The operation of the cochlea has principally been stied using simple stimuli such as tones and/or clicks. Thisprovided an enormous amount of information about tcomplicated nonlinear system. However, because the cocis nonlinear, one cannot predict the response to a nostimulus in a straightforward manner~e.g., Rhode, 1971; Sellick et al., 1982; Robleset al., 1986!. If one wishes to un-derstand processing of complex sounds more representof the acoustic ecology, it is necessary to present stimwhose spectra are more realistic than those of clickssingle tones. In particular, two-tone and multitone supprsion effects are studied here.
Although the response of the auditory nerve~AN! tovarious stimuli largely reflects basilar-membrane responsthe same stimuli, it would be beneficial to employ the sastimulus set for basilar-membrane studies that has beento study AN behavior, as a way to understand the natureAN responses. Some commonly used stimuli for the studyAN function are amplitude-modulated~AM ! signals and har-monic complexes~Javel, 1980; Joris and Yin, 1992!. Theresponses of auditory-nerve fibers~ANFs! to AM signals in-dicate an enhancement of modulation depths over a rangfrequencies and intensities.
Horst et al. ~1986, 1990! studied the representation omulticomponent~N54 to 64! octave band stimuli centereat the characteristic frequency~CF! of AN fibers in the cat.
3140 J. Acoust. Soc. Am. 110 (6), December 2001 0001-4966/2001/1
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They observed that edges of the stimulus spectrum wdominant in the response with increasingN, and response tothe center component was reduced under these conditSimilar behavior is observed at the level of basilamembrane mechanics forN as low as 3, and it appears thfrequency spacing of stimulus components is the mostportant factor~Rhode and Recio, 2001!.
Two-tone suppression has been studied at the levethe auditory nerve~e.g., Sachs, 1969; Javel, 1981; Javet al., 1978; Javelet al., 1983; Delgutte, 1990!, in cochlearmechanics~e.g., Rhode, 1977; Ruggeroet al., 1992; Cooperand Rhode, 1996b; Nuttall and Dolan, 1993!, and via hair-cell recording~Sellick and Russell, 1979; Cheatham and Dlos, 1990; Nuttall and Dolan, 1993!. Results indicate thamuch of the behavior seen in the auditory nerve arises aresult of cochlear mechanics. However, there have beenited cochlear mechanical studies using more complex stim~cf. Recio and Rhode, 2000!. To help reduce that limitationsingle-, two-, three~AM !-, five-, and seven-tone stimuli werused in the present study to further explore nonlinear effeat the level of basilar-membrane vibration.
II. METHODS
Methods are essentially those detailed in CooperRhode~1992!. Seven chinchilla cochleas were studied at aproximately the 3–4-mm location@characteristic frequency~CF!56.5–8.5 kHz#. All procedures were approved by thAnimal Care and Use Committee of the University of Wiconsin.
Each animal was anesthetized with sodium pentobarbusing a dose rate of 75 mg/kg. Additional smaller doses wadministered to maintain the animal in a deeply areflexstate. All anesthetics were administered intraperitoneallytracheotomy was performed to ensure an open airway. Athe ear was surgically removed, four screws were implanin the skull and fixed with dental cement, forming a rigbase. A bolt was then cemented at the base to providstable fixation of the skull to a head holder that has sixgrees of freedom for positioning the cochlea under thecroscope.
The bulla was opened widely and a silver ball electrowas positioned so as to touch the round window forpurpose of recording the compound action potential~CAP!of the auditory nerve in response to short-duration tones~16ms! for each animal. Because we recorded only in the hifrequency region of the cochlea, the stimulus was steppe2-kHz increments from 2 to 20 kHz. At each frequencyvisual detection threshold forN1 was determined by viewingan average of 20 repetitions as the stimulus level was vain 1-dB steps. If thresholds were above our best threshcurve by more than 20 dB, no data were collected. High Cthresholds equated to little or no compression in the hregion ~Sellick et al., 1982!. CAPs were not typically re-corded after mechanical measurements were initiated exto verify that they increased whenever mechanical sensitidecreased. Any change in mechanical sensitivity was taas a sign of a deteriorating cochlea.
The overlying cochlear bone was shaved down usinmicrochisel until the remaining tissue and/or bone debcould be removed with the microelectrode pick. Gold-coapolystyrene beads 25mm in diameter served as retrorefletors. They were placed in the perilymph and allowed to sto the basilar membrane. They have a specific gravity of 1that should minimize any loading of the basilar membraneglass cover slip was placed over the opening with no hydmechanical seal. Perilymph wicked up to the glass,beads could then be imaged without detrimental effectsthe measurements that occur through an unstable interbetween the perilymph and air.
An opening in the bony ear canal, immediately over ttympanic membrane, was made so that the probe tube12 -in. Bruel & Kjaer condenser microphone could be visalized as it was positioned parallel to the tympanic mebrane within 1 millimeter of the tip of the malleus, the prowas fixed in place with dental cement, forming a closed-fiacoustic system. The opening was sealed with a glass cafter a 45-mm bead was placed on the tympanic membranthe tip of the malleus~or umbo!. The bead was used asretroreflector for the interferometer and allowed measument of the transfer function of the malleus. The sousource was a RadioShack supertweeter dynamic phone fostimuli except two-tone stimuli. In the latter case, reverbiased1
2-in. condenser microphones that were compensafor frequency flatness were used.
A. AM stimuli
AM signals were synthesized and presented using a Tsystem~Tucker-Davis Technologies®) system@Eq. ~1!#
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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The carrier frequency,f carr, was set equal to the characteritic frequency of the basilar membrane. The modulation fquency was varied in 100-Hz steps from 100 to 1000 Hz, aabove 1 kHz was set to 1250, 1500, and 2000 Hz. The molation coefficient (m) was set to 0.5, 1.0, or 2.0 wherem52 results in three equal-amplitude sine tones. Stimulevel was varied from 0 to 90 dB SPL in 5-dB steps. Stimwere 30 ms in duration, 8 times at a rate of 10/s. A cosenvelope with 1-ms rise/fall time was used for all stimuAmplitude and phase were determined using a phase-locloop technique that computes dc and the Fourier compon~sine wave fit, abbreviated as sinfit!.
B. Multicomponent stimuli
A subset of these stimuli consisted of five equamplitude sinewaves with the first, third, or fifth componefrequency set equal to the CF of the basilar-membrane lotion under study. The frequency separation of stimulus coponents was varied in steps of 100 Hz up to 1 kHz and se1250, 1500, and 2000 Hz above 1 kHz.f mod.2000 Hz wasnot explored because ANFs do not show any AM tempocoding for this condition. Stimulus level was varied fromto 90 dB SPL in 5-dB steps. The stimulus level for mulcomponent stimuli always refers to the level of the cen~CF! component. Stimuli were 30 ms in duration, repeatetimes at a rate of 10/s. Analysis consisted of determiningamplitude of nine response components around CF usingsinfit procedure described above. The amplitude at theference frequency was also determined but was insignificor buried in the noise except for high levels and large fquency separations.
