Biology subjects

Ort, E.

Publications and source records attributed to Ort, E..

7 recordsLinked to original sources

bioRxiv2026

Role of GABA and NMDA receptors in shaping cortical timescales and large-scale network dynamics

Cortical brain regions integrate information across different timescales, ranging from fast sensory processing to longer integration windows, allowing cognitive functions like working memory. At the large-scale, brain regions organize into transient network states that rapidly switch over time and similarly contribute to cognition. Both cortical timescales and large-scale network dynamics are proposed to be determined by the balance between recurrent synaptic excitation and GABAergic inhibition. Here, we pharmacologically manipulated synaptic transmission at GABAA and NMDA receptors in 60 healthy male participants and acquired resting-state magnetoencephalography. Neuronal timescales followed a hierarchical gradient with shorter timescales in early sensory regions. Increasing GABAergic activity prolonged neuronal timescales across cortical regions. This effect was most prominent in the frontal default mode and in the dorsal attention network. Notably, dynamic network analyses revealed that the occurrence probability of the frontal default mode network increased, whereas the occurrence of the dorsal attention network was reduced. NMDA receptor modulation resulted in no significant changes. Together, these findings provide causal evidence that GABAergic inhibition is a key regulator of cortical temporal organization, linking microscale synaptic mechanisms to neuronal timescales and network dynamics that support diverse cognitive function.

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bioRxiv2024

A role for acetylcholine in reinforcement learning and decision making under uncertainty

The neuromodulator acetylcholine has been suggested to govern learning under uncertainty. Here, we investigated the role of muscarinic acetylcholine receptors in reward-guided learning and decision making under different degrees of uncertainty. We administered the muscarinic antagonist biperiden (4 mg) to healthy male participants (n = 43) in a within-subjects, placebo-controlled design. Participants performed a gambling and a learning task involving different levels of uncertainty, while magnetoencephalography (MEG) was recorded. We show that biperiden did not affect decision making in the gambling task, where no learning was required. However, in the learning task, where option values were associated with uncertainty, biperiden reduced the sensitivity to probabilities, particularly when choice-outcome-contingencies switched frequently. Reinforcement learning models revealed that the change in behaviour was caused by noisier estimates of probabilities resulting from increased learning rates for rewarded choices under biperiden. These behavioural findings were paralleled by elimination of the lateral prefrontal representation of learnt reward probability in high-beta (20 - 30 Hz) power under biperiden. Together, these findings suggest that muscarinic acetylcholine transmission controls learning in highly uncertain contexts, when the demand for carefully calibrated adjustments is highest.

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bioRxiv2022

Removal of reinforcement improves instrumental performance in humans by decreasing a general action bias rather than unmasking learnt associations

Performance during instrumental learning is commonly believed to reflect the knowledge that has been acquired up to that point. However, recent work in rodents found that instrumental performance was enhanced during periods when reinforcement was withheld, relative to periods when reinforcement was provided. This suggests that reinforcement may mask acquired knowledge and lead to impaired performance. In the present study, we investigated whether such a beneficial effect of removing reinforcement translates to humans. Specifically, we tested whether performance during learning was improved during non-reinforced relative to reinforced task periods using signal detection theory and a computational modelling approach. To this end, 60 healthy volunteers performed a novel visual go/no-go learning task with deterministic reinforcement. To probe acquired knowledge in the absence of reinforcement, we interspersed blocks without feedback. In these non-reinforced task blocks, we found an increased d, indicative of enhanced instrumental performance. However, computational modelling showed that this improvement in performance was not due to an increased sensitivity of decision making to learnt values, but to a more cautious mode of responding, as evidenced by a reduction of a general response bias. Together with an initial tendency to act, this is sufficient to drive differential changes in hit and false alarm rates that jointly lead to an increased d. To conclude, the improved instrumental performance in the absence of reinforcement observed in studies using asymmetrically reinforced go/no-go tasks may reflect a change in response bias rather than unmasking latent knowledge. Author SummaryIt appears plausible that we can only learn and improve if we are told what is right and wrong. But what if feedback overshadows our actual expertise? In many situations, people learn from immediate feedback on their choices, while the same choices are also used as a measure of their knowledge. This inevitably confounds learning and the read-out of learnt associations. Recently, it was suggested that rodents express their true knowledge of a task during periods when they are not rewarded or punished during learning. During these periods, animals displayed improved performance. We found a similar improvement of performance in the absence of feedback in human volunteers. Using a combination of computational modelling and a learning task in which humans performance was tested with and without feedback, we found that participants adjusted their response strategy. When feedback was not available, participants displayed a reduced propensity to act. Together with an asymmetric availability of information in the learning environment, this shift to a more cautious response mode was sufficient to yield improved performance. In contrast to the rodent study, our results do not suggest that feedback masks acquired knowledge. Instead, it supports a different mode of responding.

