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

Neural processing and perception of Schroeder-phase harmonic tone complexes in the gerbil: Relating single-unit neurophysiology to behavior

Schroeder-phase harmonic tone complexes have been used in physiological and psychophysical studies in several species to gain insight into cochlear function. Each pitch period of the Schroeder stimulus contains a linear frequency sweep; the duty cycle, sweep velocity, and direction are controlled by parameters of the phase spectrum. Here, responses to a range of Schroeder-phase harmonic tone complexes were studied both behaviorally and in neural recordings from the auditory nerve and inferior colliculus of Mongolian gerbils. Gerbils were able to discriminate Schroeder-phase harmonic tone complexes based on sweep direction, duty cycle, and/or velocity for fundamental frequencies up to 200 Hz. Temporal representation in neural responses based on the van Rossum spike-distance metric, with time constants of either 1 ms or related to the stimulus' period, was compared to average discharge rates. Neural responses and behavioral performance were both expressed in terms of sensitivity, d', to allow direct comparisons. Our results suggest that in the auditory nerve, stimulus fine structure is represented by spike timing while envelope is represented by rate. In the inferior colliculus, both temporal fine structure and envelope appear to be represented best by rate. However, correlations between neural d' values and behavioral sensitivity for sweep direction were strongest for both temporal metrics, for both auditory nerve and inferior colliculus. Furthermore, the high sensitivity observed in the inferior colliculus neural rate-based discrimination suggests that these neurons integrate across multiple inputs arising from the auditory periphery.

Zebra finch song is a very short-range signal in the wild: evidence from an integrated approach

Birdsong is typically seen as a long-range signal functioning in mate attraction and territory defense. Among birds, the zebra finch is the prime model organism in bioacoustics, yet almost exclusively studied in the lab. In the wild, however, zebra finch song differs strikingly from songbirds commonly studied in the wild as zebra finch males sing most after mating and in the absence of territoriality. Using data from the wild, we here provide an ecological context for a wealth of laboratory studies. By integrating calibrated sound recordings, sound transmission experiments and social ecology of zebra finches in the wild with insights from hearing physiology we show that wild zebra finch song is a very short-range signal with an audible range of about nine meters and that even the louder distance calls do not carry much farther (up to about fourteen meters). These integrated findings provide an ecological context for the interpretation of laboratory studies of this species and indicate that the vocal communication distance of the main laboratory species for avian acoustics contrasts strikingly with songbirds that use their song as a long-range advertisement signal.

Uncertainty-based informational masking in a vowel discrimination task for young and old Mongolian gerbils

Informational masking emerges with processing of complex sounds in the central auditory system and can be affected by uncertainty emerging from trial-to-trial variation of stimulus features. Uncertainty can be non-informative but confusing and thus mask otherwise salient stimulus changes resulting in increased discrimination thresholds. With increasing age, the ability for processing of such complex sound scenes degrades. Here, 6 young and 4 old gerbils were tested behaviorally in a vowel discrimination task. Animals were trained to discriminate between sequentially presented target and reference vowels of the vowel pair/I/-/i/. Reference and target vowels were generated shifting the three formants of the reference vowel in steps towards the formants of the target vowels. Non-informative but distracting uncertainty was introduced by random changes in location, level, fundamental frequency or all three features combined. Young gerbils tested with uncertainty for the target or target and reference vowels showed similar informational masking effects for both conditions. Young and old gerbils were tested with uncertainty for the target vowels only. Old gerbils showed no threshold increase discriminating vowels without uncertainty in comparison with young gerbils. Introducing uncertainty, vowel discrimination thresholds increased for young and old gerbils and vowel discrimination thresholds increased most when presenting all three uncertainty features combined. Old gerbils were more susceptible to non-informative uncertainty and their thresholds increased more than thresholds of young gerbils. Gerbils' vowel discrimination thresholds are compared to human performance in the same task (Eipert et al., 2019).

