Correlation between tissue docosahexaenoic acid levels and susceptibility to light-induced retinal degeneration

In a mouse model of acute light-induced retinal degeneration, positive correlations between the levels of DHA, the levels of n3 PUFA lipid peroxidation, and the vulnerability to photooxidative stress were observed. On the other hand, higher sensitivity of the electroretinogram a-wave response, a measure of the amplification of the phototransduction cascade, was correlated with higher retinal DHA levels. These results highlight the dual roles of DHA in cellular physiology and pathology.

Retinal sphingolipids and their very-long-chain fatty acid-containing species

PURPOSE

Recent evidence suggests that ceramide metabolism plays an important role in retinal photoreceptor cell survival and apoptosis. The purpose of this study was to characterize sphingolipids in the retina with special emphasis on the very-long-chain-containing saturated (VLC-FA) and polyunsaturated (VLC-PUFA) fatty acid-containing species. The VLC-FAs and VLC-PUFAs are synthesized by the ELOVL4 protein, which is involved in human Stargardt's macular dystrophy type 3 (STGD3).

METHODS

Total lipids were extracted from retina and other tissues, and different sphingolipid classes were isolated and purified using various combinations of liquid- and solid-phase separation. Purified sphingolipids were analyzed by high-performance thin layer chromatography (HPTLC), gas chromatography (GC), and GC-MS (GC-mass spectrometry).

RESULTS

Nonsialylated sphingolipids (NSLs) comprised approximately 3.5% of total retinal lipids of which 70% was sphingomyelin. Ceramide and glycosylceramides (GCs) constituted<or=1% of total retinal lipids. Gangliosides (GGs), on the other hand, comprised approximately 3.0% of total retinal lipids. Fatty acid analysis of retinal NSLs indicated an abundance of saturated fatty acids, with the presence of VLC-FAs but not of VLC-PUFAs beyond 24 carbons. However, GG had significant levels of unsaturated, polyunsaturated, and VLC-PUFAs. Retinal rod outer segments (ROS) contained approximately 1% each of NSL and GG, and their fatty acid profile was not very different from whole retinal NSL and GG, respectively.

CONCLUSIONS

Retina has a total of 6% to 7% fatty acids that are N-linked to a sphingosine, which would be 11 to 13 mole % in comparison to phospholipids. The presence of VLC-FAs and VLC-PUFAs in retinal sphingolipids indicates that they may play role in ELOVL4-mediated Stargardt 3.

High levels of retinal membrane docosahexaenoic acid increase susceptibility to stress-induced degeneration

The fat-1 gene cloned from C. elegans encodes an n-3 fatty acid desaturase that converts n-6 to n-3 PUFA. Mice carrying the fat-1 transgene and wild-type controls were fed an n-3-deficient/n-6-enriched diet [fat-1- safflower oil (SFO) and wt-SFO, respectively]. Fatty acid profiles of rod outer segments (ROS), cerebellum, plasma, and liver demonstrated significantly lower n-6/n-3 ratios and higher docosahexaenoic acid (DHA) levels in fat-1-SFO compared with wt-SFO. When mice were exposed to light stress: 1) the outer nuclear layer (ONL) thickness was reduced; 2) amplitudes of the electroretinogram (ERG) were lower; 3) the number of apoptotic photoreceptor cells was greater; and 4) modification of retinal proteins by 4-hydroxyhexenal (4-HHE), an end-product of n-3 PUFA oxidation was increased in both fat-1-SFO and wt mice fed a regular lab chow diet compared with wt-SFO. The results indicate a positive correlation between the level of DHA, the degree of n-3 PUFA lipid peroxidation, and the vulnerability of the retina to photooxidative stress. In mice not exposed to intense light, the reduction in DHA resulted in reduced efficacy in phototransduction gain steps, while no differences in the retinal morphology or retinal biochemistry. These results highlight the dual roles of DHA in cellular physiology and pathology.

Cochlear nerve acoustic envelope response detection is improved by the addition of random-phased tonal stimuli

We test Lowenstein's dc bias hypothesis as an alternative mechanism for the phenomenon sometimes called 'stochastic resonance'. Probe stimuli consisting of paired phase-locked tones at frequencies f(1) and f(2) (where f(2)-f(1)=800 Hz, f(1)>4.5 kHz) and at equal intensity were used to generate synchronous 800 Hz cochlear nerve activity (envelope responses). When a background tone of the same intensity, with a frequency halfway between f(1) and f(2), is presented simultaneously with the probe stimulus, the envelope response amplitude typically decreases. Consistent with Lowenstein's hypothesis, however, when the intensities of the probe and background tone are near the detection threshold of the envelope response (approximately 0-20 dB sound pressure level), the simultaneous presence of the background tone often increases the amplitude of the envelope response. At these same intensity levels, when the background tone precedes the probe stimulus, it decreases the amplitude of the response to the probe stimulus. The effects of simultaneous presentation of the probe and the background tone are frequency-dependent, becoming less pronounced or reversing as the frequency of the background tone departs from those of the probe stimuli.

Essential roles of noise in neural coding and in studies of neural coding

We present examples of results from our studies of auditory primary afferent nerve fibers and populations of such fibers in the frog and gerbil. We take advantage of the natural dithering effect of internal noise, where it is sufficient, to construct highly predictive descriptive models (based on the Wiener series with kernels derived from white-noise analysis). Where the internal noise is insufficient, we enhance dithering by applying external acoustic noise together with our stimuli. Using acoustic noise as a background sound, orthogonal to the stimulus waveform, we show that under some circumstances such background sound can enhance the ability of individual fibers and populations of fibers to encode the stimulus waveform.

Low-frequency acoustic modulations generated by the high-frequency portion of the cochlea, noninvasively recorded from the scalp of mice (Mus musculus)

Vocalizations often contain low-frequency modulations of the envelope of a high-frequency sound. The high-frequency portion of the cochlear nerve of mice (Mus musculus) generates a robust phase-locked response to these low-frequency modulations, and it can be easily recorded from the surface of the scalp. The cochlea is most sensitive to envelope modulation frequencies of approximately 500 to 2000 Hz. These responses have detection thresholds that are approximately 10 dB more sensitive than auditory brainstem responses, and they are very sharply tuned. These measurements may provide a nontraumatic means of repeatedly assessing cochlear functions involved in sound localization and perception of vocalizations.

