Jordan A. Beim

Effects of temporal stimulus properties on the perception of across-frequency asynchrony.

The role of temporal stimulus parameters in the perception of across-frequency synchrony and asynchrony was investigated using pairs of 500-ms tones consisting of a 250-Hz tone and a tone with a higher frequency of 1, 2, 4, or 6 kHz. Subjective judgments suggested veridical perception of across-frequency synchrony but with greater sensitivity to changes in asynchrony for pairs in which the lower-frequency tone was leading than for pairs in which it was lagging. Consistent with the subjective judgments, thresholds for the detection of asynchrony measured in a three-alternative forced-choice task were lower when the signal interval contained a pair with the low-frequency tone leading than a pair with a high-frequency tone leading. A similar asymmetry was observed for asynchrony discrimination when the standard asynchrony was relatively small (≤20 ms) but not for larger standard asynchronies. Independent manipulation of onset and offset ramp durations indicated a dominant role of onsets in the perception of across-frequency asynchrony. A physiologically inspired model, involving broadly tuned monaural coincidence detectors that receive inputs from frequency-selective onset detectors, was able to accurately reproduce the asymmetric distributions of synchrony judgments. The model provides testable predictions for future physiological investigations of responses to broadband stimuli with across-frequency delays.

Exploring the role of feedback-based auditory reflexes in forward masking by schroeder-phase complexes.

Several studies have postulated that psychoacoustic measures of auditory perception are influenced by efferent-induced changes in cochlear responses, but these postulations have generally remained untested. This study measured the effect of stimulus phase curvature and temporal envelope modulation on the medial olivocochlear reflex (MOCR) and on the middle-ear muscle reflex (MEMR). The role of the MOCR was tested by measuring changes in the ear-canal pressure at 6 kHz in the presence and absence of a band-limited harmonic complex tone with various phase curvatures, centered either at (on-frequency) or well below (off-frequency) the 6-kHz probe frequency. The influence of possible MEMR effects was examined by measuring phase-gradient functions for the elicitor effects and by measuring changes in the ear-canal pressure with a continuous suppressor of the 6-kHz probe. Both on- and off-frequency complex tone elicitors produced significant changes in ear canal sound pressure. However, the pattern of results was not consistent with the earlier hypotheses postulating that efferent effects produce the psychoacoustic dependence of forward-masked thresholds on masker phase curvature. The results also reveal unexpectedly long time constants associated with some efferent effects, the source of which remains unknown.

Stimulus Frequency Otoacoustic Emissions Provide No Evidence for the Role of Efferents in the Enhancement Effect

ABSTRACTAuditory enhancement refers to the perceptual phenomenon that a target sound is heard out more readily from a background sound if the background is presented alone first. Here we used stimulus-frequency otoacoustic emissions (SFOAEs) to test the hypothesis that activation of the medial olivocochlear efferent system contributes to auditory enhancement effects. The SFOAEs were used as a tool to measure changes in cochlear responses to a target component and the neighboring components of a multitone background between conditions producing enhancement and conditions producing no enhancement. In the “enhancement” condition, the target and multitone background were preceded by a precursor stimulus with a spectral notch around the signal frequency; in the control (no-enhancement) condition, the target and multitone background were presented without the precursor. In an experiment using a wideband multitone stimulus known to produce significant psychophysical enhancement effects, SFOAEs showed no changes consistent with enhancement, but some aspects of the results indicated possible contamination of the SFOAE magnitudes by the activation of the middle-ear-muscle reflex. The same SFOAE measurements performed using narrower-band stimuli at lower sound levels also showed no SFOAE changes consistent with either absolute or relative enhancement despite robust psychophysical enhancement effects observed in the same listeners with the same stimuli. The results suggest that cochlear efferent control does not play a significant role in auditory enhancement effects.

Weak Middle-Ear-Muscle Reflex in Humans with Noise-Induced Tinnitus and Normal Hearing May Reflect Cochlear Synaptopathy

Abstract Chronic tinnitus is a prevalent hearing disorder, and yet no successful treatments or objective diagnostic tests are currently available. The aim of this study was to investigate the relationship between the presence of tinnitus and the strength of the middle-ear-muscle reflex (MEMR) in humans with normal and near-normal hearing. Clicks were used as test stimuli to obtain a wideband measure of the effect of reflex activation on ear-canal sound pressure. The reflex was elicited using a contralateral broadband noise. The results show that the reflex strength is significantly reduced in individuals with noise-induced continuous tinnitus and normal or near-normal audiometric thresholds compared with no-tinnitus controls. Due to a shallower growth of the reflex strength in the tinnitus group, the difference between the two groups increased with increasing elicitor level. No significant difference in the effect of tinnitus on the strength of the middle-ear muscle reflex was found between males and females. The weaker reflex could not be accounted for by differences in audiometric hearing thresholds between the tinnitus and control groups. Similarity between our findings in humans and the findings of a reduced middle-ear muscle reflex in noise-exposed animals suggests that noise-induced tinnitus in individuals with clinically normal hearing may be a consequence of cochlear synaptopathy, a loss of synaptic connections between inner hair cells (IHCs) in the cochlea and auditory-nerve (AN) fibers that has been termed hidden hearing loss.