Another stimulus consisted of seven equal-amplitusine waves with the middle component always set equaCF. These stimuli had the same difference frequencies asfive-component stimuli and were analyzed in the same mner except that 11 components were analyzed using the stechnique. The level of the center component is specithroughout the paper.
The component amplitudes of each stimulus used~three,five, and seven-component stimuli! were compensated by thacoustic calibration.
C. Two-tone stimuli
Two-tone suppression~2TS! data were collected in ordeto provide a comparison for suppression effects due to mtiple components. These data were also important for demining whether suppression effects are compatible withlarge body of literature on 2TS in cochlear mechanics. Ttwo-tone paradigm consisted of a 30-ms probe stimulusCF and a 30-ms suppressor tone delayed 10 ms relativthe probe onset. Generally, eight repetitions of the combtion were presented at a rate of 10/s. The probe tone wasconstant while input–output~I/O! functions were collectedas the suppressor tone was varied in 5-dB SPL steps fromaximum to a minimum~usually 90 to 0 dB SPL!. Thesuppressor frequency was then changed and the proces
3141and A. Recio: Multicomponent basilar-membrane suppression
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FIG. 1. Single- and two-tone responses for a basalchlear region with CF58000 Hz. Symbol key: solidsymbol5x kHz, e.g., B52 kHz; open symbol5x dBSPL, e.g.,b52 times 10520 dB SPL.~A! Single-toneamplitude I/O functions at the indicated frequenciwith only a subset of the data shown for clarity. Symbexception:j511 kHz. ~B! Normalized isolevel curves~sensitivity5amplitude at 1 Pascal frequency transffunctions! derived from the I/O curves shown in pane~A!. A two-tone suppression study is shown in pane~C! through ~F!. ~C! Probe amplitude I/O curves thahave been normalized to 0 dB for a 20-dB SPL supprsor to emphasize probe suppression as a functionsuppressor level; probe level540 dB SPL and only athird of the I/O curves are shown.~D! Isolevel normal-ized probe amplitude curves derived from the I/curves in panel~C!. This is the normalized probe amplitude versus frequency function representation of tdata in panel~C! for the full data set.~E! Suppressoramplitude I/O curves taken at the same time as the din panel~C!. ~F! Normalized isolevel suppressor ampltude curves derived by subtracting the single-tone din panel ~B! from the isolevel suppression amplitudeobtained from the I/O functions in panel~D! for thecondition when the level of the probe is 40 dB SPThe arrow indicates the frequency of the probe tonChinchilla Ct17.
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peated. The probe level was varied in 10-dB steps. I/O futions for 20–40 suppressor frequencies were presenteeach probe level with closer frequency spacing near CF taway from it. A Hamming window~Rabiner and Gold, 1975!was applied to the response before analysis occurred.
D. Single-tone basilar-membrane and middle-earvibration measurements
Basilar-membrane input–output~I/O! functions were de-termined using 30-ms tone bursts with 1-ms raised-cosrise and fall times and presented every 100 ms. The stimlevel covered a 100-dB SPL range in 5-dB SPL stepsminimum of eight basilar-membrane and four middle-earsponses were averaged for each stimulus condition. Anaconsisted of Fourier decomposition of the steady-statetion of the averaged response at the stimulus frequency. Msurements of the basilar-membrane I/O function at CF wmade throughout the experiment to monitor preparationbility. Vibration of the ossicles was measured at the tip ofmanubrium~umbo! or at the incudo–stapedial joint or botlocations before and/or after basilar-membrane measments.
E. Displacement measurements
Mechanical responses were measured using a cusbuilt displacement-sensitive heterodyne laser interferom~Cooper and Rhode, 1992!. The laser was coupled to thpreparation using a long working distance lens~Nikon
3142 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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SLWD 5X, NA 0.1!. The laser was focused to a spot;5 mmin diameter on the reflective beads. The interferometer wnot sufficiently sensitive to measure basilar-membrane vibtion without the gold-coated beads. Instantaneous phasemeasured using two single-cycle phase meters that workequadrature. Phase meter outputs were sampled at 250and the phase was unwrapped using custom software.sponse amplitudes were corrected for the frequency respof the recording system. The noise floor was,5 pm/AHz.
III. RESULTS
A. Two-tone interactions
Two-tone studies were conducted along with threfive-, and seven-tone studies. They provide data neededetermine whether or not suppression or distortion effectsmultitone stimuli are predictable from two-tone results. Twtone studies involved the collection of displacement-le~I/O! functions during simultaneous presentation of a proand suppressor tone. The probe tone was always set tobest frequency of the basilar-membrane location under st
Prior to presentation of any complex stimulus, tsingle-tone response was collected in order to determineCF of the region under study. A portion of the I/O functionfor one study is shown in Fig. 1~A!, where the symbol at-tached to each I/O function indicates the stimulus frequein kHz. I/O functions are linear for frequencies,6 kHz.Nonlinear growth in amplitude of basilar-membrane vibrtion occurs for frequencies.6 kHz. The slope of the I/O
Rhode and A. Recio: Multicomponent basilar-membrane suppression
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function, or growth rate, equals 1 for a linear system. Growrates,1 dB/dB are characteristic of a nonlinear compresssystem, and greater than 1 for an expansive nonlinearityexpansive nonlinearity is seen at 10 kHz for leve.90 dB SPL, just beyond the notch in the I/O functiowhich is accompanied by a rapid change in the respophase~;180°, not shown!. Compression at CF (58 kHz)begins around 20 dB SPL with a growth rate of 0.36~dB/dB!between 40 and 70 dB SPL.
The complete set of I/O functions can be recast as aof transfer functions normalized by stimulus level@Fig. 1~B!#where deviation from linearity is seen by a lack of superpsition of the individual isolevel functions for frequencieabove 6 kHz. At CF, a reduction in sensitivity of;45 dBoccurs as sound pressure is increased from 30 to 100SPL. Response amplitude to a CF tone was 1.5 nm at 20SPL and was among the best of this experiment series~0.25,0.3, 0.4, 1.0, 1.4, 1.5, 1.6 nm at 20 dB SPL!. A significantportion of differences in amplitude is likely due to the vaability in the radial location of the basilar-membrane mesurement, as there is limited control on where the reflecbead falls. The maximum displacement is a function ofradial location of the bead on the basilar membrane.