Wide sensory filters underlie performance in memory-based discrimination and generalization

The way animals respond to a stimulus depends largely on an internal comparison between the current sensation and the memory of previous stimuli and outcomes. We know little about the accuracy with which the physical properties of the stimuli influence this type of memory-based discriminative decisions. Research has focused largely on discriminations between stimuli presented in quick succession, where animals can make relative inferences (same or different; higher or lower) from trial to trial. In the current study we used a memory-based task to explore how the stimulus’ physical properties, in this case tone frequency, affect auditory discrimination and generalization in mice. Mice performed ad libitum while living in groups in their home quarters. We found that the frequency distance between safe and conditioned sounds had a constraining effect on discrimination. As the safe-to-conditioned distance decreased across groups, performance deteriorated rapidly, even for frequency differences significantly larger than reported discrimination thresholds. Generalization width was influenced both by the physical distance and the previous experience of the mice, and was not accompanied by a decrease in sensory acuity. In conclusion, memory-based discriminations along a single stimulus dimension are inherently hard, reflecting a high overlap between the memory traces of the relevant stimuli. Memory-based discriminations rely therefore on wide sensory filters.

Quantifying Neuronal Information Flow in Response to Frequency and Intensity Changes in the Auditory Cortex

Studies increasingly show that behavioral relevance alters the population representation of sensory stimuli in the sensory cortices. However, the mechanisms underlying this behavior are incompletely understood. Here, we record neuronal responses in the auditory cortex while a highly trained, awake, normal-hearing gerbil listens passively to target tones of high versus low behavioral relevance. Using an information theoretic framework, we model the overall transmission chain from acoustic input stimulus to recorded cortical response as a communication channel. To quantify how much information core auditory cortex carries about high versus low relevance sound, we then compute the mutual information of the multi-unit neuronal responses. Results show that the output over the stimulus-to-response channel can be modeled as a Poisson mixture. We derive a closed-form fast approximation for the entropy of a mixture of univariate Poisson random variables. A purely rate-code based model reveals reduced information transfer for high relevance compared to low relevance tones, hinting that changes in temporal discharge pattern may encode behavioral relevance.

The Interplay Between Spike-Time and Spike-Rate Modes in the Auditory Nerve Encodes Tone-In-Noise Threshold

Auditory nerve fibers (ANFs) encode pure tones through two modes of coding, spike time and spike rate, depending on the tone frequency. In response to a low-frequency tone, ANF firing is phase locked to the sinusoidal waveform. Because time coding vanishes with an increase in the tone frequency, high-frequency tone coding relies on the spike rate of the ANFs. Adding a continuous broadband noise to a tone compresses the rate intensity function of ANFs and shifts its dynamic range toward higher intensities. Therefore, the ANFs with high-threshold/low-spontaneous rate (SR) are thought to contribute to behavioral tone detection in noise. However, this theory relies on the discharge rate of the ANFs. The direct comparison with the masking threshold through spike timing, irrespective of the spontaneous rate, has not so far been investigated. Taking advantage of a unique proxy to quantify the spike synchrony (i.e., the shuffle autocorrelogram), we show in female gerbils that high-SR ANFs are more adapted to encode low-frequency thresholds through temporal code, giving them a strong robustness in noise. By comparing behavioral thresholds measured using prepulse inhibition of the acoustical startle reflex with population thresholds calculated from ANFs pooled per octave band, we show that threshold-based spike timing provides a better estimate of behavioral thresholds in the low-frequency range, whereas the high-frequency behavioral thresholds rely on the spiking rate, particularly in noise. This emphasizes the complementarity of temporal and rate modes to code tone-in-noise thresholds over a large range of frequencies.SIGNIFICANCE STATEMENT There is a general agreement that high-threshold/low-spontaneous rate (SR) auditory nerve fibers (ANFs) are of prime importance for tone detection in noise. However, this theory is based on the discharge rate of the fibers. Comparing the behavioral thresholds and single ANF thresholds shows that this is only true in the high-frequency range of tone stimulations. In the low-frequency range of tones (up to 2.7 kHz in the gerbil), the most sensitive ANFs (high-SR fibers) carry neural information through a spike-timing mode, even for noise in which tones do not induce a noticeable increment in the spike rate. This emphasizes the interplay between spike-time and spike-rate modes in the auditory nerve to encode tone-in-noise threshold over a large range of tone frequencies.

