Biology subjects

Caie, B.

Publications and source records attributed to Caie, B..

3 recordsLinked to original sources

bioRxiv2024

Intra-individual variability in the effects of transcranial directcurrent stimulation on free choice saccade behaviour

Transcranial direct current stimulation (tDCS) is used as a tool to causally influence neural activity in humans non-invasively. Although most studies recruit a large number of participants in order to uncover population-level effects, growing evidence suggests that tDCS may be expected to induce different effects in different individuals, leading to large inter-individual variability and confounds in population-level testing. Additionally, variability may arise from intra-individual differences and confounds that are difficult to assess in studies with limited to no re-testing. Here, we performed 10 sessions of tDCS each on 5 human participants performing a free choice saccade task while neural activity was measured via EEG. Participants first underwent functional MRI to localize the human right frontal eye field (rFEF) homologue. An HD-tDCS montage was then used to focally target rFEF based on individual MRI localizations, alternating the polarity between anodal or cathodal current over repeated sessions during a 5 week period (twice weekly). On stimulation days, participants performed a free choice task prior to and after administration of tDCS while EEG activity was recorded. To quantify the likelihood that tDCS induced a causal effect on behaviour and neural activity across different levels of analysis, we developed a multilevel causal inference method based on permutation testing of a quasi-experiment (difference-in-differences). We then used this method to assess the likelihood of a causal effect at different levels of abstraction: group-level, participant-level, and paired-session level. At the group-level, we found evidence for an influence of tDCS on choice reaction times, which followed a reaction-time dependent change in EEG activity, and on how choices depended on previous trials. However, individuals showed heterogeneous effects. Further, analysis of paired-session variability often belied the pooled individual effect, suggesting that different sessions of tDCS may have produced markedly different effects in the same participants. In light of this, we discuss potential causes of this variability, and the counterfactuals that should be considered when making data-driven inferences about the effects of tDCS. Author SummaryDeveloping reliable interventions on human neural activity is important for establishing causal relationships in basic research and developing therapeutics for pathological brain states. Transcranial direct current stimulation is a promising technique to intervene on neural activity safely in humans, but it is poorly understood if tDCS reliably impacts brain and behaviour in the same way across sessions. We performed an extensive test-retest study on tDCS in humans, and developed statistical methods to assess variability across sessions. Our results provide strong evidence against a consistent effect of tDCS in the same individual across different sessions. This warrants caution in using tDCS as a predictable intervention on neural activity in research and clinical practices.

neuroscience↗

bioRxiv2023

Choice anticipation as gated accumulation of sensory expectations

Predictions are combined with sensory information when making choices. Accumulator models have conceptualized predictions as trial-by-trial updates to a baseline evidence level. These models have been successful in explaining the influence of choice history across-trials, however they do not account for how sensory information is transformed into choice evidence within-trials. Here, we derive a gated accumulator that models the onset of evidence accumulation as a combination of delayed sensory information and a prediction of sensory timing. To test how the delays interact with across-trial predictions, we designed a free choice saccade task where participants directed eye movements to either of two targets that appeared with variable delays and asynchronies. Despite instructions not to anticipate, participants responded prior to target onset on some trials. We reasoned that anticipatory responses may reflect a trade-off between inhibiting and facilitating the onset of evidence accumulation via a gating mechanism as target appearance became more likely. Using a choice history analysis, we found that anticipatory responses were more likely following repeated choices, despite task randomization, suggesting that the balance between anticipatory and sensory responses was driven by a prediction of sensory timing. By fitting the gated accumulator model to the data, we found that variance in within-trial fluctuations in baseline evidence best explained the joint increase of anticipatory responses and faster sensory-guided responses with longer delays. Thus, we conclude that a prediction of sensory timing is involved in balancing the costs of anticipation with lowering the amount of accumulated evidence required to trigger saccadic choice, which could be explained by history-dependence in the baseline dynamics of a gated accumulator. New and NoteworthyEvidence accumulation models are used to study how recent history impacts the processes underlying how we make choices. Biophysical evidence suggests that the accumulation of evidence is gated, however classic accumulator models do not account for this, so little is known about how recent history may influence the gating process. In this work, computational modeling and experimental data from a free choice saccade task argue that predictions of the timing of sensory information are important in controlling how evidence accumulation is gated, and that signatures of these predictions can be detected even in randomized task environments where history effects may not be expected.

neuroscience↗

bioRxiv2019

HD-tDCS over mIPS causally modulates online reach correction

Brain lesion and stimulation studies have suggested posterior parietal cortex and the medial intraparietal sulcus in particular as a crucial hub for online movement error corrections. However, causal evidence for this is still sparse. Indeed, lesion studies are potentially confounded by compensatory reorganization mechanisms while brain stimulation studies have produced heterogeneous results when employing transcranial magnetic stimulation. Here we designed a new complementary paradigm using fMRI-guided high-definition transcranial direct current stimulation (HD-tDCS) of the left medial intraparietal sulcus (mIPS) together with regression-based mediation analysis to re-examine the causal role of mIPS in online reach corrections to jumping targets. We obtained two independent measures of stimulation-induced changes in brain activity by modeling current flow in the brain and through EEG recordings before and after HD-tDCS stimulation. Third, to quantify behavioral effects of HD-tDCS we computed movement curvature as a measure of online correction. We demonstrate that both of our measurements of brain activity were consistent with a polarity-specific modulation of the online correction for targets jumping to the contralateral side of the stimulation. Importantly, using a mediation analysis of the relationship between stimulation current and movement curvature suggests that the induced current modifies brain activity, which then leads to the observed behavioral changes. This unique combination of methods and analysis thus provides complementary evidence for the crucial role of the posterior parietal cortex in online error correction, while at the same time setting a new methodological standard with respect to the causal influence of transcranial direct current stimulation. New & NoteworthyTranscranial direct current stimulation (tDCS) is an interesting and potentially useful tool for asking causal scientific questions and design clinical treatments. With our unique combination of highly accurate fMRI guided stimulation, current forward modeling, EEG recordings before and after the stimulation and behavioral changes we could unravel the causal structure of tDCS. Our approach naturally deals with the variability of tDCS results, increasing its potential usefulness as a tool for research and clinical applications.

neuroscience↗