Frequency difference limens of pure tones and harmonics within complex stimuli in Mongolian gerbils and humans

Frequency difference limens (FDLs) for pure tones between 200 and 6400 Hz and for the first, the second, or the eighth harmonic of an 800 Hz complex in four Mongolian gerbils (Meriones unguiculatus) were determined using a Go/NoGo-procedure. The 12 harmonics of the complex started either in sine phase or at a random phase. Gerbils showed very high pure tone FDLs ranging from 17.1% Weber fraction (200 Hz) to 6.7% (6400 Hz). They performed much better in detecting mistuning of a harmonic in the complex in the sine phase condition with FDLs decreasing from 0.07% for the first harmonic to 0.02% for the eighth harmonic. FDLs were about one order of magnitude higher when temporal cues were degraded by randomizing the starting phase of every component in the harmonic complex for every stimulus. These results are strikingly different from those obtained in four human subjects who needed about four times higher frequency shifts than gerbils for detecting a mistuned component in a sine phase complex and showed similar detection of mistuning in the random phase condition. The results are discussed in relation to possible processing mechanisms for pure tone frequency discrimination and for detecting mistuning in harmonic complex stimuli.

Effects of omni-directional noise-exposure during hearing onset and age on auditory spatial resolution in the Mongolian gerbil (Meriones unguiculatus) -- a behavioral approach

Inhibitory inputs to the binaural brainstem nuclei medial and lateral superior olives (MSO and LSO, respectively) are thought to be important for sound localization in mammals. Here, we investigate whether aged gerbils that typically exhibit degenerative changes in auditory nuclei providing inhibition to MSO and LSO show diminished localization ability. We also tested the localization ability in gerbils reared in omni-directional white noise during hearing onset, a treatment that affects the adjustment of inhibitory inputs to MSO neurons possibly resulting in weakened sensitivity to interaural time difference. Localization ability of both groups was compared to that of young gerbils raised under control conditions. Stimuli had a duration of 125 ms and were pure tones of 0.5, 1, 2, 4 and 8 kHz, 300-Hz-bands of noise centered at 0.5, 2 and 8 kHz or broad-band noise. Gerbils trained in a two-alternative-forced-choice procedure indicated if sounds were presented from the left or from the right by choosing the respective response compartment of a Y-shaped experimental setup. The minimum resolvable angle (MRA) was calculated as the minimum angle between two loudspeaker locations that a gerbil was able to discriminate. MRAs for aged gerbils were higher compared to controls, whereas MRAs of noise-reared gerbils did not differ from those of the control group. Results are discussed with respect to the progressive degeneration affecting the gerbil's auditory system, changes in the anatomical arrangement of inhibitory inputs on binaural neurons in the MSO, and hearing thresholds.

Detecting modulated signals in modulated noise: (I) behavioural auditory thresholds in a songbird

Most signals from the auditory world have temporal patterns of amplitude modulation that either emanate from the signal source or result from environmental interference (e.g. air turbulence). To investigate mechanisms associated with the segregation and processing of amplitude-modulated signals, we trained European starlings (Sturnus vulgaris) to detect a signal noise band embedded in several flanking noise bands (FBs). We manipulated the envelope correlation between the signal and FBs, the onset synchrony between signal and FBs (0 or100 ms), signal duration (60 or 400 ms) and the spectrum level of the FBs (15 or 50 dB). The lowest signal-detection thresholds were found when the envelopes of the FBs were correlated with each other but different from the signal envelope (the 'co-uncorrelated' condition). Detection thresholds were on average 7 dB higher when both the signal and the FBs had correlated envelopes (the 'all correlated' condition). Thresholds were even higher when the envelopes of all noise bands were independent (the 'all uncorrelated' condition). The difference in detection thresholds between the co-uncorrelated and the all correlated conditions is termed 'comodulation detection difference' (CDD). Differences in signal duration and masker level had significant effects on detection threshold, but not on CDD magnitudes; differences in onset synchrony had no effects. We compare data from starlings with those from previous psychoacoustic studies of humans, and discuss possible mechanisms on which these perceptual effects may rely. Our behavioural data are the reference for a companion study investigating CDD at the neuronal level of the starling [M.A. Bee et al. (2007) Eur. J. Neurosci., 26, 1979-1994].

