Enhancement of speech-in-noise comprehension through vibrotactile stimulation at the syllabic rate

Alpha-Band Brain Activity Shapes Online Perceptual Learning of Concurrent Speech Differentially in Musicians vs. Nonmusicians

Plasticity from auditory experience shapes the brain's encoding and perception of sound. Though stronger neural entrainment (i.e., brain-to-acoustic synchronization) aids speech perception, underlying oscillatory activity may uniquely interact with long-term auditory experiences (i.e., music training) and short-term plasticity during concurrent speech perception. Here, we explored oscillatory activity during rapid auditory perceptual learning of concurrent speech sounds in normal-hearing young adults who differed in their amount of self-reported music training (defined as "musicians" and "nonmusicians"). Participants learned to identify double-vowel mixtures during ~45 min training sessions with concurrent high-density EEG recordings. We analyzed alpha-band power (7-12 Hz) following a rhythmic speech-stimulus train (~9 Hz) preceding behavioral identification to determine whether increased (brain-to-speech entrainment) or decreased alpha activity (alpha-band suppression) corresponded with task success. Source and directed functional connectivity analyses of EEG data probed whether behavior was driven by group differences in auditory-motor coupling. Both groups improved in behavioral identification with training. Listeners' alpha-band power prior to target speech predicted behavioral identification performance; surprisingly, stronger alpha oscillations were observed preceding incorrect compared to correct trial responses. We also found stark hemispheric biases in auditory-motor coupling, with greater auditory-motor connectivity in right compared to left hemisphere for musicians (R > L) but not in nonmusicians (R = L). Stronger alpha activity preceding incorrect behavioral responses supports the notion that alpha-band (~10 Hz) suppression is an important modulator of trial-by-trial success in perceptual processing. Our findings suggest that neural oscillations and auditory-motor connectivity interact with music training to impact speech perception.

Touch Helps Hearing: Evidence From Continuous Audio-Tactile Stimulation

OBJECTIVES

Identifying target sounds in challenging environments is crucial for daily experiences. It is important to note that it can be enhanced by nonauditory stimuli, for example, through lip-reading in an ongoing conversation. However, how tactile stimuli affect auditory processing is still relatively unclear. Recent studies have shown that brief tactile stimuli can reliably facilitate auditory perception, while studies using longer-lasting audio-tactile stimulation yielded conflicting results. This study aimed to investigate the impact of ongoing pulsating tactile stimulation on basic auditory processing.

DESIGN

In experiment 1, the electroencephalogram (EEG) was recorded while 24 participants performed a loudness-discrimination task on a 4-Hz modulated tone-in-noise and received either in-phase, anti-phase, or no 4-Hz electrotactile stimulation above the median nerve. In experiment 2, another 24 participants were presented with the same tactile stimulation as before, but performed a tone-in-noise detection task while their selective auditory attention was manipulated.

RESULTS

We found that in-phase tactile stimulation enhanced EEG responses to the tone, whereas anti-phase tactile stimulation suppressed these responses. No corresponding tactile effects on loudness-discrimination performance were observed in experiment 1. Using a yes/no paradigm in experiment 2, we found that in-phase tactile stimulation, but not anti-phase tactile stimulation, improved detection thresholds. Selective attention also improved thresholds but did not modulate the observed benefit from in-phase tactile stimulation.

CONCLUSIONS

Our study highlights that ongoing in-phase tactile input can enhance basic auditory processing as reflected in scalp EEG and detection thresholds. This might have implications for the development of hearing enhancement technologies and interventions.

Auditory-motor entrainment and listening experience shape the perceptual learning of concurrent speech

Background

Plasticity from auditory experience shapes the brain's encoding and perception of sound. Though prior research demonstrates that neural entrainment (i.e., brain-to-acoustic synchronization) aids speech perception, how long- and short-term plasticity influence entrainment to concurrent speech has not been investigated. Here, we explored neural entrainment mechanisms and the interplay between short- and long-term neuroplasticity for rapid auditory perceptual learning of concurrent speech sounds in young, normal-hearing musicians and nonmusicians.

Method

Participants learned to identify double-vowel mixtures during ∼45 min training sessions with concurrent high-density EEG recordings. We examined the degree to which brain responses entrained to the speech-stimulus train (∼9 Hz) to investigate whether entrainment to speech prior to behavioral decision predicted task performance. Source and directed functional connectivity analyses of the EEG probed whether behavior was driven by group differences auditory-motor coupling.

