Biomedical subjects

Juergen Fell

Publications and source records attributed to Juergen Fell.

13 recordsLinked to original sources

PubMed2006

Rhinal-hippocampal coupling during declarative memory formation: dependence on item characteristics.

Lesion and imaging studies have demonstrated that encoding and retrieval of declarative memories, i.e. consciously accessible events and facts, depend on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Analysis of intracranially recorded EEG in presurgical epilepsy patients revealed that successful memory formation is accompanied within one second by a transient enhancement and later decrease of Rhinal-hippocampal phase synchronization in the gamma range, as well as enhanced connectivity in the low-frequency range. In these studies, words with a high frequency of occurrence were used as stimulus material. Here, we re-examined these effects in another group of 10 presurgical epilepsy patients, this time not only for high-frequency, but also for low-frequency words. For successfully memorized compared to later forgotten high-frequency words we again observed an early phase coupling and later decoupling within the gamma range, as well as enhanced coupling within the sub-gamma range. However, for remembered as compared to forgotten low-frequency words clear synchronization increases were only observed for the delta band, but not for the gamma band. Our data suggest, that broadband Rhinal-hippocampal coupling including the gamma range only occurs, when significant semantic associations are processed within rhinal cortex, as is the case for high-frequency words.

Adult↗

PubMed2006

Memory formation by neuronal synchronization.

Cognitive functions not only depend on the localization of neural activity, but also on the precise temporal pattern of activity in neural assemblies. Synchronization of action potential discharges provides a link between large-scale EEG recordings and cellular plasticity mechanisms. Here, we focus on the role of neuronal synchronization in different frequency domains for the subsequent stages of memory formation. Recent EEG studies suggest that synchronized neural activity in the gamma frequency range (around 30-100 Hz) plays a functional role for the formation of declarative long-term memories in humans. On the cellular level, gamma synchronization between hippocampal and parahippocampal regions may induce LTP in the CA3 region of the hippocampus. In order to encode spatial locations or sequences of multiple items and to guarantee a defined temporal order of memory processing, synchronization in the gamma frequency range has to be accompanied by a stimulus-locked phase reset of ongoing theta oscillations. Simultaneous gamma- and theta-dependent plasticity leads to complex learning rules required for realistic declarative memory formation. Subsequently, consolidation of declarative memories may occur via replay of newly acquired patterns in so-called sharp wave-ripple complexes, predominantly during slow-wave sleep. These irregular bursts induce longer lasting forms of synaptic plasticity in output regions of the hippocampus and in the neocortex. In summary, synchronization of neural assemblies in different frequency ranges induces specific forms of cellular plasticity during subsequent stages of memory formation.

Action Potentials↗

PubMed2005

Rhinal-hippocampal connectivity determines memory formation during sleep.

Compared with waking state attention, volition and semantic processing play a minor role during sleep. Thus, investigating declarative memory formation during sleep may allow us to isolate mnemonic core processes. The most feasible approach to memory formation during sleep is the analysis of dream memories. Lesion and imaging studies have demonstrated that encoding of declarative memories, i.e. consciously accessible events and facts, depends on operations within the rhinal cortex and the hippocampus, two substructures of the medial temporal lobe. Successful memory formation is accompanied by a transient rhinal-hippocampal interaction. Consequently, the ability to memorize dreams may be related to mediotemporal connectivity. Therefore, we recorded EEG during sleep from rhinal and hippocampal depth electrodes implanted in 12 epilepsy patients (eight women, mean age 41.1 +/- 6.4 years). They were awakened during rapid eye movement sleep (REM) and asked to recall their dream. Via coherence analyses we show that rhinal-hippocampal connectivity values are approximately twice as large for patients with good dream recall versus those patients with poor recall. This suggests that rhinal-hippocampal connectivity is a key factor in determining declarative memory formation.

Adult↗

PubMed2005

Language processing within the human medial temporal lobe.

Although the hippocampal formation is essential for verbal memory, it is not fully understood how it contributes to language comprehension. We recorded event-related potentials (ERPs) directly from two substructures of the medial temporal lobe (MTL), the rhinal cortex and the hippocampus proper, while epilepsy patients listened to sentences that either were correct or contained semantic or syntactic violations. Semantic violations elicited a large negative ERP response peaking at approximately 400 ms in the rhinal cortex. In contrast, syntactically incorrect sentences elicited a negative deflection of 500-800 ms in the hippocampus proper. The results suggest that functionally distinct aspects of integration in language comprehension are supported by different MTL structures: the rhinal cortex is involved in semantic integration, whereas the hippocampus proper subserves processes of syntactic integration. An analysis of phase synchronization within the gamma band between rhinal and hippocampal recording sites showed that both of the above-mentioned ERP components were preceded by an increase of phase synchronization. In contrast to these short phasic increases of phase synchronization in both violation conditions, correct sentences were associated with a long-lasting synchronization in a late time window, possibly reflecting the integration of semantic and syntactic information as required for normal comprehension.

Adult↗

PubMed2005

Phase/amplitude reset and theta-gamma interaction in the human medial temporal lobe during a continuous word recognition memory task.

