen). The bats were maintained at 37°C, the
level of anesthesia was monitored continuously, and every 40 min an additional Ketavet or Hypnorm dose was administered. At the end of the
experiment, the animals were overdosed with pentobarbitol sodium. The
middle ear was opened to expose the round window of the cochlea, which
was left intact. Tone stimuli were delivered through the auditory
meatus via a calibrated, closed sound system under computer control.
The sound system incorporated two 1/2-in driving microphones
(Bruel and Kjaer 4133) and a 1/4-in measuring microphone (Bruel
and Kjaer 4135) that was used to measure OAEs. The system was
compensated for constant sound pressure level (SPL) at the microphone
membrane. Detailed methods are published elsewhere (Kössl
1994).
Displacements of the BM were measured by the self-mixing effect of a
laser diode. This method of measuring basilar membrane vibration has
been described previously (Kössl and Russell 1995) and involves reflecting back from the BM a small proportion of the
light emitted by the laser into the laser cavity. The self-mixing effect has the advantage that it is not necessary to place reflecting targets on the BM and it is essentially linear for displacements of
<30 nm. The light emitted by the laser is monitored by an intrinsic photodiode, and the current from this is directly proportional to the
BM displacement. The bandwidth of the displacement-dependent signal was
200 kHz. Calibration of the signal was achieved by displacing the
interferometer by a known amount with a piezoelectric driver. The
system is also self-calibrating in that the limits of the transfer
characteristics of the interferometer are a quarter wavelength, which
is 167.5 nm for the laser diode used in these experiments. A schematic
of the interferometer is shown in Fig. 2.
The output of the interferometer was fed into a spectrum analyzer (3561A, Hewlet Packard) and a pair of lock-in amplifiers (5210 EG&G
Princeton) set in quadrature, and the phase and magnitude of the signal
were calculated on-line by the computer. Sound levels are given in
decibels (dB) re 20 µPa. The beam of the laser diode interferometer
was focused through the transparent round window to form a 5-µm
(diameter) spot, with a depth of focus of <2.0 µm, in the middle of
the basal half turn of the BM. At the end of each experiment, a lesion
was made with a micropipette at the location of the spot on the BM,
from which was estimated that most measurements were made from the SI
region of the cochlea (see Fig. 1). The beam was reflected almost
entirely from the bases of the outer pillar cells or Deiters' cells.
It was relatively easy to direct the beam to any position on the BM,
spiral lamina, or the secondary osseous spiral lamina (which covers the
spiral ligament), all of which were visible through the round window membrane. Tone-evoked vibrations could only be recorded from the BM.
Responses were not detected from either the rigid spiral lamina or the
secondary osseous spiral lamina. Vibrations were measured only from
structures within the narrow depth of field of the interferometer (>2.3 µm), and scattered light from other structures (e.g., the round window) did not interfere with the measurements. Direct measurements from the round window membrane showed that it did not
vibrate at the intensities and frequencies used in these experiments.
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To determine the cochlear resonance frequency of the individual bats,
SFOAEs were measured in awake animals before conducting surgery to
expose the cochlea for the BM recordings. The bats were restrained
gently in a shaped, foam cavity and by a head holder
(Kössl 1994). To elicit the SFOAEs, a pure tone
stimulus generated by a B&K 4133 microphone capsule used as a speaker
was swept upward in frequency, and the resulting frequency response at
the tympanum was recorded with a B&K 4135 microphone connected to a B&K
2670 preamplifier and a B&K 2610 measuring amplifier. At the frequency
of cochlear resonance, the tone stimulus interferes with the SFOAE to
produce a characteristic sequence of a sound pressure maximum and
minimum associated with phase changes in the frequency response (Fig.
3A) (Kössl
1994
). The transition between level maximum and minimum, where
maximum phase change occurred, was used to define the SFOAE frequency
(vertical line in Fig. 3A).
