Psychoacoustics publications

Although lizards have highly sensitive ears, it is difficult to condition them to sound, making standard psychophysical assays of hearing sensitivity impractical. This paper describes non-invasive measurements of the auditory brainstem response (ABR) in both Tokay geckos (Gekko gecko; nocturnal animals, known for their loud vocalizations) and the green anole (Anolis carolinensis, diurnal, non-vocal animals). Hearing sensitivity was measured in 5 geckos and 7 anoles. The lizards were sedated with isoflurane, and ABRs were measured at levels of 1 and 3% isoflurane. The typical ABR waveform in response to click stimulation showed one prominent and several smaller peaks occurring within 10 ms of the stimulus onset. ABRs to brief tone bursts revealed that geckos and anoles were most sensitive between 1.6–2 kHz and had similar hearing sensitivity up to about 5 kHz (thresholds typically 20–50 dB SPL). Above 5 kHz, however, anoles were more than 20 dB more sensitive than geckos and showed a wider range of sensitivity (1–7 kHz). Generally, thresholds from ABR audiograms were comparable to those of small birds. Best hearing sensitivity, however, extended over a larger frequency range in lizards than in most bird species.

The ability of native and non-native speakers to enhance intelligibility of target vowels by speaking clearly was compared across three talker groups: monolingual English speakers and native Spanish speakers with either an earlier or a later age of immersion in an English-speaking environment. Talkers produced the target syllables “bead, bid, bayed, bed, bad” and “bod” in ‘conversational’ and clear speech styles. The stimuli were presented to native English-speaking listeners in multi-talker babble with signal-to-noise ratios of −8 dB for the monolingual and early learners and −4 dB for the later learners. The monolinguals and early learners of English showed a similar average clear speech benefit, and the early learners showed equal or greater intelligibility than monolinguals for most target vowels. The 4-dB difference in signal-to-noise ratio yielded approximately equal average intelligibility for the monolinguals and later learners. The average clear speech benefit was smallest for the later learners, and a significant clear speech decrement was obtained for the target syllable “bid.” These results suggest that later learners of English as a second language may be less able than monolinguals to accommodate listeners in noisy environments, due to a reduced ability to improve intelligibility by speaking more clearly.

Temporal weighting functions (TWFs), quantifying sensitivity to interaural time differences (ITD) and interaural level differences (ILD) over the duration of brief stimuli, were measured in 6 normal hearing subjects using trains of 16 Gabor clicks centered at 4 kHz presented dichotically at 4 rates [inter-click intervals (ICI) of 10, 5, 2.5, and 1.25 ms]. Random ITD or ILD were imposed independently on each click in the train in separate conditions. The subject’s task was to discriminate the lateral position of the click train (“left” or “right”). Receiver operating characteristic (ROC) analysis was then used to quantify the effectiveness or “weight” of each click according to individual click ITD or ILD. Although individual differences were evident, onset cues appeared to dominate at high rates. Onset dominance was apparent for both ITD and ILD at 1.25 ms ICI and for ITD at 2.5 ms ICI, but for neither cue at 5 or 10 ms ICI. Onset dominance was greater on average for ITD than ILD, although TWFs were qualitatively similar for the two cues. No evidence was found for “upweighting” of late-arriving ILD [Stecker, G. C., and Hafter, E. R. (2009), J. Acoust. Soc. Am. 125, 3914–3924].

Psychophysical studies show a slower response to changes in the specifically binaural input than to changes in the monaural input (binaural sluggishness). However, there is disagreement about the time course. Tracking changes in a target yields fast time constants, while detecting a constant target against a varying background yields the slowest. Changes in the binaural properties of a target are tracked up to high rates by cells in the midbrain. Indeed cells respond rapidly to a step change and then the firing rate slowly adapts. These experiments, though, are analogues of psychophysical experiments that give the faster time constants. Sluggishness should be more apparent physiologically in a binaural masking paradigm, detecting a short tone in a noise masker with a step change in masker correlation: the small change in firing rate due to the signal must be detected against the adapting firing rate change caused by the step change in the masker. However, in 40 inferior colliculus cells in the anesthetized guinea pig, in a direct analogue of the psychophysical masking paradigm, measuring thresholds for short tones across a transition in a binaural masker (e.g., from N0S0 to NπS0) provided little evidence of sluggishness within individual cells despite masking level differences in these cells comparable with previous data. Previous studies of physiological correlates of binaural masking level difference suggested that different psychophysical thresholds arise from different populations of cells. This suggests the hypothesis that sluggishness may result from a change in focus between the different populations of cells signaling threshold in different binaural configurations rather than within the intrinsic properties of the cells themselves.