Processing of interaural phase differences in components of harmonic and mistuned complexes in the inferior colliculus of the Mongolian gerbil

Harmonicity and spatial location provide eminent cues for the perceptual grouping of sounds. In general, harmonicity is a strong grouping cue. In contrast, spatial cues such as interaural phase or time difference provide for strong grouping of stimulus sequences but weak grouping for simultaneously presented sounds. By studying the neuronal basis underlying the interaction of these cues in processing simultaneous sounds using van Rossum spike train distance measures, we aim at explaining the interaction observed in psychophysical experiments. Responses to interaural phase differences imposed on single components of harmonic and mistuned complex tones as well as noise delay functions were recorded as multiunit responses from the inferior colliculus of Mongolian gerbils. Results revealed a better representation of interaural phase differences if imposed on a harmonic rather than a mistuned frequency component of a complex tone. The representation of interaural phase differences was better for long integration-time windows approximately reflecting firing rates rather than short integration-time windows reflecting the temporal pattern of the stimulus-driven response. We found only a weak impact of interaural phase differences if combined with mistuning of a component in a harmonic tone complex.

Developmental Conductive Hearing Loss Reduces Modulation Masking Release

Hearing-impaired individuals experience difficulties in detecting or understanding speech, especially in background sounds within the same frequency range. However, normally hearing (NH) human listeners experience less difficulty detecting a target tone in background noise when the envelope of that noise is temporally gated (modulated) than when that envelope is flat across time (unmodulated). This perceptual benefit is called modulation masking release (MMR). When flanking masker energy is added well outside the frequency band of the target, and comodulated with the original modulated masker, detection thresholds improve further (MMR+). In contrast, if the flanking masker is antimodulated with the original masker, thresholds worsen (MMR−). These interactions across disparate frequency ranges are thought to require central nervous system (CNS) processing. Therefore, we explored the effect of developmental conductive hearing loss (CHL) in gerbils on MMR characteristics, as a test for putative CNS mechanisms. The detection thresholds of NH gerbils were lower in modulated noise, when compared with unmodulated noise. The addition of a comodulated flanker further improved performance, whereas an antimodulated flanker worsened performance. However, for CHL-reared gerbils, all three forms of masking release were reduced when compared with NH animals. These results suggest that developmental CHL impairs both within- and across-frequency processing and provide behavioral evidence that CNS mechanisms are affected by a peripheral hearing impairment.

Release from informational masking by auditory stream segregation: perception and its neural correlate

In the analysis of acoustic scenes, we easily miss sounds or are insensitive to sound features that are salient if presented in isolation. This insensitivity that is not due to interference in the inner ear is termed informational masking (IM). So far, the cellular mechanisms underlying IM remained elusive. Here, we apply a sequential IM paradigm to humans and gerbils using a sound level increment detection task determining the sensitivity to target tones in a background of standard (same frequency) and distracting tones (varying in level and frequency). The amount of IM that was indicated by the level increment thresholds depended on the frequency separation between the distracting and the standard and target tones. In humans and gerbils, we observed similar perceptual thresholds. A release from IM of more than 20 dB was observed in both species if the distracting tones were well segregated in frequency from the other tones. Neuronal rate responses elicited by similar sequences in gerbil inferior colliculus and auditory cortex were recorded. At both levels of the auditory pathway, the neuronal thresholds obtained with a signal-detection-theoretic approach deducing the sensitivity from the analysis of the neurons' receiver operating characteristics matched the psychophysical thresholds revealing that IM already emerges at midbrain level. By applying objective response measures in physiology and psychophysics, we demonstrated that the population of neurons has a sufficient sensitivity for explaining the perceptual level increment thresholds indicating IM. There was a good correspondence between the neuronal and perceptual release from IM being related to auditory stream segregation.

Developmental hearing loss impairs signal detection in noise: putative central mechanisms

Listeners with hearing loss have difficulty processing sounds in noisy environments. This is most noticeable for speech perception, but is reflected in a basic auditory processing task: detecting a tonal signal in a noise background, i.e., simultaneous masking. It is unresolved whether the mechanisms underlying simultaneous masking arise from the auditory periphery or from the central auditory system. Poor detection in listeners with sensorineural hearing loss (SNHL) is attributed to cochlear hair cell damage. However, hearing loss alters neural processing in the central auditory system. Additionally, both psychophysical and neurophysiological data from normally hearing and impaired listeners suggest that there are additional contributions to simultaneous masking that arise centrally. With SNHL, it is difficult to separate peripheral from central contributions to signal detection deficits. We have thus excluded peripheral contributions by using an animal model of early conductive hearing loss (CHL) that provides auditory deprivation but does not induce cochlear damage. When tested as adults, animals raised with CHL had increased thresholds for detecting tones in simultaneous noise. Furthermore, intracellular in vivo recordings in control animals revealed a cortical correlate of simultaneous masking: local cortical processing reduced tone-evoked responses in the presence of noise. This raises the possibility that altered cortical responses which occur with early CHL can influence even simple signal detection in noise.

