The salience of enhanced components within inharmonic complexes

Cues to reduce modulation informational masking

The detectability of target amplitude modulation (AM) can be reduced by masker AM in the same carrier-frequency region. It can be reduced even further, however, if the masker-AM rate is uncertain [Conroy and Kidd, J. Acoust. Soc. Am. 149, 3665-3673 (2021)]. This study examined the effectiveness of contextual cues in reducing this latter, uncertainty-related effect (modulation informational masking). Observers were tasked with detecting fixed-rate target sinusoidal amplitude modulation (SAM) in the presence of masker SAM applied simultaneously to the same broadband-noise carrier. A single-interval, two-alternative forced-choice detection procedure was used to measure sensitivity for the target SAM; masker-AM-rate uncertainty was created by randomly selecting the AM rate of the masker SAM on each trial. Relative to an uncued condition, a pretrial cue to the masker SAM significantly improved sensitivity for the target SAM; a cue to the target SAM, however, did not. The delay between the cue-interval offset and trial-interval onset did not affect the size of the masker-cue benefit, suggesting that adaptation of the masker SAM was not responsible. A simple model of within-AM-channel masking captured important trends in the psychophysical data, suggesting that reduced masker-AM-rate uncertainty may have played a relatively minor role in the masker-cue benefit.

Neural auditory contrast enhancement in humans

The perception of sensory events can be enhanced or suppressed by the surrounding spatial and temporal context in ways that facilitate the detection of novel objects and contribute to the perceptual constancy of those objects under variable conditions. In the auditory system, the phenomenon known as auditory enhancement reflects a general principle of contrast enhancement, in which a target sound embedded within a background sound becomes perceptually more salient if the background is presented first by itself. This effect is highly robust, producing an effective enhancement of the target of up to 25 dB (more than two orders of magnitude in intensity), depending on the task. Despite the importance of the effect, neural correlates of auditory contrast enhancement have yet to be identified in humans. Here, we used the auditory steady-state response to probe the neural representation of a target sound under conditions of enhancement. The probe was simultaneously modulated in amplitude with two modulation frequencies to distinguish cortical from subcortical responses. We found robust correlates for neural enhancement in the auditory cortical, but not subcortical, responses. Our findings provide empirical support for a previously unverified theory of auditory enhancement based on neural adaptation of inhibition and point to approaches for improving sensory prostheses for hearing loss, such as hearing aids and cochlear implants.

Evaluating peripheral versus central contributions to spectral context effects in speech perception

Perception of a sound is influenced by spectral properties of surrounding sounds. When frequencies are absent in a preceding acoustic context before being introduced in a subsequent target sound, detection of those frequencies is facilitated via an auditory enhancement effect (EE). When spectral composition differs across a preceding context and subsequent target sound, those differences are perceptually magnified and perception shifts via a spectral contrast effect (SCE). Each effect is thought to receive contributions from peripheral and central neural processing, but the relative contributions are unclear. The present experiments manipulated ear of presentation to elucidate the degrees to which peripheral and central processes contributed to each effect in speech perception. In Experiment 1, EE and SCE magnitudes in consonant categorization were substantially diminished through contralateral presentation of contexts and targets compared to ipsilateral or bilateral presentations. In Experiment 2, spectrally complementary contexts were presented dichotically followed by the target in only one ear. This arrangement was predicted to produce context effects peripherally and cancel them centrally, but the competing contralateral context minimally decreased effect magnitudes. Results confirm peripheral and central contributions to EEs and SCEs in speech perception, but both effects appear to be primarily due to peripheral processing.

