Behavioral Auditory Thresholds Research Articles

Cryoprobe-induced apical lesions in the chinchilla. II. Effects on behavioral auditory thresholds.

Lesions of the hair cells in the cochlear apex were produced by a miniature cryoprobe and changes in behavioral auditory thresholds were measured. Monauralized adult chinchillas were behaviorally trained using operant procedures to produce pure-tone audiograms at frequencies from 63 Hz to 40 kHz. Following collection of baseline thresholds, the apical and middle turns of the experimental ear were visualized through a hole drilled in the bulla and a copper cryoprobe that had been cooled in liquid nitrogen was placed on the apical turn of the cochlea. Post-lesion threshold shifts from two subjects showed a flat loss of approximately 20 dB restricted to frequencies below either 710 Hz or 1 kHz; thresholds were normal at higher frequencies. The cytocochleograms, prepared from the ears following completion of threshold testing, show an almost complete loss of both inner and outer hair cells in the apical-most 20% of the cochlea with an abrupt transition region to areas of normal-looking hair cell populations. The relationship between the frequencies at which hearing was impaired and the location of missing hair cells along the basilar membrane is in agreement with the frequency-place map for the chinchilla of Eldredge et al. [(1981) J. Acoust. Soc. Am. 69, 1091-1095]. The magnitude of the loss, however, is less than might be expected based on comparison with threshold shifts produced by similar pathology in the basal turns.

An electroencephalographic evoked response correlate of auditory adaptation

Normal-hearing subjects were evaluated to determine whether changes in behavioral auditory thresholds, in both a heterophonic and homophonic listening condition, would produce equal two dissimilar changes in the amplitude and latency of the auditory evoked potential. Each subject completed a preadaptation loudness balance prior to any stimulation. A 2000-Hz pure tone was then presented to the subject's right ear for a period of 7 min at a level of 60 dB SPL. During this 7-min adapting period four AER runs consisting of 32 tone pips superimposed on the adapting tone were presented. Finally, a perstimulatory loudness balance was completed while the adapting tone had been on for 7 min. Measurements of amplitude and latency of the evoked potentials were analyzed and results suggest that more adaptation occurred for the homophonic condition. However, a significant amount of decrement in the amplitude of the monaurally elicited AER was measured for both the homophonic and heterophonic condition. It was suggested that AER may be a sensitive measurement technique of auditory adaptation.

Correlations between cochlear hair cell loss and shifts of masked and absolute behavioral auditory thresholds in the house mouse.

Masked and absolute behavioral auditory thresholds were measured in kanamycin-treated C3H mice with a significant damage or loss of outer hair cells (OHCs) near the cochlear base, and in 17--24-month-old NMRI mice with a significant loss of OHCs and inner hair cells (IHCs) in the basal region and of OHCs near the apex. A loss of IHCs is correlated with an increase in absolute thresholds. A moderate damage or loss of OHCs in a cochlear region had no effect on absolute thresholds but correlated with a decrease and other changes of characteristics of masked thresholds in the respective frequency regions. OH C-damage makes masking by continuous white broad-band noise less efficient. It is concluded that masking and critical bands depend on the function of OHCs, which must be assumed to influence intracochlearly the activity of IHCs in processing the information to the brain.

Differences in Auditory Sensitivity Between Guinea Pig and Chinchilla

Species differences in (a) acoustic impedance at the tympanic membrane and (b) the ratio of sound pressure at the tympanic membrane to sound pressure in the field (Pd/Pf) contribute significantly to species differences in (1) behavioral auditory thresholds and (2) sensitivity to noise-induced cochlear fatigue. Chinchillas exposed continuously for two or more days to an octave band of noise centered at 0.5 kHz at 105 dB SPL or centered at 4.0 kHz at 80 dB SPL show shifts in behavioral auditory thresholds of more than 50 dB at appropriate frequencies and corresponding loss of sensitivity for cochlear microphonic responses. Guinea pigs show only trivial loss of CM under the same exposure conditions. CM measurements indicate that in the chinchilla the transmission ratio, bulla closed/bulla open, is approximately 0 dB, but at low frequencies in the guinea pig transmission ratios of about −18 dB corresponding to impedance changes described by Zwislocki [J. Acoust. Soc. Amer. 35, 1034 (1963)] are confirmed. Also, in the guinea pig resonances for Pd/Pf occur at 7.5 to 10 kHz in contrast to the resonances at 2.5 to 5 kHz in the chinchilla [G. von Bismarck, M. S. Thesis, MIT, Cambridge, Mass. (1967)].

