Juliana Yordanova

Bulgarian Academy of Sciences | BAS · Institute of Neurobiology

Background Preliminary data suggests that obesity might hasten the decline in mRNA vaccine-induced immunity against SARS-CoV-2. However, whether this renders individuals with obesity more susceptible to long COVID symptoms post-vaccination remains uncertain. Given sleep’s critical role in immunity, exploring the associations between obesity, probab...

Modern world of globalization and unification is of particular interest considering the rapid changes in men's and women's dispositions and engagement in societal institutions. According to these contemporary developments of the psychosocial context, cognitive differences between females and males may also be changing. The aim of this study was to...

The aim of this research was to enhance understanding of the relationship between brief music listening and working memory (WM) functions. The study extends a previous large-scale experiment in which the effects of brief exposure to music on verbal WM were explored. In the present second phase of the experiment, these effects were assessed for the...

It has been demonstrated that during motor responses, the activation of the motor cortical regions emerges in close association with the activation of the medial frontal cortex implicated with performance monitoring and cognitive control. The present study explored the oscillatory neurodynamics of response-related potentials during correct and erro...

The association between nightmare frequency (NMF) and suicidal ideation (SI) is well known, yet the impact of the COVID-19 pandemic on this relation is inconsistent. This study aimed to investigate changes in NMF, SI, and their association during the COVID-19 pandemic. Data were collected in 16 countries using a harmonised questionnaire. The sample...

Accurate measurement of habitual sleep duration (HSD) is crucial for understanding the relationship between sleep and health. This study aimed to assess the bias and agreement limits between two commonly used short HSD self-report methods, considering sleep quality (SQ) and social jetlag (SJL) as potential predictors of bias. Data from 10,268 parti...

Background Self-rated health (SRH) is widely recognized as a clinically significant predictor of subsequent mortality risk. Although COVID-19 may impair SRH, this relationship has not been extensively examined. The present study aimed to examine the correlation between habitual sleep duration, changes in sleep duration after infection, and SRH in s...

Based on previous concepts that a distributed theta network with a central “hub” in the medial frontal cortex is critically involved in movement regulation, monitoring, and control, the present study explored the involvement of this network in error processing with advancing age in humans. For that aim, the oscillatory neurodynamics of motor theta...

Study objectives: Preliminary evidence suggests that the risk of Long COVID is higher among people with pre-existing medical conditions. Based on its proven adjuvant role in immunity, habitual sleep duration may alter the risk for developing Long COVID. The objective of this study was to determine whether the odds of Long COVID are higher amongst...

Objective: Both cognitive and primary motor networks alter with advancing age in humans. The networks activated in response to external environmental stimuli supported by theta oscillations remain less well explored. The present study aimed to characterize the effects of aging on the functional connectivity of response-related theta networks during...

Based on previous concepts that a distributed theta network with a central 'hub' in the medial frontal cortex is critically involved in movement regulation, monitoring, and control, the present study explored the involvement of this network in error processing with advancing age in humans. For that aim, the oscillatory neurodynamics of motor theta...

It has been demonstrated that during motor responses, the activation of the motor cortical regions emerges in close association with the activation of the medial frontal cortex implicated with performance monitoring and cognitive control. The present study explored the oscillatory neurodynamics of response-related potentials during correct and erro...

Event-related neuroelectric oscillations have provided important tools for exploring information processing in the brain. The concept of event-related oscillations (EROs) is linked to that of event-related potentials (ERPs). Both the ERPs and EROs are derived from electroencephalographic (EEG) recordings following the appearance of an event. There...

Short nighttime sleep duration impairs the immune response to virus vaccination, and long nighttime sleep duration is associated with poor health status. Thus, we hypothesized that short (<6 h) and long (>9 h) nighttime sleepers have a higher post-COVID risk than normal nighttime sleepers, despite two doses of mRNA vaccine (which has previously bee...

