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fMRI of the Locus Coeruleus during Slow Wave Sleep.
Tonic firing in the locus coeruleus (LC), the main site of noradrenaline in the brain, is strongly reduced during sleep. However, previous studies in animals and humans have demonstrated phasic activity of the LC during slow wave activity. By reanalyzing data from a previous combined EEG/fMRI sleep study, we characterize the preferential response of the LC to one subtype of slow wave. These findings point to an intriguing role of the LC and noradrenaline in memory consolidation during sleep.
Effective connectivity during resting state after sleep deprivation.
Slow waves are a landmark of deep sleep and are thought to play a key role in preparing our brain to process new information. Slow waves have been shown to travel over the cortex mostly in a frontal-to-caudal direction and a recent study has identified the cingulate cortex as one of their favorite routes. Therefore, we hypothesized that, without the beneficial effect of slow wave activity (SWA) on the previous night, transfer of information along the cingulate cortex might be impaired and, because of the directionality in SWA, the impairment might be more pronounced in one direction than the other.
Task-induced neuronal network connectivity is reactivated during sleep: an MEG study.
Previous research has shown that sleep helps to consolidate recently acquired memory. The mechanisms responsible for this memory boost are, however, still unknown. One hypothesis suggests that connections between neurons that have been activated during a task are re-activated during subsequent sleep. Several lines of evidence have lent support to this hypothesis, but, while intracranial recordings in animals have consistently shown reactivation of recently activated neural networks, results in humans have not been consistent.
In this two-day experiment, MEG signal was acquired from twelve participants performing two tasks which are known to engage two very different memory systems: a procedural learning task (in which they were instructed to mirror-trace a trajectory presented on the screen) and a declarative memory task (in which they were asked to learn the association between a name and a face). After each task, participants were invited to take a nap of a duration of 90 minutes, roughly equivalent to the duration of a sleep cycle. Eight participants managed to reach NREM stage 2 sleep during the MEG scanning session and their data were included in the analysis.
The analysis consists of two steps: (1) identifying networks that were selectively active for one task in comparison to the other and (2) compare the activity of those networks during subsequent sleep. The analysis, which is under way, aims at finding the appropriate measure of intraregional connectivity that tags the formation of memory on a large scale.
Lying awake of insomnia: towards a neurobiology of broken nights and sound sleep (NWO project).
The neurobiology of vigilance disturbances is hardly known. Why do so many people complain of poor sleep even if their polysomnography suggests otherwise, as in 'sleep-state-misperception': Why do others sleep sound and wake up well-rested even during periods of limited sleep or stress? At what threshold of habitual sleep-restriction do health consequences like fatigue and obesity become a risk? Are genetic differences involved and, if so, in what neuronal signaling pathways? (from NWO project description).
Previous Projects
Beta oscillations correlate with probability of perceiving rivalrous visual stimuli. (.pdf, 1.8MB)
Visual illusions are the key to unravel the mechanisms underlying visual perception. A remarkable case of visual illusion is illusory motion reversal (IMR, see an example from Dr. VanRullen's webpage). Under continuous lighting, moving stimuli such as ceiling fans and car wheels can sporadically appear to move in the reverse direction—this phenomenon is known as illusory motion reversal (IMR). We have previously suggested that IMR results from the spurious activation of motion detectors tuned for the opposite direction of motion, leading to a rivalry between two possible motion percepts. To determine if this hypothesis is supported by evidence from electrophysiology, we used EEG to directly compare neural signatures in IMR and binocular rivalry (BR), a well-studied form of rivalry. We find that both IMR and BR show large changes in power in the beta range (14–30 Hz) at the time of a perceptual switch. More importantly, during a stable perception, beta power correlates with the probability of a perception. Specifically, beta power associated with veridical motion perception (experienced the majority of the time) is higher than the power during illusory motion perception (experienced a minority of the time). The BR percepts, each 50% probable, are associated with an intermediate beta amplitude. We propose that the amplitude of synchronized beta activity reflects the size of currently active neural coalitions, with less likely percepts associated with smaller coalitions.
The Time Course of Word Category Ambiguity Processing: an MEG study. (.pdf, 1.45MB)
During my intership at the F.C. Donders Centre for Cognitive Neuroimaging in Nijmegen, I worked on an MEG experiment on ambiguous word processing. Participants were presented, one word at the time, with a sentence which contained a word which could be read as a noun or as a verb. For example, I train every day versus I miss the train. However, we used Dutch, which is much more flexible than English. The ambiguous word was in the 3rd, 4th or 5th position and sentences were constructed in such a way that both (noun/verb) readings were still plausible and equally likely.
We analyzed the MEG signal with both ERFs (Event-related fields, the MEG equivalent of the ERPs) and time-frequency analysis. Results showed that oscillatory activity (in the 20-22 Hz frequency band), but not ERF amplitude, increased when participants read the ambiguous word. However, when the following word, which always disambiguated the meaning of the ambiguous sentence, appeared, we found no changes in the oscillatory activity, but a clear ERF amplitude increase for the ambiguous sentence in comparison to a non-ambiguous control one.
Diagrams and Representation of Reality in C.S. Peirce
During my Bachelor's thesis, I studied how diagrams can represent the reality so faithfully. There is little in common between a house and a sketch of it on paper. Nevertheless, architects and construction workers use this sketch to construct a perfectly solid house. How does a single piece of paper convey so much information, more than our language can?
Based on the work of C.S. Peirce, I investigated what the house and its sketch have in common. Peirce calls it iconicity, roughly defined as the similarity between the sign (the sketch) and its reference (the house). Although intuitively clear, the idea of iconicity in Peirce plays a major role in determining and creating reality, seen as the product of a converging and always-refining process (Scholastic Realism).