In a two-tone suppression~2TS! paradigm, the motion inresponse to the probe tone~40 dB SPL at CF58 kHz! isreduced as the suppressor level is increased beyond a cethreshold@a subset of I/O functions is shown in Fig. 1~C!#.Suppressor frequencies near the probe frequency exhibilowest suppression thresholds,,40 dB SPL or slightly be-low the level of the probe. As suppressor frequencycreases, suppression threshold increases and the amousuppression decreases. As the suppressor frequencycreases below CF (5probe frequency) the rate of suppresion, measured as the slope of the curves in Fig. 1~C!, ap-proaches 1 dB/dB. The rate of suppression decreasesuppressor frequency increases above CF. These lattertions are seen best in Fig. 1~D!, where probe suppressionshown as a reduction of probe amplitude versus supprefrequency with suppressor level the parame(probe level540 dB SPL). When the suppressor level is ne40 dB SPL the first signs of suppression are seen invicinity of CF. As the suppressor level is increased, the raof frequencies that suppresses the probe widens, andamount of suppression increases to;30 dB for suppressolevels .80 dB SPL. Because probe amplitude is monotocally decreasing with increasing suppressor level, it ispothesized that probe suppression would continue to incrwith higher suppressor levels than were used here. Simmaximum amounts of suppression~25–35 dB! were obtainedin the other six preparations.
An alternate method of expressing the interaction of ttones is to plot the amplitude I/O functions of the supprestones@Fig. 1~E!# corresponding to the probe I/O function@Fig. 1~C!#. Compared to the single-tone I/O [email protected]~A!# these are more linear and the notch in the 10-kfunction is not present. Mutual suppression of the probethe suppressor response components can be seen by subing the single-tone isolevel curves shown in Fig. 1~A! fromthe suppressor isolevel curves obtained from the data in
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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1~E! with the result shown in Fig. 1~F!. The 40-dB SPLprobe tone at CF is shown to have a suppressive effect onsecond tone that extends from 6 to 11 kHz, nearly an octaand can be seen to suppress the second tone by more thdB at 9.5 kHz. As suppressor level is increased, suppresdue to the probe tone is reduced in both amplitude andquency extent. The probe tone at CF has its strongestpressive effect for the cochlear region basal to its locatwith the location of the maximum effect increasing with supressor level. At this point it should be obvious that tterms, probe/suppressor, are merely a convenience sincethe probe and the suppressor produce suppression oother component with the caveat that both must reside incompressive region of the basilar membrane for mutual spression to occur.
Two-tone neural suppression in the auditory nervebeen extensively studied~e.g., Sachs, 1969; Javelet al.,1983; Delgutte, 1990!. One typical display method foauditory-nerve 2TS I/O functions is to examine the effecta suppressor tone at several levels on the I/O function atfor a nerve fiber. This AN paradigm was used in one studythe mechanical correlate of neural 2TS when the suppretone was held constant while the I/O function at CF(56.5 kHz) was measured~Fig. 2!. The typical rightwardshift observed in the rate-level AN I/O functions with increasing suppressor level is also seen in the mechanicafunctions for all suppressor frequencies. Larger shifts weach level increment occur for suppressor frequencies lothan CF@4 and 6 kHz in Figs. 2~A! and~B!, respectively#. Asthe I/O response function shifts to the right with increasisuppressor level, the probe amplitude response stays lineproportionally higher levels. In all other studies, the suppr
FIG. 2. I/O curves that are similar to those collected from the auditory nein a two-tone suppression paradigm. Probe amplitude versus probewith suppressor level varied (CF56500 Hz). The suppressor frequencyindicated as a parameter in each panel. Probe level was varied in 5-dBsteps. Responses with amplitudes less than228 dB re 1 nm are removedfrom the display since they were below the system noise level. Cb057.
3143and A. Recio: Multicomponent basilar-membrane suppression
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sor level was decremented in 5-dB steps from 90 to 0SPL to produce the I/O functions while the probe level wheld constant. Results for one of these studies shown in3 are presented in the same format as in Fig. 2, althoughprobe levels are fewer~six versus 19! and also the range olevels is smaller than that in Fig. 2~20 to 70 vs 0 to 90 dBSPL!. The rightward shift of the probe amplitude function fa 10-dB suppressor level increase is as much as 18 d3000 Hz or a rate of suppression of 1.8 dB/dB~Fig. 3,CF58 kHz!. The magnitude of the shift to the right of thprobe response curves increases with decreasing supprfrequencies. The shift is relatively constant, usua;1dB/dB, once the suppressor frequency is in the linregion of the basilar-membrane response. Probe ampliI/O functions are linear to higher levels as the suppreslevel is raised. In general, this mechanical 2TS behavclosely parallels comparable neural 2TS data.
Three views of two-tone suppression are presentedFig. 4 for three probe levels~30-, 50-, and 70 dB SPL inrows 1, 2, and 3, respectively!. Suppression thresholds of a8-kHz, 30-dB SPL probe by a near-CF suppressor areequal or even lower (;20 dB SPL) levels than probe leve@Fig. 4~A!#. With increasing suppressor level, the frequenextent of suppression increases until all lower frequencsuppress the probe. On the high-frequency side of the prsuppression occurred at least up to 15 kHz, the highestquency suppressor that was employed. It appears thatpression continues with further increases in suppressorquency, though there is a relatively small reduction ofprobe. Near CF there is a ‘‘break’’ in the isolevel curves flevels .60 dB SPL ~indicated by the solid line below thabscissa! that is a result of leakage in the frequency analy
FIG. 3. I/O curves that are similar to those collected from the auditory nein a two-tone suppression paradigm. Data were collected in the madescribed in the Methods section: Probe I/O functions were collected wthe suppressor level was stepped in 5-dB SPL increments. The supprlevel is indicated on each curve and the suppressor frequency is giveeach panel. Data are from the same experiment as the data shown inwhere CF58 kHz ~Ct17!.
3144 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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As the probe level is increased to 50 and 70 dB S@Figs. 4~B! and ~C!, respectively#, the suppressor level thajust begins to suppress the probe remains roughly 10 dBthan the probe level. The ‘‘break’’ in the 30-dB SPL isolevcurves near CF doesn’t occur when the probe is at higlevels (.40 dB SPL). To avoid possible damage of the cchlea by stimulus levels.95 dB SPL, the suppressor levenever reached 40–50 dB above the probe level, whichnecessary to see the break. For probe levels between 2030 dB SPL the suppressor level can be as much as 60greater than the probe level. Another effect of increasprobe level is that the suppressor frequency that produmaximum suppression moves to lower values.
Iso-suppression curves for 1, 10, and 20 dB of supprsion are compared to the single-tone 1-nm iso-amplitucurve @Figs. 4~D!, ~E!, and ~F!: dashed line51-nm iso-amplitude curve#. Iso-suppression curves lose their ‘‘tip’’ aprobe level increases and become nearly constant upcutoff frequency. It is also apparent that the high-frequenside of the suppressive region extends beyond the 1-nmamplitude region. The single-tone 1-nm iso-amplitude regcan be viewed as the excitatory region for an AN fiber wthe same CF, essentially a frequency threshold curve.
