Psychobiological Responses Reveal Audiovisual Noise Differentially Challenges Speech Recognition

Effects of training and using an audio-tactile sensory substitution device on speech-in-noise understanding

Understanding speech in background noise is challenging. Wearing face-masks, imposed by the COVID19-pandemics, makes it even harder. We developed a multi-sensory setup, including a sensory substitution device (SSD) that can deliver speech simultaneously through audition and as vibrations on the fingertips. The vibrations correspond to low frequencies extracted from the speech input. We trained two groups of non-native English speakers in understanding distorted speech in noise. After a short session (30-45 min) of repeating sentences, with or without concurrent matching vibrations, we showed comparable mean group improvement of 14-16 dB in Speech Reception Threshold (SRT) in two test conditions, i.e., when the participants were asked to repeat sentences only from hearing and also when matching vibrations on fingertips were present. This is a very strong effect, if one considers that a 10 dB difference corresponds to doubling of the perceived loudness. The number of sentence repetitions needed for both types of training to complete the task was comparable. Meanwhile, the mean group SNR for the audio-tactile training (14.7 ± 8.7) was significantly lower (harder) than for the auditory training (23.9 ± 11.8), which indicates a potential facilitating effect of the added vibrations. In addition, both before and after training most of the participants (70-80%) showed better performance (by mean 4-6 dB) in speech-in-noise understanding when the audio sentences were accompanied with matching vibrations. This is the same magnitude of multisensory benefit that we reported, with no training at all, in our previous study using the same experimental procedures. After training, performance in this test condition was also best in both groups (SRT ~ 2 dB). The least significant effect of both training types was found in the third test condition, i.e. when participants were repeating sentences accompanied with non-matching tactile vibrations and the performance in this condition was also poorest after training. The results indicate that both types of training may remove some level of difficulty in sound perception, which might enable a more proper use of speech inputs delivered via vibrotactile stimulation. We discuss the implications of these novel findings with respect to basic science. In particular, we show that even in adulthood, i.e. long after the classical "critical periods" of development have passed, a new pairing between a certain computation (here, speech processing) and an atypical sensory modality (here, touch) can be established and trained, and that this process can be rapid and intuitive. We further present possible applications of our training program and the SSD for auditory rehabilitation in patients with hearing (and sight) deficits, as well as healthy individuals in suboptimal acoustic situations.

Autonomic Nervous System Correlates of Speech Categorization Revealed Through Pupillometry

Human perception requires the many-to-one mapping between continuous sensory elements and discrete categorical representations. This grouping operation underlies the phenomenon of categorical perception (CP)-the experience of perceiving discrete categories rather than gradual variations in signal input. Speech perception requires CP because acoustic cues do not share constant relations with perceptual-phonetic representations. Beyond facilitating perception of unmasked speech, we reasoned CP might also aid the extraction of target speech percepts from interfering sound sources (i.e., noise) by generating additional perceptual constancy and reducing listening effort. Specifically, we investigated how noise interference impacts cognitive load and perceptual identification of unambiguous (i.e., categorical) vs. ambiguous stimuli. Listeners classified a speech vowel continuum (/u/-/a/) at various signal-to-noise ratios (SNRs [unmasked, 0 and -5 dB]). Continuous recordings of pupil dilation measured processing effort, with larger, later dilations reflecting increased listening demand. Critical comparisons were between time-locked changes in eye data in response to unambiguous (i.e., continuum endpoints) tokens vs. ambiguous tokens (i.e., continuum midpoint). Unmasked speech elicited faster responses and sharper psychometric functions, which steadily declined in noise. Noise increased pupil dilation across stimulus conditions, but not straightforwardly. Noise-masked speech modulated peak pupil size (i.e., [0 and -5 dB] > unmasked). In contrast, peak dilation latency varied with both token and SNR. Interestingly, categorical tokens elicited earlier pupil dilation relative to ambiguous tokens. Our pupillary data suggest CP reconstructs auditory percepts under challenging listening conditions through interactions between stimulus salience and listeners' internalized effort and/or arousal.

Acoustic noise and vision differentially warp the auditory categorization of speech

Speech perception requires grouping acoustic information into meaningful linguistic-phonetic units via categorical perception (CP). Beyond shrinking observers' perceptual space, CP might aid degraded speech perception if categories are more resistant to noise than surface acoustic features. Combining audiovisual (AV) cues also enhances speech recognition, particularly in noisy environments. This study investigated the degree to which visual cues from a talker (i.e., mouth movements) aid speech categorization amidst noise interference by measuring participants' identification of clear and noisy speech (0 dB signal-to-noise ratio) presented in auditory-only or combined AV modalities (i.e., A, A+noise, AV, AV+noise conditions). Auditory noise expectedly weakened (i.e., shallower identification slopes) and slowed speech categorization. Interestingly, additional viseme cues largely counteracted noise-related decrements in performance and stabilized classification speeds in both clear and noise conditions suggesting more precise acoustic-phonetic representations with multisensory information. Results are parsimoniously described under a signal detection theory framework and by a reduction (visual cues) and increase (noise) in the precision of perceptual object representation, which were not due to lapses of attention or guessing. Collectively, findings show that (i) mapping sounds to categories aids speech perception in "cocktail party" environments; (ii) visual cues help lattice formation of auditory-phonetic categories to enhance and refine speech identification.