Biomedical subjects

S S Saunders

Publications and source records attributed to S S Saunders.

27 records · Page 2Linked to original sources

PubMed1990

Response of auditory-nerve fibers to intensity increments in a multitone complex: neural correlates of profile analysis.

Recent psychophysical studies have shown that the detection of an intensity increment superimposed on the center component (1 kHz) of a multitone complex (1, 3, 7, or 11 components) improves as more components are added outside of the critical band. It has been suggested that this form of intensity discrimination is based on a change in the neural profile. To test this hypothesis, neural profiles were constructed by plotting the degree of phase locking to the 1-kHz tone as a function of each unit's characteristic frequency (CF). Neural phase-locking profile to the 1-component signal at 1 kHz had a broad peak; however, the neural profile became narrower as the number of components in the signal increased. The just detectable increment for the 1-component condition was -5 dB re: 1000-Hz component level (3.86-dB increment plus component level re: component level), whereas, for the 3-, 7-, and 11-component conditions, it was -15 dB re: component level (1.42 dB). The neural and psychophysical IDL for the chinchilla were similar for the 1-component condition. However, the overall trends in the psychophysical and neural data are different. In the psychophysical studies IDL is typically poorest in the 3-component condition and improves when more components are added. By contrast, the neural IDL was poorest in the 1-component condition and improved when more components were added. In the multicomponent conditions, units with CFs in 492-1380 Hz were found to be more sensitive in detecting the intensity increment to the 1000-Hz component.

Animals↗

PubMed1990

Evoked-response tone-on-tone masking in the chinchilla: effect of masker frequency.

Tone-on-tone masking patterns were measured at 0.5, 1, and 4 kHz using the auditory-evoked response from the inferior colliculus of the chinchilla. Masking profiles obtained with a masking level of 30 dB SPL were relatively symmetrical; however, as masker level increased, masking spread toward the high frequencies, particularly with the 0.5-kHz masker. For masker frequencies of 1 and 4 kHz, a low-threshold notch was observed in the masking profile 2/3 of an octave above the frequency of the masker. The low-threshold notch may represent a response to the distortion tone 2F1-F2. The notch was absent in the 0.5-kHz masking profile. The masking profiles obtained with the evoked response are somewhat wider, but otherwise qualitatively similar to those measured psychophysically in humans. Thus, the evoked-response procedure may provide a convenient way of evaluating the spread of masking and the presence of distortion tones in difficult-to-test subjects.

Acoustic Stimulation↗

PubMed1989

Evoked-response forward-masking functions in chinchillas with noise-induced permanent hearing loss.

Evoked-response forward-masking functions were measured by chronic electrodes in the inferior colliculus of the chinchilla before and after exposure to an intense tone that produced a permanent hearing loss. Before exposure, the forward-masking time constants ranged from 50 to 90 ms. After exposure, the forward-masking time constants increased significantly in the region of hearing loss, but not in regions where hearing was normal. The effect of the hearing loss on the time course of forward masking was most pronounced once the hearing loss exceeded 20-25 dB. These physiological changes in the evoked-response forward-masking functions appear to parallel those observed psychophysically in human listeners.

Animals↗

PubMed1987

Evoked response 'forward masking' patterns in chinchillas with temporary hearing loss.

Evoked response "forward masking" data were measured from the inferior colliculus of the chinchilla before and during a temporary threshold shift. The hearing loss was induced by a 2 kHz pure tone of 85 dB SPL presented from 5-8 days. The exposure elevated thresholds by approximately 35 dB at the mid frequencies, but had no effect on low frequency hearing. The exposure also altered the time course of the evoked response forward masking data. Time constants fitting the forward masking data increased by up to a factor of three at the frequency with the greatest loss, but remained within normal limits at the low frequencies where hearing was normal. The increase in the forward masking time constants became most noticeable once the hearing loss exceeded 25 dB. These physiological results are consistent with psychophysical forward masking data from hearing impaired listeners.

Adaptation, Physiological↗

PubMed1987

Evoked response 'forward masking' functions in chinchillas.

Evoked potentials were obtained from the inferior colliculus of awake chinchillas using a 'forward masking' stimulus paradigm. The response was elicited using a brief probe tone presented 10 dB above threshold. Forward masking functions were then measured by determining the intensity of the masker needed to produce a 50% reduction in evoked response amplitude over a range of time intervals between masker offset and probe onset. The time course of forward masking was evaluated by fitting an exponential function to the results. The average time constant values of forward masking functions ranged from 50 to 90 ms. Although the constants at 0.5 and 8.0 kHz were somewhat longer than those at the midfrequencies, the differences were not statistically significant. The evoked response forward masking functions were quite similar to those obtained psychophysically; this suggests that it may be an efficient method of estimating the time course of forward masking in difficult to test subjects.