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Distortion product otoacoustic emissions (DPOAEs) were recorded to
monitor changes in cochlear mechanics during the anesthesia and
surgery. To obtain DPOAE data, the ear was stimulated with two pure
tone stimuli (f1, f2). Two B&K 4133 microphone capsules served as
speakers and the 2f1-f2 DPOAE was measured with the B&K 4135 microphone. The stimuli wave-forms were generated by using a dual HP
8904A synthesizer, and the recorded response was fed into a HP 3561A
dynamic signal analyzer for fast Fourier Transform (FFT) analysis.
Figure 3B shows representative examples of so-called "distortion audiograms" measured before and during anesthesia and
after opening the middle ear to gain access to the cochlea. To measure
audiograms of the 2f1-f2 DPOAE, the two stimuli were delivered with a
constant level difference, with the level of f1 10 dB above that of f2,
and both stimulus frequencies were increased stepwise over the tested
frequency range. During this procedure, the frequency ratio f2/f1 was
kept constant at a small value between 1.001 and 1.04 which, for the
mustached bat, is known to elicit maximum levels of the 2f1-f2 DPOAE
(Kössl 1994). There were clear influences of
anesthesia and of middle-ear opening on the DPOAE levels. In the
frequency range between 60 and 70 kHz, the DPOAE levels decreased by
3-15 dB, and for higher frequencies the loss of DPOAE levels could
amount
50 dB. Furthermore in anesthetized animals, the cochlea
resonance frequency, as measured by the SFOAEs, shifts by a few hundred
hertz to lower frequencies.
In addition to the preceding measurements, we recorded DPOAEs in six awake bats that were not used for surgery to determine the phase and group delay behavior of these emissions for comparison with the BM data. To be able to measure the DPOAE phase, both stimuli have to be presented at a constant phase relationship; this is not possible with the HP synthesizer. Therefore we used two D/A output ports of a Microstar Dap 3200/e400 digital signal processing board sampling at a rate of 250-400 kHz to produce the stimuli. The microphone response was recorded with an A/D channel of the same board and FFT analysis was performed onboard. Software for data acquisition and processing was written in Asyst (Keithley).
The phase behavior of the 2f1-f2 DPOAE was measured by keeping the f2
stimulus constant at a certain frequency and the f1 frequency was
varied stepwise. As a result, the 2f1-f2 frequency was swept across
the frequency range to be tested. The measured phase of the 2f1-f2
DPOAE (2f1-f2) was corrected for the stimuli phases according to Mills and Rubel (1997)
. The
corrected DPOAE phase angle (
2f1-f2) is given
by
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The corrected DPOAE phase angles for successive f1 and DPOAE
frequencies then were unwrapped by a computer program and displayed. In
some cases, for lower primary levels, the phase change between two
consecutive data points was >180° and therefore the unwrapping is
ambiguous. If in such cases a more gradual phase change could be
observed for higher sound levels, the low level data were unwrapped to
yield an overall phase change similar to that observed at higher levels. The group delay of the 2f1-f2 DPOAE
(T2f1-f2) was calculated from the
phase change (2f1-f2) versus frequency
change (
f2f1-f2) of successive data
points (Kimberley et al. 1993
; Mills and Rubel 1997
), where
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RESULTS |
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Frequency dependence of basilar membrane displacement at constant SPL
BM displacement measurements were made in nine individuals of
P. parnellii by directing the beam of the interferometer
through the transparent round-window membrane. From lesions made after each experiment, it was discovered that the measurements reported in
the present study were made from the SI region of the cochlea. Examples
of tone-evoked BM displacement recorded from a position between 35 and
40% of the BM length with respect to the base of the cochlea (see Fig.
