Auditory Enhancement Effect Research Articles

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.

Spectral contrast effects and auditory enhancement under normal and impaired hearing.

We are generally able to identify sounds and understand speech with ease, despite the large variations in the acoustics of each sound, which occur due to factors such as different talkers, background noise, and room acoustics. This form of perceptual constancy is likely to be mediated in part by the auditory system's ability to adapt to the ongoing environment or context in which sounds are presented. Auditory context effects have been studied under different names, such as spectral contrast effects in speech and auditory enhancement effects in psychoacoustics, but they share some important properties and may be mediated by similar underlying neural mechanisms. This review provides a survey of recent studies from our laboratory that investigate the mechanisms of speech spectral contrast effects and auditory enhancement in people with normal hearing, hearing loss, and cochlear implants. We argue that a better understanding of such context effects in people with normal hearing may allow us to restore some of these important effects for people with hearing loss via signal processing in hearing aids and cochlear implants, thereby potentially improving auditory and speech perception in the complex and variable everyday acoustic backgrounds that surround us.

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 enhancement and other context effects in normal, impaired, and electric hearing

Neal Viemeister’s contributions to our understanding of auditory enhancement stood out because they combined novel and intriguing empirical data with a testable theoretical framework to explain the results. Thirty-five years later, the resultant “adaptation of inhibition” hypothesis remains the default explanation for auditory enhancement effects, and it can be used to account for both basic auditory context effects as well as speech context effects. Recent work in our lab has focused on comparing speech and non-speech context effects in people with normal and impaired hearing, as well as cochlear implants, in an attempt to elucidate the underlying neural mechanisms and to work towards compensating for any loss of context effects in clinical populations via signal processing. This presentation will provide a survey of recent progress in this area with examples from simultaneous masking, forward masking, and speech discrimination experiments. [Work supported by NIH grant R01DC012262.]

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.

Erratum to: The Auditory Enhancement Effect is Not Reflected in the 80-Hz Auditory Steady-State Response

Erratum to: JARO DOI 10.1007/s10162-014-0455-y The y-axis scale in Fig. 3 was incorrectly shifted upwards because of a coding error, and the unit of measure in the y-axis label was incorrectly reported as μV instead of μV2. The correct figure is shown below. FIG. 3 A ASSR levels for the background components as a function of precursor type. B ASSR levels for the target component as a function of precursor type. Each colored point represents an individual listener. The color code is the same as in Figure 2. The ...

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 80Hz. 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.

Evidence of the enhancement effect in electrical stimulation via electrode matching (L)

The ability to match a pulsing electrode during multi-electrode stimulation through a research interface was measured in seven cochlear-implant (CI) users. Five listeners were relatively good at the task and two could not perform the task. Performance did not vary as a function of the number of electrodes or stimulation level. Performance on the matching task was not correlated to performance on an electrode-discrimination task. The listeners may have experienced the auditory enhancement effect, and this may have implications for speech recognition in noise for CI users.

Peripheral and central locus of a nonspeech phonetic context effect

Previous work has demonstrated that nonspeech sounds with the appropriate spectral characteristics can affect the identification of speech sounds [Lotto and Kluender, Percept. Psychophys. 60, 602–619 (1998)]. It has been proposed that these spectral context effects are due to interactions in the peripheral auditory system. For example, they could be the result of masking at the auditory nerve or of auditory enhancement effects that have been demonstrated to be monaural [Summerfield and Assmann, Percept. Psychophys. 45, 529–536 (1989)]. To examine the locus of the context effect, synthesized syllables varying from /da/ to /ga/ were preceded by single-formant stimuli that mimicked the third formant of the syllables /al/ and /ar/. The nonspeech stimulus was presented either to the same or opposite ear as the target speech stimulus. Subjects speech identifications were shifted as a function of context in predicted directions for both presentation conditions. However, the size of the shift was smaller when the context was in the ear contralateral to the target syllable. These results agree well with similar results for speech contexts. The data suggest that the context effects occur at multiple levels of the auditory system and are not simply examples of masking or auditory enhancement.

Evidence that adaptation of suppression cannot account for auditory enhancement or enhanced forward masking.

