Biology subjects

Keshavarzi, M.

Publications and source records attributed to Keshavarzi, M..

13 recordsLinked to original sources

bioRxiv2026

Impaired cortical encoding of prosodic prominence in single multisyllabic words for adults with dyslexia

Developmental dyslexia is associated with impaired cortical tracking of continuous speech and difficulties in perceiving within-word prosodic structure in children and adults. For adults and continuous speech, both low-frequency delta- and theta-band impairments and high-frequency gamma- and beta-band differences are found. To date, perceptual difficulties with intra-word prosody have not been studied in direct relation to neural speech encoding. Here neurotypical adults and adults with dyslexia listened to multisyllabic words. EEG was recorded, and cortical encoding was quantified using temporal response function (TRF) modelling. Four speech representations were explored: lexical stress pattern (intra-word prosodic prominence), the low-frequency speech envelope, word onsets and syllable onsets. Adults with dyslexia showed significantly reduced encoding of intra-word prosodic prominence (delta band) and of the speech envelope (delta and theta bands). In contrast, encoding of word and syllable onsets was preserved. Pinpointing delta-band cortical tracking as central to encoding intra-word prosodic prominence opens new avenues for research.

neuroscience↗

bioRxiv2026

Computing the geometry of phase-amplitude coupling: A new moment-based framework

Phase-amplitude coupling (PAC) is widely used to quantify interactions between neural oscillations, yet cannot establish whether two coupling profiles share the same geometry, phase polarity or modal structure. Here we demonstrate that commonly-used PAC metrics constitute many-to-one mappings from phase-amplitude distributions to low-dimensional summaries. This results in distinct coupling geometries yielding identical PAC values. We propose formalising PAC as a hierarchy of representations, introducing a circular-moment decomposition that preserves richer geometric information. Simulations demonstrate that distinct coupling geometries produce identical Modulation Index (MI) values. Analyses of EEG data acquired during adult language listening reveal substantial geometric variability among PAC profiles with matched MI values, validating the moment-based approach. Applied to a child-language dataset, the framework identifies differences in phase geometry of delta-low gamma PAC in children with developmental language disorder, potentially uniting competing neuroscientific theories. We propose a representation-selection framework for future PAC research.

neuroscience↗

bioRxiv2026

Modulation index-based phase-amplitude coupling does not encode temporal polarity

Phase-amplitude coupling (PAC) is widely used to quantify interactions between neural oscillations across timescales and is often interpreted as reflecting temporally meaningful coordination between slow and fast neural activity. Combining empirical EEG data with mathematical analysis, we show that the modulation index (MI), a widely used PAC metric, remains unchanged when the phase of the low-frequency oscillation is inverted by 180{degrees}. By contrast, preferred phase rotates by exactly 180{degrees}, as expected. Thus, opposite temporal organisations of cross-frequency coupling can yield identical MI values. This finding defines a fundamental interpretational limit of MI-based PAC: MI quantifies the strength of phase-dependent amplitude modulation, but not its temporal polarity. Accordingly, MI alone cannot distinguish between temporally aligned and temporally inverted coupling configurations or support inferences about temporal alignment, phase polarity, or directionality. Because PAC is widely used across neuroscience, these results establish an important boundary on what one of the fields most common cross-frequency coupling measures can validly reveal about neural coordination.

neuroscience↗

bioRxiv2026

Slow-Rate Temporal Sampling Deficits During Naturalistic Speech Listening in Children with Developmental Language Disorder

Neural tracking of slow temporal modulations in speech supports extraction of prosodic and syllabic structure critical for speech comprehension, yet whether these automatic cortical tracking mechanisms are altered in children with developmental language disorder (DLD) remains unclear. We recorded MEG while children with and without DLD listened to a story, and quantified source-level lagged speech-brain coherence and frequency-specific cortical functional connectivity across bilateral cortical regions. Children with DLD showed significantly reduced coherence in the 0.9-2.5 Hz range associated with prosodic information in the story, spanning bilateral auditory and speech-related cortex. In the 2.5-5 Hz range, linked to syllabic-rate modulations in the story, group differences were right-lateralised. No reliable differences were observed at higher modulation rates (5-9 Hz or 12-40 Hz). These coherence reductions were accompanied by altered functional connectivity between cortical regions across all frequency bands, indicating disrupted large-scale coordination within speech-processing networks in DLD.

neuroscience↗

bioRxiv2026

Atypical cortical encoding of the low-frequency temporal dynamics of natural speech identifies children with Developmental Language Disorder

