Relations between frequency selectivity and two-tone rate suppression in lizard cochlear-nerve fibers

Tuning properties of turtle auditory nerve fibers: evidence for suppression and adaptation

Second-order reverse correlation (second-order Wiener-kernel analysis) was carried out between spike responses in single afferent units from the basilar papilla of the red-eared turtle and band limited white noise auditory stimuli. For units with best excitatory frequencies (BEFs) below approximately 500 Hz, the analysis revealed suppression similar to that observed previously in anuran amphibians. For units with higher BEFs, the analysis revealed dc response with narrow-band tuning centered about the BEF, combined with broad-band ac response at lower frequencies. For all units, the analysis revealed the relative timing and tuning of excitation and various forms of inhibitory or suppressive effects.

Functional recovery in the avian ear after hair cell regeneration

Trauma to the inner ear in birds, due to acoustic overstimulation or ototoxic aminoglycosides, can lead to hair cell loss which is followed by regeneration of new hair cells. These processes are paralleled by hearing loss followed by significant functional recovery. After acoustic trauma, functional recovery is rapid and nearly complete. The early and major part of functional recovery after sound trauma occurs before regenerated hair cells become functional. Even very intense sound trauma causes loss of only a proportion of the hair cell population, mainly so-called short hair cells residing on the abneural mobile part of the avian basilar membrane. Uncoupling of the tectorial membrane from the hair cells during sound overexposure may serve as a protection mechanism. The rapid functional recovery after sound trauma appears not to be associated with regeneration of the lost hair cells, but with repair processes involving the surviving hair cells. Small residual functional deficits after recovery are most likely associated with the missing upper fibrous layer of the tectorial membrane which fails to regenerate after sound trauma. After aminoglycoside trauma, functional recovery is slower and parallels the structural regeneration more closely. Aminoglycosides cause damage to both types of hair cells, starting at the basal (high frequency) part of the basilar papilla. However, functional hearing loss and recovery also occur at lower frequencies, associated with areas of the papilla where hair cells survive. Functional recovery in these low frequency areas is complete, whereas functional recovery in high frequency areas with complete hair cell loss is incomplete, despite regeneration of the hair cells. Permanent residual functional deficits remain. This indicates that in low frequency regions functional recovery after aminoglycosides involves repair of nonlethal injury to hair cells and/or hair cell-neural synapses. In the high frequency regions functional recovery involves regenerated hair cells. The permanent functional deficits after the regeneration process in these areas are most likely associated with functional deficits in the regenerated hair cells or shortcomings in the synaptic reconnections of nerve fibers with the regenerated hair cells. In conclusion, the avian inner ear appears to be much more resistant to trauma than the mammalian ear and possesses a considerable capacity for functional recovery based on repair processes along with its capacity to regenerate hair cells. The functional recovery in areas with regenerated hair cells is considerable but incomplete.

Two-tone rate suppression boundaries of cochlear ganglion neurons in chickens following acoustic trauma

The purpose of the present study was to examine the effects of acoustic trauma and hair cell loss and regeneration on the two-tone rate suppression (TTRS) boundaries of cochlear ganglion neurons in chickens. Chickens were exposed for 48 hours to a 525-Hz, 120-dB SPL tone which destroyed the hair cells and tectorial membrane in a crescent-shaped patch along the abneural side of the basilar papilla. Afterwards, TTRS boundaries were recorded from cochlear ganglion neurons at 0-1, 5, 14, and 28 days postexposure. Acoustic trauma reduced the percentage of neurons with TTRS boundaries below CF (TTRSb) (52.6% to 8.2%) and above CF (TTRSa) (88.4% to 46.6%). In addition, the exposure reduced TTRS boundary slopes, elevated best suppression threshold (BST), and increased the frequency separation between the tips of the TTRS boundaries and CF. All the TTRS measures started to recover by 5 days postexposure and by 14 days and 28 days postexposure, most measures had recovered to normal levels. However, the BST, TTRS slopes, and the frequency separation of TTRSb boundaries from CF were still slightly abnormal near the exposure frequency. In addition, the percentage of neurons with TTRS below CF was reduced significantly. The partial recovery of TTRS boundaries is presumably due to the regeneration of hair cells and the lower honeycomb layer of the tectorial membrane. The residual TTRS deficits observed 28 days postexposure were most closely associated with the missing upper fibrous layer of the tectorial membrane.

