Biology subjects

Sergent, C.

Publications and source records attributed to Sergent, C..

6 recordsLinked to original sources

bioRxiv2026

Briefer than brief - uncovering the true temporal resolution of the visual system

The temporal resolution of vision is seriously limited. However, the response to a very brief flash, called the impulse response, is already quite sluggish at the earliest stages of vision, potentially obscuring the true temporal resolution of the rest of the visual system. Faster monitors that produce briefer flashes are subject to diminishing returns because, by definition, they cannot elicit responses that are any briefer than the impulse response. Here, taking inspiration from previous attempts, we develop a novel technique for presenting flashes that elicit 'briefer-than-brief' visual responses. Using a simple deconvolution technique, we reverse-engineer the visual response and estimate the form that the stimulus should take to elicit the response that a faster visual system would produce to a normal flash. Using psychophysics on human observers, we demonstrate that these 'briefer-than-brief' (BTB) flashes partially bypass the temporal limits presumably imposed by the early visual system using two paradigms: one that requires temporal segregation and one that requires temporal integration of sequential flashes. With BTB flashes, human observers successfully isolated two successive flashes at shorter intervals than with conventional flashes, improving temporal resolution by around 16%. We found that BTB stimuli not only improved temporal resolution, but also induced poorer performance on tasks requiring temporal integration, suggesting that the visual responses elicited by BTB flashes overlap less in time due to their briefer duration. In sum, our findings suggest that, using reverse-engineered stimuli, we can alleviate a temporal bottleneck that probably originates from the earliest stages of vision. In doing so, we allow higher visual areas to operate at a higher temporal resolution than previously thought possible.

neuroscience↗

bioRxiv2026

Bifurcation dynamics in a shared network beyond sensory areas characterize conscious auditory perception independently of report

The neural correlates of conscious perception remain debated, particularly regarding the role of extra-sensory regions such as the prefrontal cortex. One promising approach is to study the dynamical properties of neural processing in task-related and task-free contexts. A previous EEG study showed that conscious perception is associated with all-or-none late activations, giving rise to bifurcation dynamics even without a task. Here we used fMRI and near-threshold auditory stimulation to ask which brain networks give rise to these bifurcations, and how they differ depending on task. In both contexts, stimulus intensity modulated activity within broad networks spanning sensory and extra-sensory regions, including the prefrontal cortex. These networks showed both shared and distinct components. Single-trial modelling further revealed bifurcation dynamics beyond primary sensory cortices and enabled single-trial prediction of conscious perception in both contexts. These findings resolve previous conflicting results and reveal common networks and dynamics underlying conscious perception irrespective of task.

neuroscience↗

bioRxiv2026

Effects of expectation, attention, and NMDA receptor blockade on feedforward and feedback processing

Perception is increasingly viewed as an inferential process wherein sensory inputs are integrated with prior expectations. We employed time-resolved decoding on electroencephalography (EEG) data (n = 30 male participants) to investigate how expectations modulate sensory processing across varying levels of stimulus complexity, and tested the effect of attention and NMDA receptor blockade. We designed a visual stimulus containing features of different complexity whose processing relies on distinct neural mechanisms: local contrast, collinearity, and the Kanizsa illusion, involving primarily feedforward, lateral, and feedback processes, respectively. EEG decoding revealed that expectations modulated lateral and feedback processing (better decoding for unexpected stimuli) but not feedforward processing. These expectation effects were confined to attended (task-relevant) features and were not observed for task-irrelevant features. The NMDA receptor antagonist memantine selectively enhanced decoding of the Kanizsa illusion, implicating NMDA-mediated feedback mechanisms in perceptual inference, but it did not modulate the effects of expectation or attention. These findings highlight the differential impact of expectations across different stages of sensory processing and reveal a distinct role of NMDA receptor-mediated feedback mechanisms. Significance statementPerception integrates sensory inputs with prior expectations. Using EEG decoding, we examined how expectations shape sensory processing at different levels of complexity and tested the effects of attention and NMDA receptor blockade. Our visual stimuli were designed to capture EEG markers of feedforward, lateral, and feedback mechanisms. Expectations influenced lateral and feedback processing (better decoding for unexpected stimuli) but not feedforward processing, and these effects were selective to task-relevant stimulus features. Memantine, an NMDA receptor antagonist, selectively improved decoding of the Kanizsa illusion, implicating NMDA-mediated feedback in perceptual inference, but it did not shape expectation or attention effects.

