Allen Ryan

Hearing sensitivity of the mongolian gerbil, Meriones unguiculatis

Mongolian gerbils were trained to respond to pure tones in a shock‐avoidance task. The animals proved to be readily trainable and behaviorally stable. Thresholds were obtained by a method of limits and, in the case of one animal, by generating psychrometric functions. The gerbil responds to frequencies between 0.1 and 60.0 kHz. For pure tones between 1.0 and 16.0 kHz, the threshold of hearing averages 4.6 dB SPL. Between 1.0 and 0.1 kHz, sensitivity declines at a rate of 12 dB/octave. Between 16.0 and 40.0 kHz, threshold increases at a rate of 13 dB/octave, and above 40.0 kHz at a rate of 66 dB/octave. Thresholds derived from psychometric functions were essentially identical to those obtained by the method of limits. Hearing in this species is consistent with its size and anatomical characteristics.Subject Classification: [43]80.50; [43]65.50.

Behavioral, compound action potential, and single unit thresholds: Relationship in normal and abnormal ears

Comparisons were made for two species (chinchilla and mongolian gerbil) among mean behavioral audiogram, mean just detectable action potential (AP) responses to tone bursts, and single-fiber response thresholds at the characteristic frequency, averaged in one-octave bands. In normal animals and in a group of Kayamycin-treated chinchillas, these mean measures appear to have a well-ordered relationship. Unit and AP thresholds are within 10 dB from one another throughout the frequency range. Behavioral thresholds are usually 15--20 dB more sensitive, but the three curves are roughly parallel except at the highest frequencies, where the behavioral threshold begins to increase approximately one-half octave above the physiological ones. Individual examples for four gerbils and four chinchillas having hair cell losses due to Kanamycin intoxication reinforce the notion based on mean data that in most cases AP thresholds can serve to predict the behavioral threshold configuration.

Psychophysical tuning curves and auditory thresholds after hair cell damage in the chinchilla

Chinchillas were treated with kanamycin sulfate (150--200 mg/kg/day) to produce high-frequency hearing loss extending to about 4.0 kHz. Thresholds and psychophysical tuning curves (PTCs) were obtained before and after treatment, utilizing a shuttlebox avoidance procedure, and cochlear hair cells were evaluated under phase contrast microscopy. Hair cell loss resulting from kanamycin treatment varied from restricted lesions of the outer hair cells (OHCs) in the cochlear base, with no loss of inner hair cells (IHCs), to more extensive lesions involving both OHCs and IHCs. Threshold shift of at least 40 dB was always associated with OHC loss. PTCs obtained from frequency regions exhibiting 40--50 dB of threshold shift were normal in shape. With threshold shift in excess of 50 dB, PTCs were progressively distorted, with truncation of the tip segment and in some cases increased sensitivity of the tail segment. The results suggest that the threshold of optimally functional IHCs after kanamycin-induced OHC loss is about 40 dB higher than normal. Threshold shift in excess of 40 dB may represent IHC damage. IHCs are capable of transducing the fine-frequency information necessary for generating normally sharp PTCs in the absence of OHCs. However, with threshold shift in excess of approximately 50 dB, this frequency resolution is increasingly compromised.

Psychophysical tuning curves of chinchillas

Psychophysical tuning curves were obtained on normal chinchillas using a shock‐avoidance technique. Curves were measured at several levels and frequencies of the signal, and both simultaneous and forward masking were employed. The chinchilla curves were found to be similar to those obtained with human subjects by other researchers. Also, a comparison of the chinchilla psychophysical tuning curves with cat physiological tuning curves shows that the two are similar in shape and sharpness of tuning.Subject Classification: [43]65.58, [43]65.68, [43]65.35, [43]65.75; [43]80.50.

Effects of behavior performance on single‐unit firing patterns in the inferior colliculus of the Rhesus monkey: additional observations

In a previous report [J. Acoust. Soc. Am. 55, 468 (1974)], it was noted that performance in an auditory reaction‐time task tended to increase both the evoked firing rate and initial latency of single‐unit responses to acoustic stimuli in the inferior colliculus and medial geniculate nucleus of rhesus monkeys, while the spontaneous rates of these units were unaffected. Our sample of chronically recorded collicular units has been expanded and our earlier findings were in general confirmed. However, more specific effects of performance were noted. “Off” responses were consistently, and sometimes strikingly, suppressed in the performance condition when compared to responses to identical stimuli delivered while the animal sat passively. Moreover, in some units increases in evoked firing rate were confined to specific temporal portions of the response pattern, and might be offset by suppression of firing in other portions. Also, the timing of some neural events could be related to specific aspects of reaction t...