Perception of breathy voice quality is cued by a number of acoustic changes including an increase in aspiration noise level (AH) and spectral slope. Changes in AH in a vowel may be evaluated through measures such as the harmonic-to-noise ratio, cepstral peak prominence (CPP), or via auditory measures such as the partial loudness of harmonic energy and loudness of aspiration noise. Although a number of experiments have reported high correlation between such measures and ratings of perceived breathiness, a formal model to predict breathiness of a vowel has not been proposed. This research describes two computational models to predict changes in breathiness resulting from variations in AH. One model uses auditory measures, whereas the other uses CPP as independent variables to predict breathiness. For both cases, a translated and truncated power function is required to predict breathiness. Some parameters in both of these models were observed to be pitch dependent. The "unified" model based on auditory measures was observed to be more accurate than one based on CPP.

Background: Salicylate and quinine have been shown to reliably induce short-term tinnitus when administered at high doses. The present study compared salicylate and quinine-induced tinnitus in rats using the gap prepulse inhibition of acoustic startle (GPIAS). Methods: Twenty-four rats were divided into 2 groups; the first group (n = 12) was injected with salicylate (300 mg kg d), whereas the second (n = 12) was treated with quinine orally at a dose of 200 mg kg d. Animals were treated daily for 4 consecutive days. All rats were tested for tinnitus and hearing loss before and 2, 24, 48, 72, and 96 hours after the first drug administration. Tinnitus was assessed using GPIAS; hearing function was measured with distortion product otoacoustic emissions (DPOAEs) and auditory brainstem response. Results: Salicylate treatment induced transient tinnitus with a pitch near 16 kHz starting 2 hours posttreatment, persisting over the 4-day treatment period and disappearing 24 hours later. Animals in the quinine group showed GPIAS changes at a higher pitch (20 kHz); however, changes were more variable among animals, and the mean data were not statistically significant. Hearing function varied across treatments. In the salicylate group, high-level DPOAEs were slightly affected; most changes occurred 2 hours posttreatment. Low-level DPOAEs were affected at all frequencies with a progressive dose-dependent effect. In the quinine group, only high-level DPOAEs were affected, mainly at 16 kHz. Conclusion: The present study highlights the similarities and differences in the frequency and the time course of tinnitus and hypoacusis induced by salicylate and quinine. Transient tinnitus was reliably induced pharmacologically with salicylate, whereas hearing loss remained subclinical with only minor changes in DPOAEs.

In humans, auditory perception reaches maturity over a broad age range, extending through adolescence. Despite this slow maturation, children are considered to be outstanding learners, suggesting that immature perceptual skills might actually be advantageous to improvement on an acoustic task as a result of training (perceptual learning). Previous non‐human studies have not employed an identical task when comparing perceptual performance of young and mature subjects, making it difficult to assess learning. Here, we used an identical procedure on juvenile and adult gerbils to examine the perception of amplitude modulation (AM), a stimulus feature that is an important component of most natural sounds. On average, Adult animals could detect smaller fluctuations in amplitude (i.e., smaller modulation depths) than Juveniles, indicating immature perceptual skills in Juveniles. However, the population variance was much greater for Juveniles, a few animals displaying adult‐like AM detection. To determine whether immature perceptual skills facilitated learning, we compared naïve performance on the AM detection task with the amount of improvement following additional training. The amount of improvement in Adults correlated with naïve performance: those with the poorest naïve performance improved the most. In contrast, the naïve performance of Juveniles did not predict the amount of learning. Those Juveniles with immature AM detection thresholds did not display greater learning than Adults. Furthermore, for several of the Juveniles with adult‐like thresholds, AM detection deteriorated with repeated testing. Thus, immature perceptual skills in young animals were not associated with greater learning. © 2010 Wiley Periodicals, Inc. Develop Neurobiol 70: 636–648, 2010

Objective: To determine whether phonophobia and dynamic mechanical (brush) allodynia are associated in episodic migraine (EM). Methods: Adult patients with EM were prospectively recruited. A structured questionnaire was used to obtain demographic and migraine related data. Phonophobia was tested quantitatively using a real time sound processor and psychoacoustic software. Sound stimuli were pure tones at frequencies of 1000 Hz, 4000 Hz and 8000 Hz, delivered to both ears at increasing intensities, until an aversive level was reached. Allodynia was assessed by brushing the patient's skin with a gauze pad at different areas. Patients were tested both between and during acute attacks. Sound aversion thresholds (SATs) in allodynic and non-allodynic patients were compared. Results: Between attacks, SATs were lower in allodynic compared with non-allodynic patients, with an average difference of -5.7 dB (p=0.04). During acute attacks, the corresponding average SAT difference (allodynic-non-allodynic) was -15.7 dB (p=0.0008). There was a significant negative correlation between allodynia scores and SATs, both within and between attacks. Conclusions: The results support an association between phonophobia and cutaneous allodynia in migraine.

