Skyler G. Jennings

Evaluating Adaptation and Olivocochlear Efferent Feedback as Potential Explanations of Psychophysical Overshoot

Masked detection threshold for a short tone in noise improves as the tone’s onset is delayed from the masker’s onset. This improvement, known as “overshoot,” is maximal at mid-masker levels and is reduced by temporary and permanent cochlear hearing loss. Computational modeling was used in the present study to evaluate proposed physiological mechanisms of overshoot, including classic firing rate adaptation and medial olivocochlear (MOC) feedback, for both normal hearing and cochlear hearing loss conditions. These theories were tested using an established model of the auditory periphery and signal detection theory techniques. The influence of several analysis variables on predicted tone-pip detection in broadband noise was evaluated, including: auditory nerve fiber spontaneous-rate (SR) pooling, range of characteristic frequencies, number of synapses per characteristic frequency, analysis window duration, and detection rule. The results revealed that overshoot similar to perceptual data in terms of both magnitude and level dependence could be predicted when the effects of MOC efferent feedback were included in the auditory nerve model. Conversely, simulations without MOC feedback effects never produced overshoot despite the model’s ability to account for classic firing rate adaptation and dynamic range adaptation in auditory nerve responses. Cochlear hearing loss was predicted to reduce the size of overshoot only for model versions that included the effects of MOC efferent feedback. These findings suggest that overshoot in normal and hearing-impaired listeners is mediated by some form of dynamic range adaptation other than what is observed in the auditory nerve of anesthetized animals. Mechanisms for this adaptation may occur at several levels along the auditory pathway. Among these mechanisms, the MOC reflex may play a leading role.

Auditory filter tuning inferred with short sinusoidal and notched-noise maskers

The physiology of the medial olivocochlear reflex suggests that a sufficiently long stimulus (>100 ms) may reduce cochlear gain and result in broadened frequency selectivity. The current study attempted to avoid gain reduction by using short maskers (20 ms) to measure psychophysical tuning curves (PTCs) and notched-noise tuning characteristics, with a 4-kHz signal. The influence of off-frequency listening on PTCs was evaluated using two types of background noise. Iso-level curves were derived using an estimate of the cochlear input/output (I/O) function, which was obtained using an off-frequency masker as a linear reference. The influence of masker duration on PTCs was assessed using a model that assumed long maskers (>20 ms) evoked gain reduction. The results suggested that the off-frequency masker was a valid linear reference when deriving I/O functions and that off-frequency listening may have occurred in auditory filters apical to the signal place. The iso-level curves from this growth-of-masking study were consistent with those from a temporal-masking-curve study by Eustaquio-Martin and Lopez-Poveda [J. Assoc. Res. Otolaryngol. 12, 281-299. (2011)], suggesting that either approach may be used to derive iso-level curves. Finally, model simulations suggested that masker duration may not influence estimates of frequency selectivity.

PsyAcoustX: A flexible MATLAB® package for psychoacoustics research

The demands of modern psychophysical studies require precise stimulus delivery and flexible platforms for experimental control. Here, we describe PsyAcoustX, a new, freely available suite of software tools written in the MATLAB® environment to conduct psychoacoustics research on a standard PC. PsyAcoustX provides a flexible platform to generate and present auditory stimuli in real time and record users’ behavioral responses. Data are automatically logged by stimulus condition and aggregated in an exported spreadsheet for offline analysis. Detection thresholds can be measured adaptively under basic and complex auditory masking tasks and other paradigms (e.g., amplitude modulation detection) within minutes. The flexibility of the module offers experimenters access to nearly every conceivable combination of stimulus parameters (e.g., probe-masker relations). Example behavioral applications are highlighted including the measurement of audiometric thresholds, basic simultaneous and non-simultaneous (i.e., forward and backward) masking paradigms, gap detection, and amplitude modulation detection. Examples of these measurements are provided including the psychoacoustic phenomena of temporal overshoot, psychophysical tuning curves, and temporal modulation transfer functions. Importantly, the core design of PsyAcoustX is easily modifiable, allowing users the ability to adapt its basic structure and create additional modules for measuring discrimination/detection thresholds for other auditory attributes (e.g., pitch, intensity, etc.) or binaural paradigms.