Comparison of auditory thresholds determined behaviorally and electrocochleographically in normal and kanamycin‐treated gerbils

The measurements of two electrical responses of the auditory periphery have been proposed for the objective assessment of the auditory sensitivity. The first is the classical method of measuring cochlear microphonic (CM) isopotential curves. The second is obtaining thresholds of whole‐nerve action potentials (AP) by using shaped tone bursts. To determine the validity and shortcomings of both methods, measurements have been conducted on normal and kanamycin‐treated gerbils and they are compared with behavioral audiometry. The study shows that between 500 and 24 000 Hz, AP threshold curves compare favorably with behavioral thresholds whereas CM isopotential curves generally do not. At very high frequencies, both responses fail to approximate the behavioral response pattern. Studies with kanamycin also support the idea that the whole‐nerve action potentials can be objectively used for the measurement of audiometric patterns. Such a proposal is also consistent with the observations that AP generated by low‐in...

Mechanisms of auditory masking

Psychephysical “tuning curves” [e.g., A.M. Small Jr., J. Acoust. Soc. Am. 31, 1619–1625 (1959)] were collected from chinchillas trained with the shock‐avoidance method. Tonal maskers were used in both a simultaneous and forward‐masking paradigm. The experiments were performed before and after administration of kanamycin. The drug dosage was controlled so that relatively pure outer hair cell lesions were obtained in the basal region of the cochlea, resulting in a high‐frequency threshold shift of about 40 dB while low‐frequency hearing remained normal. That portion of a two‐tone masking curve which was located within the area of threshold shift was shifted proportionally. That portion outside of the lesion had a configuration which indicated that the amount of masking was dependent on the threshold shift of the masker, rather than the maskee. This finding implies that masking is not mediated by that segment of the cochlea alone that responds to the maskee, but that either longitudinal cochlear interactions...

Frequency selectivity mediated by inner hair cells alone

A well‐recognized measure of frequency selectivity can be obtained by a tone‐on‐tone masking paradigm, which generates a so‐called “psychophysical tuning curve” [e.g., A. M. Small Jr., J. Acoust. Soc. Am. 31, 1619–1625 (1959); E. Zwicker in Facts and Models in Hearing (Springer‐Verlag, New York, 1974), pp. 132–139]. This procedure was adapted to chinchillas trained with the shock‐avoidance method. In order to eliminate outer hair cells from controlled regions of the cochlea, kanamycin was administered. We have demonstrated previously that this method is suitable for producing relatively pure outer hair cell losses in the basal part of the cochlea, with the immediate psychophysical correlate of an approximately 40‐dB threshold shift [A. Ryan and P. Dallos, Nature (in press, 1975)]. Measurements of the “tuning curves” both before and after kanamycin treatment indicated that (1) for maskee frequencies in the unaffected frequency region the “tuning curves” are unchanged, and (2) when the maskee is in the affected region, the segment of the “tuning curve” also within that region is simply shifted proportional to the threshold shift. The curves do not become wider. This suggests that the sharpness of tuning in the auditory system can be determined by inner hair cells, acting without significant influence by the outer hair cells. [Work supported by grants from the NINDS. We thank T. McGee and C. Sanes for their contribution to this research.]

Noise‐induced threshold shifts in the Mongolian gerbil

Mongolian gerbils were trained in a shuttlebox avoidance task and their auditory thresholds determined. Groups of six subjects each were exposed to a two‐octave (1414–5656 Hz) band of noise for one hour at intensities of 100, 110, and 120 dB SPL. Thresholds between 0.1 and 16.0 kHz were determined 0.5, 3.0, 6.0, and 12.0 hours after exposure, and daily for 27 days. Final threshold determinations were made at least two months postexposure. A temporary threshold shift (TTS) was observed in all groups, the extent of which increased with increasing intensity of exposure. Recovery of thresholds followed an exponential course, and for extensive TTS lasted throughout the 28‐day observation period. In some cases additional recovery was noted upon final threshold determination. No significant permanent threshold shift (PTS) was seen in the 100‐dB exposure group, but PTS did occur in the 110‐and 120‐dB groups. PTS was limited to a relatively narrow band of frequencies, and exhibited a half‐octave shift toward higher frequencies. In terms of both TTS and PTS, the gerbil appears to be less sensitive to noise than chinchillas exposed to the same stimulus. [Work supported by NINCDS and the Medical Research Service of the Verterans Administration.]

Behaviorally determined auditory sensitivity of the mongolian gerbil

Thirteen mongolian gerbils (Meriones unguiculatis) were trained in a shock avoidance task based on that developed by Miller [J. Acoust. Soc. Am. 48, 513–523 (1970)] for the chinchilla. The gerbil proved to be readily trainable and behaviorally stable. Thresholds were obtained by a method of limits, and in the case of one animal, by generating psychometric functions, from 0.1 to 60.0 kHz. For pure tones between 1.0 and 16.0 kHz, the threshold of the mongolian gerbil is about 4.6 dB SPL. Between 1.0 and 0.1 kHz, sensitivity declines at a rate of 12 dB/octave. Between 16.0 and 40.0 kHz, threshold increases at a rate of 13 dB/octave, and above 40.0 kHz at a rate of 66 dB/octave. Thresholds derived from psychometric functions were slightly lower than those obtained by the method of limits. [Work supported by grants from the NINDS. I thank Carol Sanes for her contribution to this research.]