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bioRxiv2019

Humans can efficiently look for, but not select multiple visual objects

The human brain recurrently prioritizes task-relevant over task-irrelevant visual information. A central, question is whether multiple objects can be prioritized simultaneously. To answer this, we let observers search for two colored targets among distractors. Crucially, we independently varied the number of target colors that observers anticipated, and the number of target colors actually used to distinguish the targets in the display. This enabled us to dissociate the preparation of selection mechanisms from the actual engagement of such mechanisms. Multivariate classification of electroencephalographic activity allowed us to track selection of each target separately across time. The results revealed only small neural and behavioral costs associated with preparing for selecting two objects, but substantial costs when engaging in selection. Further analyses suggest this cost is the consequence of neural competition resulting in limited parallel processing, rather than a serial bottleneck. The findings bridge diverging theoretical perspectives on capacity limitations of feature-based attention.

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bioRxiv2019

Anticipatory distractor suppression elicited by statistical regularities in visual search

Salient yet irrelevant objects often capture our attention and interfere with our daily tasks. Distraction by salient objects can be reduced by suppressing the location where they are likely to appear. The question we addressed here was whether suppression of frequent distractors is already implemented beforehand, in anticipation of the stimulus. Using electroencephalography (EEG), we recorded cortical activity of human participants searching for a target while ignoring a salient distractor. The distractor was presented more often at one location than at any other location. We found reduced capture for distractors at frequent locations, indicating that participants learned to avoid distraction. Critically, we found evidence for proactive suppression as already prior to display onset, there was enhanced power in parieto-occipital alpha oscillations contralateral to the frequent distractor location - a signal known to occur in anticipation of irrelevant information. Locked to display onset, event-related potentials analysis showed a distractor-suppression-related PD component for this location. Importantly, this PD was found regardless of whether distracting information was presented at the frequent location. In addition, there was an early PD component representing an early attentional index of the frequent distractor location. Our results are show anticipatory (proactive) suppression of frequent distractor locations in visual search already starting prior to display onset.

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bioRxiv2019

The Role of Proactive and Reactive Cognitive Control for Target Selection in Multiple-Target Search

Cognitive control can involve proactive (preparatory) and reactive (corrective) mechanisms. Using a gaze-contingent eye tracking paradigm combined with fMRI, we investigated the involvement of these different modes of control and their underlying neural networks, when switching between different targets in multiple-target search. Participants simultaneously searched for two possible targets presented among distractors, and selected one of them. In one condition, only one of the targets was available in each display, so that the choice was imposed, and reactive control would be required. In the other condition, both targets were present, giving observers free choice over target selection, and allowing for proactive control. Switch costs emerged only when targets were imposed and not when target selection was free. We found differential levels of activity in the frontoparietal control network depending on whether target switches were free or imposed. Furthermore, we observed core regions of the default mode network to be active during target repetitions, indicating reduced control on these trials. Free and imposed switches jointly activated parietal and posterior frontal cortices, while free switches additionally activated anterior frontal cortices. These findings highlight unique contributions of proactive and reactive control during visual search.

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bioRxiv2018

Beta and theta oscillations differentially support free versus forced control over target selection in visual search

Many important situations require human observers to simultaneously search for more than one object. Despite a long history of research into visual search, the behavioral and neural mechanisms associated with multiple-target search are poorly understood. Here we test the novel theory that the efficiency of looking for multiple targets critically depends on the mode of cognitive control the environment affords to the observer. We used an innovative combination of EEG and eye tracking while participants searched for two targets, within two different contexts: Either both targets were present in the search display and observers were free to prioritize either one of them, thus enabling proactive control over selection; or only one of the two targets would be present in each search display, which requires reactive control to reconfigure selection when the wrong target is prioritized. During proactive control, both univariate and multivariate signals of beta-band (15-35 Hz) power suppression prior to display onset predicted switches between target selections. This signal originated over midfrontal and sensorimotor regions and has previously been associated with endogenous state changes. In contrast, imposed target selections requiring reactive control elicited prefrontal power enhancements in the delta/theta-band (2-8 Hz), but only after display onset. This signal predicted individual differences in associated oculomotor switch costs, reflecting reactive reconfiguration of target selection. The results provide compelling evidence that multiple target representations are differentially prioritized during visual search, and for the first time reveal distinct neural mechanisms underlying proactive and reactive control over multiple-target search.\n\nSignificance StatementSearching for more than one object in complex visual scenes can be detrimental for search performance. While perhaps annoying in daily life, this can have severe consequences in professional settings such as medical and security screening. Previous research has not yet resolved whether multiple-target search involves changing priorities in what people attend to, and how such changes are controlled. We approached these questions by concurrently measuring cortical activity and eye movements using EEG and eye tracking, while observers searched for multiple possible targets. Our findings provide the first unequivocal support for the existence of two modes of control during multiple-target search, which are expressed in qualitatively distinct time-frequency signatures of the EEG both before and after visual selection.

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