Biology subjects

Wasade, V. S.

Publications and source records attributed to Wasade, V. S..

5 recordsLinked to original sources

bioRxiv2026

Human iEEG recordings during audiovisual speech violations support and extend predictive routing models

Rather than constantly reconstructing the world solely through sensory input, predictive processing theories posit that the brain utilizes predictions maintained as internal representations to guide sensory processing. An emerging model, predictive routing, proposes that an interplay between neuronal gamma (40-150 Hz) and alpha-beta (10-30 Hz) rhythms implements a dynamic mechanism for predictive processing. Namely, predictions enhance alpha-beta oscillations that suppress gamma activity during the processing of predicted inputs. While this model is supported by animal studies, its validity has not been fully explored within the human brain. The present study addresses this by leveraging human intracranial EEG (iEEG) recordings across superior temporal and prefrontal/premotor areas of seven presurgical epilepsy patients during an audiovisual speech paradigm. We calculated iEEG power in the theta, alpha-beta, and high gamma bands during simple auditory deviance, multisensory integration, and a condition where an incongruent visual cue (lip movements) clashed with an auditory stimulus (a syllable), creating audiovisual incongruence that can elicit the McGurk illusion. Results revealed increased high-gamma and decreased alpha-beta power in the superior temporal cortex in response to auditory deviance and the McGurk condition, while frontal areas, despite reflecting a similar decrease in alpha-beta power, did not present this high gamma prediction error signal. Instead, the theta band revealed a strong prediction error signal in the McGurk condition, consistent with theories on the role of theta in reorganizing cortical networks. These results both support and extend existing predictive routing models for human-specific forms of predictive processing.

neuroscience↗

bioRxiv2026

Visual speech enhances phoneme separability in human superior temporal gyrus

Visual speech, such as lipreading, facilitates spoken word recognition, but the neural mechanisms underlying audiovisual speech perception remain poorly understood. Visual cues may disambiguate fine-grained articulatory features during early perceptual stages or instead integrate with speech at more categorical, phoneme-level stages. To test how speech representations are modulated by visual input, we analyzed intracranial electroencephalography (iEEG) signals recorded from 12 epilepsy patients performing an audiovisual speech perception task. Participants perceived 16 monosyllabic words presented in auditory-only, visual-only, or congruent audiovisual formats. Words were constructed from four onset consonants (/b/, /g/, /m/, /n/) and four rimes (vowel nucleus and any coda consonants). We examined event-related potentials (ERP) in superior temporal gyrus (STG) and trained support vector machine (SVM) classifiers to decode word identity from neural activity at individual electrodes. Discrete and continuous confusion matrices captured complementary changes in classification accuracy and normalized inverse classification loss, a continuous proxy for classifier confidence. Decoding performance was hierarchically evaluated at the word, phoneme, and phonetic feature levels to determine the representations affected by visual speech. Congruent audiovisual speech increased classifier confidence for phoneme-level representations and improved decoding accuracy at both the word and phoneme level, without corresponding effects on phonetic features. Time-resolved analyses further revealed earlier successful decoding for audiovisual than auditory-only speech, with audiovisual enhancement primarily observed for onset consonants rather than rimes. Together, these findings suggest that visual speech sharpens primarily categorical phoneme representations in STG, with accelerated speech processing and improved word recognition emerging as downstream consequences of phoneme-level enhancement.

neuroscience↗

bioRxiv2024

Visual speech enhances auditory onset timing and envelope tracking through distinct mechanisms

