Juliana Fell - Academia.edu (original) (raw)

Papers by Juliana Fell

Research paper thumbnail of The neural dynamics of memory consolidation in the human brain: direct evidence by intracranial EEG

Frontiers in Human Neuroscience, 2008

Research paper thumbnail of Intracranial electroencephalography power and phase synchronization changes during monaural and binaural beat stimulation

The European journal of neuroscience, 2015

Auditory stimulation with monaural or binaural auditory beats (i.e. sine waves with nearby freque... more Auditory stimulation with monaural or binaural auditory beats (i.e. sine waves with nearby frequencies presented either to both ears or to each ear separately) represents a non-invasive approach to influence electrical brain activity. It is still unclear exactly which brain sites are affected by beat stimulation. In particular, an impact of beat stimulation on mediotemporal brain areas could possibly provide new options for memory enhancement or seizure control. Therefore, we examined how electroencephalography (EEG) power and phase synchronization are modulated by auditory stimulation with beat frequencies corresponding to dominant EEG rhythms based on intracranial recordings in presurgical epilepsy patients. Monaural and binaural beat stimuli with beat frequencies of 5, 10, 40 and 80 Hz and non-superposed control signals were administered with low amplitudes (60 dB SPL) and for short durations (5 s). EEG power was intracranially recorded from mediotemporal, temporo-basal and tempo...

Research paper thumbnail of Retrieval from the Brain's Perspective

Frontiers in human neuroscience, 2012

HUMAN NEUROSCIENCE that implicit memory can drive behavioral responses on explicit memory tests a... more HUMAN NEUROSCIENCE that implicit memory can drive behavioral responses on explicit memory tests aiming to measure recollection and familiarity, which may cause distorted results. More specifically, apparent familiarity signals may in many cases be actually due to conceptual priming. This corrupts the interpretation of activation patterns during familiarity judgments and renders reverse inference problematic (Poldrack, 2006). Other researchers have pointed to the fact that a separation between brain structures supporting recollection and familiarity is often confounded by memory strength (e.g., Wixted and Squire, 2010). It may be argued that, for instance, the categories of item vs. source memory can be more clearly operationalized and may better distinguish between involved brain structures and neural processes (e.g., Staresina and Davachi, 2008). While the assessment of recollection and familiarity typically relies on introspective reports that are inherently subjective, source memory can be objectively assessed. Recently, Henke (2010) moved beyond this methodological point and argued that in principle, memory operations should be characterized by different processing modes, and not by the involvement of consciousness (explicit vs. implicit) or other phenomenal criteria. This line of reasoning was strongly motivated by neuroimaging studies suggesting that the criterion for the recruitment of a specific brain structure is related to its computational role rather than to the phenomenal experiences during the memory task. According to her new taxonomy, the hippocampus, for instance, accomplishes rapid encoding of flexible associations, while the parahippocampal gyrus supports rapid encoding of single items. Compared to the explicit vs. implicit dichotomy, the advantage of this classification is that its relation to different brain structures is less ambiguous.

Research paper thumbnail of Discrimination of sleep stages: a comparison between spectral and nonlinear EEG measures

Electroencephalography and clinical neurophysiology, 1996

During recent years, methods from nonlinear dynamics were introduced into the analysis of EEG sig... more During recent years, methods from nonlinear dynamics were introduced into the analysis of EEG signals. Although from a theoretical point of view nonlinear measures quantify properties being independent from conventional spectral measures, it is a crucial question whether in practice nonlinear EEG measures yield additional information, which is not redundant to the information gained by spectral analysis. Therefore, we compared the ability of several spectral and nonlinear measures to discriminate different sleep stages. We evaluated spectral measures (relative delta power, spectral edge, spectral entropy and first spectral moment), and nonlinear measures (correlation dimension D2, largest Lyapunov exponent LI, and approximated Kolmogorof entropy K2), and additionally the stochastic time domain based measure entropy of amplitudes. For 12 healthy subjects these measures were calculated from sleep EEG segments of 2:44 min duration, each segment unambiguously corresponding to one of the...