Auditory enhancement under forward masking in normal-hearing and hearing-impaired listeners

A target within a spectrally notched masker can be enhanced by a preceding copy of the masker. Enhancement can also increase the effectiveness of the target as a forward masker. Enhancement has been reported in hearing-impaired listeners under simultaneous but not forward masking. However, previous studies of enhancement under forward masking did not fully assess the potential effect of differences in sensation level or spectral resolution between the normal-hearing and hearing-impaired listeners. This study measured enhancement via forward masking in hearing-impaired and age-matched normal-hearing listeners with different spectral notches in the masker, to account for potential differences in frequency selectivity, and with levels equated by adding a background masking noise to equate both sensation level and sound pressure level or by reducing the sound pressure level of the stimuli to equate sensation level. Hearing-impaired listeners showed no significant enhancement, regardless of spectral notch width. Normal-hearing listeners showed enhancement at high levels, but showed less enhancement when sensation levels were reduced to match those of the hearing-impaired group, either by reducing sound levels or by adding a masking noise. The results confirm a lack of forward-masked enhancement in hearing-impaired listeners but suggest this may be partly due to reduced sensation level.

Auditory enhancement and spectral contrast effects in speech perception

The auditory system is remarkably sensitive to changes in the acoustic environment. This is exemplified by two classic effects of preceding spectral context on perception. In auditory enhancement effects (EEs), the absence and subsequent insertion of a frequency component increases its salience. In spectral contrast effects (SCEs), spectral differences between earlier and later (target) sounds are perceptually magnified, biasing target sound categorization. These effects have been suggested to be related, but have largely been studied separately. Here, EEs and SCEs are demonstrated using the same speech materials. In Experiment 1, listeners categorized vowels (/ɪ/-/ɛ/) or consonants (/d/-/g/) following a sentence processed by a bandpass or bandstop filter (vowel tasks: 100-400 or 550-850 Hz; consonant tasks: 1700-2700 or 2700-3700 Hz). Bandpass filtering produced SCEs and bandstop filtering produced EEs, with effect magnitudes significantly correlated at the individual differences level. In Experiment 2, context sentences were processed by variable-depth notch filters in these frequency regions (-5 to -20 dB). EE magnitudes increased at larger notch depths, growing linearly in consonant categorization. This parallels previous research where SCEs increased linearly for larger spectral peaks in the context sentence. These results link EEs and SCEs, as both shape speech categorization in orderly ways.

Auditory enhancement and the role of spectral resolution in normal-hearing listeners and cochlear-implant users

Detection of a target tone in a simultaneous multi-tone masker can be improved by preceding the stimulus with the masker alone. The mechanisms underlying this auditory enhancement effect may enable the efficient detection of new acoustic events and may help to produce perceptual constancy under varying acoustic conditions. Previous work in cochlear-implant (CI) users has suggested reduced or absent enhancement, due perhaps to poor spatial resolution in the cochlea. This study used a supra-threshold enhancement paradigm that in normal-hearing listeners results in large enhancement effects, exceeding 20 dB. Results from vocoder simulations using normal-hearing listeners showed that near-normal enhancement was observed if the simulated spread of excitation was limited to spectral slopes no shallower than 24 dB/oct. No significant enhancement was observed on average in CI users with their clinical monopolar stimulation strategy. The variability in enhancement between CI users, and between electrodes in a single CI user, could not be explained by the spread of excitation, as estimated from auditory nerve evoked potentials. Enhancement remained small, but did reach statistical significance, under the narrower partial-tripolar stimulation strategy. The results suggest that enhancement may be at least partially restored by improvements in the spatial resolution of current CIs.

Auditory sequential accumulation of spectral information

In many listening situations, information about the spectral content of a target sound may be distributed over time, and estimating the target spectrum requires efficient sequential processing. Listeners' ability to estimate the spectrum of a random-frequency, six-tone complex was investigated and the spectral content of the complex was revealed using a sequence of bursts. Whether each of the six tones was presented within each burst was determined at random according to a presentation probability. In separate conditions, the presentation probabilities (p) ranged from 0.2 to 1, the total number of bursts varied from 1 to 16, and the inter-burst interval was either 0 or 200 ms. To evaluate the information acquired by the listener, the burst sequence was followed, after a 500-ms silent interval, by the six-tone complex acting as an informational masker and the listener was required to detect a pure-tone target presented simultaneously with the masker. Greater performance in this task indicates more accurate estimation of the spectrum of the complex by the listener. Evidence for integration of information across bursts was observed, and the integration process did not significantly depend on inter-burst interval.