Developmental Language Disorder (DLD) is a neurodevelopmental condition that causes significant difficulties in understanding and using spoken language. Here we use electroencephalography (EEG) recorded during natural speech listening with 9-year-old children to identify dynamic neural processing patterns that characterize children with DLD compared to typically-developing age-matched controls. We applied Principal Components Analysis (PCA) to identify spatial ensembles of channels that represented distinct (uncorrelated) sources of cortical activity, and explored phase-amplitude coupling (PAC) between delta (0.5-4 Hz), theta (4-8 Hz) and low-gamma (25-40 Hz) oscillations. We then isolated EEG common spatial patterns (CSP) that identified children with DLD. The PAC analyses identified the delta band as a key source of group differences, and only delta-low gamma PAC differed significantly between participant groups. The CSP analyses also identified the delta band as a key mechanistic source of group differences. The findings are suggestive of distinct atypical low-frequency neural dynamics during speech encoding for children with DLD, which could be targeted by novel interventions.

neuroscience↗

bioRxiv2026

Neural processing of natural speech by adults with and without dyslexia: Evidence for atypical cortical decoding of speech information in the delta and theta EEG bands

Neural oscillations in the delta (0.5-4 Hz) and theta (4-8 Hz) bands play a key role in tracking the temporal structure of speech. According to Temporal Sampling (TS) theory, dyslexia arises from atypical entrainment of these low-frequency oscillations to speech during infancy and childhood, which is particularly disruptive regarding phonological encoding. However, studies of adults with dyslexia have rarely examined both delta and theta cortical tracking under naturalistic listening conditions, and have not measured delta-band cortical tracking. Using EEG, here we focused on delta and theta band cortical tracking continuous natural speech by adults with and without dyslexia, applying a decoding analysis previously used with dyslexic children. Forty-eight English-speaking adults (24 dyslexic, 24 control) listened to a 16-minute continuous spoken narrative while EEG was recorded. Neural decoding of the speech envelope was quantified using backward multivariate Temporal Response Function (mTRF) models applied at two levels: a between-group analysis evaluating group-level differences in neural representation patterns, and a within-participant analysis assessing individual decoding accuracy. Cerebro-acoustic coherence was computed in parallel to provide a complementary measure of neural-speech synchronisation. Additional analyses examined band power, cross-frequency phase-amplitude coupling (PAC), and cross-frequency phase-phase coupling (PPC). Dyslexic adults exhibited less accurate delta- and theta-band decoding in the between-group analysis and reduced theta-band decoding accuracy in the within-participant analysis, alongside reduced coherence in both bands and increased delta-band power, particularly over the right temporal region. No group differences were found for PAC or PPC. HighlightsO_LIAdults with dyslexia showed reduced delta- and theta-band speech decoding C_LIO_LICerebro-acoustic coherence was reduced in delta and theta bands in dyslexia group C_LIO_LIDelta-band power was increased in dyslexia, especially over right temporal region C_LIO_LICross-frequency coupling did not differ between adults with and without dyslexia C_LI

neuroscience↗

bioRxiv2025

Atypical low-frequency and high-frequency neural entrainment to rhythmic audiovisual speech in adults with dyslexia

Developmental dyslexia has been linked to atypical neural processing of the temporal dynamics of speech, but there has been disagreement concerning whether faster or slower dynamics are impaired. According to the Temporal Sampling (TS) theory, dyslexia arises from impaired entrainment of low-frequency neural oscillations - particularly in the delta (1-4 Hz) and theta (4-8 Hz) bands - to the rhythmic modulations of speech. This hypothesis was tested for adults with and without dyslexia using electroencephalography (EEG) during a rhythmic audiovisual speech paradigm, previously delivered to children. Participants viewed a "talking head" repeating the syllable "ba" at a 2-Hz rate. Measures were neural phase entrainment and band power in the delta, theta, beta (15-25 Hz), and low gamma (25-40 Hz) bands, and broadband event-related potentials (ERPs), focusing on the N1 and P2 components. Phase-amplitude coupling (PAC) and phase-phase coupling (PPC) were also assessed for delta-theta, delta-beta, theta-beta, delta-gamma, and theta-gamma interactions. Both groups exhibited significant delta- and theta-band phase entrainment; however, the two groups differed significantly in the preferred phase for the theta band. While the control group showed consistent beta- and low gamma-band phase entrainment, this was not observed for the dyslexic group. There was significantly greater delta-band power for the dyslexic group across the whole brain and in the right temporal region. Additionally, the P2 ERP component differed significantly between groups. The data are interpreted with respect to TS theory.

neuroscience↗

bioRxiv2025

Neural processing of natural speech by children with developmental language disorder (DLD): EEG speech decoding, power and classifier investigations