neuroscience↗

bioRxiv2024

Dynamic connectivity profiles characteristic of conscious states are associated with enhanced conscious processing of external stimuli

One of the goals of the neuroscience of consciousness is to identify neural markers capable of distinguishing brain dynamics in awake, healthy individuals from unconscious conditions. This problem also has a clinical diagnostic interest in disorders of consciousness. Recent research has shown that brain connectivity patterns characterized by long-range interactions and anticorrelations are associated with conscious states and diminish with loss of consciousness in human and non-human primates. However, the precise contribution of these patterns to conscious processing and subjective experience formation remains unclear. In this study, we investigated the functional role of these brain patterns in shaping conscious content by examining their influence on participants ability to process external information during wakefulness. Participants underwent fMRI recordings during an auditory detection task. Phase coherence-based functional connectivity and k-means clustering confirmed that the ongoing dynamics were underpinned by brain patterns consistent with those identified in previous research, including the "high pattern" characteristic of conscious states. We found that the detection of auditory stimuli at threshold was specifically improved when the connectivity pattern at the time of presentation corresponded to this high-pattern. In return, the occurrence of the high-pattern increased after detection, indicating that participants were more likely to transition to a high-pattern following stimulus detection. Our findings suggest that ongoing brain dynamics and conscious perception mutually influence each other and that certain brain configurations are more favorable for conscious processing of external stimuli. In the future, targeting these moments of favorable patterns in patients with disorders of consciousness may help us identify windows of greater receptivity to the external world, paving the way for developing individualized patient care protocols.

neuroscience↗

bioRxiv2022

A connectome-based model of conscious access in monkey cortex

A growing body of evidence suggests that conscious perception of a sensory stimulus coincides with all-or-none activity across multiple cortical areas, a phenomenon called ignition. In contrast, the same stimulus, when undetected, induces only transient activity. In this work, we report a large-scale model of the macaque cortex based on recently quantified structural mesoscopic connectome data. We use this model to simulate a detection task, and demonstrate how a dynamical bifurcation mechanism produces ignition-like events in the model network. The model predicts that feedforward excitatory transmission is primarily mediated by the fast AMPA receptors to ensure rapid signal propagation from sensory to associative areas. In contrast, a greater proportion of the inter-areal feedback projections and local recurrent excitation depend on the slow NMDA receptors, to ensure ignition of distributed frontoparietal activity. Our model predicts, counterintuitively, that fast-responding sensory areas contain a higher ratio of NMDA to AMPA receptors compared to association cortical areas that show slow, sustained activity. We validate this prediction using cortex-wide in-vitro receptor autoradiography data. Finally, we show how this model can account for various behavioral and physiological effects linked to consciousness. Together, these findings clarify the neurophysiological mechanisms of conscious access in the primate cortex and support the concept that gradients of receptor densities along the cortical hierarchy contribute to distributed cognitive functions.

neuroscience↗

bioRxiv2021

Consciously detecting and recognizing a stimulus without knowing what it looks like

What is the role of sensory processing in conscious perception? Current theories of consciousness are divided on this question. Some propose that conscious perception arises during the buildup of sensory representations. Other argue for a secondary process that broadcasts these representations to higher-level areas. This second view makes a counter-intuitive prediction: one could consciously perceive abstract representations untied to any low-level sensory feature. We tested this prediction by combining visual masking with retrospective cueing. We found that when visually masked words were followed by a semantically related auditory word, participants were better at detecting this past word and reporting its identity, but were strikingly unable to report its visual features (letter casing or position on screen). This suggests that retro-cueing can help a semantic representation reach awareness even after the associated sensory information has been masked. The mechanisms of conscious access might thus be largely independent from early sensory build-up.

neuroscience↗