Research paper thumbnail of Nonlinear EEG dynamics during sleep in depression and schizophrenia

The International journal of neuroscience, 1994

The article deals with the question, whether nonlinear analysis of the sleep EEG may give clues t... more The article deals with the question, whether nonlinear analysis of the sleep EEG may give clues to understanding of disturbed information processing in psychiatric diseases. We point out how the nonlinear approach to EEG dynamics is motivated and review recent investigations concerning nonlinear dynamical properties of physiological systems. We present calculations of the correlation dimension D2 and the principal Lyapunov-exponent lambda 1 for sleep EEG data from 9 depressive and 11 schizophrenic inpatients compared to healthy controls. Combining the findings for D2 and lambda 1 our results point to altered nonlinear brain dynamics mainly during slow wave sleep in depression and during REM sleep in schizophrenia. Finally, possible interpretations of altered nonlinear EEG dynamics during sleep concerning disturbed information processing in depression and schizophrenia are discussed.

Research paper thumbnail of The calculation of the first positive Lyapunov exponent in sleep EEG data

Electroencephalography and clinical neurophysiology, 1993

To help determine if the EEG is quasiperiodic or chaotic we performed a new analysis by calculati... more To help determine if the EEG is quasiperiodic or chaotic we performed a new analysis by calculating the first positive Lyapunov exponent L1 from sleep EEG data. Lyapunov exponents measure the mean exponential expansion or contraction of a flow in phase space. L1 is zero for periodic as well as quasiperiodic processes, but positive in case of chaotic processes expressing the sensitive dependence on initial conditions. We calculated L1 for sleep EEG segments of 15 healthy male subjects corresponding to sleep stages I, II, III, IV and REM (according to Rechtschaffen and Kales). Our investigations support the assumption that EEG signals are neither quasiperiodic waves nor simple noise. Moreover, we found statistically significant differences between the values of L1 for different sleep stages.

Research paper thumbnail of Gamma power reductions accompany stimulus-specific representations of dynamic events

Current biology : CB, Jan 2, 2015

Neural representations of specific stimuli rely on activity patterns in distributed neural assemb... more Neural representations of specific stimuli rely on activity patterns in distributed neural assemblies [1-4]. According to one influential view, these assemblies are characterized by synchronized gamma-band activity (GBA) [5-11] that reflects stimulus-specific representations [12-14]. However, recent studies have shown that GBA is closely correlated with the overall amount of cellular activity and may be detrimental for precise representations of specific stimuli [15, 16]. Until now, the role of GBA for the formation of dynamically changing representations has been unknown. Here, we applied representational similarity analysis (RSA) [17] to intracranial electroencephalogram (iEEG) data from ten presurgical epilepsy patients to identify stimulus-specific neural representations. Patients first learned and then retrieved their paths through virtual houses. Dynamic representations were identified by the rapidly changing distributions of frequency-specific global (spatial) activity patter...

Research paper thumbnail of P436 Malignancies in children receiving infliximab and other inflammatory bowel disease therapies: an inflammatory bowel disease multicenter, prospective, long-term registry of pediatric patients (DEVELOP) registry data

Poster presentations relapses. These findings support the importance of maintenance therapies in ... more Poster presentations relapses. These findings support the importance of maintenance therapies in UC that reduce or prevent relapses. Quantifying the relationship between relapse rate and costs will inform future health economic studies.

Research paper thumbnail of Directed functional connectivity between neocortex and mediotemporal lobe during different sleep stages

Research paper thumbnail of Working memory processes in the medial temporal lobe

Research paper thumbnail of An in vitro investigation into the suitability of pH-dependent polymers for colonic targeting

International Journal of Pharmaceutics, 1993

... International Journal of Pharmaceutics, 91 (1993) 241-245 241 1993 Elsevier Science Publisher... more ... International Journal of Pharmaceutics, 91 (1993) 241-245 241 1993 Elsevier Science Publishers BV All rights reserved 0378-5173/93 ... for colonic targeting Marianne Ashford a, John T. Fell a, David Attwood a and Philip J. Woodhead b " Department of Pharmacy, University of ...

