Two-tone suppression in cochlear mechanicsNigel P. Coopera)Department of Neurophysiology, University of Wisconsin—Madison, 275 Medical Sciences Building,1300 University Avenue, Madison, Wisconsin 53706~Received 28 February 1995; accepted for publication 15 January 1996!Mechanical responses to one- and two-tone stimuli were recorded from the basilar membrane ~BM!in the hook region of the guinea-pig cochlea. The most sensitive or ‘‘best’’ frequencies ~BFs! for thesites studied were ;25–30 kHz. Two-tone suppression ~2TS! of the responses to near BF probetones was noted using suppressor tones either above or below the BF. Rates of growth of 2TS werehighest ~;1 dB/dB! when the suppressor tones were presented below the BF. Below-BFsuppression thresholds ~the suppressor intensities causing ;10% reduction in the probe-evokedresponses! corresponded to BM displacements of ;1–5 nm. Above-BF suppression thresholdscorresponded to much smaller displacements at the location studied. Both above- and below-BFsuppressor tones changed the phase of the probe tone responses in the same way that increases inthe probe tone intensity did ~they evoked small phase-lags for below-BF probes, and smallphase-leads for near- and above-BF probes!. Low-frequency suppressor tones ~,;7 kHz! evokeda frequency- and intensity-dependent mixture of phasic ~ac! and tonic ~dc! suppression. Peak ~ac!suppression was observed around the times of peak BM displacement ~not velocity!. These findingsare discussed in relation to those of other workers. © 1996 Acoustical Society of America.PACS numbers: 43.64.Kc, 43.64.NfINTRODUCTIONThe peripheral auditory system transduces airbornesounds into a robust and meaningful neural code ~for anoverview see Pickles, 1992!. Many of the mechanisms in-volved in the sound-transduction process perform nonlinearsignal transformations, and some show a strong dependenceon the frequency of the stimuli being processed ~for reviewsee Patuzzi and Robertson, 1988!. The combination of strongfrequency-dependence and nonlinearity can lead to complexinteractions in the system’s response to many sounds. Onesuch interaction is demonstrated in the phenomenon of two-tone suppression ~2TS!—the responses evoked by one tonecan be reduced quite substantially when a second tone ispresented at the same time. Nonlinear interactions of thistype are of potential importance in many areas of hearingresearch ~e.g., in the peripheral encoding of speech stimuli—see Sachs and Young, 1980; and in determining the maskingeffects of background noise—see Rhode et al., 1978!.The most detailed studies of 2TS in the auditory periph-ery have been performed at the level of the cochlear nerve~originally by Sachs and Kiang, 1968; for more recent refer-ences see Delgutte, 1990!. However, 2TS has also been ob-served in the electrical responses of individual inner andouter hair cells ~Sellick and Russell, 1979; Cheatham andDallos, 1989, 1990, 1992; Russell and Ko¨ssl, 1992; Nuttalland Dolan, 1993!, and in a limited number of cochlear me-chanical studies ~Rhode and Robles, 1974; Rhode, 1977;Patuzzi et al., 1984; Robles et al., 1989; Ruggero et al.,1992; Nuttall and Dolan, 1993; Rhode and Cooper, 1993;Cooper and Rhode, 1995!. These observations ~combinedwith others! have led some researchers to the view that many~if not all! of the frequency specific nonlinearities of theperipheral auditory system originate in the mechanics of thecochlear partition ~see Ruggero, 1992 for review!.The present study provides further data relating to the2TS which occurs in cochlear mechanics. The major goal isto clarify two discrepancies in the existing literature on basi-lar membrane ~BM! 2TS, and hence to further our under-standing of the cochlea in general.The first discrepancy concerns the changes in responsephase which occur when suppressor tones are presentedabove and below a preparation’s best frequency ~BF; themost sensitive frequency for a particular location on theBM!. Nuttall and Dolan ~1993! reported that below-BF sup-pressors induced phase leads in the BM’s response to anear-BF probe tone, while above-BF suppressors inducedphase lags. Robles et al. ~1989! and Ruggero et al. ~1992!also reported small phase-lags at the BF when usingabove-BF suppressors. In contrast, Rhode and Cooper ~1993!reported both above- and below-BF suppressors to inducephase leads ~Ruggero et al., 1992, also reported phase leadsto occur in a few experiments, but only for above-BF sup-pressors of above-BF tones!. Most of the probe tones used inthe experiments performed to date have been presented at afrequency ~nominally referred to as the BF! where an in-crease in the intensity of the probe-tone ~alone! would haveled to a slight increase in the phase lead of the responses.Hence, as pointed out by Nuttall and Dolan ~1993!,iftheeffect of the various suppressor tones was simply to reducethe apparent intensity of the BF tones, then the only phasechanges that should occur ~at the BF! should be small phaselags ~as seen by both Robles et al. and Nuttall and Dolanusing above-BF suppressors!. If the suppression was reallydue to a mechanism which ‘‘turned down the gain’’ of thecochlea, however ~e.g., as happens when the intensity of aBF tone is increased—not decreased!, then small phase leadsa!E-mail: [email protected] 3087J. Acoust. Soc. Am. 99 (5), May 1996 0001-4966/96/99(5)/3087/12/$6.00 © 1996 Acoustical Society of AmericaDownloaded¬10¬Feb¬2011¬to¬65.112.5.1.¬Redistribution¬subject¬to¬ASA¬license¬or¬copyright;¬see¬http://asadl.org/journals/doc/ASALIB-home/info/terms.jspmight be expected at the preparation’s BF ~as reported byRhode and Cooper, 1993; the reasoning behind this state-ment is expanded in Sec. III B!. The present report providesfurther evidence that small phase leads do accompany the2TS caused by either above- or below-BF tones ~at leastwhen the probe tone is sufficiently close to the preparation’sBF!. It also provides an explicit demonstration that thesephase changes are consistent ~both qualitatively and, to alarge extent, quantitatively! with those which accompany ap-propriate increases in the intensity of the probe tones ~eitherbelow-, near-, or above-BF!.The second discrepancy in the existing literature relatesto the question of whether the displacements or the velocitiesof the mechanical responses to a suppressive stimulus pro-vide the most effective cues for 2TS on the BM. BothPatuzzi et al.