The Frequency Selectivity of Gain Reduction Masking: Analysis Using Two Equally-Effective Maskers

The “temporal effect” occurs when masked threshold is shifted as a result of the signal being preceded by sound (i.e., a “precursor”) instead of silence. Several authors have suggested that the temporal effect may be mediated by the medial olivocochlear reflex (MOCR), which reduces the gain of the cochlear amplifier. We recently measured an analogous temporal effect in forward masking (Jennings et al., J Acoust Soc Am 125:2172-2181, 2009). This study estimated the basilar membrane input-output (I/O) function using psychophysical methods. When an on-frequency precursor was present, the gain of the I/O function decreased, consistent with the MOCR hypothesis. Here, we present data on the same forward masking temporal effect, but with specific interest on the tuning of the precursor’s effect. In experiment 1, off-frequency GOM was measured to estimate the I/O function. In experiment 2, psychophysical tuning curves (PTCs) were measured to estimate the tuning of the precursor for two signal levels. Finally, in experiment 3 we combined the masker levels from experiment 1 and the precursor levels from experiment 2 to measure the shift in signal threshold. In the discussion, we model these data in terms of gain reduction and additivity of masking. The results and modeling suggest that the precursor reduced the gain of the I/O function and that the resulting PTCs reflect the tuning of the gain reduction mechanism.

Computational Modeling of Individual Differences in Behavioral Estimates of Cochlear Nonlinearities

Temporal masking curves (TMCs) are often used to estimate cochlear compression in individuals with normal and impaired hearing. These estimates may yield a wide range of individual differences, even among subjects with similar quiet thresholds. This study used an auditory model to assess potential sources of variance in TMCs from 51 listeners in Poling et al. [J Assoc Res Otolaryngol, 13:91–108 (2012)]. These sources included threshold elevation, the contribution of outer and inner hair cell dysfunction to threshold elevation, compression of the off-frequency linear reference, and detection efficiency. Simulations suggest that detection efficiency is a primary factor contributing to individual differences in TMCs measured in normal-hearing subjects, while threshold elevation and the contribution of outer and inner hair cell dysfunction are primary factors in hearing-impaired subjects. Approximating the most compressive growth rate of the cochlear response from TMCs was achieved only in subjects with the highest detection efficiency. Simulations included off-frequency nonlinearity in basilar membrane and inner hair cell processing; however, this nonlinearity did not improve predictions, suggesting that other sources, such as the decay of masking and the strength of the medial olivocochlear reflex, may mimic off-frequency nonlinearity. Findings from this study suggest that sources of individual differences can play a strong role in behavioral estimates of compression, and these sources should be considered when using forward masking to study cochlear function in individual listeners or across groups of listeners.

Effects of a precursor on amplitude modulation detection are consistent with efferent feedback

The acoustic waveform of speech is characterized by slowly varying amplitude fluctuations (i.e., envelope) and an accurate representation of the envelope is essential for speech understanding. The post-cochlear representation of the contrast between peaks and valleys of the envelope (peak-to-valley contrast) may be reduced by cochlear compression. This study tested (1) whether amplitude modulation (AM) detection thresholds are consistent with cochlear compression and (2) whether the introduction of a precursor before the carrier results in improved AM thresholds, consistent with a decompressed cochlear response via the medial olivocochlear reflex (MOCR). In the no precursor condition, AM thresholds worsen at mid-levels, consistent with reduced peak-to-valley contrast from cochlear compression. In the precursor condition, AM thresholds improved at low modulation frequencies and mid-to-high levels, consistent with a reduction in cochlear amplifier gain and decompression of the cochlear input/output function...