Research paper thumbnail of P104 a Nationwide Audit of Adalimumab Use and Safety in Children with Crohn's Disease

Journal of Crohn's and Colitis Supplements, 2009

Research paper thumbnail of Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling

NATURE NEUROSCIENCE, 2001

Research paper thumbnail of Human scalp recorded sigma activity is modulated by slow EEG oscillations during deep sleep

INTERNATIONAL JOURNAL OF NEUROSCIENCE, 2002

The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and... more The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and below 1 Hz which can be distinguished from other delta (0.5-3.5 Hz) activity. Intracranial studies showed that this slow oscillation triggers and groups cortical network firing. In the present study, we examined whether the phases of the slow oscillation during sleep stage 4 are correlated with the magnitude of sigma (12-16 Hz) and gamma (>20 Hz) scalp activity. For this purpose, 10-min segments of uninterrupted stage 4 sleep EEG from 9 subjects were analyzed by applying wavelet techniques. We found that scalp recorded sigma, but not gamma, activity is modulated by the phases of the slow oscillation during deep sleep. Enhancement of sigma activity was observed to be triggered by the peak of the surface positive slow wave component, whereas reduction of sigma activity started around the peak of the negative component.

Research paper thumbnail of Independent delta/theta rhythms in the human hippocampus and entorhinal cortex

Frontiers in Human Neuroscience, 2008

Theta oscillations in the medial temporal lobe (MTL) of mammals are involved in various functions... more Theta oscillations in the medial temporal lobe (MTL) of mammals are involved in various functions such as spatial navigation, sensorimotor integration, and cognitive processing. While the theta rhythm was originally assumed to originate in the medial septum, more recent studies suggest autonomous theta generation in the MTL. Although coherence between entorhinal and hippocampal theta activity has been found to infl uence memory formation, it remains unclear whether these two structures can generate theta independently. In this study we analyzed intracranial electroencephalographic (EEG) recordings from 22 patients with unilateral hippocampal sclerosis undergoing presurgical evaluation prior to resection of the epileptic focus. Using a wavelet-based, frequency-band-specifi c measure of phase synchronization, we quantifi ed synchrony between 10 different recording sites along the longitudinal axis of the hippocampal formation in the non-epileptic brain hemisphere. We compared EEG synchrony between adjacent recording sites (i) within the entorhinal cortex, (ii) within the hippocampus, and (iii) between the hippocampus and entorhinal cortex. We observed a signifi cant interregional gap in synchrony for the delta and theta band, indicating the existence of independent delta/theta rhythms in different subregions of the human MTL. The interaction of these rhythms could represent the temporal basis for the information processing required for mnemonic encoding and retrieval.

Research paper thumbnail of Local Category-Specific Gamma Band Responses in the Visual Cortex Do Not Reflect Conscious Perception

Journal of Neuroscience, 2012

Which neural processes underlie our conscious experience? One theoretical view argues that the ne... more Which neural processes underlie our conscious experience? One theoretical view argues that the neural correlates of consciousness (NCC) reside in local activity in sensory cortices. Accordingly, local category-specific gamma band responses in visual cortex correlate with conscious perception. However, as most studies manipulated conscious perception by altering the amount of sensory evidence, it is possible that they reflect prerequisites or consequences of consciousness rather than the actual NCC. Here we directly address this issue by developing a new experimental paradigm in which conscious perception is modulated either by sensory evidence or by previous exposure of the images while recording intracranial EEG from the higher-order visual cortex of human epilepsy patients. A clear prediction is that neural processes directly reflecting conscious perception should be present regardless of how it comes about. In contrast, we observed that although subjective reports were modulated both by sensory evidence and by previous exposure, gamma band responses solely reflected sensory evidence. This result contradicts the proposal that local gamma band responses in the higher-order visual cortex reflect conscious perception.

Research paper thumbnail of Interactions between Medial Temporal Lobe, Prefrontal Cortex, and Inferior Temporal Regions during Visual Working Memory: A Combined Intracranial EEG and Functional Magnetic Resonance Imaging Study

