Biology subjects

Gehmacher, Q.

Publications and source records attributed to Gehmacher, Q..

8 recordsLinked to original sources

bioRxiv2026

The dynamic relationship between pupil dilation and neural surprise in natural language comprehension

Predictive processing accounts hold that the brain continuously generates predictions and updates internal models from error signals, but it remains unclear whether prediction representation and model-updating reflect a single graded computation or computationally distinct operations implemented in dissociable systems. We recorded magnetoencephalography (MEG) and pupillometry while participants listened to continuous natural speech, quantifying word-by-word lexical surprise and semantic prediction error with a GPT-2 model. Broadly speaking, lexical surprise was tracked in a predominantly graded fashion, whereas the response to semantic error was better captured by a rectified linear (ReLU) gating function - i.e., that responds only when the error exceeds a recent contextual baseline. There were also some interesting dissociations in the strength of these relative effects. The auditory cortex tracked lexical surprise in a graded, continuous manner, with later cortical stages reflecting semantic error. By contrast, tonic pupil diameter and source-localised brainstem responses were predominantly captured by a gated response to semantic error. A pupil-coupling analysis confirmed that this gating signature was statistically shared between pupil-linked arousal and brainstem-localised, but not cortical, responses. Together, these findings reveal a division of labour in which the cortex maintains a continuous, high-fidelity map of predictive information while a pupil-linked model-update system acts as a selective gate, engaged specifically by meaning-disrupting events that have crossed a threshold level of error. This asymmetry suggests that signals serving continuous parsing of prediction error and signals serving model revision may be computationally dissociated, with a range of implications for our understanding of the intrinsic interactions between mechanisms serving perception and learning.

neuroscience↗

bioRxiv2025

Tracking visual rhythms: a concert of sensory and motor simulation

When will I see something and what will it be? Temporal predictions are crucial for adaptive interaction with the environment and are typically accompanied by predictions about sensory content, yet these two types of what and when predictions are usually studied separately. Specifically, oscillatory phase-coupling (or entrainment) has been proposed to align our neural sensitivity with likely moments of stimulus appearance, however these accounts ignore that predictions about when something will appear are usually accompanied by predictions about what it will be. Thus, temporal predictions may not enhance all sensory processing but rather modulate particular channels encoding predicted content. We here demonstrate oscillatory phase-coupling in vision and show how it relates to content-specific encoding. In a magnetoencephalography (MEG) study, participants observed rhythmic Gabors at 1.33 or 2 Hz with predictable orientations. They judged the timing or orientation of a delayed probe which manipulated the requirement to covertly maintain the sequence rhythm. We found sustained oscillatory phase-coupling to the entrained rhythm in motor areas specifically when participants judged stimulus timing, where its extent was associated with perceptual performance. Meanwhile, neural decoding revealed content predictions in early visual areas ( what) that fluctuated in line with temporal predictions ( when). These temporally-specific content predictions appeared regardless of task instruction but were correlated and phase-aligned with the motor phase-coupling during timing judgements. These findings suggest that temporal predictions may be derived from motor readouts of temporally-specific sensory predictions, with broad implications for our understanding of entrainment and prediction, and how we represent time more generally.

neuroscience↗

bioRxiv2024

The "Ocular Response Function" for encoding and decoding oculomotor related neural activity

Oculomotor activity provides critical insights into cognition and health, with growing evidence demonstrating its involvement in various cognitive functions such as attention, memory, and sensory processing, and that it is a significant indicator of psychopathologies and neurological disorders. Despite its crucial importance across domains, the neural mechanisms supporting oculomotion have been underexplored, largely because eye movements are typically treated as artefacts to be removed from the neural signal. While useful for data cleaning, this approach risks discarding valuable information about oculomotor control, and there has been recent interest in modelling these signals instead to understand them. Using time-resolved regression methods with magnetoencephalography (MEG) and eye tracking during the resting state, we thus here sought to model Ocular Response Functions (ORFs), that characterise the neural signatures of distinct oculomotor events, specifically saccades, blinks, and pupil dilation. We demonstrate the relationships between ocular action and neural activity (encoding), revealing a range of sensory cortical, cerebellar and frontal signatures preceding and following such ocular events. We conversely show how we can reconstruct ocular events from brain activity, and further apply resting-state derived ORFs to a passive listening task - demonstrating how some neural markers interpreted as directly related to sensation may in principle indirectly result from oculomotor contributions to task-related neural processing. By providing an accessible framework for examining the interplay between eye movements and neural processes, we offer a range of insights with potential applications across cognitive and clinical neuroscience.

