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PubMed1979

A model of loudness summation applied to noise-induced hearing loss.

The main contention of this paper is that Zwicker's model of loudness summation is applicable to observers with noise-induced hearing loss when certain parameters of the model are modified. Two types of measurement were obtained in observers with normal hearing and noise-induced hearing loss: loudness summation as a function of level and narrow-band masking. These measurements provided a basis for modifying the parameters of the model. Results suggest that the model of loudness summation is applicable to observers with noise-induced hearing loss when the presence of recruitment and reduced frequency selectivity is taken into account.

Hearing Loss, Noise-Induced↗
PubMed1982

Intensity coding in the auditory periphery of the cat: responses of cochlear nerve and cochlear nucleus neurons to signals in the presence of bandstop masking noise.

The dynamic range over which fine intensity discrimination is possible has been reported to be largely unaffected by limitation of the spread of neuronal activity to neighbouring frequency regions by bandstop noise masking. We have therefore examined the responses of cochlear nerve and nucleus neurons to tone and noise signals in the presence of a bandstop masking noise designed to be comparable to that employed in the psychophysical experiments. Under these conditions, the vast majority of cochlear nerve fibres were saturated by sound levels at which some 50% of our sample of cochlear nucleus neurons still responded to signal level differences. The extended dynamic ranges of these cochlear nucleus neurons was shown to be a result of activation, by the masking noise, of the lateral inhibitory side-bands 'biassing' the neuron's discharge. A small proportion of cochlear fibres, having low spontaneous discharge rates and showing strong two-tone suppression effects, demonstrated analogous but not so pronounced effects. It is unclear in what form information on the level of stimuli under these conditions is transmitted by the majority of apparently saturated cochlear nerve fibres, but several possible mechanisms are discussed.

Animals↗
PubMed1983

N1 latency following acute pure-tone trauma.

The latency of the N1 component of tone burst evoked compound action potentials was examined in chinchilla following acute pure-tone trauma. At and below the trauma frequency (4 kHz) the N1 latency at threshold generally increased, while above the trauma frequency it decreased; tonotopically paralleling pitch shifts observed in humans following pure-tone trauma. When N1 latency at threshold is considered across animals as a linear function of dB SPL at threshold, after trauma a high degree of linear correlation was found at 6 and 8 kHz, while a low degree of linear correlation was found at 4 kHz. An interpretation and the significance of the data are discussed.

Animals↗
PubMed1983

Frequency selectivity of phase-locking of complex sounds in the auditory nerve of the rat.

Frequency selectivity of single auditory nerve fibers in the auditory nerve of the rat was studied using pseudorandom noise as the stimulus. The noise was lowpass filtered ternary m-sequences. Period histograms of the discharges of single auditory nerve fibers, locked to the periodicity of the noise, were cross-correlated with one period of the noise to obtain estimates of the impulse response. These cross-correlograms were subsequently Fourier transformed to obtain estimates of the frequency transfer functions. Earlier results obtained using noise that was based on binary sequences as the stimulus showed a systematic dependence on stimulus intensity of the bandwidth and center frequency of the computer transfer functions. The results of the present study confirmed this dependence and showed that a linear model based upon first-order cross-correlations fit the histograms of response. It is concluded that phase-locked activity of single auditory nerve fibers accurately reproduces the half-wave rectified motion of the basilar membrane over a large range of sound intensities.

Animals↗
PubMed1983

Neural correlates of cubic difference tones in the medial geniculate body of the cat.

Single unit responses to the cubic difference tone CDT (2f1 - f2 = CF) and the difference tone DT (f2 - f1 = CF) were studied in the medial geniculate body (MGB) of the cat. Out of 66 units tested with CDT stimuli and having characteristic frequencies (CF) below 10 kHz, 77% gave a response to the two-tone combination stimulus. The component tones when presented alone evoked no responses, or in some cases a response pattern that was different from the one observed for the combination tone. The CDT response pattern was always similar to that seen for a pure tone at the CF. The threshold of response for the CDT was 10-70 dB higher than for a pure tone stimulus at the CF. The few units which were phase-locked could be synchronised with the CF, CDT, or DT, depending on the particular stimulus conditions. The index of synchrony was in many cases found to be higher for CDT responses than for a pure tone at CF.

Animals↗
PubMed1983

Auditory-nerve correlates of loudness summation with stimulus bandwidth, in normal and pathological cochleae.