Auditory Enhancement in Cochlear-Implant Users Under Simultaneous and Forward Masking

Auditory enhancement is the phenomenon whereby the salience or detectability of a target sound within a masker is enhanced by the prior presentation of the masker alone. Enhancement has been demonstrated using both simultaneous and forward masking in normal-hearing listeners and may play an important role in auditory and speech perception within complex and time-varying acoustic environments. The few studies of enhancement in hearing-impaired listeners have reported reduced or absent enhancement effects under forward masking, suggesting a potentially peripheral locus of the effect. Here, auditory enhancement was measured in eight cochlear-implant (CI) users with direct stimulation. Masked thresholds were measured under simultaneous and forward masking as a function of the number of masking electrodes, and the electrode spacing between the maskers and the target. Evidence for auditory enhancement was obtained under simultaneous masking, qualitatively consistent with results from normal-hearing listeners. However, no significant enhancement was observed under forward masking, in contrast to earlier results with normal-hearing listeners. The results suggest that the normal effects of auditory enhancement are partially but not fully experienced by CI users. To the extent that the CI users' results differ from normal, it may be possible to apply signal processing to restore the missing aspects of enhancement.

New perspectives on the measurement and time course of auditory enhancement

A target sound can become more audible and may "pop out" from a simultaneously presented masker if the masker is presented first by itself, as a precursor. This phenomenon, known as auditory enhancement, may reflect the general perceptual principle of contrast enhancement, which facilitates adaptation to ongoing acoustic conditions and the detection of new events. Little is known about the mechanisms underlying enhancement, and potential confounding factors have made the size of the effect and its time course a point of contention. Here we measured enhancement as a function of precursor duration and delay between precursor offset and target onset, using 2 single-interval pitch comparison tasks, which involve either same-different or up-down judgments, to avoid the potential confounds of earlier studies. Although these 2 tasks elicit different levels of performance and may reflect different underlying mechanisms, they produced similar amounts of enhancement. The effect decreased with decreasing precursor duration, but remained present for precursors as short as 62.5 ms, and decreased with increasing gap between the precursor and target, but remained measurable 1 s after the precursor. Additional conditions, examining the effect of precursor/masker similarity and the possible role of grouping and cueing, suggest multiple sources of auditory enhancement.

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

Auditory 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.

The auditory enhancement effect is not reflected in the 80-Hz auditory steady-state response

The perceptual salience of a target tone presented in a multitone background is increased by the presentation of a precursor sound consisting of the multitone background alone. It has been proposed that this "enhancement" phenomenon results from an effective amplification of the neural response to the target tone. In this study, we tested this hypothesis in humans, by comparing the auditory steady-state response (ASSR) to a target tone that was enhanced by a precursor sound with the ASSR to a target tone that was not enhanced. In order to record neural responses originating in the brainstem, the ASSR was elicited by amplitude modulating the target tone at a frequency close to 80 Hz. The results did not show evidence of an amplified neural response to enhanced tones. In a control condition, we measured the ASSR to a target tone that, instead of being perceptually enhanced by a precursor sound, was acoustically increased in level. This level increase matched the magnitude of enhancement estimated psychophysically with a forward masking paradigm in a previous experimental phase. We found that the ASSR to the tone acoustically increased in level was significantly greater than the ASSR to the tone enhanced by the precursor sound. Overall, our results suggest that the enhancement effect cannot be explained by an amplified neural response at the level of the brainstem. However, an alternative possibility is that brainstem neurons with enhanced responses do not contribute to the scalp-recorded ASSR.