Many people report suffering from post‐acute sequelae of COVID‐19 or “long‐COVID”, but there are still open questions on what actually constitutes long‐COVID and how prevalent it is. The current definition of post‐acute sequelae of COVID‐19 is based on voting using the Delphi‐method by the WHO post‐COVID‐19 working group. It emphasizes long‐lasting...

Gender can affect nervous system functioning at multiple levels – from genetics to behaviour. In the present review, effects of gender on neurophysiologic mechanisms of mental rotation, efficiency of language processing, emotions and sensory processes were studied in adults by exploring event-related brain potentials (ERPs). Since these processes w...

Objectives According to the core Buddhist psychology models of the “two arrows of pain” and “co-dependent origination,” pain is the resultant of bodily and mental factors, which can be regulated by meditation states and traits. Here we investigated how pain and the related aversion and identification (self-involvement) experiences are modulated by...

This protocol paper describes the second survey produced by the International Covid Sleep Study (ICOSS) group with the aim to examine the associations between SARS-CoV-2 infection and sleep, sleepiness, and circadian problems as potential predisposing factors for more severe COVID-19 disease profile and for development of Long-COVID in the general...

Background: Multi-tasking is usually impaired in older people. In multi-tasking, a fixed order of sub-tasks can improve performance by promoting a time-structured preparation of sub-tasks. How proactive control prioritizes the pre-activation or inhibition of complex tasks in older people has received no sufficient clarification so far. Objective: T...

Research on aesthetic descriptors of art in different languages is scarce. The aim of the present study was to elucidate the conceptual structure of aesthetic experiences of three forms of art (music, visual arts and literature) in the Greek language, which has not been explored so far. It was further aimed to study if biological and cognitive fact...

Meditation practice is suggested to engage training of cognitive control systems in the brain. To evaluate the functional involvement of attentional and cognitive monitoring processes during meditation, the present study analysed the electroencephalographic synchronization of fronto-parietal (FP) and medial-frontal (MF) brain networks in highly exp...

Meditation has been integrated into different therapeutic interventions. To inform the evidence-based selection of specific meditation types it is crucial to understand the neural processes associated with different meditation practices. Here we explore commonalities and differences in electroencephalographic oscillatory spatial synchronisation pat...

It has been suggested that a distributed oscillatory system in the brain operating in the theta (3.5–7 Hz) frequency range plays a major role in coordinating motor actions. The major objective of the present study was to explore the effects of human aging on the neurodynamics of motor-related EEG theta activity during correct motor response generat...

OBJECTIVE: To investigate the prevalence of burnout in high-risk professionals and its association with personal resilience; to determine relationships among resilience level, three burnout dimensions, and environmental stress load; and to establish demographic differences in psychological variables evaluated. METHODS: Data were collected using ps...

Conference paper

Proactive cognition is characterized by the formation and active maintenance of an internal task representation. The aim of this study was to explore if the internal task representation might affect the processing of incoming stimuli. For that aim, the effects of proactive and reactive modes of processing on sensory and cognitive information proces...

Dynamic coupling between slow waves and sleep spindles depends on the sleep stages of NREM sleep (SWS and S2) Juliana Yordanova1,2 *, Roumen Kirov2, Rolf Verleger1,3, Vasil Kolev1,2 1 Department of Neurology, University of Lübeck, Germany 2 Institute of Neurobiology, Bulgarian Academy of Sciences, Sofia, Bulgaria 3 Institute of Psychology II, Uni...

Brief exposure to music has been reported to lead to transient improvement of cognitive functions in no-music domains. Regarding the possible roles of working memory, processing of acoustic regularities, arousal and emotions in mediating the effects of music on subsequent cognition, the present study explored if brief listening to music might produ...

According to the implicit-explicit model of language (Ullman [²]) implicit memory plays a critical role for language acquisition. The model focuses on phonological, morphological and syntactic aspects of language, where structured patterns dominate by postulating that such regular patterns are implicitly incorporated in language functions. In the p...

Co-existent sleep spindles and slow waves have been viewed as a mechanism for offline information processing. Here we explored if the temporal synchronization between slow waves and spindle activity during slow wave sleep (SWS) in humans was modulated by preceding functional activations during pre-sleep learning. We activated differentially the lef...