Delaying the onset of a signal relative to the onset of a simultaneous notched masker often improves the ability of listeners to 'hear out' the signal at both threshold and suprathreshold levels. Viemeister & Bacon (J. acoust. Soc. Am., 71, 1502-1507 (1982)) suggested that such auditory enhancement effects could be accounted for if the suppression produced by the masker on the signal frequency adapted, thereby releasing the signal from suppression. In support of their hypothesis, Viemeister & Bacon reported that a masker preceded by an enhancer having no component at the signal frequency produced more forward masking than did the masker by itself. Here evidence is provided from five new experiments showing that adaptation of psychophysical two-tone suppression is inadequate to account either for auditory enhancement effects or for the enhanced forward masking demonstrated by Viemeister & Bacon.

Performance of hearing-impaired persons on auditory enhancement tasks

An auditory enhancement effect was evaluated in normal and hearing-impaired persons using a paradigm similar to that used by Viemeister and Bacon [J. Acoust. Soc. Am. 71, 1502-1507 (1982)]. Thresholds for a 2000-Hz probe were obtained in two forward-masking conditions: (1) the standard condition in which the masker was a four-component harmonic complex including 2000 Hz, and (2) the enhancing condition in which the same harmonic complex except for the exclusion of the 2000-Hz component preceded the four-component masker. In addition, enhancement for speech was evaluated by asking subjects to identify flat-spectrum harmonic complexes that were preceded by inverse vowel spectra. Finally, suppression effects were evaluated by measuring forward-masked thresholds for a 2000-Hz probe as a function of suppressor frequency added to a 2000-Hz masker. Across all subjects, there was evidence of enhancement and better vowel recognition in those persons who also demonstrated evidence of suppression; however, two of the normal-hearing persons demonstrated reduced enhancement yet normal suppression effects.

Evaluation of auditory enhancement effect in normal and hearing-impaired listeners

The auditory enhancement effect was evaluated in normal and hearing-impaired listeners using a forward masking paradigm similar to that used by Viemeister and Bacon [J. Acoust. Soc. Am. 71, 1502–1507 (1982)]. Masked thresholds for a 2000-Hz probe were compared between a standard condition in which the masker was a four-component harmonic complex including 2000 Hz, and an enhancing condition in which a three-component harmonic complex not including 2000 Hz preceded the four-component masker. The spectrum level of the masker was held constant at 85 dB SPL. For the normal listeners, the thresholds in the enhancing condition were approximately 30 dB greater than in the standard condition. For the hearing-impaired listeners, the enhancing condition resulted in only an 8- to 18-dB shift in threshold relative to the standard condition. One explanation for the enhancement effect in this case involves the adaptation of suppression in components surrounding the unadapted component such that there is actually an increase in intensity in the unadapted region. Suppression effects were also evaluated in both groups by measuring forward masked thresholds for a 2000-Hz probe as a function of a variable frequency suppressor tone that was added to a 2000-Hz masker. There was evidence of suppression for the normal listeners, particularly when the suppressor component was above the masker in frequency. However, for the hearing-impaired listeners, there was limited evidence of suppression as there was little change in masked threshold as a function of suppressor frequency.

Auditory enhancement of changes in spectral amplitude.

An auditory enhancement effect occurs when one component of a harmonic series is omitted for a few hundred milliseconds and then reintroduced: The reintroduced harmonic stands out perceptually. Three experiments are reported that studied a version of this effect in which several components of a harmonic series are enhanced to define the formants of a vowel. Using the accuracy of vowel identification to measure the prominence of the formant peaks in the effective auditory representation, forms of the effect were identified that are qualitatively similar to the incremental and decremental responses seen in primary auditory-nerve fibers. These results are compatible with an origin for the enhancement effect in peripheral auditory adaptation. However, an additional mechanism is required to account for the demonstration [Viemeister and Bacon, J. Acoust. Soc. Am. 71, 1502-1507 (1982)] that enhancement can involve a true gain in the frequency region of the reintroduced component. These effects demonstrate one way in which the auditory system may attenuate the prominence of background noises while preserving the ability to represent changes in spectral amplitude produced by newly arriving signals.