Killer whale (Orcinus orca) hearing: auditory brainstem response and behavioral audiograms

Killer whale (Orcinus orca) audiograms were measured using behavioral responses and auditory evoked potentials (AEPs) from two trained adult females. The mean auditory brainstem response (ABR) audiogram to tones between 1 and 100 kHz was 12 dB (re 1 mu Pa) less sensitive than behavioral audiograms from the same individuals (+/- 8 dB). The ABR and behavioral audiogram curves had shapes that were generally consistent and had the best threshold agreement (5 dB) in the most sensitive range 18-42 kHz, and the least (22 dB) at higher frequencies 60-100 kHz. The most sensitive frequency in the mean Orcinus audiogram was 20 kHz (36 dB), a frequency lower than many other odontocetes, but one that matches peak spectral energy reported for wild killer whale echolocation clicks. A previously reported audiogram of a male Orcinus had greatest sensitivity in this range (15 kHz, approximately 35 dB). Both whales reliably responded to 100-kHz tones (95 dB), and one whale to a 120-kHz tone, a variation from an earlier reported high-frequency limit of 32 kHz for a male Orcinus. Despite smaller amplitude ABRs than smaller delphinids, the results demonstrated that ABR audiometry can provide a useful suprathreshold estimate of hearing range in toothed whales.

Noise improves transfer of near-threshold, phase-locked activity of the cochlear nerve: evidence for stochastic resonance?

Stochastic resonance can be described as improved detection of weak periodic stimuli by a dynamic nonlinear system, resulting from the simultaneous presentation of a restricted dynamic range of low-intensity noise. This property has been reported in simple physical and biological activities. The present study describes data consistent with the interpretation that stochastic resonance can be observed in the response of cochlear neurons. These experiments utilized low levels (-5 to 25 dB SPL) of stimuli and noise (5 to 30 dB SPL). Stimuli consisted of simultaneously presented 8 kHz (F1) and 8.8 kHz (F2) tone bursts, which generated an 800 Hz F2-F1 cochlear nerve envelope ensemble response in the gerbil. The mean response threshold was approximately -3 dB SPL. Simultaneous presentation of a low-intensity wideband noise increased the amplitude of this response. This was observed with tonal stimuli having intensities of 0-5 dB SPL; responses to stimulus levels > 10 dB were attenuated by noise. Response amplitude was increased by noise levels of 10-15 dB; the amplitude was unaffected by lower levels of noise, and decreased in the presence of higher noise levels. These properties are compatible with those of stochastic resonance.

Temporal factors associated with cochlear nerve tuning to dual and single tones: a qualitative study

The simultaneous presentation of a 10- and 10.86-kHz tone produces an 860-Hz cochlear nerve difference tone (DT) response in the gerbil which persists for the duration of the stimulus. Forward masking shows this response is generated by neurons sharply tuned to the stimulus frequencies. When compared with the DT response, the cochlear nerve compound action potential (CAP) to a single tone is smaller in amplitude, has a higher nonmasked threshold, and produces a less sensitive tuning curve (TC). Forward maskers can also produce amplitude enhancement of the CAP, but this was not observed for the onset portion of the DT response. The CAP TC is as sharply tuned as the TC of either the DT onset response or the entire DT response. A comparison was made of tuning of the DT response to the onset, the first half and second half of the 23-ms duration probe stimulus, using either a 5- or 15-ms masker-probe interval. An increase of the tip threshold of the TC to all three portions of the stimulus occurred as the interval was increased between the end of the masker and the midpoint of the portion of the stimulus under question. The 15-ms masker-probe interval produced sharper TCs.

Cochlear tuning in the gerbil: a comparison of responses to sinusoidal amplitude modulation and difference tone stimuli

Vocalizations often have periodic variations of their acoustic waveform envelope. Two simultaneously presented frequencies have an envelope fluctuation with a frequency equal to their difference tone (DT = F2-F1). Sinusoidal amplitude modulation (SAM) of a carrier frequency also produces an envelope fluctuation. Electrical ensemble responses to DT and SAM stimuli were recorded from the gerbil's round window. The predominant frequency of the response to the DT stimuli is F2-F1; to the SAM stimuli, it is the modulation frequency. Both responses are spectrally, temporally, and dynamically non-linear. Forward masking of a low-frequency DT response produced a tuning curve (TC) with a tip at the high-stimulus frequency. Forward masker TCs of a low-frequency SAM ensemble response had tips at the high frequency of the carrier. Tip thresholds and sharpness of tuning of DT and SAM TCs are quite similar, with cochlear neurons having high characteristic frequencies providing sharply tuned information about low frequency acoustic envelope periodicities.

Sharply tuned cochlear nerve ensemble periodicity responses to sonic and ultrasonic frequencies

Vertebrates are able to perceive the pitch of a series of harmonics, even when the fundamental frequency has been removed from the acoustic stimulus. Neural periodicity responses corresponding to the "missing fundamental" frequency of sonic stimuli have been observed in the auditory system of several animal species, including our own. This paper examines periodic cochlear neural responses of the gerbil. Periodicity responses to both sonic and ultrasonic stimuli originate within the cochlea of this animal. Acoustic stimuli, consisting of 2-12 successive harmonic frequencies, were used to generate an ensemble cochlear nerve periodicity response that was recorded from the round window of the cochlea. This response had a frequency equal to that of the missing fundamental, and not to those of the harmonic stimuli. Forward masking of the stimuli used to produce the periodicity response was used to generate sharp tuning curves, with tip frequencies corresponding to the harmonics and not to the periodicities. The sharpness of these functions increased as the frequencies of the harmonics increased, up to at least 38 kHz. This property could be related to reception of ultrasonic vocalizations utilized by many rodent species.

Auditory nerve neurophonic tuning curves produced by masking of round window responses

In response to low-intensity, low-frequency, phase-locked tonal stimuli with non-alternating polarity, the time-average round window (RW) response of the gerbil is a mixture of the auditory nerve neurophonic (ANN) and cochlear microphonic (CM), with the former often being of equal or greater magnitude than the latter. Forward masking (using a conservative 25% amplitude reduction criterion) can be used to generate ANN tuning curves (TC). Most of these TCs are sharply tuned V-shaped functions. Harmonic distortion is often present in the ANN, especially in response to the lower-frequency (< or = 1 kz) or higher-intensity (> or = 50 dB) stimuli. The TCs created by forward masking of the harmonics are similar in appearance to those generated by masking the fundamental frequency of the ANN. When lower-frequency probe stimuli (< or = approximately equal to 1 kHz) are used, the frequency of the TC tip tends to be higher than that of the probe; with higher probe frequencies, the tip tends to be lower. Regardless of the frequency of the probe, the TC tip threshold occurs at an intensity level lower than that of the probe. The sharpness of these TCs generally increases as a function of the frequency of the probe stimulus and the values of Q10dB are comparable to those of FTCs of cochlear nerve fibers of the gerbil. The amplitude of the ANN is often enhanced in response to a limited intensity range of forward maskers over a restricted range of frequencies that are outside the high-frequency boundary of the forward masker TC. By alternating the polarity of the probe stimulus, the CM can be canceled, allowing the effects of simultaneous maskers to be evaluated.