4, inset) are shown in Fig. 4
for frequencies between 20 and 90 kHz and levels between 50 and 100 dB
SPL. This region is near the apical extent of the SI region, and
primary afferent neurons in this region have characteristic frequencies between 63 and 65 kHz (Kössl and Vater 1985a). For
low-level stimuli (50 dB SPL), the response bandwidth is very narrow
and centered around the resonance frequency of 61.125 kHz
of the individual bat. This is slightly lower than the SFOAE of 61.99 kHz in the awake animal. At 70 dB SPL, a second peak appears at the
characteristic frequency of the recording location (64.2 kHz). With
increasing level, the region of response extends toward higher
frequencies with peak responses to both 61 and 65 kHz. At the highest
levels, there is a sharp high-frequency cutoff ~74 kHz. In addition,
for levels <100 dB SPL, there is a sharp low-frequency cutoff of the response close to 60 kHz. The region of insensitivity extends down to
~50 kHz. BM displacements were recorded in response to frequencies
<50 kHz and extended at least down to 20 kHz, which was the lowest
stimulus frequency used in these experiments. In response to tones at
100 dB SPL, the low-frequency cutoff and the region of insensitivity
disappeared.
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Basilar membrane displacement as a function of sound level
It soon became apparent that each measurement point in the SI
region responded sharply and sensitively to the SFOAE frequency and to
a higher frequency (characteristic frequency) at which the response
threshold reached a second minimum. The frequency of this second
minimum corresponds to the characteristic cochlear frequency place as
determined from a cochlear frequency map that is based on horseradish
peroxidase labeling of the afferent innervation of IHCs
(Kössl and Vater 1985a) (Fig. 1A). The
displacement of the BM was plotted as a function of sound level for
tones at frequencies close to and at the SFOAE frequency as an initial
step in constructing BM iso-displacement frequency-tuning curves.
Examples of the compressive nonlinear level-functions are shown in Fig.
5 for a single preparation for
frequencies around the resonance frequency (61.25 kHz; Fig. 5A) and the characteristic frequency (64.2 kHz; Fig.
5B). The level functions are typically saturating, nonlinear
curves with initial slopes close to unity and are most sensitive for
frequencies close to the resonance frequency and the characteristic
frequency. They are relatively insensitive and do not saturate for
levels <90 dB SPL for frequencies ~1 kHz above and below the
resonance frequency (Fig. 5A). The curves become compressive
again 2 kHz below the characteristic frequency and remain compressive
for frequencies 2 kHz above the characteristic frequency (Fig.
5B).
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Basilar membrane frequency tuning curves
Iso-response frequency-tuning curves for four different animals are shown in Fig. 6. They are derived from displacement-level functions such as those shown in Fig. 5. The response criteria are different for each preparation and are taken just above the noise floor of the measurements. Three of the isoresponse curves (Fig. 6, A-C) are characterized by a sharp threshold minimum at the resonance frequency, which was between 60.45 and 61.56 kHz in six preparations. At frequencies within 1 kHz below the frequency at the tip of the tuning curve there is a strongly insensitive region. Within 1 kHz above the frequency of the tip there is a high-frequency shoulder, which may appear as a distinct lobe (see Fig. 6, A and B). Where extended frequency measurements have been possible (Fig. 6, A-D), distinct threshold minima were observed at frequencies corresponding to the characteristic frequencies of the recording locations, which were between 63 and 67 kHz in these experiments. The tuning curve shown in Fig. 6D was from a relatively insensitive preparation and was measured at a higher response criterion than the other curves. The characteristic resonance peak was absent, and the curve probably reflects the mechanical frequency response of the BM at the 67-kHz recording site.
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Tuning curves Fig. 6, A-C, have sharp peaks for frequencies close to the SFOAE with very steep high and low frequency slopes. The sharpness of tuning is defined by the Q10 dB (the center frequency divided by the bandwidth 10 dB above the tip). For measurements of the resonance in five preparations in the SI region, the Q10 dB varied between 102 and 430. In four preparations where such measurements were possible, the low frequency slope of the tuning curve varied between 2,042 and 6,892 dB/octave (mean = 4,657 dB/octave) and the high-frequency slope was less steep and varied between 1,960 and 3,706 dB/octave (mean = 2,735 dB/octave). It was possible to measure the Q10 dB of the characteristic frequency from three preparations and these were 11 (CF = 65.0 kHz), 25 (CF = 64.2 kHz), and 6 (CF = 67 kHz).