Slopes of the probe I/O functions@cf. Fig. 1~C!# versussuppressor frequency are shown with suppressor levelparameter@Figs. 4~G!, ~H!, and~I!#. These slopes define thrate of suppression and~for the convenience of using thsame ordinate scale! are compared to the negative of thsingle-tone I/O growth rates where rates,1 dB/dB definethe nonlinear or compressive region@column 3, Figs. 4~G!,~H!, and~I!, single-tone data at 70 dB SPL indicated by tdashed line#. Suppression rates approach21 dB/dB for suf-ficiently high suppressor levels in the frequency regiwhere single-tone growth rates are 1 dB/dB, i.e., the linregion of basilar-membrane single-tone response. Incompressive region around CF, the suppression rate isdB/dB and decreases to 0.2 dB/dB at 11.5 kHz. Suppresrate decreases as growth rate decreases and is not eqzero even at 15 kHz, a frequency that is beyond the sintone response region. It is noteworthy that suppressioncurs for frequencies.11 kHz, even though the single-tondisplacement is less than 0.1 nm for levels,70 dB SPL.Above 70 dB SPL the single-tone vibration is complicatedthe fact that there is a notch in the 10-kHz I/O function, athe compression slope is negative, zero, or greater thadB/dB depending on the stimulus level. The fact that theresuppression in what has traditionally been labeled the plaregion of the basilar-membrane transfer function~e.g.,Rhode, 1971; Cooper and Rhode, 1996a! suggests that thevibration of the basilar membrane is influenced by tonnearly an octave above CF and possibly more.
Perhaps a more telling relationship is the relatisuppressor-to-probe amplitude that results in a fixed redtion of the probe across both frequency and probe level~Fig.5!. This relation shows that the suppressor amplitudequired to produce 1 dB of suppression is near that ofprobe amplitude for frequencies up to CF and is nearly in
eerlesorin. 1
Rhode and A. Recio: Multicomponent basilar-membrane suppression
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FIG. 4. Suppression from several perspectives. Column 1: Isolevel suppression curves for 5-dB increments in level~10-dB levels are indicated with anumbered symbol: e.g., 30 dB SPL510 timesc while the probe is at 30, 50, and 70 dB SPL in rows 1, 2, and 3, respectively. Column 2: Suppressornecessary to reduce the probe amplitude21, 210, or220 dB ~labeled solid lines!. The 1-nm iso-amplitude curve for a single tone is repeated in each p~dashed line!. Column 3: The suppression rates for the three probe levels at 10-dB SPL increments~solid lines and numbered symbols!. The negative of theslope of single-tone I/O curves~defined as compression! at 70 dB SPL is superimposed in each panel~dashed line,G! where21 dB/dB is linear and 0 dB/dBis complete compression~Ct17!.
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pendent of the probe amplitude for frequencies,CF/2. Be-yond CF, the required suppressor amplitude decreasesidly and can be as small as 1% of the probe amplitude. Tlatter number is intriguing and suggests that the probeaffected by what is happening in the region basal to 8 kThis is consistent with the fact that there is suppression ewhen the suppressor frequency is beyond the single-toneper cutoff frequency for a particular place. In order to reduthe probe amplitude by 12 dB, the suppressor amplitudeto be increased over 20 dB. The suppressor-to-probe relaas a function of frequency is seen to be similar for all prolevels employed.
B. Multitone suppression
Suppression effects in BM responses to AM stimwere studied by analyzing the amplitude of the CF comnent of the basilar-membrane response to an AM signal rtive to the single-tone response. From results shown in6, it is clear that there is a systematic increase in probe spression with increasing modulation depth. There is onlmodest amount of suppression~1–2 dB! at 50% modulation,for which the amplitudes of sideband components are 12less than the carrier amplitude@Fig. 6~A!#. However, sup-
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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pression increases for 100% modulation, in which casesideband components amplitudes are 6 dB less than caamplitude @Fig. 6~B!#. While suppression is dependent oboth the stimulus level and the modulation frequency,
FIG. 5. Suppression amplitude is given in dB relative to probe amplitude1- and 12-dB suppression of the probe. Probe level was varied from 30 tdB SPL ~thin lines and indicated by symbols!. The 30- and 70-dB SPLsingle-tone isolevel curves are superimposed~thick lines! for comparison.CF58 kHz, Ct17.
3145and A. Recio: Multicomponent basilar-membrane suppression
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relation is not straightforward across stimulus level. Thisclearest when the AM carrier and sidebands are equaamplitude~200% modulation! where a nonmonotonic relation between suppression and level exists for modulationquencies,1 kHz @Fig. 6~C!#. While suppression increasewith level from 20 to 70 dB SPL, suppression decrearegardless of modulation frequency for stimulus levels ab70 dB SPL~Fig. 7!. This inflection in I/O curves at CF maresult from a linearization of the I/O functions with lev.75 dB SPL.
Suppression of the CF component during presentatiofive-component stimuli behaves somewhat similarly to tfor AM stimuli. In this instance, there is a strong dependenon which of the five components is positioned at CF~Fig. 8!.
FIG. 6. Suppression of the CF component versus modulation frequwhen the stimulus is an amplitude-modulated signal with modulation deof 50%, 100%, and 200% for panels~A!, ~B!, and~C!, respectively. Ct17.
FIG. 7. Probe suppression I/O curves when the modulation depth is 200a function of the level of the center~carrier! component.Fmod is indicated bythe symbols, e.g.,c5300 Hz; A51500 Hz. Line thickness increases witincreasingf mod.
3146 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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With the first component placed at CF and the remainfour components higher in frequency than CF, suppressiorelatively small,,4 dB for small frequency separation of thcomponents at 60-dB SPL stimulus level. Maximum supression occurs when the third~center! component5CF @Fig.8~B!#. In this case, suppression can reach 10 dB for modtion frequencies as high as 800 Hz with the maximum occring at 70 dB SPL. Suppression decreases for frequeseparations .800 Hz. Finally, when the fifthcomponent5CF and the remaining four components alower in frequency than CF, suppression reaches 7.5 dBfrequency separation of 400 Hz and decreases rapidlylarger frequency separations@Fig. 8~C!#. Suppression is in-creased somewhat over the case when the first componeset to CF but less than when the third componfrequency5CF. Suppression as a function of level when tstimulus is centered on CF is similar to that of the AM stimlus ~Fig. 9!. Suppression increases with stimulus levels up70 dB SPL and decreases for higher levels. Suppressioinversely proportional to frequency separation of the comnents. These results replicate those from previous studiesuppression in that energy on the low side of CF is meffective in suppressing CF tones than energy of higherquency.
Mutual suppression effects are present whenever themore than one stimulus component. Such effects aresented in Fig. 10 for the case when component frequeseparation is 500 Hz for two, three, five, and sevecomponent equal-amplitude stimuli. The amplitude of eacomponent is shown in relation to the single-tone amplitu~dot-dash line!. At the 30-dB SPL level~c!, stimuli are inthe linear response region and track the single-tone cu
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asFIG. 8. Suppression of the CF component is shown when the stimconsisted of five-equal amplitude sinewaves. The first, third, or fifth coponent is set equal to the CF in~A!, ~B!, and~C!, respectively. Ct17.