Responses of peripheral auditory neurons to two-tone stimuli during development: I. Correlation with frequency selectivity

The responses of peripheral auditory neurons to two-tone stimuli were used to inferentially examine the nature of cochlear processing during development. Rate suppression was not seen in the youngest animals, and was first observed at 77 gestational days, in units exhibiting adultlike frequency selectivity. Suppression was highly correlated with the degree of tuning, and neurons were segregated into three classes based on these responses. Broadly tuned neurons (type IB) with low characteristic frequencies (CFs) did not exhibit suppression, and were observed early in postnatal life. Sharply tuned, but still immature neurons (type IS) exhibited suppression, but to a lesser degree than mature neurons (type M). One interpretation of these results is that basilar membrane mechanics are linear during the final stages of cochlear development, indicating that the immature signal transduction process is fundamentally different from that of adults.

Representation of acoustic signals in the eighth nerve of the Tokay gecko: I. Pure tones

A systematic study of the encoding properties of 146 auditory nerve fibers in the Tokay gecko (Gekko gecko, L) was conducted with respect to pure tones and two-tone rate suppression. Our aim was a comprehensive understanding of the peripheral encoding of simple tonal stimuli and their representation by temporal synchronization and spike rate codes as a prelude to subsequent studies of more complex signals. Auditory nerve fibers in the Tokay gecko have asymmetrical, V-shaped excitatory tuning curves with best excitatory frequencies that range from 200-5100 Hz and thresholds between 4-35 dB SPL. A low-frequency excitatory 'tail' extends far into the low-frequency range and two-tone suppression is present only on the high frequency side of the tuning curve. The response properties to pure tones at different loci within a tuning curve can differ greatly, due to evident interactions between the representations of temporal, spectral and intensity stimulus features. For frequencies below 1250 Hz, pure tones are encoded by both temporal synchronization and spike rate codes, whereas above this frequency a fiber's ability to encode the waveform periodicity is lost and only a rate code predominates. These complimentary representations within a tuning curve raise fundamental issues which need to be addressed in interpreting how more complex, bioacoustic communication signals are represented in the peripheral and central auditory system. And since auditory nerve fibers in the Tokay gecko exhibit tonal sensitivity, selective frequency tuning, and iso-intensity and iso-frequency contours that seem comparable to similar measures in birds and mammals, these issues likely apply to most higher vertebrates in general. The simpler wiring diagram of the reptilian auditory system, coupled with the Tokay gecko's remarkable vocalizations, make this animal a good evolutionary model in which to experimentally explore the encoding of more complex sounds of communicative significance.

Auditory nerve of the normal and jaundiced rat. II. Frequency selectivity and two-tone rate suppression

This study is the continuation of the functional probing of the auditory periphery in the normal and jaundiced rat. Threshold tuning curves from normal rat auditory nerve fibers were comparable to those reported in other mammals. Life-long unconjugated hyperbilirubinemia does not appear to have a widespread, demonstrable effect on cochlear frequency selectivity and sensitivity as measured by the shapes of FTCs of single auditory nerve fibers. Most fibers from the jj Gunn rats had threshold tuning curves as sharp as those from control animals (Jj Gunn and Long-Evans). Any difference seems to lie in a greater threshold variability, particularly for the high-SR fibers, for the Gunn rat strain. Two-tone rate suppression, particularly above CF, was detected in most fibers from the three groups of rats. The optimal suppression frequency (SF) as a function of CF displayed the same progression. Suppression thresholds at any given CF were generally higher for high-SR fibers than for low-SR fibers for all three groups of animals.

Curious oddments of auditory-nerve studies

Three interesting theoretical issues are presented to illustrate how certain isolated observations on auditory-nerve activity can be puzzling until other, seemingly unrelated phenomena are documented. The issues are (1) disinhibition; (2) 'peak-splitting'; and (3) independence of spike generation in primary neurons innervating the same inner-hair cell. (1) The issue of disinhibition is important for theories of lateral inhibition. For auditory-nerve fibers, the question can he phrased, 'If the rate of discharge to a tone at the characteristic frequency (CF) of a unit can he reduced by adding a second tone off the CF, is it possible to suppress this reduction by adding a third tone, even further off the CF?' The data are insufficient to conclude that disinhibition is found for auditory-nerve fibers and other explanations are available to account for the results of three-tone experiments. (2) Normally, only a single peak in the histogram of responses to low tones is phase-locked, but at high stimulus levels, the histograms will show two, or even three, peaks per stimulus cycle ('peak-splitting'). At still higher levels, the histograms again show only a single peak, but it is phase-shifted from the original peak for low stimulus levels. This complex sequence of events can be accounted for by simple models. (3) Although simultaneous recordings from pairs of auditory-nerve fibers have failed to show non-stimulus related correlations between spike trains, it has not been directly demonstrated that any two recorded fibers innervate the same hair cell. However, an indirect argument is offered to support the idea that fibers innervating a single inner-hair cell must have independent spike generators.