Basilar membrane iso-level magnitude and phase responses
Iso-level magnitude and phase functions at levels between 55 dB SPL and 95 dB SPL for a single preparation are shown in Fig. 7. With increasing level, the peak response to the resonant frequency becomes larger and broader and moves toward lower frequencies as is also apparent in Fig. 4. The response envelope tends to expand toward higher frequencies and toward the region of the characteristic frequency of the measurement site. The resonance is associated with a phase transition (a phase lag), which is seen in the accumulated phase curves in Fig. 7 right. The phase lag, measured from the curves shown in Fig. 7 is 181.7 ± 10.3° (mean ± SE). Similar measurements in a second preparation yielded 183.0 ± 17.9°. The rate of change of phase in degrees per kilohertz is largest for low sound levels and amounts to 480°/kHz at 55 dB SPL. With increasing levels, the rate of phase change decreases and is 150°/kHz at a level of 95 dB SPL. If one extrapolates the slope of the phase change down to lower sound pressure levels, then at 20 dB SPL, the slope values would be ~1,200°/kHz.
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The phase of BM displacement as a function of frequency for different levels is shown in Fig. 8A, and BM phase as a function of level for frequencies around the resonance frequency is shown in Fig. 8B for the preparation shown in Fig. 7. It is apparent from both Fig. 8, A and B, that the low-level responses lead high level responses for frequencies below the resonance frequency, that the phase of low-level responses tends to lag the higher-level responses at the resonance frequency (61.6 kHz) and at frequencies above.
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DPOAE measurements
Phase changes associated with mechanical cochlear resonance are apparent in the SFOAE recordings (Fig. 3A). In these measurements, however, the observed phase change in the interference pattern is subject to interaction between the OAE and the stimulus. Therefore the full range of phase change around the resonance frequency cannot be obtained unambiguously from SFOAE data.
If one uses a signal that is generated by nonlinear transduction in the
cochlea itself and that does not coincide with the stimulus
frequencies, then the phase behavior of the mechanical resonance is
determined more easily. For this reason, we have used the 2f1-f2 DPOAE
to probe cochlear mechanics. In the examples given in Fig.
9, the frequency of f2 was adjusted to a
value between 63 and 70 kHz, which, according to the BM frequency map
of the mustached bat, should be represented in the specialized SI
region. By changing f1, the DPOAE frequency was swept across the
frequency range of the cochlear resonance. Close to the SFOAE
frequency, maxima and minima of the DPOAE levels coincided with a steep
phase change of approximately 360° (Fig. 9, bottom). For
f2 frequencies between 62 and 66 kHz and for low stimulus levels (f1,
f2 at 30/20 or 40/30 dB SPL), the steepest changes of distortion phase
ranged between 1,600 and 2,900°/kHz. For louder stimulus levels (f1, f2 at 50/40 or 60/50 dB SPL), the phase changes were more gradual and
amounted to 800-1,500°/kHz. The frequency of the steepest phase
transition moved to lower values with increasing stimulus levels.
Within the range of levels used in the experiments (30/20-60/50 dB
SPL), this shift of the frequency of steepest phase change could reach
maximum values of 370 Hz (Fig. 9A). The same behavior is
also evident in the group delay maxima associated with the steep phase
transitions. For stimulus levels of 40/30 dB SPL, group delay maxima
of
8 ms were measured (Fig. 9, top). This compares with
average values of 0,15-0,87 ms outside the range of the steep phase
transitions.
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Level-dependent changes in the resonance frequency, determined from the
phase transitions and the group delay maxima, could be observed for f2
frequencies between 62 and 64 kHz. For f2 frequencies in the range
between 65 and 66 kHz, the measured resonance frequency was level
independent and stayed constant within the measurement frequency
resolution of 122 Hz. In addition, for f2 frequencies of 74 kHz (not
shown), phase transitions could occur when the distortion was close to
the SFOAE frequency that were similar to those observed at lower f2
frequencies. Such discontinuities associated with the SFOAE could not
be measured for f2 frequencies >74 kHz.
