Biology subjects

Philiastides, M. G.

Publications and source records attributed to Philiastides, M. G..

6 recordsLinked to original sources

bioRxiv2025

Early Arousal Signals Drive Reward Learning and Subsequent Choice Behaviour

Learning to reinforce rewarding actions and avoid repeated mistakes is crucial for survival in dynamic environments. Yet, it remains unclear how distinct neural signals coordinate to implement reward-based decision-making and behavioural adjustment. We obtained simultaneous electroencephalography (EEG) and pupillometry during a probabilistic reversal learning task. Leveraging single-trial EEG, we first replicate the presence of two feedback-locked neural representations; an early signal previously linked to alertness and switching behaviours following negative feedback and a late signal associated with value updating and reward learning. Using single-trial pupillometry, we then show that differences in feedback-evoked pupil responses between positive and negative feedback are driven primarily by negative feedback encoding. Jointly examining these EEG and pupillometry signatures, we show that following negative feedback, increased trial-by-trial coupling between the pupil response and the early, but not the late, EEG signal is linked to increased uncertainty and exploration tendency as well as reduced accuracy and evidence accumulation on the next trial. Consistent with previous research implicating the locus-coeruleus-noradrenaline system in uncertainty signalling and network resets, we propose that when internal estimates of contextual uncertainty are high following negative feedback, an early signal, likely regulated by locus coeruleus activity, implements a network reset in reward learning structures of a later learning signal. This interruption may simultaneously increase the neural gain related to the processing of novel information and decrease the influence of existing representations in reward learning structures, in turn improving performance by creating new, more accurate internal representations of the external world. Significance StatementThe current study jointly examines EEG and pupillometry signatures associated with reversal-learning during reward-based learning. It suggests that when internal estimates of contextual uncertainty are high following negative feedback, an early neural signal, likely regulated by locus coeruleus activity, implements a network reset in reward learning structures of a later learning signal. This interruption may simultaneously increase the neural gain related to the processing of novel information and decrease the influence of existing representations in reward learning structures, in turn improving performance by creating new, more accurate internal representations of the external world.

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bioRxiv2023

Distinct basal ganglia contributions to learning from implicit and explicit value signals in perceptual decision-making

Metacognitive evaluations of confidence provide an estimate of decision accuracy that could guide learning in the absence of explicit feedback. We examine how humans might learn from this implicit feedback in direct comparison with that of explicit feedback, using simultaneous EEG-fMRI. Participants performed a motion direction discrimination task where stimulus difficulty was increased to maintain performance, with intermixed explicit- and no-feedback trials. We isolate single-trial estimates of post-decision confidence using EEG decoding, and find these neural signatures re-emerge at the time of feedback together with separable signatures of explicit feedback. We identified these signatures of implicit versus explicit feedback along a dorsal-ventral gradient in the striatum, a finding uniquely enabled by an EEG-fMRI fusion. These two signals are then integrated into an aggregate representation in the external globus pallidus, which broadcasts updates to improve cortical decision processing via the thalamus and insular cortex, irrespective of the source of feedback.

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bioRxiv2022

Sensorimotor decisions rely on the entanglement of evidence and motor accumulation processes

Most contemporary theories of sensorimotor decision-making formalize the process leading up to a decision as a gradual accumulation of noisy stimulus information over time. The resulting evidence signal is thought to be continuously tested against an internal criterion representing the amount of evidence required to make the decision. In the currently prevailing view, the amount of accumulated evidence required for a decision is independent of the amount of sensory evidence presented by the stimulus, and once that level is reached, a choice is categorically communicated to the motor system to execute an overt response. Recent experimental research casts doubts on both of these assumptions. Using a Leaky Integrating Threshold model, we relax these assumptions specifying both an evidence accumulation and a motor accumulation process. The evidence accumulation signal feeds into a leaky motor accumulator, and it is on the level of the motor accumulation that the final decision criterion is set. This adaptation results in a markedly better description of choice-RT data, especially when it comes to urgency manipulations. We show that this alternative theory, which proposes that sensory evidence is doubly integrated before final evaluation, does not only describe the behavioral data better, but its neural correlates can also be readily derived from EEG signatures involving systems of both evidence and motor accumulation.