Journal of Neuroscience, 2008

It is a fundamental question whether the medial temporal lobe (MTL) supports only long-term memor... more It is a fundamental question whether the medial temporal lobe (MTL) supports only long-term memory encoding, or contributes to working memory (WM) processes as well. Recent data suggest that the MTL is activated whenever multiple items or item features are being maintained in WM. This may rely on interactions between the MTL or the prefrontal cortex (PFC) and content-specific areas in the inferior temporal (IT) cortex. Here, we investigated the neural mechanism through which the MTL, PFC, and IT cortex interact during WM maintenance. First, we quantified phase synchronization of intracranial EEG data in epilepsy patients with electrodes in both regions. Second, we used directional coupling analysis to study whether oscillatory activity in the IT cortex drives the MTL or vice versa. Finally, we investigated functional connectivity in functional magnetic resonance imaging data of healthy subjects with seeds in the MTL and PFC. With increasing load, EEG phase synchronization between the IT cortex and anterior parahippocampal gyrus and within the MTL increased. Coupling was bidirectional in all load conditions, but changed toward an increased top-down (anterior parahippocampal gyrus 3 IT) coupling in the high gamma range (51-75 Hz) with increasing load. Functional connectivity between the MTL seed and the visual association cortex increased with load, but activity within the MTL and the PFC correlated with fewer voxels, suggesting that more specific neural networks were engaged. These data indicate that WM for multiple items depends on an increased strength of top-down control of activity within the IT cortex by the MTL.

Research paper thumbnail of Medial Temporal Theta/Alpha Power Enhancement Precedes Successful Memory Encoding: Evidence Based on Intracranial EEG

Journal of Neuroscience, 2011

Not only poststimulus, but also prestimulus neural activity has been shown to be predictive for l... more Not only poststimulus, but also prestimulus neural activity has been shown to be predictive for later successful memory encoding. However, it is still not clear which medial temporal lobe processes precede effective memory formation. Here, our aim was to investigate whether such prestimulus markers for successful memory encoding can be specified based on intracranial recordings directly from the hippocampus and rhinal cortex. For this purpose, we analyzed subsequent memory effects during a continuous word recognition paradigm in 31 presurgical epilepsy patients. We found that rhinal and hippocampal theta and successive alpha power enhancement before word presentation predicted successful memory encoding. Previous studies suggest that stimulus-triggered hippocampal theta activity is particularly related to memory retrieval and activation of a mnemonic context, whereas the alpha rhythm reflects inhibitory top-down control of task processing and executive functioning. In line with these suggestions, we propose that the observed medial temporal theta and alpha power increases before stimulus presentation reflect activation of contextual information and inhibitory top-down control processes preparing for stimulus-triggered memory processing.

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

Research paper thumbnail of The Interaction of Rhinal Cortex and Hippocampus in Human Declarative Memory Formation

Reviews in the Neurosciences, 2002

Human declarative memory formation crucially depends on processes within the medial temporal lobe... more Human declarative memory formation crucially depends on processes within the medial temporal lobe (MTL). These processes can be monitored in real-time by recordings from depth electrodes implanted in the MTL of patients with epilepsy who undergo presurgical evaluation. In our studies, patients performed a word memorization task during depth EEG recording. Afterwards, the difference between event-related potentials (ERPs) corresponding to subsequently remembered versus forgotten words was analyzed. These kind of studies revealed that successful memory encoding is characterized by an early process generated by the rhinal cortex within 300 ms following stimulus onset. This rhinal process precedes a hippocampal process, which starts about 200 ms later. Further investigation revealed that the rhinal process seems to be a correlate of semantic preprocessing which supports memory formation, whereas the hippocampal process appears to be a correlate of an exclusively mnemonic operation. These studies yielded only indirect evidence for an interaction of rhinal cortex and hippocampus. Direct evidence for a memory related cooperation between both structures, however, has been found in a study analyzing so called gamma activity, EEG oscillations of around 40 Hz. This investigation showed that successful as opposed to unsuccessful memory formation is accompanied by an initial enhancement of rhinal-hippocampal phase synchronization, which is followed by a later desynchronization. Present knowledge about the function of phase synchronized gamma activity suggests that this phase coupling and decoupling initiates and later terminates communication between the two MTL structures. Phase synchronized rhinal-hippocampal gamma activity may, moreover, accomplish Hebbian synaptic modifications and thus provide an initial step of declarative memory formation on the synaptic level.