neuroscience↗

bioRxiv2024

Eye Movements in Silent Visual Speech track Unheard Acoustic Signals and Relate to Hearing Experience

Behavioral and neuroscientific studies have shown that watching a speakers lip movements aids speech comprehension. Intriguingly, even when videos of speakers are presented silently, various cortical regions track auditory features, such as the envelope. Recently, we demonstrated that eye movements track low-level acoustic information when attentively listening to speech. In this study we investigated whether ocular speech tracking occurs during visual speech and how it influences cortical silent speech tracking. Furthermore, we compared the data of hearing individuals with congenitally deaf individuals, and those with acquired deafness or hearing loss (DHH; Deaf or hard of hearing) to assess how auditory deprivation (early vs. late onset) affects neural and ocular speech tracking during silent lip-reading. Using magnetoencephalography (MEG), we examined ocular and neural speech tracking of 75 participants observing silent videos of a speaker played forward and backward. Our main finding is a clear ocular unheard speech tracking effect with a dominance <1 Hz, which was not present for the lip movements. Similarly, we observed a <=1.3 Hz effect of neural unheard speech tracking in temporal regions for hearing participants. Importantly, neural tracking was not directly linked to ocular tracking in this study. Strikingly, across different listening groups, deaf participants with auditory listening experience showed higher ocular speech tracking than hearing participants, while no ocular speech tracking effect was revealed for congenitally deaf participants in a very small sample. This study extends our previous work by demonstrating the involvement of eye movements in speech processing, even in the absence of acoustic input.

neuroscience↗

bioRxiv2023

Prediction tendency, eye movements, and attention in a unified framework of neural speech tracking

Auditory speech comprehension is a multi-faceted process in which attention, prediction, and sensorimotor integration (via active sensing) interact with or complement each other. Although different conceptual models that focus on one of these aspects exist, we still lack a unified understanding of their role in speech processing. Here, we first replicated two recently published studies from our lab, confirming 1) a positive relationship between individual prediction tendencies and neural speech tracking, and 2) the phenomenon of ocular speech tracking - the tracking of attended speech by eye movements - and its shared contribution with neural activity to speech processing. In addition, we extended these findings with complementary analyses and investigated these phenomena in relation to each other in a multi-speaker paradigm with continuous, narrative speech. Importantly, prediction tendency and ocular speech tracking seem to be unrelated. In contrast to the shared contributions of oculomotor and neural activity to speech processing over a distributed set of brain regions that are critical for attention, individual prediction tendency and its relation to neural speech tracking seem to be largely independent of attention. Based on these findings, we propose a framework that aims to bridge the gaps between attention, prediction, and active (ocular) sensing in order to contribute to a holistic understanding of neural speech processing. In this speculative framework for listening, auditory inflow is, on a basic level, temporally modulated via active ocular sensing, and incoming information is interpreted based on probabilistic assumptions.

neuroscience↗

bioRxiv2023

Eye movements track prioritized auditory features in selective attention to natural speech