Detecting modulated signals in modulated noise: (II) neural thresholds in the songbird forebrain

Sounds in the real world fluctuate in amplitude. The vertebrate auditory system exploits patterns of amplitude fluctuations to improve signal detection in noise. One experimental paradigm demonstrating these general effects has been used in psychophysical studies of 'comodulation detection difference' (CDD). The CDD effect refers to the fact that thresholds for detecting a modulated, narrowband noise signal are lower when the envelopes of flanking bands of modulated noise are comodulated with each other, but fluctuate independently of the signal compared with conditions in which the envelopes of the signal and flanking bands are all comodulated. Here, we report results from a study of the neural correlates of CDD in European starlings (Sturnus vulgaris). We manipulated: (i) the envelope correlations between a narrowband noise signal and a masker comprised of six flanking bands of noise; (ii) the signal onset delay relative to masker onset; (iii) signal duration; and (iv) masker spectrum level. Masked detection thresholds were determined from neural responses using signal detection theory. Across conditions, the magnitude of neural CDD ranged between 2 and 8 dB, which is similar to that reported in a companion psychophysical study of starlings [U. Langemann & G.M. Klump (2007) Eur. J. Neurosci., 26, 1969-1978]. We found little evidence to suggest that neural CDD resulted from the across-channel processing of auditory grouping cues related to common envelope fluctuations and synchronous onsets between the signal and flanking bands. We discuss a within-channel model of peripheral processing that explains many of our results.

Auditory short-term memory persistence for tonal signals in a songbird

This paper presents an animal model for studying the persistence of auditory memory for tonal signals. Five European starlings (Sturnus vulgaris) were trained in a Go/NoGo delayed nonmatching-to-sample task to discriminate between a series of identical "sample stimuli" and a single "test stimulus." Frequencies of sample and test stimuli should be classified as being either the same or different. The performance of the birds is measured as the percentage of correct classifications. Three parameters were varied: The delay between sample and test stimuli, the number of sample stimuli presented before the test stimulus, and the salience of the difference between sample and test stimuli. Auditory memory persistence time was estimated as a function of the delay between the last sample stimulus and the test stimulus. The performance of the birds deteriorated with increasing delay before the test stimulus. Increasing the number of sample stimuli in the series of tones presented before the test stimulus improved performance. Performance was also better for more salient differences between sample and test stimuli. The individual auditory memory persistence time varied between 4 and 20 s.

Temporal integration in the gerbil: the effects of age, hearing loss and temporally unmodulated and modulated speech-like masker noises

We characterized temporal integration for 2k Hz pure tones with durations between 10 and 1000 ms in young, normal hearing old and old gerbils with a small hearing loss. Thresholds determined in silence increased for durations below 300 ms and were on average more than 10 dB higher for the 10 ms signal than asymptotic thresholds for the long signals. The amount of temporal integration tended to be less in gerbils with hearing loss. Threshold determination was repeated in the same individuals in the presence of speech-like unmodulated and modulated masking noises. Threshold shift due to the maskers was inversely related to the threshold in silence resulting in a reduced inter-individual variability of thresholds in both masking conditions. Thresholds differed systematically between both masker types in a duration dependent fashion. For long signal durations (300 and 1000 ms) thresholds were on average 2dB lower and for the 10 ms signal 1.9 dB higher in the presence of the modulated masker. These differences in threshold obtained with the two maskers were significant. One hypothesis is that long signals can be detected in the troughs of the modulated masker, while peaks interfere with the detection of short signals.

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.

Asymmetry of masking in the European starling: behavioural auditory thresholds

Psychophysical studies of simultaneous masking with human observers exhibit an asymmetry in the amount of masking that depends on the relative bandwidths of signals and maskers. For noise bands up to the bandwidth of one auditory filter, masked auditory thresholds are considerably lower when the bandwidth of the signal exceeds that of the masker compared to the reversed condition. We investigate asymmetry of masking in an animal model, that will allow to study the mechanisms associated with the asymmetry of masking effect. European starlings (Sturnus vulgaris) were trained in a Go/NoGo paradigm to report the detection of a 500 ms noise signal centred in a 700 ms noise masker. Signals and maskers with centre frequencies of 2 kHz had bandwidths of 4 Hz or 256 Hz. Thresholds for detecting the 256 Hz wide-band signal in a 4 Hz narrow-band masker were considerably lower compared to detecting the 4 Hz narrow-band signal in a 256 Hz wide masker and compared to all other conditions. The asymmetry of masking in starlings was on average 15 and 17 dB for 40 and 70 dB SPL overall masker level, respectively. Our animal model thus proved perceptual abilities similar to human subjects. The results are discussed with respect to the importance of both intensity and temporal cues for signal detection.