The sensory/neural Temporal Sampling (TS) theory of developmental language disorder (DLD) is based on the sensory and linguistic impairments in rhythm processing that are found in children with both developmental dyslexia (DD) and DLD. These sensory/linguistic impairments include decreased sensitivity to amplitude rise times (ARTs, the sensory triggers related to automatic cortical speech tracking), syllable stress patterns and speech rhythm. At the neural level TS theory predicts impairments in the cortical tracking of different rates of amplitude modulation (AM) in the speech signal <10Hz. To date, the accuracy of low-frequency cortical tracking in natural continuous speech has not been measured in children with DLD. Here, EEG was recorded during story listening from children with and without DLD aged around 9 years, and decoding analyses in the delta, theta and alpha (control) bands were carried out. EEG power was computed in the delta, theta and gamma bands, and phase-amplitude coupling and phase-phase coupling (PAC, PPC) were also computed between bands. Whole-brain analyses showed that the accuracy of low-frequency decoding (delta, theta) did not differ between groups. However, region-specific analyses revealed significantly reduced delta-band speech tracking in the right temporal cortex in the children with DLD. PAC and PPC dynamics did not differ between groups. The data suggest that low-frequency cortical tracking impairments in DLD may be spatially constrained to the right hemisphere rather than globally present as in DD. The data are discussed using TS theory. HighlightsO_LICortical tracking of natural speech is measured in children with and without DLD C_LIO_LIDelta-band tracking is impaired in right temporal regions in children with DLD C_LIO_LICross-frequency coupling dynamics (PAC, PPC) did not differ between groups C_LI

neuroscience↗

bioRxiv2024

Investigating the 'Atypical Rhythm Risk' hypothesis in children with developmental language disorder using an EEG rhythmic speech paradigm

Sensitivity to rhythmic and prosodic cues in speech has been described as a precursor of language acquisition. Consequently, atypical rhythmic processing during infancy and early childhood has been considered a risk factor for developmental language disorders. Despite many behavioural studies, the neural processing of rhythmic speech has not yet been explored in children with developmental language disorder (DLD). Here we utilise EEG to investigate the neural processing of rhythmic speech by 9-year-old children with and without DLD. In the current study, we investigate phase entrainment, angular velocity, power, event related potentials (ERPs), phase-amplitude coupling (PAC) and phase-phase coupling (PPC), at three frequency bands selected on the basis of the prior literature, delta, theta and low gamma. We predicted a different phase of entrainment in the delta band in children with DLD, and also greater theta power, atypical cross-frequency coupling and possibly atypical gamma-band responses. Contrary to prediction, children with DLD demonstrated significant and equivalent phase entrainment in the delta and theta bands to control children. However, only the control children showed significant phase entrainment in the low gamma band. The children with DLD also exhibited significantly more theta and low gamma power compared to the control children, and there was a significant gamma-band difference in angular velocity between the two groups. Finally, group resultant phase analyses showed that low-frequency phase (delta and theta) affected gamma oscillations differently by group. These EEG data show important differences between children with and without DLD in the neural mechanisms underpinning the processing of rhythmic speech. The findings are discussed in terms of auditory theories of DLD, particularly Temporal Sampling theory.

neuroscience↗

bioRxiv2023

Atypical beta-band effects in children with dyslexia in response to rhythmic audio-visual speech

Children with dyslexia are known to show impairments in perceiving speech rhythm, which impact their phonological development. Neural rhythmic speech studies have reported atypical delta phase in children with dyslexia, but beta band effects have not yet been studied. It is known that delta phase modulates the amplitude of the beta band response during rhythmic tasks via delta-beta phase-amplitude coupling (PAC). Accordingly, the atypical delta band effects reported for children with dyslexia may imply related atypical beta band effects. Here we analyse EEG data collected during a rhythmic speech paradigm from 51 children (21 typically-developing; 30 with dyslexia) who attended to a talking head repeating "ba" at 2Hz. Phase entrainment in the beta band, angular velocity in the beta band, power responses in the beta band and delta-beta PAC were assessed for each child and each group. Phase entrainment in the beta band was only significant for children without dyslexia. Children with dyslexia did not exhibit any phase consistency, and beta-band angular velocity was significantly faster compared to control children. Power in the beta band was significantly greater in the children with dyslexia. Delta-beta PAC was significant in both groups. The data are interpreted with respect to temporal sampling theory.

neuroscience↗

bioRxiv2022

Atypical speech production of multisyllabic words by children with developmental dyslexia

