Biology subjects

Akyurek, E. G.

Publications and source records attributed to Akyurek, E. G..

8 recordsLinked to original sources

bioRxiv2025

Sustained alpha oscillations serve attentional prioritization in working memory, not maintenance

Recent theory on the neural basis of working memory (WM) has attributed an important role to "activity-silent" or -quiescent mechanisms, suggesting that sustained neural activity might not be essential in the retention of information. This idea has been challenged by reports of ongoing neural activity in the alpha band during WM maintenance, however. The precise role of these alpha oscillations is unclear: Do they reflect attentional prioritization of stored information, or do they serve as a general maintenance mechanism, for instance to periodically refresh synaptic traces? To address this, we designed a visual WM task involving two memory items, one of which was prioritized by being tested first for recall. The task included both short (1 s) and a long (3 s) delay intervals between encoding and retrieval. The long delay condition allowed us to test whether the alpha-based decoding effects persist beyond the early delay period, thereby putting accounts that attribute alpha activity to generic maintenance processes to the test. Time-resolved decoding analyses revealed that both tested-first and tested-second items were initially decodable following stimulus presentation. However, only the tested-first item exhibited sustained decodability throughout the delay, particularly in the long delay condition, where it transitioned into a stable coding scheme. This prolonged representation was selectively supported by induced alpha power, which reliably tracked the prioritized tested-first item, but not the deprioritized tested-second one. Impulse-based decoding further confirmed this asymmetry, showing a selective increase in readout for the tested-second item only when it became immediately task-relevant. Together, these findings suggest that sustained alpha-band activity primarily reflects attentional prioritization, rather than general memory maintenance. Unattended, deprioritized items appear to transition into an activity-quiescent state, consistent with models of synaptic storage in WM.

neuroscience↗

bioRxiv2024

The acute effects of cocoa flavanols on cognitive control and response inhibition: A randomised crossover trial

In this pre-registered study, we investigated the effects of acute cocoa flavanol (CF) consumption on cognitive control and response inhibition processes, at two different dosage levels. This study was randomised, placebo-controlled, gender-balanced, double-blind, and utilised a crossover design. Participants consumed three different drinks across three separate sessions: A placebo drink with alkalised cocoa powder, a low dosage (415 mg), and a medium dosage (623 mg) of cocoa flavanols from flavanol-rich cocoa powder. Following the administration of these treatment conditions, participants were tested in the Flanker, Simon, and Go/No-go tasks in a counterbalanced order in each session. We analysed accuracy and response times from incongruent and congruent trials of the Simon and Flanker tasks, and commission errors, omission errors, and response times for the Go/No-go task. In addition to these main measurements, we considered interference and sequence effects, accounting for the influence of previous trials in Simon and Flanker tasks. The acute effects of CF on cognitive control and response inhibition were examined using (Generalised) Linear Mixed Model analysis, which included random intercepts, fixed effects, and random slopes. Analysis results revealed that neither dose of cocoa flavanols consumption acutely improved accuracies, interferences, errors, or response times in these three tasks. Furthermore, neither the gender of participants nor BMI scores predicted their cognitive control and response inhibition functions in addition to the treatment conditions. Our findings suggest that acute consumption of cocoa flavanols does not significantly enhance cognitive control or response inhibition in healthy young adults.

neuroscience↗

bioRxiv2023

Behaviorally irrelevant feature matching increases neural and behavioral working memory readout: Support for activity-silent working memory