Operant methods for mouse psychoacoustics

Tone detection thresholds for a 10-kHz tone in NMRI mice were determined in psychoacoustic experiments using both a constant-stimuli procedure and a two-down/one-up adaptive-tracking procedure in the same subjects and applying identical threshold criteria (70.7% response probability). Constant-stimuli thresholds were on average 24 dB lower than adaptive-tracking thresholds, and there was a trend indicating that constant-stimuli thresholds were less variable than adaptive-tracking thresholds. Furthermore, in the constant-stimuli procedure the number of trials constituting the psychometric function could be reduced from 100 to 50 trials without a large loss of accuracy of threshold determination. In the constant-stimuli procedure, the threshold value was affected by the threshold criteria. The lowest and least variable constant-stimuli thresholds were obtained by applying signal detection theory and a criterion of d' = 1. Thus, the constant-stimuli procedure in combination with signal detection theory appears to be better suited than the adaptive-tracking procedure to determine auditory sensory thresholds.

Resolution in azimuth sound localization in the Mongolian gerbil (Meriones unguiculatus)

Minimum resolvable angles (MRAs) for sound localization in azimuth in the gerbil were determined in a behavioral study using tones, 300-Hz bands of noise centered at frequencies between 500 Hz and 8 kHz and broad-band noise of on average 60 dB SPL overall level. Using the method of constant stimuli, seven gerbils were trained in a two-alternative-forced-choice procedure to indicate if sounds were presented to them from the left or from the right by choosing the left or right arm of a Y-shaped cage. The MRA is the minimum angle between two loudspeaker locations that the gerbils discriminated. Animals were either stimulated from the front (N=4) or from the back (N=3). The MRA for broad-band noise randomly varying in level by +/- 6 dB was 23 degrees and 45 degrees for gerbils stimulated from the front or back, respectively. Generally a gerbil's MRA for tones declined up to 2 kHz reaching 20 degrees and 31 degrees for gerbils stimulated from the front or back, respectively, and the MRA was generally increased above this frequency. Results for narrow-band noise stimuli were similar. Results are discussed with respect to the available interaural cues and physiological mechanisms of sound localization in the gerbil.

Auditory stream segregation in the songbird forebrain: effects of time intervals on responses to interleaved tone sequences

For both humans and other animals, the abilities to integrate separate sound elements over time into coherent perceptual representations, or 'auditory streams', and to segregate these auditory streams from other interleaved sounds are critical for hearing and vocal communication. In humans and European starlings (Sturnus vulgaris) the ability to perceptually segregate a simple interleaved tone sequence comprised of two alternating tones differing in frequency (ABA-ABA-ABA-...) into separate auditory streams of A and B tones is promoted at larger frequency separations (DeltaF) between the A and B tones. In humans, segregating A and B tones into different streams also appears to be promoted at shorter interstimulus intervals (ISI) between tones within a stream (e.g., between consecutive A tones). Here, we used the ABA experimental paradigm to investigate the influence of different time intervals between A and B tones in repeated ABA triplets on neural responses in the starling forebrain. The main finding from the study is that a DeltaF-dependent effect of ISI had a large influence on the relative responses to A and B tones. Responses to B tones were suppressed, relative to A-tone responses, when the A and B tones were more similar in frequency (smaller DeltaFs) and occurred at shorter ISIs. We attribute these suppressive effects to physiological forward masking and suggest that forward masking functions as a mechanism for segregating neural responses to interleaved tones in tonotopic space. We discuss the relevance of our physiological data with respect to previous electrophysiological studies of auditory stream segregation in mammals and previous perceptual studies in humans.

Analysis of spectral shape in the barn owl auditory system

In a behavioral experiment, we investigated how efficiently barn owls (Tyto alba) could detect changes in the spectral profile of multi-component auditory signals with stochastic envelope patterns. Signals consisted of one or five bands of noise (bandwidth 4, 16, or 64 Hz each; center frequencies 1.02, 1.43, 2.0, 2.8, 3.92 kHz). We determined increment thresholds for the 2 kHz component for three conditions: single-band condition (only the 2 kHz component), all five noise bands with the envelope fluctuations of the bands being either correlated or uncorrelated. Noise bandwidth had no significant effect on increment detection. Increment thresholds for the different conditions, however, differed significantly. Thresholds in correlated conditions were generally the lowest of all conditions, whereas, thresholds in uncorrelated conditions mostly resulted in the highest thresholds. This can be interpreted as evidence for comodulation masking release in barn owls. If the increment in the 2 kHz component is balanced by decrementing the four flanking bands in amplitude, increment detection thresholds are not affected. The data suggest that the barn owls used information from simultaneous spectral comparison across different frequency channels to detect spectral changes in multi-component noise signals rather than sequential comparison of overall stimulus levels.