Research paper thumbnail of The neural dynamics of memory consolidation in the human brain: direct evidence by intracranial EEG

Frontiers in Human Neuroscience, 2008

Research paper thumbnail of Intracranial electroencephalography power and phase synchronization changes during monaural and binaural beat stimulation

The European journal of neuroscience, 2015

Auditory stimulation with monaural or binaural auditory beats (i.e. sine waves with nearby freque... more Auditory stimulation with monaural or binaural auditory beats (i.e. sine waves with nearby frequencies presented either to both ears or to each ear separately) represents a non-invasive approach to influence electrical brain activity. It is still unclear exactly which brain sites are affected by beat stimulation. In particular, an impact of beat stimulation on mediotemporal brain areas could possibly provide new options for memory enhancement or seizure control. Therefore, we examined how electroencephalography (EEG) power and phase synchronization are modulated by auditory stimulation with beat frequencies corresponding to dominant EEG rhythms based on intracranial recordings in presurgical epilepsy patients. Monaural and binaural beat stimuli with beat frequencies of 5, 10, 40 and 80 Hz and non-superposed control signals were administered with low amplitudes (60 dB SPL) and for short durations (5 s). EEG power was intracranially recorded from mediotemporal, temporo-basal and tempo...

Research paper thumbnail of Retrieval from the Brain's Perspective

Frontiers in human neuroscience, 2012

HUMAN NEUROSCIENCE that implicit memory can drive behavioral responses on explicit memory tests a... more HUMAN NEUROSCIENCE that implicit memory can drive behavioral responses on explicit memory tests aiming to measure recollection and familiarity, which may cause distorted results. More specifically, apparent familiarity signals may in many cases be actually due to conceptual priming. This corrupts the interpretation of activation patterns during familiarity judgments and renders reverse inference problematic (Poldrack, 2006). Other researchers have pointed to the fact that a separation between brain structures supporting recollection and familiarity is often confounded by memory strength (e.g., Wixted and Squire, 2010). It may be argued that, for instance, the categories of item vs. source memory can be more clearly operationalized and may better distinguish between involved brain structures and neural processes (e.g., Staresina and Davachi, 2008). While the assessment of recollection and familiarity typically relies on introspective reports that are inherently subjective, source memory can be objectively assessed. Recently, Henke (2010) moved beyond this methodological point and argued that in principle, memory operations should be characterized by different processing modes, and not by the involvement of consciousness (explicit vs. implicit) or other phenomenal criteria. This line of reasoning was strongly motivated by neuroimaging studies suggesting that the criterion for the recruitment of a specific brain structure is related to its computational role rather than to the phenomenal experiences during the memory task. According to her new taxonomy, the hippocampus, for instance, accomplishes rapid encoding of flexible associations, while the parahippocampal gyrus supports rapid encoding of single items. Compared to the explicit vs. implicit dichotomy, the advantage of this classification is that its relation to different brain structures is less ambiguous.

Research paper thumbnail of Discrimination of sleep stages: a comparison between spectral and nonlinear EEG measures

Electroencephalography and clinical neurophysiology, 1996

During recent years, methods from nonlinear dynamics were introduced into the analysis of EEG sig... more During recent years, methods from nonlinear dynamics were introduced into the analysis of EEG signals. Although from a theoretical point of view nonlinear measures quantify properties being independent from conventional spectral measures, it is a crucial question whether in practice nonlinear EEG measures yield additional information, which is not redundant to the information gained by spectral analysis. Therefore, we compared the ability of several spectral and nonlinear measures to discriminate different sleep stages. We evaluated spectral measures (relative delta power, spectral edge, spectral entropy and first spectral moment), and nonlinear measures (correlation dimension D2, largest Lyapunov exponent LI, and approximated Kolmogorof entropy K2), and additionally the stochastic time domain based measure entropy of amplitudes. For 12 healthy subjects these measures were calculated from sleep EEG segments of 2:44 min duration, each segment unambiguously corresponding to one of the...

Research paper thumbnail of Nonlinear EEG dynamics during sleep in depression and schizophrenia

The International journal of neuroscience, 1994

The article deals with the question, whether nonlinear analysis of the sleep EEG may give clues t... more The article deals with the question, whether nonlinear analysis of the sleep EEG may give clues to understanding of disturbed information processing in psychiatric diseases. We point out how the nonlinear approach to EEG dynamics is motivated and review recent investigations concerning nonlinear dynamical properties of physiological systems. We present calculations of the correlation dimension D2 and the principal Lyapunov-exponent lambda 1 for sleep EEG data from 9 depressive and 11 schizophrenic inpatients compared to healthy controls. Combining the findings for D2 and lambda 1 our results point to altered nonlinear brain dynamics mainly during slow wave sleep in depression and during REM sleep in schizophrenia. Finally, possible interpretations of altered nonlinear EEG dynamics during sleep concerning disturbed information processing in depression and schizophrenia are discussed.