Study Objectives: The present study explored the sleep mechanisms which may support awareness of hidden regularities. Methods: Before sleep, 53 participants learned implicitly a lateralized variant of the serial response-time task in order to localize sensorimotor encoding either in the left or right hemisphere and induce implicit regularity repres...

Study Objectives The present study explored the sleep mechanisms which may support awareness of hidden regularities. Methods Before sleep, 53 participants learned implicitly a lateralized variant of the serial response-time task in order to localize sensorimotor encoding either in the left or right hemisphere and induce implicit regularity represe...

In the neglect syndrome, the perceptual deficit for contra-lesional hemi-space is increasingly viewed as a dysfunction of fronto-parietal cortical networks, the disruption of which has been described in neuroanatomical and hemodynamic studies. Here we exploit the superior temporal resolution of electroencephalography (EEG) to study dynamic transien...

Only some, but not all, individuals who practice tasks with dual structure, overt and covert, are able to comprehend consciously a hidden regularity. The formation of implicit representations of regularity has been proposed to be critical for subsequent awareness. However, explicit knowledge also has been predicted by the activation of executive co...

Sleep has been identified as a critical brain state enhancing the probability of gaining insight into covert task regularities. Both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep have been implicated with offline re-activation and reorganization of memories supporting explicit knowledge generation. According to two-stage models of...

Whether, and how, explicit knowledge about some regularity arises from implicit sensorimotor learning by practice has been a matter of long-standing debate. Previously, we had found in the number reduction task that participants who will acquire explicit knowledge differ from other participants in their event-related potentials (ERPs) already at ta...

In recent years, vibrant research has developed on "consolidation" during sleep: To what extent are newly experienced impressions reprocessed or even restructured during sleep? We used the number reduction task (NRT) to study if and how sleep does not only reiterate new experiences but may even lead to new insights. In the NRT, covert regularities...

Previous studies have found that event-related theta and gamma oscillations elicited in an auditory selective attention task are deviant in children with attention deficit/hyperactivity disorder (ADHD). It has been suggested that these deviations are associated with deficient motor inhibition in ADHD, which may lead to increased excitability of not...

Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) have been used to study the neural correlates of reward anticipation, but the interrelation of EEG and fMRI measures remains unknown. The goal of the present study was to investigate this relationship in response to a well established reward anticipation paradigm using si...

To compare sleep electroencephalographic patterns and psychological functioning of healthy adolescents running regularly in the mornings with those of control subjects. Although several studies have shown that regular moderate-to-vigorous exercise is related to favorable sleep and psychological functioning in adolescents, research on the effectiven...

Sleep problems are a prominent feature in children with attention-deficit/hyperactivity disorder (ADHD) but their relationships to sleep structure are not consistent across studies. We aimed at further examining the sleep architecture in children with ADHD, while considering the role of the first-night effect (FNE) as a possible confounder. Twenty...

In addition to active wake, emotions are generated and experienced in a variety of functionally different states such as those of sleep, during which external stimulation and cognitive control are lacking. The neural basis of emotions can be specified by regarding the multitude of emotion-related brain states, as well as the distinct neuro- and psy...

Seeking for the mechanisms by which methylphenidate (MPH) improves behavior has demonstrated that MPH modulates excitability in the primary motor cortex. However, little is known about the influence of MPH on top-down controlled mechanisms in the sensory domain. The present study explored the effects of MPH on the activation of visual cortices in h...

The number reduction task (NRT) allows us to study the transition from implicit knowledge of hidden task regularities to explicit insight into these regularities. To identify sleep-associated neurophysiological indicators of this restructuring of knowledge representations, we measured frequency-specific power of EEG while participants slept during...

The maintenance of stable goal-directed behaviour is a hallmark of conscious executive control in humans. Notably, both correct and error human actions may have a subconscious activation-based determination. One possible source of subconscious interference may be the default mode network that, in contrast to attentional network, manifests intrinsic...