Rhode and A. Recio: Multicomponent basilar-membrane suppression
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For two tones at 30 dB SPL there is nearly no suppressioeither component regardless of whether the suppressorquency is above~solid line! or below~dashed line! the probetone ~at CF58000 Hz!. When the level of the tones is increased to 60 dB SPL~f!, there is a mutual suppression;2 dB. The low-frequency component is least reduced,expected, based on having the more linear I/O functionthe individual components. That is, the rate of growthslope of the I/O function decreases as the frequencycreases in the compressive region. When three eqamplitude stimuli are presented@200% modulation, Fig.10~B!#, a similar result occurs with the amount of mutusuppression increasing relative to the two-tone case. Thesults for the five- and seven-component cases@Figs. 10~C!
FIG. 9. Probe suppression I/O curves as a function of the level of the tcomponent when the third component of the five-component stimulupositioned at CF. Frequency spacing of the components indicated bysymbols, e.g.,c5300 Hz. Line thickness increases with increasing spacfrequency.
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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and ~D!# establish a trend of increasing mutual suppresswith an increase in the number of components.
While there will always be some AN fibers for whicCFs are located at the center of a complex stimulus,majority will have CFs displaced relative to the frequencenter of the stimulus. This latter condition was exploredpositioning the first, third, or fifth component of the fivecomponent signal at CF in order to examine the differenresponse to these conditions~Fig. 11!. Even at 30 dB SPLthere is some suppression of individual components weither the third~dashed line! or first ~thin solid line! compo-nent is at CF where suppression is measured by the deviafrom the single-tone response~dot-dash lines!. Lower-frequency (,CF) components are least suppressed in ecase. When stimulus level is increased to 60 dB SPL, theras much as 30-dB suppression of the higher-frequency cponents~first5CF, thin solid line! with the CF componentsuppressed the least. With the third-component frequeequal to CF~dashed line! there is still 5–6-dB suppression oindividual components with the lowest-frequency componnearly equal to the single-tone result. Finally, when the fifcomponent frequency equals CF~thick solid line!, mutualsuppression is the smallest of the three conditions, andlowest-frequency component is even slightly higher in aplitude than the tone-alone condition.
C. Unequal-amplitude components
Most natural stimuli are not composed of equaamplitude components. Mutual suppression effects wstimulus components were not equal were studied for Astimuli with modulation,200%. Such stimuli are analogouto a three-formant stimulus in speech signals. Whenf mod is
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FIG. 10. Component response ampltudes for four multicomponent stimulat probe levels of 30 and 60 dB SPwhen the difference frequency is 50Hz and CF58000 Hz. Single-toneisolevel functions~dash-dotted lines!also at 30 and 60 dB SPL are superimposed. Number of stimulus components is varied: two, three, five, anseven in panels~A! through ~D!, re-spectively. ~A! Response amplitudesfor the two components when the second frequency is CF1500 Hz ~solidline! and CF2500 Hz ~dashed line!.~B! Response amplitude for the threcomponents of an AM stimulus for200% modulation ~three equal-amplitude components!. ~C! Responseamplitudes for the five components othe five equal-amplitude stimuluswhen the third component5CF. ~D!Response amplitudes of the sevecomponents when the fourth component frequency5CF. ~A!, ~B!, and~C!data from Ct17.~D! data from Ct23.
3147and A. Recio: Multicomponent basilar-membrane suppression
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,200 Hz @e.g., Figs. 12~A!, ~E!, ~I!# the response to eacsideband is about equal. There is an increase in suppresof both sidebands with increasing AM level up to 70 dB Sand a decrease with further level increases. Maximum spression of sidebands by the carrier is;8 dB @Fig. 12~A!#.Suppression is reduced as modulation depth [email protected]~E!, ~I!#.
As f mod increases beyond 200 Hz there is a divergeof the fate of the upper and lower sideband responses.suppression of the lower sidebands occurred while atsame time the upper sideband response was attenuatecochlear filtering@Fig. 12~B!#. Again, there is less sidebansuppression at higher modulation depths@Figs. 12~F!, ~J!#. Ata modulation depth of 200%@Fig. 12~J!# the lower sideband
FIG. 11. Response amplitudes when the stimulus consists of five eqamplitude components at levels of 30 and 60 dB SPL. The compowhose frequency is set to CF~8000 Hz! is component one~solid thin line!,component three~dashed line!, or component five~thick solid line!. Com-ponent frequency difference is 500 Hz. Single-tone data~dashed-dottedlines! at the indicated levels are also shown.~Ct17!.
3148 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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amplitude is greater than the carrier amplitude due tosuppression of the carrier. With higherf mod the amplitude ofthe lower sideband grows faster than the other AM comnents, while the upper sideband amplitude is often 20–30less than the lower sideband. These responses essenconsist of two components: the lower sideband and therier response.
Mutual suppression of stimulus response componentdependent on the relative amplitudes, frequency content,position of the acoustic spectra relative to the characterifrequency of each cochlear location. A near-CF componhas the largest effect when it is larger than the surroundcomponents. When the AM modulation frequency is 100and carrier frequency set at CF, response amplitudes arin the linear range of basilar-membrane I/O curves for lstimulus levels. Therefore, no suppression is observed~Fig.13, 20 dB SPL!. When the stimulus level is at 70 dB SPLthe sidebands are reduced relative to their expected amtudes given the input spectra. The largest reduction occfor a modulation depth of 25%, where there is a substandeviation from the expected amplitudes. The magnitudethe deviation is reduced as modulation depth is increafrom 50% to 100%@Figs. 13~B! and ~C!#. Nevertheless, ineach instance the carrier had a suppressive effect on theband components. In effect, the response spectra w‘‘sharpened’’ up. For stimulus levels greater than 80 dB Sthe I/O curves begin to approach linearity and the respospectrum is no longer sharpened. When the input spectconsists of equal-amplitude components (m52), the carrierno longer suppresses the sideband response componenfact, for small modulation frequencies the carrier is supressed by the sidebands@Fig. 13~D!#. This spectral edgeenhancement effect similar to Mach bands in vision w
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FIG. 12. Sideband suppression by thcarrier for an AM basilar-membraneresponse is shown when the modultion depth was 50%, 100%, and 200%in columns 1, 2 and 3, respectivelyf mod is indicated in the left-hand panefor each row. The dashed lines marthe amplitudes of the sidebands relative to the carrier in the stimulus. Theamplitude difference in basilar-membrane response between thlower/upper sideband amplitudes anthe carrier amplitude is indicated bythe solid/dotted lines.
Rhode and A. Recio: Multicomponent basilar-membrane suppression
ortion ofdtortion. TheCt
FIG. 13. The response spectrum for AM signals withf mod5100 Hz, and modulation depths of 0.25, 0.5, 1.0, and in panels~A! through~D!, respectively. Thecarrier is set to CF58000 Hz. In each panel the response spectra are shown for nine harmonics of 100 Hz at two levels: 20 dB SPL in the linear pthe I/O curve and 70 dB SPL in the compressive or nonlinear region of the response.d symbols indicate the data points andj indicate the expected sidebanamplitudes based on the input spectrum. The additional three values on each side of the AM response spectrum are a result of nonlinear discochlear filter is essentially flat over this 200-Hz range of input frequencies and its equivalent linear system should not alter the input spectrum.07.