Auditory nerve neurophonic produced by the frequency difference of two simultaneously presented tones

When two phase-locked sinusoidal stimuli having frequencies of F1 and F2 are simultaneously introduced to the ear of the gerbil, a difference tone (DT) can be observed (DT = F2-F1, where F2 > F1) in the time-averaged electrical response recorded from the cochlear round window (RW). Tetrodotoxin (TTX), which blocks the axonal firing of the cochlear nerve fiber, greatly attenuates this DT response, suggesting it is primarily neural in origin. Alternating the polarity of a single phase-locked tone cancels out the RW cochlear microphonic (CM) from the time-averaged response, leaving a residual auditory nerve neurophonic (ANN) response if the stimulus frequency is low enough to result in phase-locked firing of cochlear nerve axons. Simultaneous presentation of 1 kHz (F1) and 2 kHz (F2) tones, each being phase-locked with alternating polarity, produces a small ANN in response to the original tones and a large time-averaged ANN in response to the DT. Even when the frequency of the individual tones is too high to support phase-locking, a large DT-ANN can also be measured in response to simultaneously presented tones. A robust time-averaged DT-ANN can be measured when the temporal and intensity relationships between F1 and F2 are varied widely, with the latency (but not amplitude) of the response following the stimulus envelope. The DT-ANN produced by pairs of tones having frequencies ranging from 500 Hz to 3.5 kHz is largest in response to a DT of approximately 700-1100 Hz. This is in contrast to the ANN generated in response to a single tone, which decreases in magnitude as the stimulus frequency increases from 500 to 1500 Hz. Robust DT-ANNs can be measured from the gerbil even when the F2 frequency is greater than 30 kHz.

Tuning curves of the difference tone auditory nerve neurophonic

When a pair of tonal stimuli of different frequencies (F1 and F2, where F2 > F1) are simultaneously presented to the ear, an electrical response with a frequency of F2-F1 can be recorded from the round window (RW) of the gerbil's cochlea. By using phase-locked tones of alternating polarity, the cochlear microphonics are canceled, leaving a time-averaged difference tone-auditory nerve neurophonic (DT-ANN). When the F1 frequency ranges from 1.25 to 30 kHz and F2-F1 approximately 900 Hz, a DT-ANN audiogram can be constructed which parallels (but is at least 10 dB more sensitive than) the compound action potential (CAP) audiogram. In addition to this DT response, a smaller magnitude, higher threshold response having a frequency of 2 DT can often be measured. Both the DT-ANN and the 2 DT-ANN show non-monotonic amplitude input-output functions. The DT- and 2 DT-ANN responses can be forward masked. Masking of low level (e.g., 30 dB SPL) probe stimuli results in DT- and 2 DT-ANN V-shaped tuning curves (TC) with low tip thresholds (approximately 20-30 dB SPL) and a tip frequency close to that of F1 and F2. The Q10 dB values of the forward masked DT-ANN TCs ranges from 1.54 to 20.0 for F1 frequencies varying from 2 to 20 kHz, respectively. The V-shaped DT-ANN TCs generated with simultaneous maskers are often flanked, outside their high- and low-frequency slopes, by frequency-intensity domains where the masker enhances the amplitude of the DT-ANN response. These data (1) provide evidence that, in response to low-intensity tones, the DT-ANN is generated by a restricted population of neurons that have characteristic frequencies close to F1 and F2, and (2) provide evidence for sharply tuned, phase-locked activity occurring in response to low-intensity stimuli, by cochlear axons having characteristic frequencies as high as 20 kHz.

Nonlinear effects of noise on phase-locked cochlear-nerve responses to sinusoidal stimuli

It is well known that, in a cochlear afferent axon with background spike activity, a sinusoidal stimulus (tone) of sufficiently low frequency will produce periodic modulation of the instantaneous spike rate, the alternating half cycles of which comprise excursions above and below the mean background spike rate. It also is known that if the amplitude of the stimulus is sufficiently small, the instantaneous spike rate follows very nearly a sinusoidal trajectory through these positive and negative excursions. For such cases, we define the AC responsiveness of a primary auditory afferent axon to be the amplitude of sinusoidal modulation of the instantaneous spike rate divided by the amplitude of the tone producing that modulation. In the experiments described in this paper, changes in AC responsiveness were followed during and after sudden changes in the background noise level. When the amplitude of the tone was sufficiently small relative to that of the noise, we found that the AC responsiveness can be strongly dependent on the time elapsed since the last change in noise level, while being nearly independent of the amplitude of the tone itself. Under those circumstances, after transitions between noise levels 20 dB apart, we observed changes in AC responsiveness that consistently followed time courses similar to those of the short-term mean (background) spike rate (approximating the adapting response to the noise alone), unfolding over several milliseconds or tens of milliseconds. At the time of the transition between noise levels, there was another change in AC responsiveness, which appeared to be instantaneous; as the noise level increased, the AC responsiveness immediately increased with it. This seemingly paradoxical effect and the similarity of the time courses of AC responsiveness and short-term mean spike rate both are consistent with a simple, descriptive model of spike generation involving the shifting of threshold along a bell curve.