Lower boundaries of two-tone suppression regions in the guinea pig

Lower boundaries of two-tone suppression regions were determined in single fibres of the guinea pig with a tracking algorithm as described by Schmiedt (1982). For a suppressee at CF having a level of 20 dB above the threshold of the tip, suppression at the high-frequency (hf) side of the FTC could almost always be found. With the method used, the percentage of fibres in which suppression could be found at the low-frequency (lf) side of the FTC decreased with decreasing CF. Moreover, the occurrence of lf-suppression decreased for lower suppressee levels for fibres with CF approximately 2-5 kHz. For each fibre the minimum level difference between lf-suppression boundary and tip threshold was larger than 20 dB, for the whole group of fibres the difference was 34 dB on average. The hf-suppression regions sometimes reached below the tip for fibres with CFs in the 4 kHz region. The frequency at the lowest level of the hf-suppression boundary, best suppression frequency or BSF, is related to the CF as: BSF = 0.55 + 1.13 CF. When the suppressee level increased, the lower boundary at the hf side shifted upwards with a rate greater than 1 dB/dB. On the whole the two-tone suppression data in the guinea pig agree with those found in other rodents.

Unusual discharge patterns of single fibers in the pigeon's auditory nerve

Extracellular recording from single auditory nerve fibers in the pigeon, Columba livia, revealed some unusual discharge patterns of spontaneous and evoked activity. Time interval histograms (TIHs) of spontaneous activity showed a random interval distribution in 73% of the auditory fibers (Fig. 1a). The remaining 27% revealed periodicity in the TIHs (Fig. 1b-e), determined by the characteristic frequency (CF) of a given fiber. Normally, those fibers had a CF less than 2.2 kHz. In both cases spontaneous activity was irregular. The time pattern of quasiperiodic spontaneous firing in different auditory fibers is described by three main types of autocorrelation histograms (ACHs; decaying, nondecaying, and modulated), reflecting the spontaneous oscillations of the hair cell membrane potential (Fig. 1b-d). Single-tone suppression in auditory fibers with quasi-periodic spontaneous activity was found (Figs. 2, 10) and it could be observed if the eighth nerve was cut. There was no suppressive effect in fibres with random spontaneous firing. The frequency selectivity properties of auditory fibers were studied by means of an automatic method. Both 'simple' (Fig. 4) and 'complex' (Figs. 7, 8) response maps were found. Apart from the usual excitatory area, complex response maps were characterized by suppressive areas lying either above (Fig. 7), below (Fig. 8e), or on both sides of the CF (Fig. 8a-c). Generally, complex response maps were observed for fibers showing quasiperiodic spontaneous activity (Figs. 7, 8). Input-output functions at frequencies evoking single-tone suppression were nonmonotonic, while they were always monotonic at frequencies near the CF (Fig. 12). No difference in sharpness was observed between normal frequency threshold curves (FTCs) and excitatory areas of 'complex' response maps (Fig. 9). 'On-off' responses evoked by suppressive stimuli were found (Figs. 2, 3). They had a periodic pattern determined by the CF and did not depend on the stimulus frequency (Fig. 3). Low-CF fibers were observed which changed their time discharge structure to tone levels about 45 dB lower than their thresholds at the CF (Fig. 6). The observed features of the discharge patterns of the pigeon's auditory fibers reflect the distinctive nature of the fundamental mechanisms of auditory analysis in birds that are connected with electrical tuning of the hair cells and probably with the micromechanics of the bird's cochlea.