Frequency tuning in the behaving mouse: different bandwidths for discrimination and generalization

When faced with sensory stimuli, an organism may be required to detect very small differences in a physical parameter (discrimination), while in other situations it may have to generalize over many possible values of the same physical parameter. This decision may be based both on learned information and on sensory aspects of perception. In the present study we describe frequency processing in the behaving mouse using both discrimination and generalization as two key aspects of behaviour. We used a novel naturalistic behavioural apparatus designed for mice, the Audiobox, and paradigm contingencies that were identical for both auditory discrimination and generalization, the latter measured using latent inhibition. Mice learned to discriminate between frequencies that were an octave apart in a single trial. They showed significant discrimination between tone frequencies that were as close as 4–7%, and had d' of about 1 for ΔF of around 10%. In contrast, pre-exposure frequencies that were half an octave or less below the conditioned tone elicited latent inhibition, showing a generalization bandwidth of at least half an octave. Thus, in the same apparatus and using the same general memory paradigm, mice showed generalization gradients that were considerably wider than their discrimination threshold, indicating that environmental requirements and previous experience can determine whether the same two frequencies will be considered same or different. Remarkably, generalization gradients paralleled the typical bandwidths established in the auditory periphery and midbrain, suggesting that frequencies may be considered similar when falling within the same critical band.

Behavioral evidence for auditory induction in a species of rodent: Mongolian gerbil (Meriones unguiculatus)

When a segment of sound of interest is interrupted by a loud extraneous noise, humans perceive that the missing sound continues during the intrusive noise. This restoration of auditory information occurs in perceptions of both speech and non-speech sounds (e.g., tone bursts), a phenomenon referred to as auditory induction. In this study, Mongolian gerbils were trained with standard Go/No-Go operant conditioning to discriminate continuous tone bursts (the Go stimulus) from tone bursts with a silent gap in the middle (the No-Go stimulus). Noise was added to Go and No-Go stimuli to determine the condition under which induction would occur. The Mongolian gerbils engaged in Go responses to No-Go stimuli only when the noise spectrally surrounding the tone was of the same duration as the silent portion of the No-Go stimulus; these results match those previously reported in primates (humans and macaque monkeys). The result presents not only the evidence of the auditory induction in a rodent species but also suggests that similar mechanisms for restoring missing sounds are shared among mammals. Additionally, our findings demonstrated that the rodent can serve as a valuable animal model for future studies of perceptual restoration.

Mistuning detection and onset asynchrony in harmonic complexes in Mongolian gerbils

By applying a Go/NoGo paradigm, thresholds for detecting mistuning of components of a 200 Hz complex were determined in the Mongolian gerbil and compared with thresholds obtained in a previous study with an 800 Hz complex. Frequency difference limens (FDLs) for detecting mistuning decreased with increasing harmonic frequency and harmonic number (0.5% to 0.01% Weber fraction). It was furthermore examined how starting and ending the mistuned component earlier than the remaining complex affects the FDL (duration of all components 400 ms, time shift 30 to 500 ms). Large FDLs that are similar to pure tone FDLs (between 21% and 6.7%) were found for onset asynchronies of 300 ms and more, indicating separate processing of the mistuned component. Small FDLs that are similar to FDLs of the synchronous condition were found if the temporal overlap between the mistuned component and the remaining complex was 100 ms or more. These experimental data in combination with a simulation of processing of the harmonic complexes by the gerbil's peripheral auditory filters led to the conclusion that the phase and amplitude modulations in the filter outputs can provide cues that allow gerbils a sensitive detection of mistuning across a wide range of frequencies.