Auditory nerve neurophonic recorded from the round window of the Mongolian gerbil

In the Mongolian gerbil, round window (RW) recordings of averaged responses to phase-locked acoustic stimuli which are not alternated in polarity can include both the cochlear mirophonic (CM) and auditory nerve neurophonic (ANN). The ANN can dominate the recordings when the RW electrode is referenced to some portion of the body that allows the two electrodes to straddle the auditory nerve. Concentric bipolar RW electrodes are biased in favor of the CM. When there is a substantial ANN component in the RW response, as the sinusoidal stimulus intensity increases there is a non-monotonic increase of amplitude and a pronounced change of phase of the response. When the phase-locked stimuli are alternated in polarity in order to cancel the CM, a residual response is often observed. This residual response has twice the frequency of the stimulus and is decreased in amplitude by forward masking. It also shows a pattern of amplitude decrement following the stimulus onset, resembling adaptation of the firing rate of cochlear nerve axons. Tetrodotoxin (TTX) eliminates the non-monotonic RW amplitude input-output (I/O) function, reduces the phase changes of the response as the stimulus intensity is increased, eliminates the residual non-canceled response to alternated stimuli, and the time-limited amplitude decrements which resemble adaptation. Following application of TTX, the RW response of the gerbil to stimuli with non-alternated polarity much more closely resembles the CM responses of other animals. It is concluded that the gerbil's residual response following cancellation of the CM is the ANN, and that the RW of the gerbil is a convenient site for recording measures of phase-locked cochlear axonal activity.

Dynamic changes in tuning in the gerbil cochlea

Afferent axons of the gerbil cochlear nerve were studied with reverse correlation analyses carried out with movable time windows and with noise that was modulated with a 10-Hz trapezoidal envelope that switched the noise amplitude between two levels, 20 dB apart. At the time of switching, the attributes of the axonal tuning curves derived in this manner switched very rapidly (e.g., within 10 ms) from those characteristic of lower-level stimuli to those characteristic of higher-level stimuli and vice versa. As previous investigators have shown, the attributes of tuning curves at higher levels include broader bandwidth and an accentuated low-frequency hump. Characteristic frequencies (CFs) of gerbil axons used in this study ranged from approximately 500 Hz to approximately 5 kHz. Over this range, the low-frequency hump was most pronounced in our studies for units with higher CFs, each of which showed a sharp high-frequency peak and a distinctly separate, broad low-frequency hump (reminiscent of the tip and tail of a conventional frequency-threshold tuning curve). The amplitude of the peak relative to that of the hump, and the breadth of the peak, both changed rapidly and reversibly following sudden change of noise level. Observation of such rapid changes of tuning would be difficult to achieve with conventional frequency-threshold tuning curves, derived from tonal stimuli.

Amplitude enhancement is seen in the cochlear nerve but not at, or before, the afferent synapse

The amplitude of the cochlear nerve compound action potential (CAP) produced by a moderate intensity tonal stimulus (S2) can be enhanced when S2 is preceded by a low intensity S1 of the same frequency. The presence of S1 had no observable influence on the threshold of the CAP to S2. Enhancement was not observed in the cochlear microphonics or summating potentials. Deactivation of the contralateral olivocochlear bundle did not influence enhancement. Tetrodotoxin (TTX) was applied to the round window to block cochlear nerve spike activity, resulting in a residual EPSP-like potential, as described in the guinea pig by Dolan et al. (1989). Kainic acid, in turn, eliminated this EPSP-like response. Even though some differences were found in the responses of the gerbil and their guinea pig preparation to TTX and kainic acid, enhancement was not observed in this residual potential. When enhancement was observed at the level of the CAP, it was observed at brainstem levels. It is suggested that enhancement originates within the cochlear nerve axons.

Albino and pigmented gerbil auditory function: influence of genotype and gentamicin

Auditory function of albino and pigmented gerbils was examined before and after treatment with the ototoxic aminoglycoside antibiotic gentamicin. Auditory brainstem responses (ABRs) and cochlear nerve compound action potentials (CAPs) were measured in response to pure tones having frequencies between 2 and 32 kHz. Age-matched albinos had significantly lower CAP, but not ABR, thresholds than pigmented gerbils. Gentamicin treatment elevated CAP and ABR thresholds in both genotypes, but pigmented gerbils were less severely affected. Compared to the ABR, the CAP is a more sensitive measure of ototoxicity and pigmentation differences. CAP tuning curves (TCs) were another sensitive measure of genotypic differences in susceptibility to ototoxicity. TC tip thresholds from pigmented animals given gentamicin were not as elevated as the TC tip thresholds of albinos.

Suprathreshold comparisons of derived and enhanced compound action potentials

The Derived cochlear nerve compound action potential (CAP) and the Enhanced CAP are both measures which demonstrate the ability of a forward masker to increase the amplitude of the CAP produced by a probe stimulus. Enhancement occurs whenever the amplitude of a masked CAP is larger than that of a nonmasked CAP, whereas the derived CAP is produced by the subtraction of the entire masked CAP waveform from that of the nonmasked waveform. Therefore, a derived CAP is created whenever the masker produces a difference of amplitude, latency, and/or waveform shape. The present experiments compare these two measures by observing the effects of 13 kHz maskers varying from levels of -10 to +70 db SPL on CAPs produced by 50 or 60 dB SPL, 13 kHz probe stimuli. Enhancement is characterized by a nonmonotonic increase of CAP amplitude (and sometimes a decrease of latency) as a function of increasing levels of the forward masker, whereas this pattern seldom occurs with the derived CAP. Enhancement is typically seen with forward making, but seldom seen with simultaneous masking, whereas the derived CAP is very similar under these two types of masking.

Derived and enhanced compound action potentials at near-threshold levels: forward masking increases sensitivity of audiograms and tuning curves

The amplitude of a cochlear nerve compound action potential (CAP) can be increased by forward maskers having levels close to the visual detection threshold of the CAP. This effect, termed enhancement, varies as a function of the frequency of the masker and probe stimulus, and is nonmonotonic with respect to the level of the masker. Other studies using the derived CAP have used a subtraction technique to evaluate the ability of simultaneous maskers having levels near the CAP visual detection threshold to influence the CAP produced by an above threshold tone. The present paper compares audiograms produced by the conventional nonmasked CAP visual detection threshold technique with audiograms produced by both forward masked derived CAPs and forward masked enhanced CAPs. In response to low and middle frequency stimuli, both masked CAP measures produce more sensitive audiograms than does the conventional nonmasking method. Forward masked amplitude tuning curves (TCs) were also produced, comparing the conventional 50% amplitude reduction and 20 microV amplitude reduction methods with TCs obtained with derived and enhanced CAPs. When the same criteria are used, both masked CAP measures result in sharply tuned amplitude TCs that are approximately 60 dB more sensitive than the conventional CAP technique. At near-threshold levels, the properties of forward masked enhanced and derived CAPs appear to be similar.