Four cochlear implant users, having normal hearing in the unimplanted ear, compared the pitches of electrical and acoustic stimuli presented to the two ears. Comparisons were between 1,031-pps pulse trains and pure tones or between 12 and 25-pps electric pulse trains and bandpass-filtered acoustic pulse trains of the same rate. Three methods-pitch adjustment, constant stimuli, and interleaved adaptive procedures-were used. For all methods, we showed that the results can be strongly influenced by non-sensory biases arising from the range of acoustic stimuli presented, and proposed a series of checks that should be made to alert the experimenter to those biases. We then showed that the results of comparisons that survived these checks do not deviate consistently from the predictions of a widely-used cochlear frequency-to-place formula or of a computational cochlear model. We also demonstrate that substantial range effects occur with other widely used experimental methods, even for normal-hearing listeners.

Sensory systems are known to adapt their coding strategies to the statistics of their environment, but little is still known about the perceptual implications of such adjustments. We investigated how auditory spatial processing adapts to stimulus statistics by presenting human listeners and anesthetized ferrets with noise sequences in which interaural level differences (ILD) rapidly fluctuated according to a Gaussian distribution. The mean of the distribution biased the perceived laterality of a subsequent stimulus, whereas the distribution's variance changed the listeners' spatial sensitivity. The responses of neurons in the inferior colliculus changed in line with these perceptual phenomena. Their ILD preference adjusted to match the stimulus distribution mean, resulting in large shifts in rate-ILD functions, while their gain adapted to the stimulus variance, producing pronounced changes in neural sensitivity. Our findings suggest that processing of auditory space is geared toward emphasizing relative spatial differences rather than the accurate representation of absolute position.

Several studies have investigated the relationship between click-evoked auditory brainstem responses (ABRs) and loudness growth in human listeners. While some of these studies have reported promising results, showing a correlative relationship between click ABR and loudness growth as a function of level, additional studies are necessary to determine if similar results can be obtained with frequency-specific stimuli and more specific details of the loudness function can be derived from ABR recordings. The aims of this study, therefore, were to (1) develop a fully objective procedure that segments specific features of evoked, tone-burst ABR recordings, (2) investigate the feasibility of using information derived from these recordings for estimating frequency-specific loudness-growth functions, and (3) determine to what extent the loudness-growth estimation performance through ABR can be improved by controlling for residual noise levels and parametric fitting. Results from eight normal-hearing listeners using 1- and 4-kHz stimuli show that the average mean-square error of the loudness-growth estimation obtained through the procedure is comparable to that of standard psychoacoustical procedures used to estimate loudness growth. The data set has been made publicly available at www.physionet.org.

The study measured listener sensitivity to increments of a target inter-onset interval (IOI) embedded within isochronous tone sequences that featured a single accented tonal component. The sequences consisted of six 1000-Hz tone bursts separated by silent intervals to establish equal tonal IOIs of 200 ms within the sequence. Tone burst durations within the sequences were 50 ms, except one tone had a longer duration of 100 ms to produce a perception of accent. Duration DLs in ms for increments of a single sequence IOI were measured adaptively by adjusting the duration of the silent interval between two tones. Sequence position of the target IOI differed across conditions. Listeners included young normal-hearing adults and older adults with and without hearing loss. Discrimination performance of the two older listener groups was equivalent and significantly poorer than that of the younger listeners in each discrimination condition. The age-related discrimination deficits were independent of sequence locations of both the target interval and the accented tonal component. Comparative DLs collected for target intervals in unaccented tone sequences with equal tone durations revealed that the detrimental effects of accent on temporal discrimination were primarily restricted to the older listeners.