Amplitude modulation detection with a short-duration carrier: Effects of a precursor and hearing lossa)

This study tests the hypothesis that amplitude modulation (AM) detection will be better under conditions where basilar membrane (BM) response growth is expected to be linear rather than compressive. This hypothesis was tested by (1) comparing AM detection for a tonal carrier as a function of carrier level for subjects with and without cochlear hearing impairment (HI), and by (2) comparing AM detection for carriers presented with and without an ipsilateral notched-noise precursor, under the assumption that the precursor linearizes BM responses. Average AM detection thresholds were approximately 5 dB better for subjects with HI than for subjects with normal hearing (NH) at moderate-level carriers. Average AM detection for low-to-moderate level carriers was approximately 2 dB better with the precursor than without the precursor for subjects with NH, whereas precursor effects were absent or smaller for subjects with HI. Although effect sizes were small and individual differences were noted, group differences are consistent with better AM detection for conditions where BM responses are less compressive due to cochlear hearing loss or due to a reduction in cochlear gain. These findings suggest the auditory system may quickly adjust to the local soundscape to increase effective AM depth and improve signal-to-noise ratios.

Evaluating whether increment detection at mid-to-high pedestal frequencies is consistent with cochlear compression

The ability of the auditory system to encode the amplitude fluctuations of a signal is important for processing complex stimuli, such as speech. A study by Florentine [Florentine et al., J. Acoust. Soc. Am. 81, 1528–1541 (1987)] measured intensity discrimination for multiple frequencies as a function of stimulus level. They observed that, for high frequency stimuli, difference limens were poorer at mid levels than at higher or lower levels. This mid-level “hump” is consistent with cochlear compression and suggests that compression may limit the intensity resolution of the auditory system under certain circumstances. To test the generalizability of this interpretation, the current study measured increment detection for conditions associated with the mid-level hump, based on the assumption that increment detection and intensity discrimination are determined by similar physiological processes. This study is part of a larger series of experiments in our laboratory that test whether intensity perception is con...

Perceived emotion in clear speech: Effect of simulated hearing loss

Previous research suggests that both young normal-hearing and older hearing-impaired listeners judge clear speech as sounding angry more often than conversational speech. Interestingly, older hearing-impaired listeners were less likely than young normal-hearing listeners to judge sentences as angry in both speaking styles, suggesting that age and/or hearing loss may play a role in judging talkers’ emotions. An acoustic cue that helps distinguish angry speech from emotionally neutral speech is increased high-frequency energy, which may be attenuated or rendered inaudible by age-related hearing loss. The present study tests the hypothesis that simulating such a hearing loss will decrease the perception of anger by young normal-hearing listeners. Sentences spoken clearly and conversationally were processed and filtered to simulate the average hearing loss of the older hearing-impaired listeners from a previous study. Young normal-hearing listeners were asked to assign each sentence to one of six categories (...

Modeling individual differences in overshoot: Effects of age, hearing loss, and efferent feedback

The detection of a short sinusoidal probe in simultaneous masking improves as the probes onset is delayed from the maskers onset. This “overshoot” may be mediated by the medial olivocochlear (MOC) reflex, whose pathway includes spiral ganglion neurons (SGNs), olivocochlear neurons and the outer hair cells (OHCs). Overshoot was measured in younger adults with normal hearing, older adults with normal hearing, and older adults with hearing loss to test the hypothesis that overshoot decreases as components in the MOC reflex pathway are compromised. Overshoot was significantly reduced in older adults, but only those with hearing loss, which is consistent with overshoot depending primarily on the status of the OHCs and only minimally influenced by age-related reductions in SGNs. Thresholds measured when the probe was near the masker’s onset showed large differences across listeners, resulting in appreciable individual differences in overshoot. Simulations were generated from a computational model of the audit...