Modulation of cochlear nerve spike rate by cardiac activity in the gerbil

Among primary auditory axons with characteristic frequencies (CFs) below 2500 Hz, a substantial subpopulation was found in which spike activity was driven by cardiac events. The presence of cardiac-driven activity was inferred from cycle histograms triggered on the peak of the electrocardiogram (ECG). This driven activity was either like a simple onset response (often followed by a reduction of spike activity to below background level), or as a longer lasting series of peaks and troughs. In two axons with high CFs (7 kHz and 12.5 kHz), cardiac-driven suppression was observed. Recordings made by a probe microphone revealed the presence of heart-related sound in the external ear canal. The onset of that sound coincided with the onset of cardiac-driven spike activity (and suppression).

Latency enhancement of the cochlear nerve compound action potential (CAP)

Forward maskers within two frequency-intensity domains are capable of decreasing (enhancing) CAP latency: one region flanks the low frequency tail, the other flanks the tip/high frequency slope regions of the latency tuning curve (TC). By contrast, amplitude enhancement typically does not flank the high frequency slope region.

One-tone suppression in the cochlear nerve of the gerbil

One-tone rate suppression has been reported several times for auditory nerve fibers of mammalian and non-mammalian vertebrates. Because its properties are very similar to those of two-tone rate suppression, the possibility exists that one-tone rate suppression is the result of an interaction within the inner ear of the suppressing tonal stimulus and some ongoing extraneous acoustic stimulus. For this reason, reports of one-tone rate suppression often elicit suspicions that the investigators were not sufficiently careful in controlling leaks in their acoustic barriers or in the electrical pathways to their acoustic drivers. Recent reports of one-tone rate suppression in pigeon basilar-papillar fibers and goldfish saccular fibers were accompanied by descriptions of measures taken to avoid such leaks. In this paper, we describe one-tone rate suppression in a mammal, the Mongolian gerbil; and we demonstrate that the background spike activity being suppressed is not driven by either external sounds coming from outside the acoustic isolation test chamber or by non-stimulus electrical inputs to the acoustic driver. The suppressed background spike activity evidently arises from sources within the animal. These sources may be non-acoustic, associated with spontaneous pre- or post-synaptic ion-channel activity; or they may be acoustic sources--internal sound or vibration generators.

Auditory brainstem function of the F1 offspring of the cross of CBA/CaJ and AU/SsJ inbred mice

Inbred strains of laboratory mice have several distinct advantages as models for examining conditions that influence the human auditory system, but the CBA/J mouse which has most often been used as a normal model has recently been found to have several disadvantages. This paper is the first report of the auditory brainstem responses (ABRs) of the F1 offspring of CBA/CaJ and AU/SsJ parents. At midlife, high-frequency ABR thresholds are lower in the F1 than in either parental genotype. Tuning curves obtained by forward masking of the ABR also display heterosis, i.e. they are narrower in the F1 than in either parental strain.

Noise-induced auditory loss: influence of genotype, naloxone and methyl-prednisolone

Inbred strains of mice have several advantages as models for human noise-induced hearing loss. However, the isogenic nature of inbred lines is very unlike the human condition, and may make this species less valuable as an auditory model. The present experiments start with two mouse genotypes having lifelong normal cochlear functions: The CBA/CaJ and the AUS/sJ inbred strains. These strains and their F1 hybrid offspring were examined for noise-induced elevation of the auditory brainstem response (ABR) threshold. The F1 line had an intermediate degree of loss and the most uniform high frequency cochlear loss. Methylprednisolone was found to protect the F1 from noise-induced losses, whereas naloxone did not.

The mouse as a model for human audition. A review of the literature

The mouse has several distinct advantages as an experimental model for human audition. Mice and humans express a similar presbyacusic and ototraumatic pattern. Several genetic mouse models also exist for conditions resembling human auditory disorders, such as otosclerosis. This paper reviews the strains of inbred mice (e.g. the CBA/J) which have recently been used as models for the normal human auditory system, describing their deficiencies. It is suggested that the F1 offspring of CBA/CaJ and AU/SsJ inbred mice would have advantages over existing models.

Frequency-specific enhancement of the cochlear compound action potential: influence of the forward masker

Under restricted frequency and intensity conditions, forward masking can result in the amplitude of the CAP being increased above its unmasked value (Henry, 1991). The present study provides a quantitative analysis of this enhancement effect. In response to forward maskers having the same frequency as the probe stimulus, central frequency (CF) enhancement varies as a function of the level of the forward masker: the lowest masker level at which it can reliably be detected is often well below the visual detection threshold of the CAP generated by the unmasked probe stimulus; the highest masker level at which it can reliably be detected corresponds to approximately 10 dB above the probe stimulus CAP threshold. A second low frequency (LF) enhancement region also exists, encompassing a narrow range of more intense maskers. CF enhancement can double the amplitude of the CAP, whereas LF enhancement is less pronounced. The magnitude of CF enhancement varies as a function of the duration of the forward masker, with longer durations generally increasing the magnitude of the effect. This duration effect, however, interacts with the level of the stimulus. Decreasing the interval between the end of the forward masker and the beginning of the probe increases the magnitude of CF enhancement.

Enhancement of the cochlear nerve compound action potential: sharply defined frequency-intensity domains bordering the tuning curve

Forward masking can either decrease or increase the response to a subsequent stimulus. In the gerbil, frequency-intensity domains of the maskers that decrease the amplitude of the compound action potential (CAP) can be plotted as the sharply defined CAP tuning curve (TC). Regions were also found over which masking increases (enhances) the amplitude of the CAP. Center-frequency (CF) enhancement domains were found in approximately 2/3 of the animals tested, in response to maskers having frequencies very near that of the probe stimulus, at levels ranging from below the CAP detection threshold to just below the tip threshold of the TC. Approximately 2/3 of the animals showing CF enhancement also displayed low-frequency (LF) enhancement, in response to a domain which borders the low-frequency tail of the TC.

Latency and amplitude compound action potential tuning curves for tonal stimuli with nontraditional envelopes

To evaluate the influence of the acoustic context on the latency and amplitude tuning curves (TCs) of cochlear nerve compound action potentials (CAPs), tonal stimuli were generated with a variety of amplitude modulation envelopes. CAPs were produced by intensity increases (onsets) and decreases (offsets) from the low ambient sound level and by intensity changes from a preexisting tonal level. Onset CAPs from ambient levels generated V-shaped TCs. However, when simultaneous masking was used with onset CAPs which were produced by a 5- to 12-dB increase from preexisting levels of approximately 65 dB SPL, TCs were W-shaped and similar in appearance to those produced by simultaneous masking of offset CAPs. The forward masking of this same CAP resulted in a very sharp V-shaped TC. These data suggest that the preadaptation to 10 ms of a moderate level of a tonal stimulus can increase the tuning of ensembles of cochlear neurons to subsequent transient amplitude changes.