Frequency dependence of synchronization of cochlear nerve fibers in the alligator lizard: evidence for a cochlear origin of timing and non-timing neural pathways

The dependence of synchronization of spike discharges on tone frequency was measured in cochlear nerve fibers of anesthetized alligator lizards at 21 degrees C. Synchronization measures were based on the fundamental component of a Fourier analysis of the instantaneous discharge rate in response to tone bursts. Measurements were obtained from fibers innervating hair cells in both the region of the cochlea that contains a tectorial membrane (tectorial fibers) and the region where hair-cell stereocilia are free-standing in scala media (free-standing fibers). Both rate and synchronization tuning-curves were measured automatically as a function of tone frequency. For tectorial fibers, the shapes of synchronization tuning-curves are roughly similar to the shapes of rate tuning-curves: the characteristic frequencies (CF's) of both curves are approximately equal. For free-standing fibers, the shape of synchronization tuning-curves differ markedly from those of rate tuning-curves. The CF's of synchronization and rate tuning-curves differ - the ranges are 0.2-0.6 kHz and 1-4 kHz, respectively - and the two CF's are uncorrelated. Synchronization filter-functions, which are contours of synchronization index at constant average discharge rate, were measured as a function of tone frequency for both tectorial and free-standing fibers. These synchronization filter-functions have the shapes of lowpass filters. For the populations of tectorial fibers and of free-standing fibers taken separately, these functions are independent of CF. The corner frequency of these functions is 0.50 +/- 0.038 kHz for tectorial fibers and 0.37 +/- 0.037 kHz for free-standing fibers. We conclude that these populations are characterized by different synchronization filters. For free-standing fibers, synchronization filter-functions measured at average driven discharge rates of about 20 and 40 spikes/s do not differ appreciably, and the high-frequency slope is -80 to -115 dB/decade. The results show that tectorial fibers encode timing information for low-level stimuli, whereas free-standing fibers do not. It is proposed that in the alligator lizard, neural pathways that encode timing information originate in the tectorial region and those that encode non-timing information originate in the free-standing region.

Growth of suppression in the cochlear potentials

Measurement of two-tone effects in the cochlear microphonic and summating potential indicates that the growth of suppression is different for these two cochlear potentials. Whereas the CM response to the fundamental is reduced 10 dB for each 10 dB increase in suppressor level, the SP decreases at a faster rate; approximately 20 dB per 10 dB increase. Slopes of functions for the CM response to the second harmonic are similar to those for the dc component. Since these results are consistent with the notion that suppression operates by attenuating the input to the CM generator, they are consonant with a mechanical origin of suppression.

A model for signal transmission in an ear having hair cells with free-standing stereocilia. I. Empirical basis for model structure

No adequate theory for the signal-transmission properties of the peripheral auditory system exists for any vertebrate ear. Because the mammalian ear seems to pose conceptual and technical problems that complicate the development of an adequate theory, it is worthwhile to investigate simpler ears. The ear of the alligator lizard is simpler than mammalian ears in several respects: the motion of the basilar membrane is approximately independent of longitudinal position and is approximately linearly related to the sound pressure at the tympanic membrane; in a large region of the cochlea the hair cells have free-standing stereocilia that are not in contact with a tectorial membrane; the receptor potential of these hair cells is related to the sound pressure at the tympanic membrane in a relatively simple manner; the cochlear-nerve fiber responses from this region do not exhibit two-tone rate suppression. Also, the relative accessibility of this ear has enabled measurement of several response variables: tympanic-membrane volume velocity, extracolumella velocity, basilar-membrane velocity, hair-cell stereociliary displacement, hair-cell receptor potentials, and cochlear-nerve-fiber discharges. A model is developed to represent these results in terms of underlying anatomical structures and physiological mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional properties of auditory-nerve fibers during postnatal development in the kitten

The discharges of the auditory-nerve fibers were studied in kittens between 2-40 days of age. Up to the 10th postnatal day, fibers could be divided into two main categories: fibers with spontaneous activity (SA) that respond to sound and fibers without SA but with evoked responses. A third, smaller, category, fibers having neither SA nor evoked activity, was also present. The development of SA comprises two phases. The first, lasting from birth up to the third postnatal week, shows a relatively fast increase and the second, lasting up to adulthood, a slower increase. Typical tone burst responses can be recorded at the end of the first postnatal week. Thereafter reactivity steadily increases especially after the 10th postnatal day. In young animals, rate level function is characterized by a steep segment with a low dynamic range followed by a decrease in activity that lasts until the end of the second week. At this point adult-like functions may be observed, although maximal firing still increases for some weeks. Tuning curves and threshold sensitivity tend to develop inversely at corresponding frequencies. Fibers with low characteristic frequencies reach adult threshold before that of high frequency fibers and high frequency fibers reach adult tuning before low frequency fibers. A comparison of auditory-nerve fiber activity in kittens show that maturation of most functional characteristics lasts several weeks after birth and in some cases continues after the first postnatal month.