Extrapair Fertilizations in Red Bishops (Euplectes Orix): Do Females Follow Conditional Extrapair Strategies?

Although many studies have revealed differences in patterns of extrapair paternity between species and between populations of the same species, possible differences in female extrapair behavior within populations have received far less attention. Here, we propose and test the hypothesis that females of the highly polygynous Red Bishop (Euplectes orix) follow two different extrapair strategies, with benefits of extrapair copulations depending on the quality of their social mate (”conditional-extrapair-strategy hypothesis”). In particular, we propose that females mated to low-quality males enhance the genetic quality of their offspring by performing extrapair copulations with males of higher quality than their social mate (in accordance with the good-genes hypothesis), whereas females mated to high-quality males perform extrapair copulations as insurance against temporary infertility of their social mate caused by sperm depletion due to frequent copulations (in accordance with the fertility-insurance hypothesis). Several predictions derived from our hypothesis, regarding differences in frequency and distribution of both unhatched eggs and extrapair young between high-quality and low-quality males, are tested and shown to be supported. Most importantly, hatching failures occurred more often in territories of high-quality males than in those of low-quality males, and the proportion of unhatched eggs within the territory was positively correlated to breeding synchrony for high-quality males only. Those results suggest that sperm depletion is a risk for females mated to high-quality males with many mating opportunities. The fact that broods of high-quality males without unhatched eggs had a significantly higher proportion of extrapair young than broods of high-quality males with unhatched eggs is consistent with the hypothesis that females mated to high-quality males were successful in performing EPCs as insurance against temporary infertility of their social mates. Furthermore, genetic fathers of extrapair young hatched in territories of low-quality males were found to be of higher quality than the corresponding social fathers, whereas no difference in quality was found between genetic and social fathers of extrapair young hatched in territories of high-quality males. Although we cannot exclude the possibility that the observed pattern was a result of females mating randomly from within the available pool of extrapair males, it is in accordance with our prediction that females mated to low-quality males perform extrapair copulations with males of higher quality than their social mate to enhance offspring fitness. Overall, the data presented here suggest that females within our study colony adjusted their extrapair behavior to the quality of their social mate, even if we cannot entirely rule out alternative explanations for some of the results obtained. Thus, the study provides support for the conditional-extrapair-strategy hypothesis, which states that within populations, females may follow different extrapair strategies to gain different benefits.

Fertilizaciones Extra-pareja en Euplectes orix: ¿Siguen las Hembras Estrategias Extra-pareja Condicionales?

Duration discrimination in the mouse (Mus musculus)

Detection thresholds for an increment in duration of a 10-kHz pure tone were determined in the NMRI mouse using a Go/NoGo-procedure and the method of constant stimuli. Thresholds for reference durations of 50, 100 and 200 ms were obtained presenting the signals at a fixed level or at a level varying by +/-3 dB. Thresholds were determined using signal-detection theory ( d'=1.0 or d'=1.8) and the criterion of 50% correct responses. For a fixed level, the average Weber fraction Delta T/ T (criterion of d'=1.8) significantly decreased from 1.18 or 1.23 at reference durations of 50 or 100 ms, respectively, to 0.97 at a reference duration of 200 ms. Thresholds were on average reduced by 46.8 or 55.4% for the threshold criteria d'=1 or 50% correct responses, respectively. There was no effect of randomizing the level on the discrimination threshold. Duration discrimination in the NMRI mouse does not follow Weber's law. The results are consistent with a mechanism summing up neural impulses over the duration of the stimulus. The psychoacoustic data are compared with results obtained by Brand et al. (J Acoust Soc Am 51:1291-1223, 2000) on the representation of acoustic signal duration in the mouse inferior colliculus.

AM representation in green treefrog auditory nerve fibers: neuroethological implications for pattern recognition and sound localization

In addition to spectral call components, temporal patterns in the advertisement-call envelope of green treefrog males ( Hyla cinerea) provide important cues for female mate choice. Rapid amplitude modulation (AM) with rates of 250-300 Hz is typical for this species' advertisement calls. Here we report data on the encoding of these rapid call modulations by studying the responses of single auditory nerve fibers to two-tone stimuli with envelope periodicities close to those of the natural call. The free-field response properties of 86 nerve fibers were studied from 32 anesthetized males. The accuracy of stimulus envelope coding was quantified using both a Gaussian function fit to the interspike interval histograms derived from the first seven 20-ms stimulus segments, and the vector-strength metric applied to the phase-locked responses. Often, AM encoding in the initial stimulus segment was more faithful than that in its second half. This result may explain why conspecific females prefer calls in which the initial segment is unmasked rather than masked. Both the questions of pattern recognition and localization are discussed, and the data are related to behavioral observations of female choice and localization performance in this species.