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shown previously for stimuli with 5 and 7 componen~Rhode and Recio, 2001!.
D. Phase functions
AN discharge phase has been shown to exhibit a lag/relative phase relation for stimulus frequencies below/abCF with increasing stimulus level~Andersonet al., 1971!.Basilar-membrane phase relations are largely compawith the neural observations. In the chinchilla midfrequenrange, phase relations appear to be somewhat more comAt low stimulus levels the results are compatible; phase lwith increasing level for stimulus frequencies below CF, aphase leads above CF@Fig. 14~A!#. However, for stimulusfrequencies above CF, increasing the level above 70 dBresults in increasing phase lags, as there are phase lagsstimulus frequencies at the highest stimulus levels. Phdata can also be seen for all stimulus frequencies in14~B!. For stimulus levels below 70 dB SPL there is littphase change at CF~8000 Hz!, but increasing phase lagoccurred at higher levels. As much as a 130° phase lagcurred below CF and;90° phase lead above CF.
In the presence of a second tone, phase behaviosomewhat similar but is a function of the relative levels afrequencies. A common stimulus paradigm in a 2TS expment is to place the probe tone at CF and to vary the spressor level and frequency while holding the probe fquency and level constant. The probe phase for a low-leprobe exhibits a small lead with level for most suppresfrequencies@Fig. 15~A!#. At higher probe levels, there armostly phase lags with increasing suppressor level regardof suppressor frequency@Figs. 15~B!, ~C!#. The phase of thesuppressor behaves similarly to the single-tone phasefunction of level and frequency@Figs. 15~D!, ~E!, ~F!#. One
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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difference is that as the probe level increases~.45 dB SPL!only phase lags are seen, regardless of level or frequencgeneral, there is not much difference between probe andpressor phases.
FIG. 14. Phase portion of the basilar-membrane mechanical transfer ftion where CF58000 Hz.~A! Phase I/O functions for selected frequenciefrequency51000 times the symbol number;d57500 Hz and j
58500 Hz. The phase at 25 dB SPL was used to normalize each I/O cu~B! Phase versus frequency functions at the levels indicated by the symlevel510 times symbol number. Phase at 20 dB SPL was subtractedeach curve to normalize them. Ct17.
3149and A. Recio: Multicomponent basilar-membrane suppression
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FIG. 15. Phase I/O functions for boththe probe and the suppressor tonethe level of the probe or suppressorvaried for a select set of suppressofrequencies. Symbol assignment fosuppressor frequency as in Fig. 11~A!, ~B!, and ~C!. Probe phase as afunction of frequency is shown whenthe probe level~Lp! is at 30, 50, and70 dB SPL, respectively.~D!, ~E!, and~F!. Suppressor phase I/O functiowhen the probe levels are 30, 50, an70 dB SPL, respectively.
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Phase-versus-frequency transfer functions illustratephase lag/lead relation when the suppressor is the 8-kHzat several levels@Figs. 16~A!, ~B!, ~C!#. Increasing suppressor level reduces the phase variation with probe [email protected]~C!#. There is little variation for probe levels,70 dB SPL@Figs. 16~B! and~C!#. In the right column, the probe level iheld constant at the level indicated in each panel andsuppressor level is varied. There is little phase changelow levels of the probe@Fig. 16~D!# while for high levels thelag/lead relation is present. In the latter case@Fig. 16~F!#,increasing the suppressor level reduces the phase vari~cf. the d andg curves!.
IV. DISCUSSION
A. Principal achievements of this study
~1! New two-tone suppression data are provided with gredetail than previously available.
~2! These data support the view that neural 2TS phenomlargely reflect cochlear mechanics as observed in thebration of the basilar membrane.
~3! Information is provided about mutual suppression: spression of a CF tone by a roaming suppressor andpression of a roaming probe by a fixed tone at CF.
~4! Spectral edge enhancement of multicomponentsponses was observed in the vibration of the basmembrane~cf. Horst et al., 1986!. That is, suppressionof the middle component~s! of a multicomponent stimu-lus by surrounding components for three, five, and seequal-amplitude component stimuli is described.
~5! Suppression of individual components is describedan amplitude-modulated signal when the sideband cponents are 0,26, 212, and218 dB smaller than thecarrier.
3150 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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~6! Phase relations are described that are not entirely cgruent with earlier observations.
This study was undertaken to determine the represetion of a subset of multicomponent stimuli in the motionthe basilar membrane~cf. Rhode and Recio, 2001!. Howseveral stimulus components interact and mutually suppeach other has implications for understanding the procesof complex stimuli, such as speech and music, withincochlea. Any cochlear nonlinearity, such as the I/O functfor hair cells or the half-wave rectifier at the hair cell syapse, will result in suppression. However, the principal bafor suppression is the frequency-dependent compressivelinearity in the cochlea that results in differential growrates of vibration in response to tones. The compressivegion resulting from a tone at CF extends over an octa~Rhode and Recio, 2000; Russell and Nilsen, 1997!. Re-sponse to tones above CF is increasingly compressed asquency is increased until the stimulus exits the compresregion and enters what has been labeled the plateau regiothe transfer function for the basilar membrane~Rhode, 1971;Cooper and Rhode, 1996a!. This above-CF linear region habeen associated with a second mode of vibration that cosponds to the fast wave~e.g., Olson, 1998; Rhode and Reci2001; Cooper and Rhode, 1996a!.
B. Two-tone suppression: Slopes and magnitude
Ruggeroet al. ~1992! state that neural two-tone rate supression appears to originate in mechanical phenomenthe level of the basilar membrane. However, there remasome difficulty in explaining two-tone rate suppression whthe suppressor is lower in frequency than the CF of thefiber because the sum of the individual amplitudes of basimembrane vibration in response to the two tones is alway
Rhode and A. Recio: Multicomponent basilar-membrane suppression
mber of
FIG. 16. Probe phase versus frequency transfer functions for a suppressor tone at 8000 Hz and the levels indicated in each panel.Lp5probe level andLs
5suppressor level in dB SPL.~A! 30-dB SPL suppressor at 8000 Hz. Probe level510 times the symbol number.~B! 50-dB SPL suppressor.~C! 70-dB SPLsuppressor. Right column.~D!, ~E!, and~F!. Probe phase at probe levels of 41, 61, and 81 dB SPL, respectively. 8-kHz suppressor at 10 times the nuthe symbol in dB SPL andd57500 Hz,j58500 Hz. Ct17.