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DISCUSSION |
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Measurements of BM vibrations in the SI region of the cochlea,
where the echoes produced by CF calls are preprocessed, reveal that
each measurement location is tuned to the characteristic frequency of
the location (62-72 kHz) and in addition resonates at the echo
frequency (61-62 kHz), as indicated by the frequency of the SFOAE. The
level and frequency dependency of the BM vibrations measured at
ultrasonic frequencies in the bat cochlea are very similar to those
that have been measured in the basal, high-frequency turns of the
cochleae of nonecholocating mammals to tones within their auditory
response range (e.g., Cooper and Rhode 1992;
Murugasu and Russell 1995
; Nuttall and Dolan
1996
; Rhode and Robles 1974
; Robles et
al. 1986
; Ruggero et al. 1997
; Russell
and Nilsen 1997
; Sellick et al. 1982
). Thus with
increasing sound levels, the BM responses in the bat initially grow
with a slope close to unity and saturate at high levels. BM level
functions obtained at frequencies close to the resonance or to the
characteristic frequency of the measurement location, are most
sensitive and are strongly saturating. With increasing level, the
resonance broadens, the center frequency moves to lower frequencies,
and the phase changes associated with the resonance become more gradual
(Fig. 7). These findings lead us to conclude that the SI region of the
mustached bat cochlea transduces and amplifies mechanical signals in
exactly the same way as in nonecholocating mammals. Our findings do not
resolve the current controversy over the involvement of OHC motility in the amplification, compression, and sharpening of BM vibrations (Dallos and Evans 1995
; Hudspeth 1997
;
Kolston 1995
). However, if OHC motility is the basis for
cochlear sensitivity and frequency resolution, then it appears to be
able to act at ultrasonic frequencies. From measurements in isolated
OHCs in the guinea pig, there is also disagreement over the upper
frequency limit of OHC motility. While Gale and Ashmore
(1997)
have found that OHC motility has a frequency limit of
~25 kHz, Frank et al. (1999)
have measured a corner
frequency of 79 kHz in isolated OHCs.
Mechanical versus neural responses from the SI region
The responses of brain stem neurons with characteristic
frequencies at or slightly below the SFOAE frequency are extremely narrowly tuned, with Q10 dB values
between 100 and 400, while those with characteristic frequencies within
the response range of the SI region (62-72 kHz) are more broadly tuned
and are less sensitive with Q10 dB
values of ~20 (Kössl and Vater 1990; Suga and Jen 1977
). Furthermore unlike the BM responses in the SI
region, the neurons are tuned only to the characteristic frequency and not also to the resonance. On the basis of the BM frequency-place map
derived from labeling afferent dendrites (Kössl and Vater 1985a
) and from our measurements, the BM responses at each
measurement location in the SI region are tuned to tones within the
frequency response range of the SI region. On the three occasions we
were able to determine them, the Q10
dB values of the mechanical tuning curves were similar to
those of the nerve fibers and were at 6, 11, and 25, respectively. In
addition to being tuned to their characteristic frequency, each
measurement location within the SI region was found to resonate sharply
to frequencies close to the SFOAE. The Q10
dB values of the resonance varied between 102 and 430, which is within the range of the Q10
dBs measured from the auditory nerve fibers tuned to just
above the CF2 frequency but less than the Q10
dB of 610, which was measured previously (Kössl and Russell 1995
) from a sensitive
preparation. Furthermore the slopes of the BM tuning curves and of
neurons tuned to just above the CF2 frequency are similar with the
low-frequency slope being steeper than the high-frequency slope.
Although the mechanical tuning of the BM in the SI region recorded in
anesthetized animals is as sharply tuned to the neural tuning recorded
in awake animals to tones at both the resonance frequency and to tones
within the frequency response range of the SI region, it is less
sensitive. Several factors may contribute to the insensitivity of the
micromechanical responses observed here. Anesthesia and opening the
middle ear can reduce the sensitivity of OAEs by 30 dB SPL (see Fig.
