Forward masking in gerbils: the effect of age.

We investigated forward masking in 21 gerbils as a function of age (5-47 months) using 400ms maskers at 40dB SPL and a 20ms, 2.85kHz probe presented 2.5ms after the masker. Elevated thresholds for the unmasked probe were only observed in animals older than 3 years. Unmasked thresholds showed no significant age-dependent hearing loss in animals below 3 years of age. In these animals without peripheral hearing loss, we found a significant age-dependent increase of masker-induced threshold shift. A regression analysis revealed that threshold shift increased from 23dB in 1 year old gerbils to 37dB in 3 year old animals. Increased forward masking in these animals with no sign of peripheral hearing loss points to a central processing deficit.

A quantitative analysis of psychometric functions for different auditory tasks in gerbils.

The psychometric function relates the probability of a correct response to the variation of a physical stimulus parameter. In many perceptual tasks one point on this function is defined by a more or less arbitrary threshold criterion and threshold is used to study the effects of various treatments or age. Besides threshold, the shape of the psychometric function provides additional information. The variability of internal (neural) noise and the sensorineural transduction function will affect the shape of the psychometric function and may, therefore, reveal important features in the processing of stimulus characteristics. Here we analyze the effect of age on psychometric functions from gerbils: (A) for the detection of a tone or noise pulse in silence which is generally regarded as a measure of cochlear function and (B) for a gap detection task, investigating aspects of temporal processing that involve the ascending auditory pathway. Our data show that the slope of the psychometric function for the detection of tone and noise pulses in silence is independent of age and threshold. In contrast, the steepness of the psychometric function is decreased in gerbils with impaired temporal resolution. We discuss these observations in the context of physiological data from young and old animals.

Age-dependent changes of gap detection in the Mongolian gerbil (Meriones unguiculatus).

Gap detection using broadband noise was characterized in a group of young gerbils from the breeding colony of the University of Regensburg (RB gerbils), old RB gerbils, and old gerbils from the breeding colony of the University of South Carolina (SC gerbils). Data from old RB and old SC gerbils were not significantly different and were subsequently combined for a comparison with data from the group of young RB gerbils. Level dependence of gap-detection thresholds in young and old domesticated gerbils resembled the typical mammalian pattern of level dependence. Gap-detection thresholds of old gerbils were significantly elevated at 30 dB SL and 50 dB SPL as compared with young gerbils. Compared with young gerbils tested at 30 dB SL and 50 dB SPL, the distribution of gap-detection thresholds in old gerbils was broader with a spread to higher gap-detection thresholds. Some old animals retained excellent temporal resolution, while some showed impaired gap detection. The gap-detection data collected in young and old gerbils resemble previously published data from humans of different age and confirm that gerbils are a useful model to study age-dependent changes in temporal processing.

Boosting GABA improves impaired auditory temporal resolution in the gerbil.

Elderly humans often not only experience peripheral hearing loss but also suffer from more central deficits in temporal auditory processing affecting speech perception. Impaired auditory temporal resolution has also been observed in old rodents. Other studies have demonstrated a reduction of GABAergic function in the auditory pathway of old animals. Here we test the hypothesis that deficits in the GABAergic system affect central auditory processing. Our data suggests that pharmacological augmentation of the GABAergic system ameliorates impaired temporal auditory processing in the gerbil and might be a strategy for the treatment of at least some forms of central hearing loss in humans.

Gap detection in Mongolian gerbils (Meriones unguiculatus).

Hearing thresholds for white-noise stimuli and temporal gap-detection thresholds in six Mongolian gerbils (Meriones unguiculatus) were determined in a GO/NOGO procedure using the method of constant stimuli. Gerbils were offspring of animals collected in the field and only bred in captivity for four generations or less. Hearing thresholds for 800 ms bursts of white noise ranged between -0.8 and 6.3 dB SPL. The median minimum-detectable gap centered in an 800 ms burst of white noise presented at 50 dB SPL was 2.1 ms. At levels of 40 dB SPL and above, gap-detection thresholds were independent of the sound-pressure level of the stimulus. At 30 dB SPL and below, the minimum-detectable gap increased with decreasing sound-pressure level. Near 5 dB sensation level, gap-detection thresholds ranged from 13.6 to 29.4 ms. The median threshold for the detection of gaps inserted 5 ms after the onset of an 800 ms burst of white noise of 50 dB SPL was 2.3 ms. The individuals' detection thresholds varied between 2.3 and 3.6 ms for stimuli in which the relative sound-pressure levels of the noise before and after the gap differed by up to 20 dB. The data found in the Mongolian gerbil match results from other mammal and bird species.

Behavioral and evoked-potential thresholds in young and old Mongolian gerbils (Meriones unguiculatus).

Age-dependent hearing loss has been well documented in gerbils exceeding 2 years of age using physiological methods (e.g. [Mills et al. (1990) Hear. Res. 46, 201-210]). We determined behavioral thresholds for broad-band noise and pure-tone pulses in gerbils as a function of age. Contrary to expectations based on previously published physiological data, we found no significant (broad-band noise and 10 kHz) or only a very small hearing loss (7 dB at 2 kHz) in 30-36-month-old animals. In animals over 3 years of age we observed an increased spread of thresholds and threshold shifts exceeding 20 dB in some individuals. Behavioral thresholds of old gerbils from two breeding colonies (University of Regensburg and Medical University of South Carolina) were similar. Data from individual animals where thresholds were determined physiologically and behaviorally indicate that results from auditory brainstem response measurements show no shift at 18 months while subsequent measurements at 28-29 months revealed age-dependent threshold shifts of 10-15 dB. In contrast, thresholds determined by behavioral methods in these same individuals at 31-33 months of age remained stable.