We analyzed intracranial electroencephalographic (EEG) recordings from the medial temporal lobes of 12 epilepsy patients during a continuous word recognition paradigm, contrasting trials of correctly recognized repeated words (hits) and correctly identified new words (correct rejections). Using a wavelet-based analysis, we investigated how power changes and phase clustering in different frequency bands contribute to the averaged event-related potentials (ERPs). In addition, we analyzed the actual mean phases of the different oscillations. Our analyses yielded the following results: (1) power changes contributed significantly only to the late components of the ERPs (>400 ms) (2) earlier ERP components were produced by a stimulus-related broad-band phase and amplitude reset of ongoing oscillatory activity about 190 ms after stimulus onset that involved not only the theta band, but also covered alpha and lower beta band frequencies (3) phase and amplitude reset occurred during an epoch of increased phase entrainment over time that lasted for about two oscillation periods for all involved frequencies and was more pronounced for correct rejections than for hits. The broad-band phase and amplitude reset was observed for both hits and correct rejections, and therefore, did not appear to support a specific cognitive function, but rather to act as a general facilitating factor for the processes involved in this memory task. Further analyses of synchronization between oscillations and power changes in different frequency bands revealed a task-dependent modulation of gamma activity by the entrained theta cycle, a mechanism potentially related to memory encoding and retrieval in the rhinal cortex and hippocampus, respectively.

Adult↗

PubMed2004

Phase-locking characteristics of limbic P3 responses in hippocampal sclerosis.

Amplitudes of the P3 recorded invasively from the medial temporal lobe (MTL-P3) have been reported to be reduced on the side of a mediotemporal epileptogenic focus. This reduction has been attributed to the massive cell loss within the hippocampus associated with hippocampal sclerosis. It has remained unclear how functional connectivity between the hippocampus and rhinal cortex, as well as within the hippocampus, is altered in hippocampal sclerosis. To investigate this issue, we analyzed to what extent stimulus-related phase-locking and power changes within the low-frequency range (2-30 Hz) and within the gamma band (32-48 Hz), as well as rhinal-hippocampal phase synchronization contribute to the averaged MTL-P3 potentials. Event-related responses were recorded via bilateral depth electrodes in epilepsy patients with unilateral hippocampal sclerosis, who performed a visual oddball experiment. On the contralateral (nonsclerotic) side, successful target detection was associated with an increase of power and phase locking of hippocampal activity in both the low-frequency range and in the gamma range. Besides, there were rhinal-hippocampal synchronization enhancements in the theta and gamma range. On the ipsilateral (sclerotic) side, the event-related power increase in the low-frequency range had almost disappeared, a finding likely to be explained by the loss of principle neurons. However, low-frequency phase-locking, rhinal-hippocampal synchronization, as well as event-related power changes in the gamma range persisted ipsilaterally, although there were differences in temporal and spectral characteristics. These findings support the hypothesis that functional connectivity between hippocampus and rhinal cortex, as well as intrahippocampal connectivity, are partially preserved in hippocampal sclerosis.

Adult↗

PubMed2004

Identifying neural correlates of consciousness: the state space approach.

This article sketches an idealized strategy for the identification of neural correlates of consciousness. The proposed strategy is based on a state space approach originating from the analysis of dynamical systems. The article then focuses on one constituent of consciousness, phenomenal awareness. Several rudimentary requirements for the identification of neural correlates of phenomenal awareness are suggested. These requirements are related to empirical data on selective attention, on completely intrinsic selection and on globally unconscious states. As an example, neuroscientific findings on synchronized gamma activity are categorized according to these requirements.

Attention↗

PubMed2004

Do neural correlates of consciousness cause conscious states?

Neural correlates of consciousness (NCCs) have been defined as neural systems and states of those systems, which are correlated with certain states of consciousness and which are minimally sufficient for these states of consciousness to occur. Here, it is discussed whether based on the identification of a NCC one may claim that this neural correlate causes the corresponding state of consciousness. In a strict sense, neural states and corresponding states of consciousness should occur simultaneously according to the definition of NCCs. However, two causally related events should occur consecutively in time according to the general view. Thus, within the NCC framework the question of a causal psychoneural relationship is only valid under the premise of a small and practically unobservable time shift between neural states and corresponding states of consciousness. The term causality is usually applied either in the sense of efficient causation or in the sense of explanatory causation. Efficient causality addresses the physical relation of two events and the criterion of an energy exchange between both events. Explanatory causality, on the other hand, refers to the law-like character of the conjoined occurrence of two events. Under a dualistic point of view, the interpretation that neural states effectively cause consciousness would not be justified, since effective causation, as understood in physical sciences, should be associated with energy transfer. But such an energy transfer between the domain of consciousness and the neural domain would violate presently accepted physical laws. Therefore, it is argued that one may state that neural states cause states of consciousness only in the broad sense of explanatory causality with regard to the observation of a law-like psychoneural relation. Within the viewpoint of identity theory, however, one would rather claim that certain states of consciousness basically are neural states.

Causality↗

PubMed2004

Neural bases of cognitive ERPs: more than phase reset.