Psychophysical data on hearing sensitivity and various measures of supra-threshold auditory temporal processing are presented for large groups of young (18-35 y), middle-aged (40-55 y) and older (60-89 y) adults. Hearing thresholds were measured at 500, 1414 and 4000 Hz. Measures of temporal processing included gap-detection thresholds for bands of noise centered at 1000 and 3500 Hz, stimulus onset asynchronies for monaural and dichotic temporal-order identification for brief vowels, and stimulus onset/offset asynchronies for the monaural temporal masking of vowel identification. For all temporal-processing measures, the impact of high-frequency hearing loss in older adults was minimized by a combination of low-pass filtering the stimuli and use of high presentation levels. The performance of the older adults was worse than that of the young adults on all measures except gap-detection threshold at 1000 Hz. Middle-aged adults performed significantly worse than the young adults on measures of threshold sensitivity and three of the four measures of temporal-order identification, but not for any of the measures of temporal masking. Individual differences are also examined among a group of 124 older adults. Cognition and age were found to be significant predictors, although only 10-27% of the variance could be accounted for by these predictors.

Listeners detected interaural differences of time (ITDs) or level (ILDs) carried by single 4000-Hz Gabor clicks (Gaussian-windowed tone bursts) and trains of 16 such clicks repeating at an interclick interval (ICI) of 2, 5, or 10 ms. In separate conditions, target interaural differences favored the right ear by a constant amount for all clicks (condition RR), attained their peak value at onset and diminished linearly to 0 at offset (condition R0), or grew linearly from 0 at onset to a peak value at offset (condition 0R). Threshold ITDs and ILDs were determined adaptively in separate experiments for each of these conditions and for single clicks. ITD thresholds were found to be lower for 16-click trains than for single clicks at 10-ms ICI, regardless of stimulus condition. At 2-ms ICI, thresholds in RR and R0 conditions were similar to single click thresholds at 2-ms ICI; thresholds in the 0R condition were significantly worse than for single clicks at 2-ms ICI, consistent with strong rate-dependent onset dominance in listeners’ temporal weighting of ITD. ILD thresholds, in contrast, were predominantly unaffected by ICI, suggesting little or no onset dominance for ILD of high-rate stimuli.

A growing number of studies of auditory processing are being carried out in awake, behaving animals, creating a need for precisely controlled sound delivery without restricting head movements. We have designed a system for closed-field stimulus presentation in freely moving ferrets, which comprises lightweight, adjustable headphones that can be consistently positioned over the ears via a small, skull-mounted implant. The invasiveness of the implant was minimized by simplifying its construction and using dental adhesive only for attaching it to the skull, thereby reducing the surgery required and avoiding the use of screws or other anchoring devices. Attaching the headphones to a chronic implant also reduced the amount of contact they had with the head and ears, increasing the willingness of the animals to wear them. We validated sound stimulation via the headphones in ferrets trained previously in a free-field task to localize stimuli presented from one of two loudspeakers. Noise bursts were delivered binaurally over the headphones and interaural level differences (ILDs) were introduced to allow the sound to be lateralized. Animals rapidly transferred from the free-field task to indicate the perceived location of the stimulus presented over headphones. They showed near perfect lateralization with a 5 dB ILD, matching the scores achieved in the free-field task. As expected, the ferrets' performance declined when the ILD was reduced in value. This closed-field system can easily be adapted for use in other species, and provides a reliable means of presenting closed-field stimuli whilst monitoring behavioral responses in freely moving animals.

Background: Barn owls integrate spatial information across frequency channels to localize sounds in space. Methodology/principal findings: We presented barn owls with synchronous sounds that contained different bands of frequencies (3-5 kHz and 7-9 kHz) from different locations in space. When the owls were confronted with the conflicting localization cues from two synchronous sounds of equal level, their orienting responses were dominated by one of the sounds: they oriented toward the location of the low frequency sound when the sources were separated in azimuth; in contrast, they oriented toward the location of the high frequency sound when the sources were separated in elevation. We identified neural correlates of this behavioral effect in the optic tectum (OT, superior colliculus in mammals), which contains a map of auditory space and is involved in generating orienting movements to sounds. We found that low frequency cues dominate the representation of sound azimuth in the OT space map, whereas high frequency cues dominate the representation of sound elevation. Conclusions/significance: Significance: We argue that the dominance hierarchy of localization cues reflects several factors: 1) the relative amplitude of the sound providing the cue, 2) the resolution with which the auditory system measures the value of a cue, and 3) the spatial ambiguity in interpreting the cue. These same factors may contribute to the relative weighting of sound localization cues in other species, including humans.