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least as large as that of the CF tone alone~Cooper, 1996;Geisler and Nuttall, 1997; also data reported here such afrequencies below CF in Fig. 5!. The complete mechanism otwo-tone rate suppression in the AN under these conditiremains unexplained and likely requires an additiomechanism that provides another stage of filtering atlevel of the hair cell or auditory nerve~Cai and Geisler,1996; Temchinet al., 1997!. A proposal has also been madfor direct influence of cochlear potentials on excitationafferent dendrites as a cause for suppression~Hill et al.,1989!.
For high side suppressors~i.e., suppression of a probstimulus by a tone whose frequency is higher than CF!, dis-placement of the basilar membrane in the presence of apressor tone can be less than that to the probe tone a~Ruggeroet al., 1992; Cooper and Rhode, 1996b!. Also, itwas shown here that suppression can occur when suppramplitude is only 1% of that of the probe tone~Fig. 5!. Thisresult suggests that at least high-side suppression is atially distributed phenomenon, because the suppressorhave an effect even when the suppressor tone excitationnot or barely overlaps with the region for the probe tonThis result agrees with others who have noted that for spressors above CF, a region of the cochlea is likely involv~Yateset al., 1989; Geisleret al., 1990; Geisler, 1992!. Thisresult also implies that cochlear models have to incorpo
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more than independent point representations of the cochpartition. The spatial extent and variation of the nonlinearas a function of suppressor frequency has not typically badequately addressed in these models~cf. Baker, 2000!.
Present results are largely compatible with previoussults ~Cooper, 1996; Nuttall and Dolan, 1993; Geisler aNuttall, 1997; Ruggeroet al., 1992!. Suppression magnitudes of up to 35 dB were found over the rangesuppressor/probe levels that were employed. There wereeral limitations imposed by the stimulating and measuremapparatus:~1! maximum stimulus levels were limited t,100 dB SPL either by the desire to avoid either temporor permanent threshold shifts;~2! limitations in maximumstimulus levels that could be produced; and~3! the limitedrange of the interferometer. Finally, the use of relativeshort ~20-ms overlap of the 30-ms probe and supprestones! tones limited the accuracy of analysis becausewidth of the Fourier filter results in leakage between compnents. Leakage of a large component into another analcomponent was seen whenever:~1! the amplitude of the suppressor was 25 to 30 dB greater than that of the probe;~2!the suppressor frequency was near to that of the probe;~3! at low levels of the probe. It is likely that larger suppresion magnitudes than those measured (;35 dB) occur in thecochlea, as there is no reason to believe that the monotdecreasing probe I/O functions do not continue as the s
3151and A. Recio: Multicomponent basilar-membrane suppression
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pressor level increases. For example, the probe ampliI/O functions in Fig. 3~B! exhibit a 40-dB rightward shift fora 40-dB suppressor level shift. Such shifts are in line wthose recorded in AN fibers~Javel et al., 1983; Delgutte,1990!.
A number of auditory-nerve studies have shown spression slope to vary with suppressor frequency relativprobe located at CF~Javelet al., 1983; Ruggeroet al., 1992;Delgutte, 1990!. Above CF, the neural suppression slope vies from 20.2 dB/dB and increases as the suppressorquency decreases so that when it is an octave lower thanthe average slope ranges between21 and22 dB/dB. In anextensive study of the suppression growth, Delgutte shothat it varied from 0.5 to over23 dB/dB. The larger valuesoccur for suppressors one to several octaves below CF.found up to21.8 dB/dB for a suppressor frequency,CF/2@Fig. 3~A!#. Mechanical suppression slopes appear toclosely correlated with growth rate of basilar-membrane mtion. Mechanical suppression growth rates appear to beficient to explain those observed in AN for two-tone supprsion. Because a second stimulus component inmechanical linear region produces a suppression rate21 dB/dB, the shift in the CF neural curve should be pportional to the reciprocal of the growth rate at the profrequency. For example, if the growth rate at CF was 0dB/dB, then the neural curve should shift 3 dB/dB. Thvalue is at the upper end of the neural shifts found by aone. It is likely that there are several factors that enter iproducing the scatter in the neural data, but the most pronent is the way the mechanical growth rate varies withFor CFs,1 kHz the mechanics exhibits much less comprsion than in the frequency region.5 kHz. Also, varyingneural thresholds and gain functions of the hair cells leadfurther spread in the neural growth rates.
C. Suppression thresholds
There is some controversy about the level at whichlow-side suppressor takes effect. Cooper~1996! noted thatrelatively constant suppressor amplitudes are necessaestablish the suppression threshold in guinea pig. This cclusion draws support from the studies of Schmiedt~1982! ingerbil AN, where the lower threshold boundary of a 2TS abelow CF is nearly absolute in level over a large rangefiber CFs. Temchinet al. ~1997! found AN modulation andrate thresholds to be very similar at;70 dB SPL in chin-chilla. In the present study, for a 30-dB SPL probe level,21-dB iso-suppression curve is nearly identical to the 1-isolevel curve for frequencies,CF and a suppressor level o56 dB SPL. This compares to the 1.5–3 nm found for a 1-iso-suppression in the guinea pig at suppressor levels80–90 dB SPL~Cooper, 1996!. It was also found that thesuppressor level necessary to produce a 1-dB reduction inprobe amplitude increases as the probe level increasesless than proportional rate~;12-dB increase for a 40-dBincrease in probe level!. This latter response is expected bcause the growth rate of the CF response is;0.3 dB/dB ofinput; therefore, a 40-dB change in the input results in12-dB change in the output. At low-stimulus levels, the supression amplitude to produce a 1-dB suppression is appr
3152 J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S.
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mately the same as the probe amplitude~1 nm!. It is worthnoting that suppression thresholds vary 10–20 dB ewithin an animal~Schmiedt, 1982!.
Neural rate thresholds do not appear to be explicitlylated to a fixed displacement of BM because they can varmuch as 80 dB at a given CF in individual cats~Liberman,1978; Cooper, 1996!. However, in young cats Libermafound that the spread of thresholds at a given CF was fanarrow, 10–20 dB. The spread in thresholds could be largdue to hair-cell and AN synapse morphological differencthat are correlated with spontaneous rate in ANFs~Liberman,1978!. This suggests that the underlying reason for thecrease in spread with age is alterations in postprocessinbasilar-membrane motion such as changes in the physiocal condition of hair cells. Such a hypothesis is difficultprove given the difference between the experimental contions for mechanics, hair-cell recording, and auditory-nerecording. It is entirely possible that there would be closagreement if all procedures were conducted under a siregimen.
D. Mutual suppression
Mutual suppression was shown to occur with two-tostimulation@Figs. 1~D! and~F!#. With a tone at CF, suppression occurs with a second tone varying in frequency fromarbitrary low value to as much as an octave above CF. Spression slope decreases rapidly for frequencies aboveand the maximum suppression! also decreases. When thtone at CF is considered the suppressor, suppression isited to a smaller frequency region, e.g., 6–11 kHz or roug60.5 octave. Mutual suppression observed in inner hair-recordings has been suggested as a mechanism that resusharpening the cochlear filter~Cheatham and Dallos, 1990!.