Up to now, two conflicting theories have tried to explain the genesis of averaged event-related potentials (ERPs): Whereas one hypothesis claims that ERPs originate from an event-related activation of neural assemblies distinct from background dynamics, the other hypothesis states that ERPs are produced by phase resetting of ongoing oscillatory activity. So far, this question has only been addressed for early ERP components. Late ERP components, however, are generally thought to represent superimposed activities of several anatomically distinct brain areas. Thus, the question of which mechanism underlies the genesis of late ERP components cannot be easily answered based on scalp recordings. In contrast, two well-investigated late ERP components recorded invasively from within the human medial temporal lobe (MTL) in epilepsy patients, the so-called MTL-P300 and the anterior MTL-N400 (AMTL-N400), are based on single source activity. Hence, we investigated whether the MTL-P300 and the AMTL-N400 are based on an event-related activity increase, a phase reset of ongoing oscillatory activity or both. ERPs were recorded from the hippocampus and rhinal cortex in subjects performing a visual oddball paradigm and a visual word recognition paradigm. With wavelet techniques, stimulus-related phase-locking and power changes were analyzed in a frequency range covering 2 to 48 Hz. We found that the MTL-P300 is accompanied by both phase reset and power increase and that both effects overlap partly in time. In contrast, the AMTL-N400 is initially associated with phase locking without power increase and only later during the course of the AMTL-N400 we observed an additional power increase. In conclusion, both aspects, event-related activation of neural assemblies and phase resetting of ongoing activity seem to be involved in the generation of late ERP components as recorded in cognitive tasks. Therefore, separate analysis of event-related power and phase-locking changes might reveal specific insights into the mechanisms underlying different cognitive functions.

Adult↗

PubMed2003

Is synchronized neuronal gamma activity relevant for selective attention?

Today, much evidence exists that sensory feature binding is accomplished by phase synchronization of induced neuronal gamma activity (30-80 Hz). Recent studies furthermore suggest that phase synchronization of induced gamma activity may represent a general mechanism enabling transient associations of neural assemblies and thus may play a central role in cortical information processing. Here, we describe findings indicating that synchronized gamma activity is moreover specifically involved in selective attention. While feature binding appears to depend primarily on induced gamma synchronization, attentional processes seem to involve both induced and evoked gamma oscillations. Yet it is still an open question, as to which top-down and bottom-up processes are associated with attentional modulation of gamma activity. A possible mechanism to project influences from attentional control structures to areas concerned with stimulus representation and vice versa, may be neuronal synchronization and the resulting firing rate changes of coincidence-detecting neurons in target areas.

Animals↗

PubMed2003

More than synchrony: EEG chaoticity may be necessary for conscious brain functioning.

Transient phase synchronization of EEG oscillations was reported to be essential for conscious brain activity. However, it has been argued that EEG synchrony is not the only necessary factor, but at least must be accompanied by a sufficient complexity of brain dynamics. Here, we propose that, besides complexity, EEG chaoticity may be another necessary condition for conscious brain functioning.

Brain↗

PubMed2003

Rhinal-hippocampal theta coherence during declarative memory formation: interaction with gamma synchronization?

The hippocampus and the rhinal cortex, two substructures of the medial temporal lobe, together play a crucial role in human declarative memory formation. To investigate in detail the mechanism connecting these two structures transiently during memory formation we recorded depth EEG in epilepsy patients from within the hippocampus and the rhinal cortex. During this recording, patients performed a single-trial word list-learning paradigm with a free recall memory test following a distraction task. Rhinal-hippocampal EEG coherence and spectral power at both locations in the time interval up to 2 s after onset of word presentation were analysed in the frequency range 1-19 Hz. Successful as opposed to unsuccessful memory formation was associated with a general rhinal-hippocampal coherence enhancement, but without alterations in spectral power. Coherence increases in the theta range were correlated with the previously reported memory-related changes in rhinal-hippocampal gamma phase synchronization. This correlation may suggest an interaction of the two mechanisms during declarative memory formation. While theta coherence might be associated with slowly modulated coupling related to an encoding state, rhinal-hippocampal gamma synchronization may be more closely related to actual memory processes by enabling fast coupling and decoupling of the two structures.

Adult↗

PubMed2003

Rhinal-hippocampal EEG coherence is reduced during human sleep.

The deficiency of declarative memory compared with waking state is an often overlooked characteristic of sleep. Here, we investigated whether rhinal-hippocampal coherence, an electrophysiological correlate of declarative memory formation, is significantly altered during sleep as compared with waking state. For this purpose, we analysed recordings of intracranial EEG activity during sleep obtained directly from within the medial temporal lobe in patients with unilateral temporal lobe epilepsy. We found a general reduction of rhinal-hippocampal EEG coherence during sleep compared with waking state, which was most pronounced within the upper gamma bands (average decrease up to 56%). The observed coherence changes clearly differ from findings reported for surface EEG data and thus appear to be specific for the medial temporal lobe. The decrease of rhinal-hippocampal EEG coherence from waking state towards sleep may yield an electrophysiological explanation for the sleep-related deficiency of declarative memory.

Adult↗