3), and one might expect a similar reduction in the sensitivity of BM
vibration. In all recordings we have made from the SI region, the BM
vibrates most sensitively to frequencies close to the SFOAE. The
frequency of resonance, which was estimated from the peak BM
displacements and the midpoint of the associated rapid phase-change, is
a few hundred hertz below that of the SFOAE recorded in the awake
animal. This discrepancy is probably due to anesthesia because it is
known that the SFOAE can shift downward by
500 Hz when the animal is
anesthetized (Kössl and Vater 1985a
).
Thus the most noticeable difference between the BM and neural responses
in the SI region is that the nerve fibers are tuned only to their
characteristic frequency, whereas the BM is tuned to the characteristic
frequency of the measurement location and to the resonance. One
explanation for this difference is that the specialized BM and TM in
the SI region have two modes of vibration, one of which is in the
transverse plane and the other in the radial plane. Accordingly, when
driven by tones at the resonance frequency, the TM and BM vibrate
together in the transverse plane (Steele 1997). The TM
and the BM move as a single mass, radial shear between them is minimal
(e.g., Geisler and Sang 1995
; Markin and Hudspeth 1995
) and hence there is no neural excitation. However, when
excited by tones at frequencies in the response range of the SI region, it is suggested that the TM and the BM vibrate as a two-mass system so
that there is relative shear between them. The shear displaces the IHC
and OHC sensory hair bundles which ultimately leads to neural
excitation (Davis 1965
).
Evidence for standing wave resonance in the SI region: acoustic distortion products
From measurements of the 2f1-f2 acoustic distortion product it
has been proposed that the TM in the SI region is a mechanical resonator that is sharply tuned to the SFOAE frequency throughout its
length (Kössl and Vater 1996). With the exception
of f2 frequencies between 65 and 66 kHz, the distortion phase, group
delay and resonance frequency are labile and level dependent (Fig. 9).
In the frequency range of the resonance, the distortion phase change
amounts to 360°. With increasing stimulus levels, this phase change
becomes less steep, the group delay becomes smaller and the resonance frequency shifts downwards. These findings also are reflected in the BM
resonance (Fig. 7) where the phase change is 180°. However, distortion at the resonance frequency measured when the site of distortion generation is located between the 65- and 66-kHz places on
the BM (as determined by the f2 frequency) is remarkably resistant to
level change (Fig. 9). The special characteristics of this location are
also apparent from two other observations. The SFOAE is most sensitive
to acoustic suppression by tones centered ~65 kHz (Kössl
and Vater 1985a
), and suppression tuning curves for the 2f1-f2
distortion product at the SFOAE frequency, where the primaries are
located within the SI zone (62-72 kHz), are bilobed with minima near
the SFOAE frequency and between 65 and 66 kHz (Frank and
Kössl 1995
). Thus the resonance evoked at the 65- to
66-kHz location is particularly stable as a function of level but very
sensitive to suppression by tones at the frequency of the site. These
properties might be explained if the 65- to 66-kHz location represented
a node in a standing wave resonance present in the SI zone.
Generation of a standing wave resonance in the SI zone
If one views the BM and TM as strings, spanned in the longitudinal
direction between the apical and basal extents of the SI region, then
the TM string may be regarded as being fixed at both ends because the
attachment of the TM to the spiral limbus increases abruptly at the
extreme extents of the SI region (Fig.
10). In contrast, the BM string in the
SI region may allow free movement at both ends because the arcuate
thickening, and the longitudinal fibers within it, are limited in
extent to the SI region. The longitudinal fibers also will tend to
promote longitudinal mechanical coupling between adjacent
regions of the BM in the SI zone. It is proposed that reflections are
generated at the two points of morphological, and probable mechanical
discontinuity (as indicated by the sharp response cutoffs at 60 and 74 kHz in BM iso-level responses, Fig. 4) at the apical and basal extents
in the SI region of the cochlear duct, leading to intracochlear
pressure changes that generate standing-wave-like oscillations at a
frequency close to 62 kHz in the SI region. Either the TM or the BM, or
both together, could be the primary source for the resonance. In view
of the morphological considerations described in the preceding text, one might expect standing waves to be generated differently in the two
structures (Fig. 10). On the basis of the neural tuning and the
characteristic frequency responses of the BM in the SI region (Fig. 6),
the BM in this region is tuned tonotopically to frequencies between 62 and 72 kHz, whereas the TM appears to be tuned exactly to 62 kHz over
the whole length of the SI zone. Therefore we suggest that the TM
resonance drives the basilar membrane and that OHCs are critical for
the interaction of both membranes. This view is supported by the
observations that the BM resonance is labile (Kössl
and Russell 1995) and level dependent and disappears in
insensitive preparations (Fig. 6D). Similarly, BM
displacements recorded in the SI region are insensitive to frequencies
close to 65.5 kHz (Fig. 5). These observations may indicate that a
standing wave node, either in terms of displacement or velocity, is
located in this region.