The prevalent core phonological deficit model of dyslexia proposes that the reading and spelling difficulties characterizing affected children stem from prior developmental difficulties in processing speech sound structure, for example perceiving and identifying syllable stress patterns, syllables, rhymes and phonemes. Yet spoken word production appears normal. This suggests an unexpected disconnect between speech input and speech output processes. Here we investigated the output side of this disconnect from a speech rhythm perspective by measuring the speech amplitude envelope (AE) of multisyllabic spoken phrases. The speech AE contains crucial information regarding stress patterns, speech rate, tonal contrasts and intonational information. We created a novel computerized speech copying task in which participants copied aloud familiar spoken targets like "Aladdin". Seventy-five children with and without dyslexia were tested, some of whom were also receiving an oral intervention designed to enhance multi-syllabic processing. Similarity of the childs productions to the target AE was computed using correlation and mutual information metrics. Similarity of pitch contour, another acoustic cue to speech rhythm, was used for control analyses. Children with dyslexia were significantly worse at producing the multi-syllabic targets as indexed by both similarity metrics for computing the AE. However, children with dyslexia were not different from control children in producing pitch contours. Accordingly, the spoken production of multisyllabic phrases by children with dyslexia is atypical regarding the AE. Children with dyslexia may not appear to listeners to exhibit speech production difficulties because their pitch contours are intact. Research HighlightsO_LISpeech production of syllable stress patterns is atypical in children with dyslexia. C_LIO_LIChildren with dyslexia are significantly worse at producing the amplitude envelope of multi-syllabic targets compared to both age-matched and reading-level-matched control children. C_LIO_LINo group differences were found for pitch contour production between children with dyslexia and age-matched control children. C_LIO_LIIt may be difficult to detect speech output problems in dyslexia as pitch contours are relatively accurate. C_LI

neuroscience↗

bioRxiv2022

Decoding of Speech Information using EEG in Children with Dyslexia: Less Accurate Low-Frequency Representations of Speech, Not "Noisy" Representations

The amplitude envelope of speech carries crucial low-frequency acoustic information that assists linguistic decoding. The sensory-neural Temporal Sampling (TS) theory of developmental dyslexia proposes atypical encoding of speech envelope information <10 Hz, leading to atypical phonological representations. Here a backward linear TRF model and story listening were employed to estimate the speech information encoded in the electroencephalogram in the canonical delta, theta and alpha bands by 9-year-old children with and without dyslexia. TRF decoding accuracy provided an estimate of how faithfully the childrens brains encoded low-frequency envelope information. Between-group analyses showed that the children with dyslexia exhibited impaired reconstruction of speech information in the delta band. However, when the quality of speech encoding for each child was estimated using child-by-child decoding models, then the dyslexic children did not differ from controls. This suggests that children with dyslexia encode neither "noisy" nor "normal" representations of the speech signal, but different representations.

neuroscience↗

bioRxiv2021

Cortical tracking of a background speaker modulates the comprehension of a foreground speech signal

Understanding speech in background noise is a difficult task. The tracking of speech rhythms such as the rate of syllables and words by cortical activity has emerged as a key neural mechanism for speech-in-noise comprehension. In particular, recent investigations have used transcranial alternating current stimulation (tACS) with the envelope of a speech signal to influence the cortical speech tracking, demonstrating that this type of stimulation modulates comprehension and therefore evidencing a functional role of the cortical tracking in speech processing. Cortical activity has been found to track the rhythms of a background speaker as well, but the functional significance of this neural response remains unclear. Here we employ a speech-comprehension task with a target speaker in the presence of a distractor voice to show that tACS with the speech envelope of the target voice as well as tACS with the envelope of the distractor speaker both modulate the comprehension of the target speech. Because the envelope of the distractor speech does not carry information about the target speech stream, the modulation of speech comprehension through tACS with this envelope evidences that the cortical tracking of the background speaker affects the comprehension of the foreground speech signal. The phase dependency of the resulting modulation of speech comprehension is, however, opposite to that obtained from tACS with the envelope of the target speech signal. This suggests that the cortical tracking of the ignored speech stream and that of the attended speech stream may compete for neural resources. Significance StatementLoud environments such as busy pubs or restaurants can make conversation difficult. However, they also allow us to eavesdrop into other conversations that occur in the background. In particular, we often notice when somebody else mentions our name, even if we have not been listening to that person. However, the neural mechanisms by which background speech is processed remain poorly understood. Here we employ transcranial alternating current stimulation, a technique through which neural activity in the cerebral cortex can be influenced, to show that cortical responses to rhythms in the distractor speech modulate the comprehension of the target speaker. Our results evidence that the cortical tracking of background speech rhythms plays a functional role in speech processing.

neuroscience↗