Localization dominance and the effect of frequency in the Mongolian Gerbil, Meriones unguiculatus

Due to its good low-frequency hearing, the Mongolian Gerbil (Meriones unguiculatus) has become a well-established animal model for human hearing. In humans, sound localization in reverberant environments is facilitated by the precedence effect, i.e., the perceptual suppression of spatial information carried by echoes. The current study addresses the question whether gerbils are a valid animal model for such complex spatial processing. Specifically, we quantify localization dominance, i.e., the fact that in the context of precedence, only the directional information of the sound which reaches the ear first dominates the perceived position of a sound source whereas directional information of the delayed echoes is suppressed. As localization dominance is known to be stimulus-dependent, we quantified the extent to which the spectral content of transient sounds affects localization dominance in the gerbil. The results reveal that gerbils show stable localization dominance across echo delays, well comparable to humans. Moreover, localization dominance systematically decreased with increasing center frequency, which has not been demonstrated in an animal before. These findings are consistent with an important contribution of peripheral-auditory processing to perceptual localization dominance. The data show that the gerbil is an excellent model to study the neural basis of complex spatial-auditory processing.

Influence of inhibitory synaptic kinetics on the interaural time difference sensitivity in a linear model of binaural coincidence detection

Temporal correlations between the sound waves arriving at the two ears are used to extract the azimuthal position of sound sources. Nerve cells in the mammalian medial superior olive (MSO) that extract these binaural correlations are sensitive to interaural time differences (ITDs) in the range of about 10 micros. These neurons receive inputs from the two ears via four pathways, two excitatory and two inhibitory ones. In this paper, a simple linear model is fitted to the frequency dependence of ITD sensitivity of MSO neurons, which is quantified by the two parameters, characteristic phase and characteristic delay. The fit parameters are the relative delays and the relative strengths of the two inhibitory pathways and thus specify the underlying ITD-detecting circuit assuming a non-Jeffress-like situation, i.e., no excitatory delay lines but phase-locked inhibition. The fitting procedure finds the parameters of these inhibitory pathways such that they account for a desired frequency dependence of ITD sensitivity. It is found that positive characteristic delays require a finite amount of ipsilateral inhibition that arrives at roughly the same time as ipsilateral excitation as well as contralateral inhibition that lags contralateral excitation so much that it effectively leads excitation of the next cycle.

Perception and neural representation of size-variant human vowels in the Mongolian gerbil (Meriones unguiculatus)

Humans reliably recognize spoken vowels despite the variability of the sounds caused by the across-subject variability of the speakers' vocal tract. The vocal tract serves as a resonator which imprints a spectral envelope onto the sounds generated by the vocal folds. This spectral envelope contains not only information about the type of vocalization but also about the size of the speaker: the larger the speaker, the lower the formant frequencies of the spoken vowels. In a combined psychophysical and electrophysiological study in the Mongolian gerbil (Meriones unguiculatus), we investigated the perception and neural representation of human vowels spoken by speakers of different sizes. Gerbils trained to discriminate two standard vowels, correctly assigned vowels spoken from different-sized human speakers. Complementary electrophysiological recordings from neurons in the auditory brainstem, midbrain, and primary auditory cortex show that the auditory brainstem retains a truthful representation of the frequency content of the presented vowel sounds. A small percentage of neurons in the midbrain and auditory cortex, however, showed selectivity for a certain vowel type or vocal tract length which is not related to the pure-tone, frequency response area, indicative of a preprocessing stage for auditory segregation of size and structure information.

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.

