Cynthia A. Prosen

Auditory thresholds and kanamycin‐induced hearing loss in the guinea pig assessed by a positive reinforcement procedure

Absolute thresholds from 125 Hz to 52 kHz are determined for six guinea pigs trained by a positive reinforcement method. Four to five hundred trials were conducted during daily testing sessions and little between- or within-subject variability was found. Two of the six animals were subsequently treated with kanamycin and the development of a hearing loss for the high frequencies was followed. Loss of outer and to a lesser extent inner hair cells was well correlated with the threshold shift observed. Contrary to the experience of previous investigators, this operant training procedure has proved as efficient as that for other species of experimental animals, such as the monkey and the chinchilla. It holds excellent promise for future auditory behavioral work with the guinea pig.

Formant frequency discrimination by Japanese macaques (Macaca fuscata)

These studies investigated formant frequency discrimination by Japanese macaques (Macaca fuscata) using an AX discrimination procedure and techniques of operant conditioning. Nonhuman subjects were significantly more sensitive to increments in the center frequency of either the first (F1) or second (F2) formant of single-formant complexes than to corresponding pure-tone frequency shifts. Furthermore, difference limens (DLs) for multiformant signals were not significantly different than those for single-formant stimuli. These results suggest that Japanese monkeys process formant and pure-tone frequency increments differentially and that the same mechanisms mediate formant frequency discrimination in single-formant and vowel-like complexes. The importance of two of the cues available to mediate formant frequency discrimination, changes in the phase and the amplitude spectra of the signals, was investigated by independently manipulating these two parameters. Results of the studies indicated that phase cues were not a significant feature of formant frequency discrimination by Japanese macaques. Rather, subjects attended to relative level changes in harmonics within a narrow frequency range near F1 and F2 to detect formant frequency increments. These findings are compared to human formant discrimination data and suggest that both species rely on detecting alterations in spectral shape to discriminate formant frequency shifts. Implications of the results for animal models of speech perception are discussed.

Frequency discrimination in the monkey

This study evaluated frequency discrimination ability in 11 monkeys over an extended period of time using a repeating-standard procedure and the method of constant stimuli. The intersubject variability of the difference limens for frequency (delta F) was large, as reported by other investigators, but similar in magnitude to the variability of the difference limens for intensity (delta I) from three of the same subjects in an intensity discrimination experiment. Continued training generally resulted in a rapid decrease in delta Fs, followed by a longer-term, slower decrease. For one subject delta Fs slowly decreased throughout a 190-week time period. This long-term training effect was specific to frequency discrimination; a similar effect was not observed for the same subject tested in an intensity discrimination experiment. Finally, delta Fs from the well-trained monkeys of this study were larger than monkey delta Fs from this laboratory reported in an earlier study, and than human delta Fs. An anatomical explanation for the human/monkey delta F magnitude difference is explored.

Apical hair cells and hearing

This study assessed the contribution of the apical hair cells to hearing. Guinea pigs, chinchillas and monkeys were behaviorally trained using positive reinforcement to respond to pure-tone stimuli. When a stable audiogram had been determined, each subject received one of three experimental treatments: ototoxic drug administration, low-frequency noise exposure, or the application of a cryoprobe to the bony wall of the cochlear apex. After post-treatment audiograms stabilized, subjects were euthanized and the percentage of hair cells remaining was assessed by light microscopy. Results indicate that a redundancy of encoding mechanisms exist in the mammalian cochlea for low-frequency stimuli. They also suggest that a very small percentage of apical hair cells are sufficient for some low-frequency hearing. Finally, data from this and other studies suggest that the low-frequency threshold shift caused by the loss of a certain percentage of apical hair cells is less pronounced than the high-frequency threshold shift caused by the loss of a comparable percentage of basal hair cells. These data agree with anatomical and electrophysiological evidence that functional as well as anatomical differences may exist between the apex and base of the cochlea.