Cochlear nonlinearities inferred from two-tone distortion products in the ear canal of the alligator lizard

Distortion products ( DPs ) evoked by two-tone stimuli at frequencies F1 and F2 were measured in the ear-canal sound pressure of the alligator lizard. The largest sound pressures measured, other than those at F1 and F2, where at the cubic difference frequencies 2F1-F2 and 2F2-F1. All cubic DPs were greatly reduced by destruction of the basilar membrane, which suggests that its nonlinear properties are the source of the DPs . Measurements following acoustic overstimulation show a complex relationship between the magnitude of DPs and cochlear state, as assessed by measurements of cochlear potential, and indicate the existence of multiple nonlinear sources within the inner ear. Relative magnitudes of the DPs and their dependence on stimulus level suggest that the inner-ear DP sources are cubic nonlinearities. The DPs are not highly sensitive to either average stimulus frequency or stimulus frequency separation, suggesting that the nonlinear processes are within the macromechanical processes of the inner ear. Contrary to some interpretations of ear-canal DP measurements in mammals, we conclude that DPs need not be associated with hair-cell processes and are not particularly useful indicators of cochlear health.

Receptor potentials of lizard cochlear hair cells with free-standing stereocilia in response to tones

Intracellular potentials were recorded with micropipettes from hair cells with free-standing stereocilia in the cochleae of anaesthetized alligator lizards. Wave forms of intracellular responses to click stimuli were classified into three types: hair cells, supporting cells, and untuned cells. We studied primarily the responses of hair cells to tonal stimuli. For most frequencies, f, and levels, P, of tone-burst stimuli, the response envelope of the receptor potential increases monotonically at the tone-burst onset, and decreases monotonically at tone-burst offset. Overshoot in the envelope of the response at the onset and offset of tone bursts is observed only for tone bursts of low f, high P, and short (approximately equal to 1 msec) rise/fall time. The steady-state response to tones consists of a positive (depolarizing) d.c. component, V0, plus a.c. components (e.g. a fundamental component, V1, second harmonic, V2, and third harmonic, V3). The magnitudes of a.c. and d.c. components are functions of f and P, and show three characteristics: frequency selectivity, non-linearity, and low-pass filtering. The receptor potential is frequency selective. The frequency selectivity of V0 and V1 components was measured by means of iso-voltage (iso-V0 and iso-V1) contours. Iso-V0 and iso-V1 contours are V-shaped: the maximum sensitivity occurs at a characteristic frequency (c.f.). The shapes of these contours near the c.f. depend on the values of V0 and V1 at which the contours were measured and are sharper for lower values of V0 and V1. The mean slopes of the low- and high-frequency sides of these contours are: -45.0 and +85.1 dB/decade for iso-V0 contours (n = 26), and -33.6 and +103.8 dB/decade for iso-V1 contours (n = 28). The receptor potential has non-linear properties. The magnitudes and phase angles of V0, V1, V2, and V3 receptor-potential components were measured as a function of P for different f. The slopes of level functions (the dependence of log V0 and log magnitude of V1 on log P) were measured at low levels for different f. For values of f differing from c.f. by more than a half-octave, the slope for V0 is between 1 and 2 with a mean of 1.3; the slope for V1 is about 1, i.e. magnitude of V1 increases approximately linearly with P. For frequencies near c.f., the slopes for V0 and V1 are approximately 0.8 and 0.5, respectively, indicating the presence of a compressive non-linearity.(ABSTRACT TRUNCATED AT 400 WORDS)

Frequency selectivity of hair cells and nerve fibres in the alligator lizard cochlea