There is an ongoing debate about whether working memory (WM) maintenance relies on persistent activity and/or short-term synaptic plasticity. This is a challenging question, because neuroimaging techniques in cognitive neuroscience measure activity only. Recently, neural perturbation techniques have been developed to tackle this issue, such as visual impulse perturbation or "pinging", which reveals (un)attended WM content during maintenance. There are contrasting explanations of how pinging reveals WM content, which is central to the debate. Pinging could reveal mnemonic representations by perturbing content-specific networks or by increasing the neural signal-to-noise ratio of active neural states. Here we tested the extent to which the neural impulse response is patterned by the WM network, by presenting two different impulse stimuli. If the impulse interacts with WM networks, the WM-specific impulse response should be enhanced by physical overlap between the initial memory item and the subsequent external perturbation stimulus. This prediction was tested in a working memory task by matching or mismatching task-irrelevant spatial frequencies between memory items and impulse stimuli, as well as probes. Matching probe spatial frequency with memory items resulted in faster behavioral response times and matching impulse spatial frequency with memory items increased the specificity of the neural impulse response as measured from EEG. Matching spatial frequencies did neither result in globally stronger neural responses nor in a larger decrease in trial-to-trial variability compared to mismatching spatial frequencies. The improved neural and behavioural readout of irrelevant feature matching provide evidence that impulse perturbation interacts directly with the memory representations.

neuroscience↗

bioRxiv2022

The effects of gamma-aminobutyric acid (GABA) on working memory and attention: A randomized, double-blind, placebo-controlled, crossover trial

Background{gamma}-aminobutyric acid (GABA) is a primary inhibitory neurotransmitter that plays a significant role in the central nervous system. Studies on both animals and humans show it has the pharmacological potential for reducing the impact of cognitive disorders, as well as enhancing cognitive functions and mood. However, its specific effects on human attention and working memory have not yet been extensively studied. AimsIn this randomised, double-blind, placebo-controlled, and crossover trial, we aimed to test whether the administration of 800 mg GABA, dissolved in a drink, acutely affected visual working memory maintenance, as well as temporal and spatial attention in healthy adults. MethodsThe participants were 32 young adults (16 females and 16 males). Working memory recall precision, spatial attention and temporal attention were measured by a delayed match-to-sample task, a visual search task, and a speeded rapid serial visual presentation task, respectively. Participants completed two experimental sessions (GABA and Placebo) in randomized and counterbalanced order. In each session, forty-five minutes after administration of the drink, they completed the all three of the aforementioned cognitive tasks. ResultsLinear mixed model analysis results showed that GABA increased visual search time, compared to the placebo, but did not affect visual search accuracy, temporal attention, nor visual working memory precision. ConclusionsThe results suggest that GABA increases visual search time but does not affect temporal attention and memory, and that previously reported effects on cognition might rely on other functions.

neuroscience↗

bioRxiv2019

A functional spiking-neuron model of activity-silent working memory in humans based on calcium-mediated short-term synaptic plasticity

In this paper, we present a functional spiking-neuron model of human working memory (WM). This model combines neural firing for encoding of information with activity-silent maintenance. While it used to be widely assumed that information in WM is maintained through persistent recurrent activity, recent studies have shown that information can be maintained without persistent firing; instead, information can be stored in activity-silent states. A candidate mechanism underlying this type of storage is short-term synaptic plasticity (STSP), by which the strength of connections between neurons rapidly changes to encode new information. To demonstrate that STSP can lead to functional behavior, we integrated STSP by means of calcium-mediated synaptic facilitation in a large-scale spiking-neuron model and added a decision mechanism. The model was used to simulate a recent study that measured behavior and EEG activity of participants in three delayed-response tasks. In these tasks, one or two visual gratings had to be maintained in WM, and compared to subsequent probes. The original study demonstrated that WM contents and its priority status could be decoded from neural activity elicited by a task-irrelevant stimulus displayed during the activity-silent maintenance period. In support of our model, we show that it can perform these tasks, and that both its behavior as well as its neural representations are in agreement with the human data. We conclude that information in WM can be effectively maintained in activity-silent states by means of calcium-mediated STSP. Author SummaryMentally maintaining information for short periods of time in working memory is crucial for human adaptive behavior. It was recently shown that the human brain does not only store information through neural firing - as was widely believed - but also maintains information in activity-silent states. Here, we present a detailed neural model of how this could happen in our brain through short-term synaptic plasticity: rapidly adapting the connection strengths between neurons in response to incoming information. By reactivating the adapted network, the stored information can be read out later. We show that our model can perform three working memory tasks as accurately as human participants can, while using similar mental representations. We conclude that our model is a plausible and effective neural implementation of human working memory.