Hypothermia differentially affects tuning curves generated by forward and by simultaneous masking

In the gerbil maintained at euthermic (37.5 degrees C) conditions, forward masking produces a compound action potential tuning curve (CAP TC) which is less sensitive but more sharply tuned than that which is generated by simultaneous masking. These differences between forward- and simultaneously-masked CAP TCs are minimized at hypothermic (30 degrees C) conditions. The unmasking effect occurs at both temperatures, suggesting that hypothermia does not exert these changes by eliminating two-tone suppression.

Latency and amplitude tuning curves of the N1 and N2 components of the cochlear nerve compound action potential

Compound action potential tuning curves (CAP TCs) generated by masking the N1 component of the CAP provide a means of assessing the ability of the cochlea to selectively tune to certain stimuli. This paper examines some of the factors which can influence this TC when a moderately intense (i.e. 40-80 dB SPL) probe stimulus is used. At these levels, each of the four corners of the trapezoidal stimulus envelope is capable of generating a CAP. Also, short stimulus rise times can merge the CAPs produced by the first two corners, but this does not appear to have a major effect on the CAP TC. It was shown that the N2 component of the CAP for the first corner of the stimulus is equally capable of producing a well-tuned TC. Another study has shown that, in addition to amplitude decrements, one can use latency increases as a criterion for CAP TCs. We have demonstrated that latency TCs are more finely tuned than amplitude TCs at high levels, especially when the stimulus rise time is short.

Cochlear nerve responses to waveform singularities and envelope corners

One way that discrete acoustic events may be signaled to the central nervous system is through spike synchrony over a subpopulation of cochlear axons. Each of the four corners of a trapezoidally modulated tone burst is such an event. Ordinarily, each corner comprises both an abrupt change in envelope slope and a singularity in the modulated waveform. In this study, in addition to stimuli of this sort, we employed a stimulus waveform in which a corner occurred without a waveform singularity. We obtained masker tuning curves for the CAPs corresponding to both kinds of corners and single-unit responses to both kinds of corners. The results suggest that the subpopulation of cochlear axons excited by the singularity component of a corner is distinct from that excited by the abrupt change in envelope slope.

Transient responses to tone bursts

Investigating theoretical conditions under which linearly-operating tuned structures produce click-like transient responses to onsets and offsets of trapezoidal tone bursts, we come to the following conclusions: (1) each of the four corners of the trapezoidal tone burst is capable of eliciting such a response; (2) the amplitude of the response and its dependence on the frequency of the modulated tone both depend on the phase of the modulated sinusoid at the time a corner occurs; (3) such responses will arise in structures having sufficiently steep band edges, provided that the frequency of the modulated tone is well outside the pass band of the structure--for a corner in cosine phase, the sustained slope of the low-frequency band edge must be greater than zero and that of the high-frequency band edge must be greater than 12 dB/Oct, for a corner in sine phase the sustained slope of the low-frequency band edge must be greater than 6 dB/Oct and that of the high-frequency band edge must be greater than 18 dB/Oct; (4) they will not arise in response to tone bursts whose frequencies fall within the pass band of the structure; (5) nor will they arise in response to a trapezoidal tone burst of any frequency applied to structures (such as simple microphones or drivers) following second-order dynamics and having both spectral zeros at infinity. We present theoretically derived relationships between the amplitude of transient responses and the tone-burst frequency, not only for the corners of trapezoidal tone bursts, but also for tone bursts of more general shapes. We conclude that, owing to its extraordinarily steep high-frequency rolloff, the filter associated with each cochlear axon is well suited to extracting temporal information from onset or offset singularities in modulated tones whose frequencies are above the characteristic frequency of the filter. Applying the theory to observed onset and offset responses to high-intensity tone bursts in auditory afferents of the Mongolian gerbil, we conclude that some of the responses we observed must have been sculpted in part by cochlear nonlinearities.

Detuning of cochlear action potential tuning curves at high sound pressure levels: influence of temporal, spectral and intensity variables

Action potential (AP) tuning curves (TCs), generated by probe stimuli of 60-65 dB SPL with short rise and decay (r&d) times, are less sensitive (have elevated tip thresholds) and are detuned (the frequency is shifted away from that of the probe stimulus, towards a middle frequency of the audiogram). These effects are more pronounced with forward than with simultaneous masking. TCs generated by masking tonal and narrow band noise stimuli are nearly identical, even though the spectrum is much wider for the noise stimulus. Decreasing r&d time has the same effect on TCs generated from both noise and tonal stimuli, even when it only measurably increases the acoustic splatter of the latter. Detuning appears to be related to a temporal-intensity interaction.

Effects of acoustic and sensory variables on masking tuning curves of the offset auditory brain-stem response in the rodent

Simultaneous sinusoidal masking of the auditory brain-stem response (ABR) which is generated by the offset of a tone produces a W-shaped masking tuning curve (TC) in the gerbil, rat, mouse, and guinea pig. The probe stimulus offset must be very rapid in most animals. Lesions of the contralateral ear, strychnine blockage of the olivocochlear bundle, or removal of the ipsilateral outer ear do not alter the basic properties of the offset masking TC. Increasing the simultaneous masker duration selectively increases the tuning of the offset masking TC peak. Masking only the latter portions of the probe stimulus does not alter the shape of the offset masking TC.

Dietary restriction and presbyacusis: periods of restriction and auditory threshold losses in the CBA/J mouse

Dietary restriction was imposed on CBA/J mice, animals which develop presbyacusis late in their lives. Animals restricted for their whole lives, as well as those restricted after midlife, had less presbyacusis than did control mice fed ad libitum. Dietary restriction did not increase the life spans of these mice. Restriction until midlife did not protect from presbyacusis, nor did it increase life span. In this genotype, dietary restriction protects against hearing loss only if it occurs at the age of most rapid decline of cochlear function.