Results

Both musicians and nonmusicians showed rapid perceptual learning in accuracy with training. Interestingly, listeners' neural entrainment strength prior to target speech mixtures predicted behavioral identification performance; stronger neural synchronization was observed preceding incorrect compared to correct trial responses. We also found stark hemispheric biases in auditory-motor coupling during speech entrainment, with greater auditory-motor connectivity in the right compared to left hemisphere for musicians (R>L) but not in nonmusicians (R=L).

Conclusions

Our findings confirm stronger neuroacoustic synchronization and auditory-motor coupling during speech processing in musicians. Stronger neural entrainment to rapid stimulus trains preceding incorrect behavioral responses supports the notion that alpha-band (∼10 Hz) arousal/suppression in brain activity is an important modulator of trial-by-trial success in perceptual processing.

Improved tactile speech perception using audio-to-tactile sensory substitution with formant frequency focusing

Haptic hearing aids, which provide speech information through tactile stimulation, could substantially improve outcomes for both cochlear implant users and for those unable to access cochlear implants. Recent advances in wide-band haptic actuator technology have made new audio-to-tactile conversion strategies viable for wearable devices. One such strategy filters the audio into eight frequency bands, which are evenly distributed across the speech frequency range. The amplitude envelopes from the eight bands modulate the amplitudes of eight low-frequency tones, which are delivered through vibration to a single site on the wrist. This tactile vocoder strategy effectively transfers some phonemic information, but vowels and obstruent consonants are poorly portrayed. In 20 participants with normal touch perception, we tested (1) whether focusing the audio filters of the tactile vocoder more densely around the first and second formant frequencies improved tactile vowel discrimination, and (2) whether focusing filters at mid-to-high frequencies improved obstruent consonant discrimination. The obstruent-focused approach was found to be ineffective. However, the formant-focused approach improved vowel discrimination by 8%, without changing overall consonant discrimination. The formant-focused tactile vocoder strategy, which can readily be implemented in real time on a compact device, could substantially improve speech perception for haptic hearing aid users.

Improved speech intelligibility in the presence of congruent vibrotactile speech input

Vibrotactile stimulation is believed to enhance auditory speech perception, offering potential benefits for cochlear implant (CI) users who may utilize compensatory sensory strategies. Our study advances previous research by directly comparing tactile speech intelligibility enhancements in normal-hearing (NH) and CI participants, using the same paradigm. Moreover, we assessed tactile enhancement considering stimulus non-specific, excitatory effects through an incongruent audio-tactile control condition that did not contain any speech-relevant information. In addition to this incongruent audio-tactile condition, we presented sentences in an auditory only and a congruent audio-tactile condition, with the congruent tactile stimulus providing low-frequency envelope information via a vibrating probe on the index fingertip. The study involved 23 NH listeners and 14 CI users. In both groups, significant tactile enhancements were observed for congruent tactile stimuli (5.3% for NH and 5.4% for CI participants), but not for incongruent tactile stimulation. These findings replicate previously observed tactile enhancement effects. Juxtaposing our study with previous research, the informational content of the tactile stimulus emerges as a modulator of intelligibility: Generally, congruent stimuli enhanced, non-matching tactile stimuli reduced, and neutral stimuli did not change test outcomes. We conclude that the temporal cues provided by congruent vibrotactile stimuli may aid in parsing continuous speech signals into syllables and words, consequently leading to the observed improvements in intelligibility.

Transcranial Alternating Current Stimulation to Modulate Alpha Activity: A Systematic Review

BACKGROUND

Transcranial alternating current stimulation (tACS) has been one of numerous investigation methods used for their potential to modulate brain oscillations; however, such investigations have given contradictory results and a lack of standardization.

OBJECTIVES

In this systematic review, we aimed to assess the potential of tACS to modulate alpha spectral power. The secondary outcome was the identification of tACS methodologic key parameters, adverse effects, and sensations.

MATERIALS AND METHODS

Studies in healthy adults who were receiving active and sham tACS intervention or any differential condition were included. The main outcome assessed was the increase/decrease of alpha spectral power through either electroencephalography or magnetoencephalography. Secondary outcomes were methodologic parameters, sensation reporting, and adverse effects. Risks of bias and the study quality were assessed with the Cochrane assessment tool.

RESULTS

We obtained 1429 references, and 20 met the selection criteria. A statistically significant alpha-power increase was observed in nine studies using continuous tACS stimulation and two using intermittent tACS stimulation set at a frequency within the alpha range. A statistically significant alpha-power increase was observed in three more studies using a stimulation frequency outside the alpha range. Heterogeneity among stimulation parameters was recognized. Reported adverse effects were mild. The implementation of double blind was identified as challenging using tACS, in part owing to electrical artifacts generated by stimulation on the recorded signal.