Although the performance of children with attention-deficit/hyperactivity disorder (ADHD) is impaired in a variety of cognitive tasks, the specific capacity of strategic readaptation after errors as a source of behavioral deficits is not sufficiently understood. This study used an extended and refined behavioral parameterization to assess performan...

Previous research has indicated that information acquired before sleep gets consolidated during sleep. This process of consolidation might be reflected after sleep in changed extent and topography of cortical activation during retrieval of information. Here, we designed an experiment to measure those changes by means of slow event-related EEG poten...

The aim of the present study was to explore the neurophysiologic origins of gender differences in auditory processing mechanisms of 7-10 year-old children by means of event-related oscillations. It was tested if the developmental changes in synchronization and magnitude of oscillations in different processing conditions depended on gender. Eighteen...

Behavioral adaptation depends on the recognition of response errors and processing of this error-information. Error processing is a specific cognitive function crucial for behavioral adaptation. Neurophysiologically, these processes are reflected by an event-related potential (ERP), the error negativity (Ne/ERN). Even though synchronization process...

Cramer et al.'s network approach reconceptualizes mental comorbidity on the basis of symptom space originating from psychometric signatures. We argue that the advantages of this approach need to be regarded in the context of the multi-level functional organization of the neural substrate, ranging from neurogenetic to psychometric. Neuroelectric osc...

Solving a task with insight has been associated with occipital and right-hemisphere activations. The present study tested the hypothesis if sleep-related alterations in functional activation states modulate the probability of insight into a hidden abstract regularity of a task. State-dependent functional activation was measured by beta and alpha el...

Effective orienting of attention towards novel events is crucial for survival, particularly if they occur in a dangerous situation. This is why stimuli with emotional value are more efficient in capturing attention than neutral stimuli, and why the processing of unexpected novel stimuli is enhanced under a negative emotional context. Here we measur...

The objective of the present study was to evaluate patterns of implicit processing in a task where the acquisition of explicit and implicit knowledge occurs simultaneously. The number reduction task (NRT) was used as having two levels of organization, overt and covert, where the covert level of processing is associated with implicit associative and...

The present study assessed the effects of awareness at encoding on off-line learning during sleep. A new framework is suggested according to which two aspects of awareness are distinguished: awareness of task information, and awareness of task processing. The number reduction task (NRT) was employed because it has two levels of organization, an ove...

Effective orienting of attention towards novel events is crucial for survival, particularly if they occur in a dangerous situation. This is why stimuli with emotional value are more efficient in capturing attention than neutral stimuli, and why the processing of unexpected novel stimuli is enhanced under a negative emotional context. Here we measur...

Grand average event-related slow potentials (SPs) for the early- (Early-NG) and late-night group (Late-NG). Time dynamics of group mean values for three regions of interest (ROIs: left fronto-temporal, LFT; left central, LC; left occipito-parietal, LOP) is presented at the two most-left panels. Amplitudes are min-max normalized and presented as per...

Supplemental information concerning the different effects of early- and late-night sleep on the spatial reorganization of slow negative potentials for unpredictable and predictable responses in the number reduction task. (0.04 MB DOC)

Background: There is evidence that slow wave sleep (SWS) promotes the consolidation of memories that are subserved by mediotemporal- and hippocampo-cortical neural networks. In contrast to implicit memories, explicit memories are accompanied by conscious (attentive and controlled) processing. Awareness at pre-sleep encoding has been recognized as...

Response processing may comprise multiple systems working in parallel at different functional levels of performance monitoring. In time-frequency decompositions of response-locked event-related potentials from adults, a subprocess operating in the delta frequency band was interpreted as an index of cognitive error monitoring, distinguishable from a...

Neuroelectric oscillations provide important tools to study information processing in the brain. In this paper, major concepts and advantages of event-related oscillations (EROs) are considered, with a focus on their relevance for developmental research. Findings from previous studies in passive and oddball conditions are summarized to demonstrate...