Forward masking and unmasking of the offset cochlear compound action potential of the gerbil: comparison with suppression areas of the onset cochlear compound action potential

Simultaneous and forward maskers were used to generate 'onset' and 'offset' compound action potential tuning curves (TCs) in the gerbil. The simultaneously masked offset TC, generated in response to a 16 kHz, 65 dB SPL probe stimulus, is W-shaped, with a low frequency tip at 11 kHz, a high frequency tip at 20 kHz, and a peak which occurs at 16 kHz. The 16 kHz forward masked onset TC has a single tip which occurs at 11 kHz. Although it lacks the finely tuned peak and high frequency tip of the stimultaneously masked offset TC, its single tip is more finely tuned than the low frequency tip of the simultaneously masked offset TC. Normalizing these two TCs [(1977) J. Acoust. Soc. Am. 62, 1048-1051] produces a figure which resembles 11 kHz onset TCs with their nonoverlapping regions which are analogs of two-tone suppression (2TS). A similar pattern occurs when probe stimuli at frequencies from 13 to 24 kHz are used to generate offset TCs; i.e., the forward masked offset TC resembles an onset TC and normalizing it to a simultaneously masked offset TC produces areas which resemble analogs of 2TS. Unmasking of the forward masked onset TCs [(1979) Hear, Res. 1, 133-154] produces regions of 2TS which are very similar to those produced by unmasking the forward masked offset TC which is generated by a higher frequency tone. These regions of 2TS for the Offset TC, as determined by unmasking, are very similar to the analogs of 2TS described above.

Compound action potential offset and onset tuning curves generated by simultaneous masking in the mongolian gerbil. Effects of varying the intensity of the probe stimulus from 55 to 85 dB SPL

Action potential tuning curves (TCs) were produced by simultaneous masking of both the onset and offset of tone bursts. Offset TCs are much more sharply tuned than onset TCs when both responses are generated by stimuli at SPLs of 55 to 75 dB. The onset TC tip and the offset TC high frequency tip thresholds accurately reflect increases of the probe SPL, but the low frequency tip and the peak of the offset TC compress this change. With increasing probe SPL, the onset TC and low frequency TC tip (but not the high frequency tip and peak) of the offset TC become progressively detuned.

Auditory physiology and behavior in RB/1bg, RB/3bg, and their F1 hybrid mice (Mus musculus): influence of genetics, age, and acoustic variables on audiogenic seizure thresholds and cochlear functions

In the early 1950s, Frings and Frings began a process of selection for audiogenic seizure susceptibility and resistance in albino mice. The present study was conducted to examine behavioral and cochlear functions in the inbred descendants of these mice. The cochlear action potential (AP) thresholds of the susceptible RB/1bg inbred mice were abnormally high, and the resistant inbred RB/3bg mice had normal AP audiograms. The F1 hybrid showed heterosis for its cochlear function. Only the RB/1bg was susceptible to audiogenic seizures on the first acoustic exposure. Thresholds for the successive components of their audiogenic seizures were determined in response to narrow bands of noise. These paralleled the AP thresholds of RB/1bg mice (r = .89). This RB/1bg mouse showed little age-related cochlear loss, which probably accounts for its robust sensitivity to audiogenic seizures over most of its lifespan. Earlier studies had demonstrated that the susceptible RB line had a robust AP, but little or no cochlear microphonic (CM). The susceptible RB/1bg had well-defined AP and CM responses at low frequencies. The nonsusceptible RB/3bg mice were more resistant to acoustic priming than another mouse (CBA/J) strain with a similar audiometric profile.

Offset tuning curves generated by simultaneous masking are more finely tuned than those generated by forward masking

The rapid ending of a tone produces an evoked potential which has different properties than that which is produced by the sudden onset of a tone. At the level of the round window, the offset N1 N2 follows the ending of the cochlear microphonic (CM) by approximately the same amount of time as does the onset N1 N2 to the onset of the CM. Both onset and offset responses are abolished with cochlear lesion. Continuous masking was used to generate tuning curves (TCs) from the NI-PI component of the evoked potential recorded from the round window of the gerbil. Those evoked potentials generated in response to the tone onset were complementary in appearance to those generated in response to the tone offset. TCs generated by continuous masking of the NI-PII component of the auditory brainstem response (ABR) of the gerbil show the same pattern. When it is generated by simultaneous masking, the midfrequency offset TC in the gerbil and mouse is W-shaped. It has two well tuned tips which occur at frequencies below and above that of the probe stimulus used to generate the TC. It also has an even better tuned peak occurring at or slightly above the probe stimulus frequency, which becomes sharper as the masker sound pressure level (SPL) is increased from 50 to over 80 dB. Because the midfrequency onset response is approximately 40 dB lower than the midfrequency offset response, probe stimuli for onset TCs are generally set at lower SPLs. When the onset probe stimulus is set to the same level as that of the offset probe, the Q10 dB of the offset TC may be up to 10 times the value of the Q10 dB of the onset TC. The offset TC generated in the CBA/J mouse by forward masking is quite different from that produced by simultaneous masking. Both forward and simultaneous conditions utilized a 40 ms duration tone to mask the PI-NI component of offset and onset ABRs of the mouse which were evoked by a 10 ms duration, 32 kHz tone, presented at an interstimulus interval of 160 ms. Forward masking (when compared with simultaneous masking) resulted in a more sharply tuned onset TC. But the offset TC was much less sharply tuned in the forward masking condition. This suggests that the offset response may reflect functions which are involved with fine tuning at moderate to high intensities in the presence of simultaneous sounds of similar spectral characteristics.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of dietary restriction on presbyacusis in the mouse

If dietary restriction can extend the human life span, it would be useful to know whether presbyacusis would continue its normal pace. This question was experimentally addressed, using the mouse as a model. Alternate-day feeding and fasting resulted in restricted mice of the AKR and AU/Ss inbred strains weighing less than their continuously fed controls. Restriction did not increase the life span or alter presbyacusis of the AKR mouse, but it improved both functions in the AU/Ss mouse. Their life spans were increased by 40%, and cochlear functions were better than controls at every age at which animals of both groups were still alive. Nonetheless, the oldest remaining restricted AU/Ss mouse had greater cochlear loss than was seen in any AU/Ss control mice. This study demonstrates that dietary restriction can slow the cochlear losses in a mammal which has a presbyacusis condition similar to that of humans.

Cochlear function and audiogenic seizures: developmental covariance in the LP/J mouse

Innate susceptibility to audiogenic seizures develops and declines at varying rates and ages, depending upon genotype and environmental conditions. Auditory dysfunctions have been experimentally produced which induce susceptibility in otherwise nonsusceptible mouse strains. In order to more closely correlate cochlear functions and audiogenic seizures in genetically susceptible mice, both measures were obtained from LP/J mice, at ages ranging from 8 to 120 days. Susceptibility to sound-produced convulsions was first noted at 12 days, was maximal from 18 to 32 days, declined rapidly by 40 days, and disappeared by 120 days of age. Cochlear nerve-evoked potential thresholds were very high at 12 days, were lowest between 18 and 32 days and increased thereafter. The correlation between susceptibility and cochlear thresholds was greatest for high frequencies (r = -.93), intermediate for midfrequency (r = -.77), and poorest for low frequencies (r = -.56). It was concluded that either genetic or environmental factors which produce an intermediate level of high frequency cochlear damage in the young mouse will produce susceptibility to audiogenic seizures.