|
Faulstich and Kössl (1997) and Kössl and
Vater (unpublished results) have observed that the effects of
aminoglycoside antibiotics and anesthetics on P. parnellii,
which are presumed to selectively interfere with OHC function or can
lead to their destruction (see Rybak 1986
for a review),
can severely attenuate DPOAEs and cochlear neural responses by
60 dB
SPL. However, during aminoglycoside treatment, evoked OAEs that are
associated with cochlear resonance decrease in bandwidth and increase
in level. Under anesthesia, these emissions even can convert to
spontaneous OAEs. This surprising observation may indicate that,
through negative feedback, the OHCs in the SI region act to stabilize
the mechanical properties of the cochlear partition and to reduce
evoked and spontaneous OAEs at the resonance frequency.
Thus it appears that at the resonance frequency the cochlear partition
in the SI region behaves as a single-mass system with feedback from
OHCs in phase with BM motion. Under these circumstances, OHC feedback
actually can damp BM vibration rather than augment it (e.g.,
Markin and Hudspeth 1995). Hence the removal of OHC feedback after treatment with aminoglycosides might be expected to
increase the amplitude and decrease the bandwidth of evoked and
spontaneous OAEs associated with the cochlear resonance.
Cochlear resonance: product of an acoustic laser?
On the basis of our measurements of the resonance in BM vibrations
and acoustic distortion, we suggest that the frequency spectrum of the
cochlea's mechanical response to the CF2 echo frequency (~62 kHz),
at the CF2 echo place, is sharpened, fine tuned, and stabilized through
feedback from a standing wave resonance generated in the SI region. The
resonant vibrations in the SI zone cause the build-up of large
intracochlear pressure changes at the CF2 echo frequency
place at the transition region between the SI zone and the
CF2 region. In this region, the spiral ligament with its tension
fibroblasts (Henson and Henson 1988) is enlarged greatly, probably to prevent damage to the organ of Corti and BM when
this energy is released (see Kössl and Vater
1996
). If vibration of the OHC hair bundles results in
electromechanical feedback of energy to the cochlear partition, as it
does elsewhere in the mammalian cochlea (reviewed by Dallos
1992
), then this would contribute to local frequency tuning at
the CF2 echo place. In addition, the local electromechanical feedback
of energy by the OHCs also could pump energy into the cochlear
resonance in the SI region, which in turn feeds back to the CF2 echo
place. This pumping of energy is analogous to that produced by a diode laser with an extended cavity where the pumping of energy into the
cavity provides frequency-selective feedback to reduce the laser's
line width and to improve its tunability (Bosheir et al. 1991
). Fine structural features in the SI region may be
responsible for setting the subtle frequency differences related to the
age and sex in calls and SFOAEs of the mustached bat.
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ACKNOWLEDGMENTS |
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We thank J. Hartley for designing and constructing electronic apparatus and M. Faustich, A. Lukashkin, G. Richardson, and M. Vater for discussion and critical comments of an early version of the manuscript. We are grateful to the Jamaican Natural Resources Conservation Authority for their permission to export mustached bats.
This work was supported by grants from the Deutsche Forschungsgemeinschaft, the Medical Research Council, and the Royal Society.
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FOOTNOTES |
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Address reprint requests to I. J. Russell.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 4 January 1999; accepted in final form 26 March 1999.
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REFERENCES |
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