Research paper thumbnail of The calculation of the first positive Lyapunov exponent in sleep EEG data

Electroencephalography and clinical neurophysiology, 1993

To help determine if the EEG is quasiperiodic or chaotic we performed a new analysis by calculati... more To help determine if the EEG is quasiperiodic or chaotic we performed a new analysis by calculating the first positive Lyapunov exponent L1 from sleep EEG data. Lyapunov exponents measure the mean exponential expansion or contraction of a flow in phase space. L1 is zero for periodic as well as quasiperiodic processes, but positive in case of chaotic processes expressing the sensitive dependence on initial conditions. We calculated L1 for sleep EEG segments of 15 healthy male subjects corresponding to sleep stages I, II, III, IV and REM (according to Rechtschaffen and Kales). Our investigations support the assumption that EEG signals are neither quasiperiodic waves nor simple noise. Moreover, we found statistically significant differences between the values of L1 for different sleep stages.

Research paper thumbnail of Gamma power reductions accompany stimulus-specific representations of dynamic events

Current biology : CB, Jan 2, 2015

Neural representations of specific stimuli rely on activity patterns in distributed neural assemb... more Neural representations of specific stimuli rely on activity patterns in distributed neural assemblies [1-4]. According to one influential view, these assemblies are characterized by synchronized gamma-band activity (GBA) [5-11] that reflects stimulus-specific representations [12-14]. However, recent studies have shown that GBA is closely correlated with the overall amount of cellular activity and may be detrimental for precise representations of specific stimuli [15, 16]. Until now, the role of GBA for the formation of dynamically changing representations has been unknown. Here, we applied representational similarity analysis (RSA) [17] to intracranial electroencephalogram (iEEG) data from ten presurgical epilepsy patients to identify stimulus-specific neural representations. Patients first learned and then retrieved their paths through virtual houses. Dynamic representations were identified by the rapidly changing distributions of frequency-specific global (spatial) activity patter...

Research paper thumbnail of P436 Malignancies in children receiving infliximab and other inflammatory bowel disease therapies: an inflammatory bowel disease multicenter, prospective, long-term registry of pediatric patients (DEVELOP) registry data

Poster presentations relapses. These findings support the importance of maintenance therapies in ... more Poster presentations relapses. These findings support the importance of maintenance therapies in UC that reduce or prevent relapses. Quantifying the relationship between relapse rate and costs will inform future health economic studies.

Research paper thumbnail of Directed functional connectivity between neocortex and mediotemporal lobe during different sleep stages

Research paper thumbnail of Working memory processes in the medial temporal lobe

Research paper thumbnail of An in vitro investigation into the suitability of pH-dependent polymers for colonic targeting

International Journal of Pharmaceutics, 1993

... International Journal of Pharmaceutics, 91 (1993) 241-245 241 1993 Elsevier Science Publisher... more ... International Journal of Pharmaceutics, 91 (1993) 241-245 241 1993 Elsevier Science Publishers BV All rights reserved 0378-5173/93 ... for colonic targeting Marianne Ashford a, John T. Fell a, David Attwood a and Philip J. Woodhead b " Department of Pharmacy, University of ...

Research paper thumbnail of P104 a Nationwide Audit of Adalimumab Use and Safety in Children with Crohn's Disease

Journal of Crohn's and Colitis Supplements, 2009

Research paper thumbnail of Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling

NATURE NEUROSCIENCE, 2001

Research paper thumbnail of Human scalp recorded sigma activity is modulated by slow EEG oscillations during deep sleep

INTERNATIONAL JOURNAL OF NEUROSCIENCE, 2002

The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and... more The EEG during deep sleep exhibits a distinct cortically generated slow oscillation of around and below 1 Hz which can be distinguished from other delta (0.5-3.5 Hz) activity. Intracranial studies showed that this slow oscillation triggers and groups cortical network firing. In the present study, we examined whether the phases of the slow oscillation during sleep stage 4 are correlated with the magnitude of sigma (12-16 Hz) and gamma (>20 Hz) scalp activity. For this purpose, 10-min segments of uninterrupted stage 4 sleep EEG from 9 subjects were analyzed by applying wavelet techniques. We found that scalp recorded sigma, but not gamma, activity is modulated by the phases of the slow oscillation during deep sleep. Enhancement of sigma activity was observed to be triggered by the peak of the surface positive slow wave component, whereas reduction of sigma activity started around the peak of the negative component.