Seeing a speakers face facilitates speech recognition in challenging listening environments. Prior work has shown that visual speech contains timing information to aid auditory speech processing, yet how these signals are integrated within the auditory system during audiovisual speech perception remains poorly understood. Observation of preparatory mouth movements may initiate phase reset of intrinsic oscillations, potentially sensitizing the auditory system for receptive speech processing, while observation of mouth movements post speech onset may facilitate entrainment to the speech envelope. Yet, little work has been done to test whether visual speech enhances encoding of auditory speech onset, speech envelope tracking, or both, and through independent or overlapping mechanisms. To investigate this, we examined the ways in which visual speech timing information alters theta band power and phase using human intracranial electroencephalography (iEEG) recordings in a large group of patients with epilepsy (n = 21). Prior to speech onset, preparatory mouth movements elicited theta phase reset (increased inter-trial phase coherence; ITPC) throughout the superior temporal gyrus (STG), which is thought to enhance speech onset encoding. Following speech onset, visual speech modulated theta ITPC only at anterior STG electrodes while theta power was modulated at posterior STG electrodes. Pre- and post-speech onset were spatially and temporally dissociated, consistent with the hypothesis that audiovisual speech onset encoding and envelope tracking mechanisms are partially distinct. Crucially, congruent and incongruent visual speech, designed here to have identical visual timing information about speech onset time, but different visual mouth evolution, produced only a small difference in the phase of theta band oscillations in the anterior STG, highlighting a more restricted role of visual speech in ongoing auditory entrainment. These results support the hypothesis that visual speech improves the precision of auditory speech encoding through two separate mechanisms, with auditory speech onset encoded throughout the entire STG and ongoing speech envelope tracking within anterior STG.

neuroscience↗

bioRxiv2022

Auditory cortex encodes lipreading information through spatially distributed activity

Watching a speakers face improves speech perception accuracy. These benefits are owed, in part, to implicit lipreading abilities present in the general population. While it is established that lipreading can alter the perception of a heard word, it is unknown how information that is extracted from lipread words is transformed into a neural code that the auditory system can use. One influential, but untested, hypothesis is that visual speech modulates the population coded representations of phonetic and phonemic features in the auditory system. This model is largely supported by data showing that silent lipreading evokes activity in auditory cortex, but these activations could alternatively reflect general effects of arousal or attention, or the encoding of non-linguistic features such as visual timing information. This gap limits our understanding of how vision supports speech perception processes. To test the hypothesis that the auditory system encodes visual speech information, we acquired fMRI data from healthy adults and intracranial recordings from electrodes implanted in patients with epilepsy during auditory and visual speech perception tasks. Across both methods, linear classifiers successfully decoded the identity of silently lipread words using the spatial pattern of auditory cortex responses. Examining the time-course of classification using intracranial recordings, lipread words were classified at significantly earlier time-points relative to heard words, suggesting a predictive mechanism for facilitating speech. These results support a model in which the auditory system combines the joint neural distributions evoked by heard and lipread words to generate a more precise estimate of what was said. Significance StatementWhen we listen to someone speak in a noisy environment, watching their face can help us understand them better, largely due to automatic lipreading abilities. However, it unknown how lipreading information is transformed into a neural code that the auditory system can use. We used fMRI and intracranial recordings in patients to study how the brain processes silently lipread words and found that the auditory system encodes the identity of lipread words through spatially distributed activity. These results suggest that the auditory system combines information from both lipreading and hearing to generate more precise estimates of what is said, potentially by both activating the corresponding representation of the heard word and suppressing incorrect phonemic representations.

neuroscience↗

bioRxiv2020

Visual speech differentially modulates beta, theta, and high gamma bands in auditory cortex

Speech perception is a central component of social communication. While principally an auditory process, accurate speech perception in everyday settings is supported by meaningful information extracted from visual cues (e.g., speech content, timing, and speaker identity). Previous research has shown that visual speech modulates activity in cortical areas subserving auditory speech perception, including the superior temporal gyrus (STG), potentially through feedback connections from the multisensory posterior superior temporal sulcus (pSTS). However, it is unknown whether visual modulation of auditory processing in the STG is a unitary phenomenon or, rather, consists of multiple temporally, spatially, or functionally distinct processes. To explore these questions, we examined neural responses to audiovisual speech measured from intracranially implanted electrodes within the temporal cortex of 21 patients undergoing clinical monitoring for epilepsy. We found that visual speech modulates auditory processes in the STG in multiple ways, eliciting temporally and spatially distinct patterns of activity that differ across theta, beta, and high-gamma frequency bands. Before speech onset, visual information increased high-gamma power in the posterior STG and suppressed beta power in mid-STG regions, suggesting crossmodal prediction of speech signals in these areas. After sound onset, visual speech decreased theta power in the middle and posterior STG, potentially reflecting a decrease in sustained feedforward auditory activity. These results are consistent with models that posit multiple distinct mechanisms supporting audiovisual speech perception and provide a crucial map for subsequent studies to identify the types of visual features that are encoded by these separate mechanisms.

neuroscience↗