Origin of suppression of otoacoustic emissions evoked by two-tone bursts

Otoacoustic emission (OAE) data recorded for tone bursts presented separately and as a two-tone burst complex, that had been reported previously [Yoshikawa, H., Smurzynski, J., Probst R., 2000. Suppression of tone burst evoked otoacoustic emissions in relation to frequency separation. Hear. Res. 148, 95-106], were re-processed using the method of adaptive approximations by matching pursuit (MP). Two types of stimuli were applied to record tone burst OAEs (TBOAEs): (a) cosine-windowed tone bursts of 5-ms duration with center frequencies of 1, 1.5, 2 and 3kHz, (b) complex stimuli consisting of a digital addition of the 1-kHz tone burst together with either the 1.5-, 2- or 3-kHz tone burst. The MP method allowed decomposition of signals into waveforms of defined frequency, latency, time span, and amplitude. This approach provided a high time-frequency (t-f) resolution and identified patterns of resonance modes that were characteristic for TBOAEs recorded in each individual ear. Individual responses to single-tone bursts were processed off-line to form 'sum of singles' responses. The results confirmed linear superposition behavior for a frequency separation of two-tone bursts of 2kHz (the 1-kHz and 3-kHz condition). For the 1, 1.5-kHz condition, the MP results revealed the existence of closely positioned resonance modes associated with responses recorded individually with the stimuli differing in frequency by 500Hz. Then, the differences between t-f distributions calculated for dual (two-tone bursts) and sum-of-singles conditions exhibited mutual suppression of resonance modes common to both stimuli. The degree of attenuation depended on the individual pattern of characteristic resonance modes, i.e., suppression occurred when two resonant modes excited by both stimuli overlapped. It was postulated that the suppression observed in case of dual stimuli with closely-spaced components is due to mutual attenuation of the overlapping resonance modes.

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.

Reducing individual differences in the external-ear transfer functions of the Mongolian gerbil

This study examines individual differences in the directional transfer functions (DTFs), the directional components of head-related transfer functions of gerbils, and seeks a method for reducing these differences. The difference between the DTFs of a given animal pair was quantified by the intersubject spectral difference (ISSD), which is the variance in the difference spectra of DTFs for frequencies between 5 and 45 kHz and for 361 source directions. An attempt was made to reduce the ISSD by scaling the DTFs of one animal in frequency and/or rotating the DTFs along the source coordinate sphere. The ISSD was reduced by a median of 12% after optimal frequency scaling alone, by a median of 19% after optimal spatial rotation alone, and by a median of 36% after simultaneous frequency scaling and spatial rotation. The optimal scaling factor (OSF) and the optimal coordinate rotation (OCR) correlated strongly with differences in head width and pinna angles (i.e., pinna inclination around the vertical and front-back axes), respectively. Thus, linear equations were derived to estimate the OSF and OCR from these anatomical measurements. The ISSD could be reduced by a median of 22% based on the estimated OSF and OCR.

Dependency of the interaural phase difference sensitivities of inferior collicular neurons on a preceding tone and its implications in neural population coding

This study examined the sensitivities of the neuronal responses in the inferior colliculus (IC) to the interaural phase difference (IPD) of a preceding tone, and explored its implications in the neural-population representation of the IPD. Single-unit responses were recorded from the IC of anesthetized gerbils. The stimulus was a dichotic tone sequence with a common frequency (typically the unit's best frequency) and level (10-20 dB relative to the threshold), consisting of a conditioner (200 ms) followed by a probe (50 ms) with a silent gap (5-100 ms) between them. The IPDs of the 2 tones were varied independently. The presence of a conditioner generally suppressed the probe-driven responses; the effect size increased as the conditioner IPD approached the unit's most responsive IPD. The units' preferred IPDs were relatively invariant with the conditioner IPD. Two types of models were used to evaluate the effects of a conditioner on the IPD representation at the level of IC neural population. One was a version of the population-vector model. The other was the hemispheric-channel model, which assumed that the stimulus IPD is represented by the activities of 2 broadly tuned hemispheric channels. Both models predicted that, in the presence of a conditioner, the IPD representation would shift in a direction away from the conditioner IPD. This appears to emphasize the difference between the conditioner and the probe IPDs. The results indicate that in the IC, neural processes for IPD adapt to the stimulus history to enhance the representational contrast between successive sounds.

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.

Auditory sensory memory for random waveforms in the Mongolian gerbil

Mongolian gerbils (N = 21) were trained to discriminate between continuous and repeated auditory white noise. While for periods up to 40 ms of the repeated noise spectral effects make this a perceptual task, longer periods require auditory sensory memory to solve the task. Short periods (20 ms) could easily be discriminated by naive gerbils (discrimination performance, i.e. hit rate minus false alarm rate >80% after 8 days of training). Discrimination was more difficult for longer periods (100 ms: discrimination performance approximately 50% after 18 days of training). By long-term training (156 days) using an optimized training paradigm two further gerbils learned to discriminate up to a period length of 360 ms but could not proceed at 400 ms. While this falls short of human performance, it demonstrates for the first time sensory memory for random waveforms in animals.