Primitive auditory stream segregation: a neurophysiological study in the songbird forebrain

Auditory stream segregation refers to the perceptual grouping of sounds, to form coherent representations of objects in the acoustic scene, and is a fundamental aspect of hearing and speech perception. The perceptual segregation of simple interleaved tone sequences has been studied in humans and European starlings (Sturnus vulgaris) using sequences of 2 alternating tones differing in frequency (ABA-ABA-ABA-...). The segregation of A and B tones into separate auditory streams is believed to be promoted by preattentive auditory processes that increase the separation of excitation patterns along a tonotopic gradient. We tested the hypothesis that frequency selectivity and forward masking operate as 2 preattentive processes in sequential stream segregation by recording neural responses in the auditory forebrain of awake starlings to repeated ABA- sequences in which we varied the frequency separation (DeltaF) between the A and B tones and the tone repetition time (TRT). The A tones were presented at the neurons' characteristic frequency (CF), and B tones differed from the CF over a one-octave range. Larger DeltaF values and shorter TRTs promote the perceptual segregation of alternating tone sequences in humans and also resulted in larger differences in neural responses to alternating CF (A) and non-CF (B) tones. Our results are consistent with the hypothesis that preattentive auditory processes, such as frequency selectivity and forward masking, contribute to the perceptual segregation of sequential acoustic events having different frequencies into separate auditory streams, but also suggest that additional processes may be required to account for all known perceptual effects related to sequential auditory stream segregation.

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.

Within- and across-channel processing in auditory masking: a physiological study in the songbird forebrain

Synchronous envelope fluctuations in different frequency ranges of an acoustic background enhance the detection of signals in background noise. This effect, termed comodulation masking release (CMR), is attributed to both processing within one frequency channel of the auditory system and comparisons across separate frequency channels. Here we present data on CMR from a study in field L2 of the auditory forebrain of the European starling (Sturnus vulgaris) using two 25-Hz-wide bands of masking noise that provide the opportunity to distinguish between within-channel and across-channel effects. Acoustically evoked responses were recorded from unrestrained birds via radio telemetry. The signal was a 800 msec pure tone presented at the most sensitive frequency of the units in a previously determined frequency-tuning curve (FTC). One band of masking noise was centered on the signal frequency while the flanking band of noise was presented either within the limits of the excitatory FTC (i.e., within the same frequency channel as the on-frequency masker) or in the suppression area of the FTC (i.e., in a separate channel). For flanking bands inside the excitatory FTC, signal detection thresholds based on the rate code were lower in noise maskers with identical envelope fluctuations (comodulated) than in maskers with uncorrelated envelopes resulting in a neural CMR of approximately 4-7 dB. For flanking bands inside the suppression areas, the neural CMR was reduced. Although the average neural CMR was below the behaviorally determined CMR, a subsample of between 11 and 26% of the recording sites resembled the behavioral performance.

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.

An estimate of the auditory-filter bandwidth in the Mongolian gerbil

Critical ratios (CRs) and the critical band (CB) were determined in six Mongolian gerbils (Meriones unguiculatus) using a GO/NOGO procedure and the method of constant stimuli. The test-tone frequencies were 0.5, 1, 2, 4 and 8 kHz in the CR measurements and 2 kHz in the CB measurement. Critical ratios were independent of the level of the white-noise masker. The lowest CR with a mean of 24.8 dB was found at 1 kHz for maskers with spectrum levels of 20 and 40 dB SPL. The CR increased on average by 2.1 dB per octave between the frequencies 1 and 4 kHz. The CR bandwidth at 2 kHz and at a masker spectrum level of 40 dB SPL was 417 Hz. Using the band-narrowing procedure, we determined a mean CB of 216 Hz at a test-tone frequency of 2 kHz. The results are discussed in comparison to psychophysical data from other rodent species and humans. Finally, the relation of CR and CB bandwidths to the bandwidth of tuning curves of gerbil auditory-nerve fibers and the gerbil's cochlear frequency map are explored.