E. Phase effects
Suppression by a high- or low-side suppressor resulta phase change in the BM response to a CF probe. Thesome controversy in the literature as to behavior of phasthe probe tone as a function of suppressor tone level~Cooper,1996!. There are studies showing phase leads with increasuppressor level~Rhode and Cooper, 1993; Cooper, 199!;however, others show phase lags~Ruggeroet al., 1992!. Fig-ure 14 showed that the relative phase change seen inprobe phase is nearly always an increasing phase lag of u90° as the suppressor level increases regardless of supprfrequency. However, for relatively low probe levels and supressor frequencies just below the probe frequency (5CF),there is initially a phase lead, never.90°, that turns into aphase lag as high suppressor levels are attained. Concomwith the changes in probe phase are changes in the phathe suppressor that are also a function of probe levelsuppressor frequency~Fig. 15, column 2!. The increase inphase for probe level at 30 db SPL occurs for suppresfrequencies above CF, while all tones with frequencies beCF show phase lags with increasing suppressor level.probe levels.40 dB SPL, there is only an increasing phalag with increasing suppressor level regardless of freque~Fig. 14!.
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Cooper~1996! reported phase leads with increasing supressor level in the vicinity of CF~;26-kHz cochlear regionin the guinea pig! of similar magnitude to those presentehere, although at higher suppressor lev(.80 dB SPL). He also found that 2TS phase changes wsimilar to those that occurred when single-tone levels wincreased by an amount that caused an appropriate decin BM response sensitivity. This difference could be duespecies difference and/or cochlear location.
AN studies of 2TS have demonstrated that rate/lefunctions shift to the right with increasing suppressor le~e.g., Javelet al., 1983!. Based on cochlear mechanical stuies, the underlying explanation is likely that the cochlepartition exhibits the same behavior~Figs. 3 and 4 in Nuttalland Dolan, 1993!. These observations address the hypothethat suppression is equivalent to a simple attenuation ofmotion of the basilar membrane; however, one has to csider phase behavior of the two tones. For high-side suppsors Nuttall and Dolan found that the hypothesis is valid,they noted that this conclusion differs from that obtainedhair-cell recording in the apex of the guinea pig, wheCheatham and Dallos~1989, 1990! found a phase lead during suppression, opposite to observations in the base. Itsuggested that there could be different suppression menisms in apex and base. The mechanics in these two reghave considerable similarity~Cooper and Rhode, 1992Rhode and Cooper, 1996! as there is a compressive nonliearity in each region and isolevel vibration curves are vsimilar to auditory-nerve frequency threshold curves. Inapex, however, there is a smaller amount of cochlear amfication that exists over the entire frequency response ofapical region.
F. Multiple tones and suppression
Suppression has been demonstrated psychophysiusing the pulsation threshold method with vowel-like soun~Houtgast, 1974!, for multiple-component stimuli~4–64! inan octave centered at the characteristic frequency ofauditory-nerve fiber~Horst et al., 1990!, and for multicom-ponent stimuli such as speech in the auditory nerve~Sachsand Young, 1980!. It has been suggested that suppresscould be a way for the formants in speech signals toenhanced relative to the neighboring components. Here,pression in BM responses to AM stimuli is shown todependent on modulation frequency, modulation depth,stimulus level. The amount of suppression of CF responincreased with level and modulation depth. At any of tmodulation depths studied, the largest amount of suppresoccurred for stimuli with modulation frequencies arou1500 Hz, and was smallest at either the lowest~100 Hz! orlargest ~2000 Hz! modulation frequencies. However, thmagnitude of carrier suppression does not vary greatly afunction of modulation frequency.
There are prominent mutual suppression effects thatcur for small modulation frequencies~see Fig. 13!. Thegreatest suppression of the sideband response occurredf mod5100 Hz and for small modulation depths~m,0.5!.This had the effect of sharpening the response spectwhen the response components were in the nonlinear po
J. Acoust. Soc. Am., Vol. 110, No. 6, December 2001 W. S. Rhode
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of the basilar-membrane I/O curve. It was also shown whAM components were all equal~m52! that sideband re-sponse amplitudes were greater than the carrier. This speedge enhancement also occurs when the number of comnents is either five or seven and the component frequeseparation is small~,300 Hz; see Rhode and Recio, 2001!.The effect is analogous to the Mach band in vision~Carter-etteet al., 1969!.
The fact that the response amplitudes are a compfunction of frequency separation, modulation depth, astimulus level was demonstrated in Fig. 12. Withf mod and/orlevel increases the lower sideband suppresses the carriein effect results in an overmodulated response~Rhode andRecio, 2001!. This is due to the fact that the lower sidebais located in the linear portion of the cochlear response whthe growth rate is 1 dB/dB and the carrier is at CF and isthe compressive region where growth rate is;0.3 dB/dB.Therefore, the lower sideband grows faster than the carand must drive the outer hair cell into its saturated reg~Geisler, 1992!. The result is that the CF response componis reduced. The upper sideband of the AM stimulus is largeliminated by the cochlear filter and hence does not plasignificant role.
As the number of components increases@Fig. 8~B!#, theamount of suppression measured in responses tocomponent stimuli with a frequency separation of 1500 Hzapproximately the same as the one measured using 2AM stimuli @Fig. 6~C!#. However, five-component stimulproduced greater suppression at lower frequency separa~700 Hz! than measured in AM responses with similar coponent frequency separation. Greatest suppression ocwhen more stimulus components are located relatively clto CF ~but not too close!.
Multitone suppression can result in larger suppresseffects than those produced by two tones when componare symmetrically placed around CF. Results suggestmutual suppression between signal components is nolarge as might be expected based on summing two-tonepression amplitudes. In fact, the net result of multiple tonon suppression of the probe tone can be less than fortones in some circumstances. Suppression is dependenthe number, level, and frequency composition of the stimlus. Low-frequency~,CF! suppressors at high levels produce the greatest suppression. At low stimulus levels, thecomponent has the strongest suppressive effect possiblycause all the other components are in the linear regionbasilar-membrane vibration.
V. SUMMARY
There is overall agreement that suppression largoriginates in cochlear mechanics. The present results rforce this conclusion and further show the basilar-membrrole in the processing of multitone stimuli. The effect is pevasive and a complex function of stimulus features: comnent amplitudes, number of components, frequency seption, and distribution of the components relativecharacteristic frequency. This effect has implications for hcomplex signals such as speech are processed even bthey enter the central auditory system.
3153and A. Recio: Multicomponent basilar-membrane suppression
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ACKNOWLEDGMENTS
This work was supported by the National InstituteDeafness and Communications Disorders, Grant No. RDC 01910. Special thanks are given to C. Dan Geisler, MaRuggero, and Keith Kluender for reviewing an earlier vsion of this manuscript. We also thank Wiebe Horst andanonymous reviewer for their comments.
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Rhode and A. Recio: Multicomponent basilar-membrane suppression