The firing of guinea pig auditory nerve fibres was measured in response to bands of noise of many different bandwidths, centre frequencies and intensities. The total amount of activity in the auditory nerve fibre array was calculated as a function of the bandwidth and intensity of the stimulus. As the bandwidth of the stimulus increased, while its net intensity was kept constant, the total firing rate increased steadily. There was no sign of a breakpoint corresponding to the critical bandwidth, seen in psychophysical judgements of loudness with similar stimuli. Moreover, the slope of the relation between total activity and stimulus bandwidth was particularly shallow in fibres with shallow tuning curves. The results suggest that (i) while loudness may in broad terms be related to the total amount of activity in the auditory nerve, this is not true in detail, (ii) the critical bandwidth in loudness summation does not relate to the resolution bandwidth of the cochlea, and (iii) the slope of the psychophysical loudness summation function may be able to give information about the sharpness of neural tuning.

Acoustic Stimulation↗
PubMed1983

Multichannel electrical stimulation of the auditory nerve in man. II. Channel interaction.

A multichannel cochlear implant can be an effective prosthesis only if its channels are independent of each other. Presumably independence is achieved by stimulating different populations of surviving neurons. Two types of interaction might occur between channels: electrical current field summation peripheral to stimulation of the nerves and neural-perceptual interaction following stimulation. Two psychophysical techniques to assess channel independence are discussed. In one technique a masker is presented on one channel in order to adapt the nerves responding to that channel. The forward masked threshold of a signal is then measured on all other channels and elevation of threshold is assumed to indicate overlapping neural populations. In the second procedure channel interaction is evaluated by measuring the loudness summation of stimuli presented simultaneously to two channels. The magnitude, distribution, and phasic components of the loudness summation are measures of interaction between channels. Data from two subjects suggests that monopolar stimulation produces broader interaction patterns than bipolar stimulation as a function of electrode separation. Considerable differences in the extent of channel interaction were observed between the two subjects, possibly because of the difference in the absolute current levels needed for equivalent sensation levels.

Auditory Threshold↗
PubMed1984

Dynamic aspects of guinea pig inner hair cell receptor potentials with transient asphyxia.

DC and AC receptor potentials of cochlear inner hair cells in response to tone bursts of various frequencies and intensities were continuously measured during and following periods of transient asphyxia. The effects of asphyxia were most pronounced for low sound pressure level (SPL) acoustic stimuli near the characteristic frequency (CF) of the inner hair cell, leading to vulnerability of the 'tip' of the cell's frequency tuning curve (FTC). The resulting changes in the shape of the FTC are, first, a reduction in tip criterion sensitivity of 10-20 dB without significant loss in sharpness of tuning. Later, when the full effect of 30-45 s asphyxia occurs, tip sensitivity loss between 30 and 65 dB is accompanied by greatly broadened tuning and a shift downward in frequency of the CF by greater than 1/4 octave. The CF shift is due to a progressive loss of high frequency sensitivity. The linear segment of the input-output (intensity) function, plotted as log DC receptor potential versus SPL (at the original CF), becomes longer during the early phase asphyxia, and the slope of the segment declines by 50%. At high SPLs, for all frequencies, the time course of the receptor potential change was similar in shape to that exhibited by the endocochlear potential (EP). In particular, for high sound levels, the recovery of response matches the EP while for low level tip frequency sounds recovery is protracted. No difference between the decline of the AC and DC receptor potentials at CF was observed. Inner hair cell resting membrane potential (Em) hyperpolarized during asphyxia by 2-6 mV, correlating with the change in EP according to a ratio of 1/10 (Em/EP).

Animals↗
PubMed1984

Single-neuron labeling and chronic cochlear pathology. III. Stereocilia damage and alterations of threshold tuning curves.