Investigating the role of the medial olivocochlear reflex in selective attention

Selective attention can dramatically attenuate the cortical responses to unattended stimuli relative to attended stimuli. The mechanisms behind this attenuation and their locus in the auditory pathways are not yet understood. Recent animal work suggests that changes may begin to occur as early as the cochlea itself, potentially through the medial olivocochlear (MOC) reflex. Since measuring MOC activity directly is too invasive to be done in humans, otoacoustic emissions (OAEs) are used to examine changes in cochlear function. Previous investigations of attention-related efferent effects on cochlear processing using OAEs have generally found small and inconsistent effects. The previous lack of effects could be due in part to shortcomings in OAE and MOC-reflex measurement methodology. A new experimental paradigm designed to overcome these shortcomings was used here to examine the effects of selectively attending to visual or auditory stimuli. Participants directed attention to either low-frequency tones, hi...

Perception of Across-Frequency Asynchrony by Listeners with Cochlear Hearing Loss

Cochlear hearing loss is often associated with broader tuning of the cochlear filters. Cochlear response latencies are dependent on the filter bandwidths, so hearing loss may affect the relationship between latencies across different characteristic frequencies. This prediction was tested by investigating the perception of synchrony between two tones exciting different regions of the cochlea in listeners with hearing loss. Subjective judgments of synchrony were compared with thresholds for asynchrony discrimination in a three-alternative forced-choice task. In contrast to earlier data from normal-hearing (NH) listeners, the synchronous-response functions obtained from the hearing-impaired (HI) listeners differed in patterns of symmetry and often had a very low peak (i.e., maximum proportion of “synchronous” responses). Also in contrast to data from NH listeners, the quantitative and qualitative correspondence between the data from the subjective and the forced-choice tasks was often poor. The results do not provide strong evidence for the influence of changes in cochlear mechanics on the perception of synchrony in HI listeners, and it remains possible that age, independent of hearing loss, plays an important role in temporal synchrony and asynchrony perception.

The effect of spectral asynchrony on speech recognition in demanding listening conditions

Speech recognition is remarkably robust to overall spectral asynchronies as large as 160-180 ms when performance is measured in quiet. One reason for such robustness may be substantial redundancy in the speech stimulus. This study investigated the effect of spectral asynchrony on speech recognition in listening conditions yielding 80% or less correct recognition score for synchronized speech (baseline condition). IEEE sentences mixed with a speech-shaped noise or two-talker babble were divided into 1/3-octave bands. Successive bands were progressively delayed from low to high, or from high to low, center frequency. The overall delay between the lowest and highest-frequency channels was varied between 0 and 160 ms. The difficulty of the task in the synchronous condition was determined by different signal-to-noise ratios and by presenting either the full stimulus or a lowpass-filtered version. The results show that performance is less robust to spectral asynchrony when speech is presented with an interferin...

Examining the role of a medial olivocochlear reflex elicitor on the attentional modulation of cochlear function

Selective attention has been shown to modulate cortical and subcortical neural representations of sound in the auditory systems of humans and research animals. Neuroanatomy of the auditory system suggests that cortical activity is capable of modulating cochlear responses to sound via corticofugal projections to the medial olivocochlear (MOC) efferent system. Several human studies using otoacoustic emissions (OAEs) suggest that selective attention can modulate cochlear responses to sound, but results across studies typically show small and inconsistent effects. Recent work in our laboratory has demonstrated much larger effects of cross-modal selective attention on cochlear processing than previously reported, likely due to improved methods for measuring MOC effects. One unanswered question is whether selective attention can modulate cochlear function directly, or whether it only modulates stimulus-elicited MOC activity. Our current study compares OAE magnitudes measured while participants attend to auditor...

Effect of low-intensity highpass noise on stimulus frequency otoacoustic emission group delays for low frequency evoking tones

Stimulus-frequency otoacoustic emissions (SFOAEs) have been used in previous research to estimate cochlear tuning in humans. These estimates of tuning rely on the theory that SFOAEs arise from coherent reflections from the place on the basilar membrane (BM) with the characteristic frequency (CF) of the tone evoking the emission. Theories underlying SFOAE generation are still the subject of much debate, and several recent studies have shown evidence supporting an alternative theory postulating that generators of SFOAE are distributed basally to the CF place on the BM. Basally distributed emission generators could explain why SFOAE-derived group delays measured in the chinchilla cochlea for low-frequency tones were shorter than predicted by coherent-reflection hypothesis. The aim of the current study is to look for potential effects of basal emission generators in SFOAE-derived measurements of group delay at low frequencies in humans. SFOAE group delays were measured at 0.5, 0.75 kHz with and without a low intensity highpass noise, used to perturb basally distributed emission generation sites. Results show that the presence of the highpass noise leads to increased group delays estimated from SFOAEs, consistent with the hypothesis that basal generators of emission influence estimates of cochlear group delay at low frequencies.

Challenges in exploring the role of medial olivocochlear efferents in auditory tasks via otoacoustic emissions

A number of recent psychophysical studies have hypothesized that the activation of medial olivocochlear (MOC) efferents plays a significant role in forward masking. These hypotheses are based on general similarities between spectral and temporal characteristics exhibited by some psychophysical forward-masking results and by effects of efferent activation measured using physiological methods. In humans, noninvasive physiological measurements of otoacoustic emissions have been used to probe changes in cochlear responses due to MOC efferent activation. The aim of this study was to verify our earlier efferent-based hypothesis regarding the dependence of psychophysical forward masking of a 6-kHz probe on the phase curvature of harmonic-complex maskers. The ear-canal pressure for a continuous 6-kHz probe was measured in the presence and absence of Schroeder-phase complexes used as forward maskers in our previous psychophysical study. Changes in the ear-canal pressure were analyzed using methods for estimating t...