Receptor potentials of hair cells and spike discharges of cochlear nerve fibres were recorded with micropipettes from the free-standing region of the basilar papilla of anaesthetized alligator lizards in response to tones. In this region the hair-cell stereocilia are free-standing, i.e. they protrude directly into endolymph and are not in contact with a tectorial membrane. The frequency selectivity of hair-cell responses was measured by means of isovoltage contours of the d.c. (V0) and fundamental-a.c. (V1) component of the receptor potential, i.e. iso-V0 and iso-V1 contours. The frequency selectivity of the nerve-fibre discharge was measured by iso-rate (iso-V0) contours. Iso-V0, iso-V1 and iso-V0 contours are basically V-shaped with a characteristic frequency (c.f.) defined as the frequency at which minimum sound pressure (Pmin) is required to evoke the criterion value of the response. Receptor potential iso-V0 contours and neural iso-V0 contours have similar slopes: the mean slopes of the low-frequency sides (dB/decade) are -43.0 and -44.3; the slopes of the high-frequency sides are 85.0 and 80.2. The band widths of iso-V0 and iso-V0 contours away from c.f. are similar (mean values of Q30dB are 0.40 and 0.53, respectively). The band widths of iso-V0 contours near c.f. are narrower than those of iso-V0 contours (mean values of Q10dB are 2.34 and 1.20, respectively). However, the shapes of the contours near c.f. depend on the iso-response criteria, and we have not determined whether or not iso-V0 and iso-V0 contours are similar near c.f. The shapes of iso-V1 contours differ from those of iso-V0 and iso-V0 contours. Nerve fibre c.f.s are tonotopically organized in the nerve, with lowest c.f.s recorded from fibres innervating the border of free-standing and tectorial regions, a region in which hair-cell stereocilia are longest, and the highest c.f.s recorded from fibres innervating the end of the free-standing region in which hair-cell stereocilia are shortest. The c.f. of nerve-fibre response (and by implication hair-cell response) is, therefore, correlated with the height of the stereociliary tuft. The shapes of iso-V0 contours vary systematically with c.f. and, therefore, tonotopically with nerve position.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous and impulsively evoked otoacoustic emissions: indicators of cochlear pathology?

The first author's right ear produces a spontaneous otoacoustic emission (SOAE) at 7529 Hz and 16 dB SPL. An external continuous tone is able to suppress the SOAE. The 3 dB iso-suppression curve is broadly tuned and displaced, relative to the SOAE, toward higher frequencies. An audiogram notch exists at frequencies just below that of the SOAE. We explain the occurrence of both spontaneous and impulsively evoked OAEs in terms of disruption of active feedback mechanisms of the OHCs upon basilar membrane vibration. According to this hypothesis, each segment of the organ of Corti feeds back positively upon its segment of basilar membrane and negatively upon adjacent segments. If a patch of OHC loss exists, adjacent segments of the basilar membrane are released from the negative feedback and respond to an impulsive stimulus with exaggerated oscillations at their resonance frequencies, thus producing OAEs. At particularly sharp transitions between normal and abnormal regions of the organ of Corti SOAEs may be generated.

Boundaries of two-tone rate suppression of cochlear-nerve activity

Two-tone rate suppression was examined in the responses of single cochlear-nerve fibers in Mongolian gerbils. The iso-rate tracking algorithm developed by Kiang and Moxon (Kiang, N.Y.-S. and Moxon, E.C. (1974): J. Acoust Soc. Am. 55, 620-630) for obtaining tuning curves was modified to track iso-rate suppression boundaries as a function of frequency with the excitor tone fixed at the characteristic frequency (CF) of the fiber. Lower threshold boundaries of the areas of suppression flanking the tuning curve above and below CF were outlined for fibers over a large CF range. It was found that the boundaries of rate suppression obtained below CF were very stable in their absolute positions on the intensity-frequency plane. This stability was evident both as a function of fiber CF (0.6-15 kHz) and as a function of the shape of the tuning curve at a given CF. In other words, the suppression boundary obtained below CF was largely independent of the tuning curve. In a second series of experiments tuning curves were taken in the presence of a fixed tone placed in the suppression area located above the fiber CF. The fixed tone by itself was not excitatory. These tuning curves were compared to tuning curves obtained with a single tone. It was found that frequencies around the fiber CF were most affected (suppressed) by the presence of the second tone, and that the low-frequency tail of the tuning curve tended to shift toward the boundary of the suppression area below CF. Because this suppression boundary lies below the threshold of the normal tail of the tuning curve for many mid- and high-CF fibers, these fibers often became hypersensitive at low frequencies in the presence of the second tone above CF.