Tuning curves were obtained from 100 to 150 auditory-nerve fibers spanning the range of characteristic frequencies (CFs) in each of eight cases of permanent noise-induced and three cases of permanent kanamycin-induced threshold shift. In each ear, from one to six neurons were intracellularly labeled with horseradish peroxidase. Locating the labeled terminals in plastic-embedded surface preparations of the cochlea enabled us to accurately correlate particular tuning-curve abnormalities with the condition of the sensory cells generating them. The correlations between structural and functional changes suggest that a normal tuning-curve tip requires that the stereocilia on both the IHCs and OHCs (especially those from the first row) be normal. Selective damage to the OHCs is associated with elevation of the tips and hypersensitivity of the tuning-curve tails. This tuning-curve pattern also originates from cochlear regions at the basal border of hair cell lesions where the local hair cells (and their stereocilia) appear completely normal at the light-microscopic level. Total destruction of the OHCs in a region in which the IHCs appear normal (as can happen in cases of kanamycin poisoning) is associated with bowl-shaped tuning curves which appear to lack a tip. Combined damage to the IHCs and OHCs (as typically happens in cases of acoustic trauma) is invariably associated with elevation of both tips and tails on the tuning curve. A framework for the interpretation of the results is suggested in which the activity of the OHCs is transmitted via the tectorial membrane to the tall row of stereocilia on the IHCs.

Animals↗
PubMed1984

Single-neuron labeling and chronic cochlear pathology. IV. Stereocilia damage and alterations in rate- and phase-level functions.

The rate and phase of auditory-nerve response to tone bursts were studied as a function of stimulus level in normal and acoustically traumatized animals. The rate- and phase-level functions of normal auditory-nerve fibers are often separable into a low-intensity component (component I) and high-intensity component (component II), as defined by a dip in the rate function and a simultaneous abrupt shift in the phase function at stimulus levels near 90 dB SPL [10,12,9]. Baseline data are established by defining the relation between stimulus frequency and the characteristic frequency and spontaneous discharge rate of a fiber normally required for the appearance of these two components in the response. Abnormalities of the level functions are shown to occur in acoustically traumatized ears. Noise-induced threshold shift is often characterized by selective attenuation of component I. In some instances, it appears that component I has been eliminated, leaving a response which is identical in threshold, phase and maximum discharge rate to a normal component II. Results of single-unit labeling in such a case suggest that the selective attenuation of component I is associated with selective loss of the tallest row of stereocilia on the inner hair cells (IHCs). It is suggested that component I is normally generated through an interaction between the outer hair cells and the tall row of IHC stereocilia, while component II requires only the shorter row of IHC stereocilia.

Animals↗
PubMed1984

Frequency threshold curves and simultaneous masking functions in high-threshold, broadly-tuned, fibres of the guinea pig auditory nerve.

Tuning curves for simultaneous masking were measured electrophysiologically, in single fibres of the auditory nerve. The recordings were made in guinea pigs with cochlear hearing losses. The masking paradigm was an analogy of that used in the determination of psychophysical tuning curves in man. Below the probe frequency, the slope of the masking function was similar to that of the frequency-threshold curve. Here, changes in the slope of the frequency-threshold curve with hearing loss were closely mirrored by changes in the slope of the masking function. Above the probe frequency, however, the masking function for low-threshold fibres had a shallower slope than the frequency-threshold curve. When the high-frequency slope of the frequency-threshold curve became shallower with hearing loss, the changes were not mirrored in the masking function, until the threshold was raised to 70-80 dB SPL. The results are discussed in terms of the influence of cochlear nonlinearity on frequency resolution, and the vulnerability of the nonlinearity.

Animals↗
PubMed1984

Profile analysis and level variation.

This study examines the effects of random level variation, a method used in studies of profile analysis [3-6,14,15]. Presentation levels for a complex of sinusoids are varied randomly on each interval of a two-interval, forced-choice detection task in which subjects are required to detect an increment on one of the sinusoidal components of the complex. Three experiments are reported. The first experiment examines the effect of the range of level variation. The second is concerned with the effects of the median level about which the presentation levels vary. The third experiment is designed to provide a within-trial analysis of the effect of the differences in presentation levels. As the range of level variation is increased, ability to detect the increment decreases. The results indicate that detection performance is best at moderate intensity levels and decreases at lower and higher levels. Finally, the difference in levels within a single trial has little if any effect.

Acoustic Stimulation↗
PubMed1984

Baclofen reduces tone-evoked activity of cochlear nucleus neurons.