Acoustic Stimulation↗

PubMed1987

Auditory intensity discrimination in the chinchilla.

A positive reinforcement conditioning procedure was used to train chinchillas to respond to intensity differences between successively occurring tone bursts. Intensity difference limens were measured at 0.5, 1, 4, and 8 kHz at five intensities ranging from 10- to 55-dB sensation level. The intensity difference limen decreased from approximately 8 dB near threshold to approximately 3.5 dB at the highest level. The intensity difference limens for the chinchilla were considerably larger than those for humans as well as several other mammals; however, the results were similar to those obtained for the parakeet. The present results from intensity discrimination appeared to be related to previous data for the discrimination of amplitude modulated noise.

Acoustic Stimulation↗

PubMed1987

Evoked potentials: computer-automated threshold-tracking procedure using an objective detection criterion.

A computer-automated, threshold-tracking procedure was developed for measuring evoked response thresholds on-line. The procedure utilizes an objective detection criterion (correlation) to evaluate whether a response is absent or present. Sound intensity is adjusted according to a modified PEST (parameter estimation by sequential testing) procedure in order to estimate threshold. The computer-automated procedures were used to obtain objective estimates of evoked response thresholds in normal and hearing-impaired chinchillas. The evoked response waveforms stored in the computer were also used to obtain visual judgments of threshold. The objective thresholds determined by the computer were virtually identical to the visual detection thresholds. Thus, the computer-automated procedure provides a reliable, objective, and efficient method of estimating evoked response thresholds.

Animals↗

PubMed1985

Behavioral thresholds for electrical stimulation applied to auditory brainstem nuclei in cat are altered by injurious and noninjurious sound.

Each of three young-adult female cats with normal hearing received a total of eight permanent electrodes which were implanted bilaterally in cochlear nucleus (CN) and inferior colliculus (IC). Three experiments were performed using behaviorally measured thresholds for electrical stimulation of CN and IC. In Expt. 1, electrical stimulation thresholds (in dB re 1.0 microA) were obtained in the presence of a continuous tone of moderate intensity and in quiet. In comparison with quiet, electrical stimulation thresholds measured during tone were lower by as much as 15 dB (stimulation hypersensitivity). In Expt. 2, a brief exposure to an intense sound produced a temporary threshold shift (TTS) for acoustic stimuli but only produced small changes in electrical stimulation threshold. The acoustic stimuli used in Expts. 1 and 2 were termed noninjurious since no permanent hearing loss was produced. Expt. 3 employed an exposure to a white noise that resulted in a mean permanent threshold shift (PTS) of 34.1 dB for acoustic stimulation. The PTS was accompanied by a mean stimulation hypersensitivity of 9.6 dB. Comparing Expts. 1 and 3, it was shown that the transient hypersensitivity produced by the noninjurious continuous tone correlated strongly with the permanent hypersensitivity that was produced by the PTS. In regard to the origin of stimulation hypersensitivity, the suggestion is made that it is an indication of a physiological change localizable perhaps in the auditory nuclei of the upper brainstem.

Acoustic Stimulation↗

PubMed1984

Hypersensitivity to electrical stimulation of auditory nuclei follows hearing loss in cats.

The purpose of the study was to determine if permanent, sound-induced hearing loss altered behaviorally measured thresholds for the detection of electrical stimulation applied to auditory nuclei. Electrodes were placed in cochlear nucleus and inferior colliculus in four cats. Behaviorally measured thresholds for the detection of brief trains of electrical pulses were determined before and after a 48 h exposure to a 1 kHz tone of approximately 110 dB SPL. The mean decrease in electrical stimulation threshold as a result of the sound exposure was 10.4 dB. The ongoing electrical activity (in microV, rms) recorded from the electrodes showed a mean 2.2 dB decrease after the sound exposure. In some electrodes, there was partial recovery towards pre-exposure levels for stimulation threshold and for ongoing activity, but typically, the changes persisted until the animals were terminated 30 days later. The magnitudes of the decreases in stimulation threshold and background activity proved not to be highly correlated. The permanent auditory threshold shift across all cats and all frequencies was 19 dB. This mild hearing loss produced a marked alteration in certain characteristics of the central auditory mechanisms.

Animals↗