Cochlear potentials and electron microscopy applied to the study of small cochlear lesions.

Many investigators have attempted to define the relations among behavioral auditory thresholds, cochlear potentials and cochlear pathology after excessive exposure to noise. We have observed that some noise exposed chinchillas have fewer missing sensory cells yet lower cochlear potentials than nonexposed controls. By comparing the cochleae of animals with the above characteristics, we found that the noise exposed ear had many misshapen outer hair cells, possible loss of some of the peripherally arranged mitochondria in the same cells and an increase in vesicles, vacuoles and smooth endoplasmic reticulum in the afferent nerve fibers in the region of the organ of Corti from which the low cochlear potentials were recorded. It is clear from these findings that hair cell counts alone do not always give a true indication of the functional status of the cochlea. In describing the permanent anatomical effects of a particular noise exposure on the inner ear, the condition of the cells and nerve fibers which remain after injury must be determined in addition to the number of sensory cells and nerve fibers which were lost.

Temporary threshold shifts in chinchilla: electrophysiological correlates.

Carder and Miller [Trans. Amer. Acad. Ophthalmol. Otolaryngol. 75, 1346–1354 (1972)] exposed chinchillas for seven days to an octave band of noise centered at 500 Hz and at 95 dB SPL to produce temporary shifts of behavioral auditory thresholds (TTS) of about 48 dB which required four to seven days to recover to normal. In the present study physiological potentials were measured about 5, 24, and 48 h after exposures to the same noise for two or three days. Cochlear microphonic (CM) responses and dc endocochlear potentials were measured in each of the three cochlear turns. Input-output functions for whole-nerve action potential (AP) responses to clicks and visual detection levels (VDL) for early averaged evoked responses (AER) arising in the brain stem were also measured. Endocochlear potentials were unchanged. Shifts of sensitivity for CM in the second and third turns showed the closest numerical correspondence to behavioral TTS. Loss of sensitivity for AP was greater. Changes in VDL for the AER were consistent with behavioral TTS. This pattern of physiological changes was consistent with a disorder in the mechano-electric modulation by the hair cells plus failure to synchronize primary neural responses. Just-detectable loss of outer hair cells was also observed anatomically.

Cochlear and Neural Auditory Mechanisms in the Raccoon

Electrophysiological measures of sensory and neural auditory responses were recorded in adult raccoons, from which behavioral auditory thresholds had previously been obtained. The characteristics of the peripheral auditory mechanism were evaluated through ac cochlear potential measures of frequency and intensity recorded from the round window. Neural processes were investigated over a range of intensities by measuring the N1 and N2 components of the click evoked potential recorded at the round window. The auditory areas of the cerebral cortex were mapped for click stimuli, and an intensity function was obtained from the most active location. The similarity between the behavioral thresholds and the frequency spectrum of the cochlear potential measures, and, the relationship of the electrophysiological data to that of other mammalian species will be discussed. [This research was supported by NIMH, Public Health Service, and Higgins funds to E.G. Wever, Department of Psychology, Princeton University.]

Absolute Behavioral Auditory Thresholds in Mice

The experiment was designed to obtain behavioral measures of absolute auditory thresholds and to compare them with cochlear potentials and with single-unit eighth-nerve-fiber thresholds. Mice were selected because certain strains display pathological conditions that appear similar to some congenital auditory defects found in humans. The present study deals only with normal mice. Results of avoidance and escape-conditioning techniques are compared, as are results of the psychophysical method of limits and the method of constant stimuli. The animal was trained to avoid shock (in avoidance-conditioning) by responding to the tone alone, or to terminate shock (in escape-conditioning) by making one response to tone plus shock and another to shock alone. The range of frequencies tested was 500–32 000 cps. The test chamber was placed inside a sound-shielded box. Tones were transmitted from one oscillator outside the box. The intensity of all tones and the noise level in the test chamber were measured by a wave analyzer. Shock was administered through the grid floor. Results are discussed in terms of the minimum sound intensity required at the various frequencies, response latency, and response variability. Some problems inherent in the training techniques are described.