Frequency difference limens in normal and sensorineural hearing impaired chinchillas

This study assessed normal frequency discrimination ability in the chinchilla and determined how this ability changes as a function of an experimentally induced sensorineural hearing loss. Four chinchillas were trained by the methods of positive reinforcement to report absolute thresholds and frequency difference limens (FDLs). Subjects were then treated with the aminoglycosidic antibiotic amikacin until a 30-dB hearing loss was measured at 10.0 kHz. Absolute and frequency difference thresholds were determined during and after drug treatment. When post-drug thresholds had stabilized, subjects were sacrificed and their cochleas stained, embedded in plastic, microdissected, and viewed with phase contrast microscopy to permit examination of the cochlear tissue. Post-drug data suggest that frequency discrimination at a high frequency is unaffected by a 40- to 45-dB sensorineural hearing loss, considerable hair cell damage, and the resultant disruption of the cochlear micromechanics. The data, in concert with previously published reports, suggest that FDLs may be less affected by a high-frequency sensorineural hearing loss than by a low-frequency sensorineural hearing loss.

Low-frequency detection and discrimination following apical hair cell destruction

This study assessed the effects of apical hair cell destruction on the detection and discrimination of low-frequency stimuli. Monauralized chinchillas were trained using operant conditioning and positive reinforcement to respond to pure-tone stimuli in the absence and the presence of a high-pass noise masker. Following the collection of the baseline absolute thresholds, psychophysical tuning curves (PTCs) also were determined at low and high frequencies. Apical hair cells in the experimental ear of each subject then were destroyed by applying a liquid-nitrogen-cooled miniature cryoprobe to the bony wall of the cochlea. Post-cryosurgery, unmasked and masked absolute thresholds and psychophysical tuning curves were re-evaluated. Following cryosurgery, low-frequency absolute thresholds increased by 30-50 dB. High-pass masking data suggested that receptors that were unaffected by the masking noise were responsible for the remaining low-frequency hearing. Low-frequency tuning, monitored by assessing changes in PTCs, was significantly altered following apical receptor cell loss, with the most effective maskers located several octaves above the test tone frequency. Following these determinations, one control and two experimental subjects then participated in a third experiment assessing low-frequency discrimination acuity. Some discrimination ability was retained after the cryosurgery; however, these post-lesion difference limens increased when a high-pass noise masker was added to the test environment. At the termination of the behavioral experiment, subjects were euthanized and their cochleae dissected to correlate behavioral and histopathological data. The data suggest that receptors located in frequency regions of the cochlea normally responsive to middle and high frequencies may be responsible for detection and discrimination of low-frequency stimuli in apically damaged cochleae. These data are consistent with other reports which indicate that redundant mechanisms are available for the detection of low-frequency stimuli, and they provide new information regarding how low-frequency stimuli are discriminated throughout the cochlea.

Permanent threshold shift and cochlear hair cell loss in the kanamycin-treated guinea pig.

The differential contribution of the inner hair cells (IHC) and the outer hair cells (OHC) in the mammalian cochlea to hearing sensitivity was assessed in six behaviorally-trained guinea pigs by comparing audiograms preadministration and postadministration of kanamycin, an antibiotic that predominantly destroys guinea pig OHC while leaving the IHC structurally unchanged. The results support the hypothesis that only the IHC of the cochlea responds to tones approximately 50 to 60 dB above the threshold of the intact cochlea.

Rise‐time difference thresholds in the monkey

Difference thresholds for changes in rise time were measured in monkeys at two standard stimulus durations, 250 and 500 ms, and with a fixed‐ or a variable‐duration standard stimulus. Data suggested that (1) rise‐time difference thresholds were larger with a variable‐ than a fixed‐duration standard stimulus, (2) stimulus duration had a minor effect on thresholds, (3) the Weber fractions determined by the data were consistent with Weber fractions in the literature, and (4) monkey rise‐time difference thresholds were larger than human rise‐time difference thresholds.

Auditory thresholds in the guinea pig determined by positive reinforcement methods

Auditory thresholds of six guinea pigs were determined employing operant conditioning techniques. Each subject was trained to nose‐press a transluscent response disk (the observing response) mounted on the wall of the experimental chamber just above the floor. A tone was presented through a speaker on the chamber ceiling directly over the subjects head on a variable‐interval schedule. The subject reported the tone by pressing once on a second response disk (the reporting response) mounted near the first. A small pellet of food was delivered following reporting responses made in the presence of a tone while a 15‐sec time out followed false alarms. Thresholds from 125 to 45 000 Hz were determined by the method of constant stimuli. All subjects were most sensitive to test stimuli located between 4000 and 8000 Hz where the threshold ranged from −10 to 0 dBre 2×10−5 N/m2. A brief training period and the lack of significant variability between or within subjects indicate that the guinea pig is a reliable speci...