Ossicular and otic capsular lesions in LP/J mice

Otosclerosis is an inherited disease in which abnormal bone growth results in ossicular fixation and hearing loss. Although the disease affects up to 10% of humans, it has not been observed in other animals. The LP/J inbred mouse has been found to develop abnormal bony lesions of the middle ear which resemble human otosclerosis. In this study of 113 temporal bones from LP/J mice, we found that the lesions develop after puberty and involve only the ossicles and the otic capsule. The most frequent site of involvement was the incus (46.9%), followed by the anterior crus of the stapes (31.3%), the malleus (14.2%), the otic capsule (14.2%), and the stapes footplate (8%). Cochlear hair cell loss was progressive throughout the lifespan of these animals. Although this disease is not identical in histologic appearance to human otosclerosis, understanding its disease process may shed light upon the pathophysiology of the human disease.

ON and OFF components of the auditory brainstem response have different frequency- and intensity-specific properties

When the ear of the mouse is stimulated with a tone burst of sufficiently long duration, a stimulus offset evoked potential is generated which mirrors, in some respects, the onset auditory brainstem response (ABR). The general waveform and interpeak latencies suggest this offset response is generated within the cochlea and auditory brainstem. But when visual detection threshold audiograms are made for these two responses, their shapes are not similar. The onset ABR thresholds reflect the behavioral detection thresholds, being lowest at midfrequency, while the offset thresholds are highest at midfrequency in the normal hearing CBA/J and RB3/bg mice. The LP/J mouse, with a mixed (conductive and sensory) dysfunction, shows a different relationship between its onset and offset thresholds. The onset- and offset-ABRs of the normal mouse also differ from each other in the slopes of their amplitude input-output functions.

Extra-high-frequency auditory thresholds: fine structure, reliability, temporal integration and relation to ear canal resonance

Three trained subjects were repeatedly tested for detection of auditory thresholds at 27 frequencies, from 2 to 18 kHz. A double-blind procedure was used with the method of adjustment, and the quasi-free-field delivery system of Osterhammel et al. [Scand. Audiol. 6:91-95, 1977] was used to monaurally stimulate the ear. A very reliable series of high- and low-threshold changes, termed the fine structure of the extra-high-frequency audiogram, was observed from 8 to 16 kHz. All 6 ears had a major low-threshold region at or near 13.5 kHz, and up to 3 other replicable sensitive frequencies along this one octave range. Thresholds were invariant over 3 conditions of stimulus presentation (constant tone; 500 ms duration, 25 ms rise and decay, 1 s interstimulus interval; and 100 ms duration, 1.5 ms rise and decay, 200 ms interstimulus interval). As the duration of the tone was decreased from 100 to 5 ms, temporal integration (an average of approximately 2.5 dB threshold increase as the duration was halved) was noted. Although temporal integration was slightly less at higher frequencies, the shape of the audiograms remained essentially unchanged as the stimulus duration was decreased to 5 ms. There was no consistent relationship between the maximally sensitive high frequencies and the amount of temporal integration. But there was a general association between the maximally sensitive high frequencies and the resonant frequency characteristics of that particular ear.

Tuning of the auditory brainstem OFF responses is complementary to tuning of the auditory brainstem ON response

Continuous masking studies show a complementary pattern of effects on the auditory brainstem responses (ABRs) which are generated by the onset and by the offset of a midfrequency tone. The masking profiles of the two responses are almost opposite with a probe stimulus frequency of 32 kHz (16-32 kHz is the midfrequency region for the CBA/J mouse). The Offset and Onset ABR tuning curves (TCs) also reveal very different properties at the midfrequencies of 16, 20, 24 and 32 kHz. The Offset TC is exquisitely sensitive to masking by very low intensity stimuli at a narrow range of frequencies which are lower than the probe stimulus frequency. Continuous masking produces a well-tuned low frequency tip to the Offset TC. For Offset TCs generated in response to midfrequency tones, the Q+10 dB of this tip averages 8.3. Masking at this low frequency tip of the Offset TC has no observable effect on the Onset ABR. The Offset ABR is also sensitive to masking by a narrow range of frequencies which are higher than the probe stimulus frequency. This occurs at an intensity which also has no observable effect on the Onset ABR. The Q+10 dB of this high frequency tip averages 9.2. The average frequencies where these Offset TC tips occur fit the cubic difference formula (2f1-f2), which describes a distortion product of two-tone suppression. At low probe stimulus frequencies, there is only a high frequency Offset TC tip; at high stimulus frequencies, only a low frequency tip. The high frequency tip has a higher threshold and appears more susceptible to metabolic disturbance. The Offset ABR TC also has a peak which corresponds to the probe stimulus frequency. Continuous masking with the stimulus frequency produces nonmonotonic enhancement of the Offset ABR, while it simultaneously reduces the magnitude of the Onset ABR. The tuning of this Offset TC peak (measured as Q-10 dB) is almost always much sharper than the corresponding Onset TC tip in the same mouse. These values for midfrequency stimuli average 6.2 for the Onset, and 13.6 for the Offset TCs. This fine tuning of the Offset TC at the probe stimulus frequency occurs at SPLs from 50 to more than 90 dB.

Extra-high-frequency noise remotely masks and alters temporal integration at lower frequencies

A quasi-free-field technique was used to assess the effects of extra-high-frequency-band masking on detection threshold of middle and high (1-7 kHz) frequencies. At an SPL of 60 dB, the 10- to 20-kHz masker produced a slight amount of masking. Increasing the masking level by another 5 dB produced a disproportionate increase of the detection thresholds. This increase was greatest in response to 4- and 5-kHz stimuli, and was detected with both constant and pulsed tones. Decreasing the bandwidth of the masker reduced the magnitude of the effect, but not the frequency-specific pattern of the remote masking. Extra-high-frequency masking also increased the amount of temporal integration at middle and high frequencies. The data are discussed in reference to peripheral and central neural changes associated with sensorineural hearing loss.