CONCLUSIONS

Most assessed studies reported that tACS has the potential to modulate brain alpha power. The optimization of this noninvasive brain stimulation method is of interest mostly for its potential clinical applications with neurological conditions associated with perturbations in alpha brain activity. However, more research efforts are needed to standardize optimal parameters to achieve lasting modulation effects, develop methodologic alternatives to reduce experimental bias, and improve the quality of studies using tACS to modulate brain activity.

Audiotactile Stimulation Can Improve Syllable Discrimination through Multisensory Integration in the Theta Frequency Band

Syllables are an essential building block of speech. We recently showed that tactile stimuli linked to the perceptual centers of syllables in continuous speech can improve speech comprehension. The rate of syllables lies in the theta frequency range, between 4 and 8 Hz, and the behavioral effect appears linked to multisensory integration in this frequency band. Because this neural activity may be oscillatory, we hypothesized that a behavioral effect may also occur not only while but also after this activity has been evoked or entrained through vibrotactile pulses. Here, we show that audiotactile integration regarding the perception of single syllables, both on the neural and on the behavioral level, is consistent with this hypothesis. We first stimulated participants with a series of vibrotactile pulses and then presented them with a syllable in background noise. We show that, at a delay of 200 msec after the last vibrotactile pulse, audiotactile integration still occurred in the theta band and syllable discrimination was enhanced. Moreover, the dependence of both the neural multisensory integration as well as of the behavioral discrimination on the delay of the audio signal with respect to the last tactile pulse was consistent with a damped oscillation. In addition, the multisensory gain is correlated with the syllable discrimination score. Our results therefore evidence the role of the theta band in audiotactile integration and provide evidence that these effects may involve oscillatory activity that still persists after the tactile stimulation.

Speech-derived haptic stimulation enhances speech recognition in a multi-talker background

Speech understanding, while effortless in quiet conditions, is challenging in noisy environments. Previous studies have revealed that a feasible approach to supplement speech-in-noise (SiN) perception consists in presenting speech-derived signals as haptic input. In the current study, we investigated whether the presentation of a vibrotactile signal derived from the speech temporal envelope can improve SiN intelligibility in a multi-talker background for untrained, normal-hearing listeners. We also determined if vibrotactile sensitivity, evaluated using vibrotactile detection thresholds, modulates the extent of audio-tactile SiN improvement. In practice, we measured participants' speech recognition in a multi-talker noise without (audio-only) and with (audio-tactile) concurrent vibrotactile stimulation delivered in three schemes: to the left or right palm, or to both. Averaged across the three stimulation delivery schemes, the vibrotactile stimulation led to a significant improvement of 0.41 dB in SiN recognition when compared to the audio-only condition. Notably, there were no significant differences observed between the improvements in these delivery schemes. In addition, audio-tactile SiN benefit was significantly predicted by participants' vibrotactile threshold levels and unimodal (audio-only) SiN performance. The extent of the improvement afforded by speech-envelope-derived vibrotactile stimulation was in line with previously uncovered vibrotactile enhancements of SiN perception in untrained listeners with no known hearing impairment. Overall, these results highlight the potential of concurrent vibrotactile stimulation to improve SiN recognition, especially in individuals with poor SiN perception abilities, and tentatively more so with increasing tactile sensitivity. Moreover, they lend support to the multimodal accounts of speech perception and research on tactile speech aid devices.

Effects of continuous tactile stimulation on auditory-evoked cortical responses depend on the audio-tactile phase

Auditory perception can benefit from stimuli in non-auditory sensory modalities, as for example in lip-reading. Compared with such visual influences, tactile influences are still poorly understood. It has been shown that single tactile pulses can enhance the perception of auditory stimuli depending on their relative timing, but whether and how such brief auditory enhancements can be stretched in time with more sustained, phase-specific periodic tactile stimulation is still unclear. To address this question, we presented tactile stimulation that fluctuated coherently and continuously at 4Hz with an auditory noise (either in-phase or anti-phase) and assessed its effect on the cortical processing and perception of an auditory signal embedded in that noise. Scalp-electroencephalography recordings revealed an enhancing effect of in-phase tactile stimulation on cortical responses phase-locked to the noise and a suppressive effect of anti-phase tactile stimulation on responses evoked by the auditory signal. Although these effects appeared to follow well-known principles of multisensory integration of discrete audio-tactile events, they were not accompanied by corresponding effects on behavioral measures of auditory signal perception. Our results indicate that continuous periodic tactile stimulation can enhance cortical processing of acoustically-induced fluctuations and mask cortical responses to an ongoing auditory signal. They further suggest that such sustained cortical effects can be insufficient for inducing sustained bottom-up auditory benefits.