Psychophysical forward masking is an increase in threshold of detection of a sound (probe) when it is preceded by another sound (masker). This is reminiscent of the reduction in neuronal responses to a sound following prior stimulation. Studies in the auditory nerve and cochlear nucleus using signal detection theory techniques to derive neuronal thresholds showed that in centrally projecting neurons, increases in masked thresholds were significantly smaller than the changes measured psychophysically. Larger threshold shifts have been reported in the inferior colliculus of awake marmoset. The present study investigated the magnitude of forward masking in primary auditory cortical neurons of anaesthetised guinea-pigs. Responses of cortical neurons to unmasked and forward masked tones were measured and probe detection thresholds estimated using signal detection theory methods. Threshold shifts were larger than in the auditory nerve, cochlear nucleus and inferior colliculus. The larger threshold shifts suggest that central, and probably cortical, processes contribute to forward masking. However, although methodological differences make comparisons difficult, the threshold shifts in cortical neurons were, in contrast to subcortical nuclei, actually larger than those observed psychophysically. Masking was largely attributable to a reduction in the responses to the probe, rather than either a persistence of the masker responses or an increase in the variability of probe responses.

We measured the responses of neurons in auditory cortex of male and female ferrets to artificial vowels of varying fundamental frequency ( f 0 ), or periodicity, and compared these with the performance of animals trained to discriminate the periodicity of these sounds. Sensitivity to f 0 was found in all five auditory cortical fields examined, with most of those neurons exhibiting either low-pass or high-pass response functions. Only rarely was the stimulus dependence of individual neuron discharges sufficient to account for the discrimination performance of the ferrets. In contrast, when analyzed with a simple classifier, responses of small ensembles, comprising 3-61 simultaneously recorded neurons, often discriminated periodicity changes as well as the animals did. We examined four potential strategies for decoding ensemble responses: spike counts, relative first-spike latencies, a binary “spike or no-spike” code, and a spike-order code. All four codes represented stimulus periodicity effectively, and, surprisingly, the spike count and relative latency codes enabled an equally rapid readout, within 75 ms of stimulus onset. Thus, relative latency codes do not necessarily facilitate faster discrimination judgments. A joint spike count plus relative latency code was more informative than either code alone, indicating that the information captured by each measure was not wholly redundant. The responses of neural ensembles, but not of single neurons, reliably encoded f 0 changes even when stimulus intensity was varied randomly over a 20 dB range. Because trained animals can discriminate stimulus periodicity across different sound levels, this implies that ensemble codes are better suited to account for behavioral performance.

Localizing a sound source involves the detection and integration of various spatial cues present in the sound waves at each ear. Previous studies indicate that the brain circuits underlying sound localization are calibrated by experience of the cues available to each individual. Plasticity in spatial hearing is most pronounced during development but can also be demonstrated during adulthood under certain circumstances. Investigations into whether adult humans can adjust to reduced input in one ear and learn a new correspondence between interaural differences cues and directions in space have produced conflicting results. Here we show that humans of both sexes can relearn to localize broadband sounds with a flat spectrum in the horizontal plane after altering the spatial cues available by plugging one ear. In subjects who received daily training, localization accuracy progressively shifted back toward their pre-plug performance after 1 week of earplugging, whereas no improvement was seen if all trials were performed on the same day. However, localization performance did not improve on a task that used stimuli in which the source spectrum was randomized from trial to trial, indicating that monaural spectral cues are needed for plasticity. We also characterized the effects of the earplug on sensitivity to interaural time and level differences and found no clear evidence for adaptation to these cues as the free-field localization performance improved. These findings suggest that the mature auditory system can accommodate abnormal inputs and maintain a stable spatial percept by reweighting different cues according to how informative they are.

Objectives: To examine the impact of hearing impairment on a listener's ability to process simultaneous spoken messages. Design: Nine young listeners with bilateral sensorineural hearing loss and nine young listeners with normal hearing participated in this study. Two messages of equal level were presented separately to the two ears. The messages were systematically degraded by adding speech-shaped noise. Listeners performed a single task in which report of one message was required and a dual task in which report of both messages was required. Results: As the level of the added noise was increased, performance on both single and dual tasks declined. In the dual task, performance on the message reported second was poorer and more sensitive to the noise level than performance on the message reported first. When compared to listeners with normal hearing, listeners with hearing loss showed a larger deficit in recall of the second message than the first. This difference disappeared when performance of the hearing loss group was compared to that of the normal-hearing group at a poorer signal to noise ratio. Conclusions: A listener's ability to process a secondary message is more sensitive to noise and hearing impairment than the ability to process a primary message. Tasks involving the processing of simultaneous messages may be useful for assessing hearing handicap and the benefits of rehabilitation in realistic listening scenarios.