Acoustic Context Alters Vowel Categorization in Perception of Noise-Vocoded Speech

Brain plasticity and hearing disorders

Permanently changed sensory stimulation can modify functional connectivity patterns in the healthy brain and in pathology. In the pathology case, these adaptive modifications of the brain are referred to as compensation, and the subsequent configurations of functional connectivity are called compensatory plasticity. The variability and extent of auditory deficits due to the impairments in the hearing system determine the related brain reorganization and rehabilitation. In this review, we consider cross-modal and intra-modal brain plasticity related to bilateral and unilateral hearing loss and their restoration using cochlear implantation. Cross-modal brain plasticity may have both beneficial and detrimental effects on hearing disorders. It has a beneficial effect when it serves to improve a patient's adaptation to the visuo-auditory environment. However, the occupation of the auditory cortex by visual functions may be a negative factor for the restoration of hearing with cochlear implants. In what concerns intra-modal plasticity, the loss of interhemispheric asymmetry in asymmetric hearing loss is deleterious for the auditory spatial localization. Research on brain plasticity in hearing disorders can advance our understanding of brain plasticity and improve the rehabilitation of the patients using prognostic, evidence-based approaches from cognitive neuroscience combined with post-rehabilitation objective biomarkers of this plasticity utilizing neuroimaging.

Spectral Contrast Effects Reveal Different Acoustic Cues for Vowel Recognition in Cochlear-Implant Users

OBJECTIVES

The identity of a speech sound can be affected by the spectrum of a preceding stimulus in a contrastive manner. Although such aftereffects are often reduced in people with hearing loss and cochlear implants (CIs), one recent study demonstrated larger spectral contrast effects in CI users than in normal-hearing (NH) listeners. The present study aimed to shed light on this puzzling finding. We hypothesized that poorer spectral resolution leads CI users to rely on different acoustic cues not only to identify speech sounds but also to adapt to the context.

DESIGN

Thirteen postlingually deafened adult CI users and 33 NH participants (listening to either vocoded or unprocessed speech) participated in this study. Psychometric functions were estimated in a vowel categorization task along the /I/ to /ε/ (as in "bit" and "bet") continuum following a context sentence, the long-term average spectrum of which was manipulated at the level of either fine-grained local spectral cues or coarser global spectral cues.

RESULTS

In NH listeners with unprocessed speech, the aftereffect was determined solely by the fine-grained local spectral cues, resulting in a surprising insensitivity to the larger, global spectral cues utilized by CI users. Restricting the spectral resolution available to NH listeners via vocoding resulted in patterns of responses more similar to those found in CI users. However, the size of the contrast aftereffect remained smaller in NH listeners than in CI users.

CONCLUSIONS

Only the spectral contrasts used by listeners contributed to the spectral contrast effects in vowel identification. These results explain why CI users can experience larger-than-normal context effects under specific conditions. The results also suggest that adaptation to new spectral cues can be very rapid for vowel discrimination, but may follow a longer time course to influence spectral contrast effects.

Accommodation of gender-related phonetic differences by listeners with cochlear implants and in a variety of vocoder simulations

Speech perception requires accommodation of a wide range of acoustic variability across talkers. A classic example is the perception of "sh" and "s" fricative sounds, which are categorized according to spectral details of the consonant itself, and also by the context of the voice producing it. Because women's and men's voices occupy different frequency ranges, a listener is required to make a corresponding adjustment of acoustic-phonetic category space for these phonemes when hearing different talkers. This pattern is commonplace in everyday speech communication, and yet might not be captured in accuracy scores for whole words, especially when word lists are spoken by a single talker. Phonetic accommodation for fricatives "s" and "sh" was measured in 20 cochlear implant (CI) users and in a variety of vocoder simulations, including those with noise carriers with and without peak picking, simulated spread of excitation, and pulsatile carriers. CI listeners showed strong phonetic accommodation as a group. Each vocoder produced phonetic accommodation except the 8-channel noise vocoder, despite its historically good match with CI users in word intelligibility. Phonetic accommodation is largely independent of linguistic factors and thus might offer information complementary to speech intelligibility tests which are partially affected by language processing.

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.

Variability in talkers' fundamental frequencies shapes context effects in speech perception

The perception of any given sound is influenced by surrounding sounds. When successive sounds differ in their spectral compositions, these differences may be perceptually magnified, resulting in spectral contrast effects (SCEs). For example, listeners are more likely to perceive /ɪ/ (low F₁) following sentences with higher F₁ frequencies; listeners are also more likely to perceive /ɛ/ (high F₁) following sentences with lower F₁ frequencies. Previous research showed that SCEs for vowel categorization were attenuated when sentence contexts were spoken by different talkers [Assgari and Stilp. (2015). J. Acoust. Soc. Am. 138(5), 3023-3032], but the locus of this diminished contextual influence was not specified. Here, three experiments examined implications of variable talker acoustics for SCEs in the categorization of /ɪ/ and /ɛ/. The results showed that SCEs were smaller when the mean fundamental frequency (f0) of context sentences was highly variable across talkers compared to when mean f0 was more consistent, even when talker gender was held constant. In contrast, SCE magnitudes were not influenced by variability in mean F1. These findings suggest that talker variability attenuates SCEs due to diminished consistency of f0 as a contextual influence. Connections between these results and talker normalization are considered.

Spectral contrast effects produced by competing speech contexts

The long-term spectrum of a preceding sentence can alter the perception of a following speech sound in a contrastive manner. This speech context effect contributes to our ability to extract reliable spectral characteristics of the surrounding acoustic environment and to compensate for the voice characteristics of different speakers or spectral colorations in different listening environments to maintain perceptual constancy. The extent to which such effects are mediated by low-level "automatic" processes, or require directed attention, remains unknown. This study investigated spectral context effects by measuring the effects of two competing sentences on the phoneme category boundary between /i/ and /ε/ in a following target word, while directing listeners' attention to one or the other context sentence. Spatial separation of the context sentences was achieved either by presenting them to different ears, or by presenting them to both ears but imposing an interaural time difference (ITD) between the ears. The results confirmed large context effects based on ear of presentation. Smaller effects were observed based on either ITD or attention. The results, combined with predictions from a two-stage model, suggest that ear-specific factors dominate speech context effects but that the effects can be modulated by higher-level features, such as perceived location, and by attention. (PsycINFO Database Record

Effects of spectral resolution on spectral contrast effects in cochlear-implant users

The identity of a speech sound can be affected by the long-term spectrum of a preceding stimulus. Poor spectral resolution of cochlear implants (CIs) may affect such context effects. Here, spectral contrast effects on a phoneme category boundary were investigated in CI users and normal-hearing (NH) listeners. Surprisingly, larger contrast effects were observed in CI users than in NH listeners, even when spectral resolution in NH listeners was limited via vocoder processing. The results may reflect a different weighting of spectral cues by CI users, based on poorer spectral resolution, which in turn may enhance some spectral contrast effects.