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bioRxiv2021

A common neural currency account for social and non-social decisions

To date, social and non-social decisions have been studied in isolation. Consequently, the extent to which social and non-social forms of decision uncertainty are integrated using shared neurocomputational resources remains elusive. Here, we address this question using simultaneous EEG-fMRI and a task in which decision evidence in social and non-social contexts varies along comparable scales. First, we identify comparable time-resolved build-up of activity in the EEG, akin to a process of evidence accumulation. We then use the endogenous trial-by-trial variability in the slopes of these accumulating signals to construct parametric fMRI predictors. We show that a region of the posterior-medial frontal cortex (pMFC) uniquely explains trial-wise variability in the process of evidence accumulation in both the social and non-social contexts. We further demonstrate a task-dependent coupling between the pMFC and regions of the human valuation system in dorso- and ventro-medial prefrontal cortex (dmPFC/vmPFC) across both contexts. Finally, we report domain-specific representations in regions known to encode the early decision evidence for each context. These results are suggestive of a domain-general decision-making architecture, whereupon domain-specific information is likely converted into a "common currency" in the dmPFC/vmPFC and accumulated for the decision in the pMFC.

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bioRxiv2018

Neural dynamics of the impact of font style on lexical decision making in adult dyslexia

Good reading comprehension is indispensable in many situations including contract-based transactions that have become so prevalent in our everyday lives. People with dyslexia often exhibit impairments in this important cognitive process. Although the effects of italics - a commonly used style for highlighting important content in a range of documents - and font in general, have been explored with behavioural measures, their impact on human brain dynamics remains poorly understood. Here, we used electroencephalography (EEG) to investigate the specific effects of italics in a sentence reading lexical decision task in adult dyslexics and an age matched non-dyslexia group. Overall, the performance of dyslexics was worse. Cluster-based event-related potential (ERP) analysis revealed that brain responses within the first 300 ms following the decision stimulus differed in amplitude and spatial distribution between dyslexics and non-dyslexics when processing italicised text. An initial ERP component over occipitotemporal electrode sites started to differ between the groups as early as 167 ms following the onset of short italicised decision words. A subsequent ERP component over centrofrontal electrodes showed differences lasting until about 300 ms post-stimulus onset. Inter-individual amplitude differences in this centrofrontal neural signal were predictive of behavioural performance across participants, further highlighting the role of fast post-sensory linguistic processes in lexical decision making. Crucially, our findings emphasise the importance of choosing font style carefully to optimise word processing and reading comprehension by everyone.

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bioRxiv2017

Human VMPFC encodes early signatures of confidence in perceptual decisions

Choice confidence, an individuals internal estimate of judgment accuracy, plays a critical role in adaptive behaviour. Despite its importance, the early (decisional) stages of confidence processing remain underexplored. Here, we recorded simultaneous EEG/fMRI while participants performed a direction discrimination task and rated their confidence on each trial. Using multivariate single-trial discriminant analysis of the EEG, we identified a stimulus- and accuracy-independent component encoding confidence, appearing prior to subjects choice and explicit confidence report. The trial-to-trial variability in this EEG-derived confidence signal was uniquely associated with fMRI responses in the ventromedial prefrontal cortex (VMPFC), a region not typically associated with confidence for perceptual decisions. Furthermore, we showed that the VMPFC was functionally coupled with regions of the prefrontal cortex that support neural representations of confidence during explicit metacognitive report. Our results suggest that the VMPFC encodes an early confidence readout, preceding and potentially informing metacognitive evaluation and learning, by acting as an implicit value/reward signal.

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