Recent evidence suggests that an excitant amino acid may be a neurotransmitter at acoustic nerve synapses in cochlear nucleus (CN). Release of excitant amino acids is reportedly reduced by baclofen, a lipophilic GABA-mimetic used to treat the spasticity of multiple sclerosis and spinal injury. Microiontophoresis of (-)baclofen suppressed spontaneous and tone-evoked activity in CN neurons. GABA inhibited the responses of most neurons responsive to (-)baclofen. However, iontophoresis of these two substances onto the same CN neuron resulted in dramatic differences in time course to maximum effect and to recovery. Onset and offset of (-)baclofen-induced firing reduction were gradual at all doses (currents), but even the highest doses rarely caused total suppression of firing. Inhibition of firing by GABA was abrupt, and total suppression was frequently observed over the range of doses used. GABA desensitization (fading) commonly occurred while the (-)baclofen response never faded. The same CN neurons were also suppressed by D-alpha-aminoadipate, which blocks certain excitatory amino acid receptors, while the GABA antagonist bicuculline had no effect on the (-)baclofen response. These findings support the hypothesis that an excitant amino acid may be a transmitter at acoustic nerve synapses in CN.

2-Aminoadipic Acid↗
PubMed1984

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.

Acoustic Stimulation↗
PubMed1985

Threshold and loudness functions for pulsatile stimulation of cochlear implants.

Thresholds and loudness estimates were measured for biphasic pulsatile electrical stimulation of the auditory nerve. Measures were collected as a function of the parameters: pulse duration, and pulse rate. The results indicate that the sensations of threshold and loudness are determined by a complex function of the stimulating current waveform. For stimuli with the same charge, maximum loudness is seen at the shortest pulse durations, and a secondary maximum is seen at pulse durations of 2-3 ms/phase. It is possible that the secondary peak in the loudness function and the slow growth of loudness just above threshold for long pulses are indications of dendrite survival near the electrode. If this interpretation is valid, these measures could lead to perceptual tests of peripheral nerve viability. In addition, a speech processor device could use these measures to equalize the loudness of stimuli with different pulse durations and pulse rates.

Aged↗
PubMed1985

Characteristics of tone-pip response patterns in relationship to spontaneous rate in cat auditory nerve fibers.

The responses of single auditory nerve (AN) fibers in the cat were recorded in response to 25 ms tone pips. Peristimulus time histograms (PSTH) of discharge patterns recorded from fibers with high spontaneous rates (high SRs), show that the discharge rate rapidly adapts to a much lower steady-state level over a 15 ms period with shorter times for units with best frequencies (CFs) greater than 5 kHz. The PSTHs of auditory nerve fibers with low SRs do not show this pattern of rapid adaptation. Differences between the high and low SR populations include higher thresholds, better tuning, and longer latency in the low SR population. The peak-to-steady-state discharge ratio is an increasing function of SR and CF; it varies from 1.0 for fibers with SR = 0 to over 8 for fibers with high SRs and CFs near 10 kHz. This ratio increases with increasing stimulus intensity and stimulus recovery time. The high SR population shows a number of responses to transients which are weak or absent in the low SR population. Increasing the recovery time shortened the latency of both high and low SR AN fibers by as much as 1 ms. A number of other response properties of AN fibers are also reported that are important when interpreting the responses of cochlear nucleus neurons to tone pips.

Acoustic Stimulation↗
PubMed1985

Origin of latency shift of cochlear nerve potentials with sound intensity.

Compound action potentials were recorded from the round window in anesthetized rats in response to tonebursts and to continuous tones. The responses to tonebursts were compared to cross-correlograms of the responses to continuous tones that were amplitude modulated by pseudorandom noise. The cross-correlograms were similar in shape to the responses to tonebursts, but the latencies of the two cross-correlogram peaks decreased less when the sound intensity was increased from near threshold values than did the latencies of the peaks in the responses to tonebursts. When an unmodulated tone was added to these stimuli, the latencies of peaks in the response to tonebursts increased as the intensity of the unmodulated tone was increased. However, the effect of an unmodulated tone on the latencies of the peaks in the cross-correlograms was more complex: when the frequency of the unmodulated tone was below that of the modulated tone, there was a decrease in the latency of the peaks in cross-correlograms. This decrease in latency was a function of the intensity of the unmodulated tone, and was similar to the decrease in latency seen when the intensity of a single modulated tone is increased. These results are interpreted as supporting the findings of earlier research, which showed that the cross-correlogram of the unit response to amplitude-modulated tones has a latency that is nearly independent of the stimulus intensity. In addition, the data indicate that the decrease in the latency of the peaks in the cross-correlograms of the gross response from the round window is a result of nonlinearity of the cochlear frequency analyzer and is not directly related to neural excitation in the cochlea.

Animals↗
PubMed1985

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.

Animals↗