Over the last decades, cognitive neuroscience has identified a distributed set of brain regions that are critical for attention - one of the key principles of adaptive behavior. A strong anatomical overlap with brain regions critical for oculomotor processes suggests a joint network for attention and eye movements. However, the role of this shared network in complex, naturalistic environments remains understudied. Here, we investigated eye movements in relation to (un)attended sentences of natural speech in simultaneously recorded eye tracking and magnetoencephalographic (MEG) data. Using temporal response functions (TRF), we show that eye gaze tracks acoustic features (envelope and acoustic onsets) of attended speech, a phenomenon we termed ocular speech tracking. Ocular speech envelope tracking even differentiates a target from a distractor in a multi speaker context and is further related to intelligibility. Moreover, we provide evidence for its contribution to neural differences in speech processing, emphasizing the necessity to consider oculomotor activity in future research and in the interpretation of neural differences in auditory cognition. Our results extend previous findings of a joint network of attention and eye movement control as well as motor theories of speech. They provide valuable new directions for research into the neurobiological mechanisms of the phenomenon, its dependence on learning and plasticity, and its functional implications in social communication.

neuroscience↗

bioRxiv2022

Individual prediction tendencies facilitate cortical speech tracking

Listening can be conceptualized as a process of active inference, in which the brain forms internal models to predict and integrate auditory information in a complex interaction of bottom-up and top-down processes. Whether inter-individual "prediction tendencies" shape listening experiences of real-world stimuli such as speech is, however, unknown. In the current study, we used a passive paradigm presenting tone sequences of varying entropy level, to independently quantify auditory prediction tendency (as the tendency to anticipate low-level acoustic features according to their contextual probability) for each individual. This measure was then used to predict the magnitude of cortical speech (envelope) tracking in a multi speaker listening task, where participants listened to audiobooks narrated by a target speaker in isolation or interfered by 1 or 2 distractors. Furthermore, rare semantic violations were introduced into the story, enabling us to also examine effects of word surprisal during continuous speech processing. Our results show that individual prediction tendency facilitates cortical speech tracking. Furthermore, we find interactions between individual prediction tendency and background noise as well as word surprisal in disparate brain regions. In sum, our findings suggest that individual prediction tendencies are generalizable across different listening situations and may serve as a valuable element to explain interindividual differences in natural listening experience.

neuroscience↗

bioRxiv2021

Direct cochlear recordings show human hearing nerve activity is modulated by selective attention

The architecture of the efferent auditory system enables prioritization of strongly overlapping spatiotemporal cochlear activation patterns elicited by relevant and irrelevant inputs. So far, attempts at finding such attentional modulations of cochlear activity delivered indirect insights in humans or required direct recordings in animals. The extent to which spiral ganglion cells forming the human auditory nerve are sensitive to selective attention remains largely unknown. We investigated this question by testing the effects of attending to either the auditory or visual modality in human cochlear implant (CI) users (3 female, 13 male). Auditory nerve activity was directly recorded with standard CIs during a silent (anticipatory) cue-target interval. When attending the upcoming auditory input, ongoing auditory nerve activity within the theta range (5-8 Hz) was enhanced. Crucially, using the broadband signal (4-25 Hz), a classifier was even able to decode the attended modality from single-trial data. Follow-up analysis showed that the effect was not driven by a narrow frequency in particular. Using direct cochlear recordings from deaf individuals, our findings suggest that cochlear spiral ganglion cells are sensitive to top-down attentional modulations. Given the putatively broad hair-cell degeneration of these individuals, the effects are likely mediated by alternative efferent pathways as compared to previous studies using otoacoustic emissions. Successful classification of single-trial data could additionally have a significant impact on future closed-loop CI developments that incorporate real-time optimization of CI parameters based on the current mental state of the user. Significance StatementThe efferent auditory system in principle allows top-down modulation of auditory nerve activity, however evidence for this is lacking in humans. Using cochlear recordings in participants performing an audiovisual attention task, we show that ongoing auditory nerve activity in the silent cue-target period is directly modulated by selective attention. Specifically, ongoing auditory nerve activity is enhanced within the theta range when attending upcoming auditory input. Furthermore, over a broader frequency range, the attended modality can be decoded from single-trial data. Demonstrating this direct top-down influence on auditory nerve activity substantially extends previous works that focus on outer hair cell activity. Generally, our work could promote the use of standard cochlear implant electrodes to study cognitive neuroscientific questions.

neuroscience↗