neuroscience↗

bioRxiv2019

Two faces of the attentional blink: Gradual and discrete loss of perceptual awareness

In a series of experiments, the nature of perceptual awareness during the attentional blink was investigated. Previous work has considered the attentional blink as a discrete, all-or-none phenomenon, indicative of general access to conscious awareness. Using continuous report measures in combination with mixture modeling, the outcomes showed that perceptual awareness during the attentional blink can be a gradual phenomenon. Awareness was not exclusively discrete, but also exhibited a gradual characteristic whenever the spatial extent of attention induced by the first target spanned more than a single location. Under these circumstances, mental representations of blinked targets were impoverished, but did approach the actual identities of the targets. Conversely, when the focus of attention covered only a single location, there was no evidence for any partial knowledge of blinked targets. These two different faces of awareness during the attentional blink challenge current theories of both awareness and temporal attention, which cannot explain the existence of gradual awareness of targets during the attentional blink. To account for the current outcomes, an adaptive gating model is proposed that casts awareness on a continuum between gradual and discrete, rather than as being of either single kind.

neuroscience↗

bioRxiv2019

Drifting codes within a stable coding scheme for working memory

Working memory (WM) is important to maintain information over short time periods to provide some stability in a constantly changing environment. However, brain activity is inherently dynamic, raising a challenge for maintaining stable mental states. To investigate the relationship between WM stability and neural dynamics, we used electroencephalography to measure the neural response to impulse stimuli during a WM delay. Multivariate pattern analysis revealed representations were both stable and dynamic: there was a clear difference in neural states between time-specific impulse responses, reflecting dynamic changes, yet the coding scheme for memorized orientations was stable. This suggests that a stable subcomponent in WM enables stable maintenance within a dynamic system. A stable coding scheme simplifies readout for WM-guided behaviour, whereas the low-dimensional dynamic component could provide additional temporal information. Despite having a stable subspace, WM is clearly not perfect - memory performance still degrades over time. Indeed, we find that even within the stable coding scheme, memories drift during maintenance. When averaged across trials, such drift contributes to the width of the error distribution.

neuroscience↗

bioRxiv2019

Impulse responses reveal unimodal and bimodal access to visual and auditory working memory

It is unclear to what extent sensory processing areas are involved in the maintenance of sensory information in working memory (WM). Previous studies have thus far relied on finding neural activity in the corresponding sensory cortices, neglecting potential activity-silent mechanisms such as connectivity-dependent encoding. It has recently been found that visual stimulation during visual WM maintenance reveals WM-dependent changes through a bottom-up neural response. Here, we test whether this impulse response is uniquely visual and sensory-specific. Human participants (both sexes) completed visual and auditory WM tasks while electroencephalography was recorded. During the maintenance period, the WM network was perturbed serially with fixed and task-neutral auditory and visual stimuli. We show that a neutral auditory impulse-stimulus presented during the maintenance of a pure tone resulted in a WM-dependent neural response, providing evidence for the auditory counterpart to the visual WM findings reported previously. Interestingly, visual stimulation also resulted in an auditory WM-dependent impulse response, implicating the visual cortex in the maintenance of auditory information, either directly, or indirectly as a pathway to the neural auditory WM representations elsewhere. In contrast, during visual WM maintenance only the impulse response to visual stimulation was content-specific, suggesting that visual information is maintained in a sensory-specific neural network, separated from auditory processing areas.\n\nSignificance StatementWorking memory is a crucial component of intelligent, adaptive behaviour. Our understanding of the neural mechanisms that support it has recently shifted: rather than being dependent on an unbroken chain of neural activity, working memory may rely on transient changes in neuronal connectivity, which can be maintained efficiently in activity-silent brain states. Previous work using a visual impulse stimulus to perturb the memory network has implicated such silent states in the retention of line orientations in visual working memory. Here, we show that auditory working memory similarly retains auditory information. We also observed a sensory-specific impulse response in visual working memory, while auditory memory responded bi-modally to both visual and auditory impulses, possibly reflecting visual dominance of working memory.

neuroscience↗