Research paper thumbnail of Independent delta/theta rhythms in the human hippocampus and entorhinal cortex

Frontiers in Human Neuroscience, 2008

Theta oscillations in the medial temporal lobe (MTL) of mammals are involved in various functions... more Theta oscillations in the medial temporal lobe (MTL) of mammals are involved in various functions such as spatial navigation, sensorimotor integration, and cognitive processing. While the theta rhythm was originally assumed to originate in the medial septum, more recent studies suggest autonomous theta generation in the MTL. Although coherence between entorhinal and hippocampal theta activity has been found to infl uence memory formation, it remains unclear whether these two structures can generate theta independently. In this study we analyzed intracranial electroencephalographic (EEG) recordings from 22 patients with unilateral hippocampal sclerosis undergoing presurgical evaluation prior to resection of the epileptic focus. Using a wavelet-based, frequency-band-specifi c measure of phase synchronization, we quantifi ed synchrony between 10 different recording sites along the longitudinal axis of the hippocampal formation in the non-epileptic brain hemisphere. We compared EEG synchrony between adjacent recording sites (i) within the entorhinal cortex, (ii) within the hippocampus, and (iii) between the hippocampus and entorhinal cortex. We observed a signifi cant interregional gap in synchrony for the delta and theta band, indicating the existence of independent delta/theta rhythms in different subregions of the human MTL. The interaction of these rhythms could represent the temporal basis for the information processing required for mnemonic encoding and retrieval.

Research paper thumbnail of Local Category-Specific Gamma Band Responses in the Visual Cortex Do Not Reflect Conscious Perception

Journal of Neuroscience, 2012

Which neural processes underlie our conscious experience? One theoretical view argues that the ne... more Which neural processes underlie our conscious experience? One theoretical view argues that the neural correlates of consciousness (NCC) reside in local activity in sensory cortices. Accordingly, local category-specific gamma band responses in visual cortex correlate with conscious perception. However, as most studies manipulated conscious perception by altering the amount of sensory evidence, it is possible that they reflect prerequisites or consequences of consciousness rather than the actual NCC. Here we directly address this issue by developing a new experimental paradigm in which conscious perception is modulated either by sensory evidence or by previous exposure of the images while recording intracranial EEG from the higher-order visual cortex of human epilepsy patients. A clear prediction is that neural processes directly reflecting conscious perception should be present regardless of how it comes about. In contrast, we observed that although subjective reports were modulated both by sensory evidence and by previous exposure, gamma band responses solely reflected sensory evidence. This result contradicts the proposal that local gamma band responses in the higher-order visual cortex reflect conscious perception.

Research paper thumbnail of Interactions between Medial Temporal Lobe, Prefrontal Cortex, and Inferior Temporal Regions during Visual Working Memory: A Combined Intracranial EEG and Functional Magnetic Resonance Imaging Study

Journal of Neuroscience, 2008

It is a fundamental question whether the medial temporal lobe (MTL) supports only long-term memor... more It is a fundamental question whether the medial temporal lobe (MTL) supports only long-term memory encoding, or contributes to working memory (WM) processes as well. Recent data suggest that the MTL is activated whenever multiple items or item features are being maintained in WM. This may rely on interactions between the MTL or the prefrontal cortex (PFC) and content-specific areas in the inferior temporal (IT) cortex. Here, we investigated the neural mechanism through which the MTL, PFC, and IT cortex interact during WM maintenance. First, we quantified phase synchronization of intracranial EEG data in epilepsy patients with electrodes in both regions. Second, we used directional coupling analysis to study whether oscillatory activity in the IT cortex drives the MTL or vice versa. Finally, we investigated functional connectivity in functional magnetic resonance imaging data of healthy subjects with seeds in the MTL and PFC. With increasing load, EEG phase synchronization between the IT cortex and anterior parahippocampal gyrus and within the MTL increased. Coupling was bidirectional in all load conditions, but changed toward an increased top-down (anterior parahippocampal gyrus 3 IT) coupling in the high gamma range (51-75 Hz) with increasing load. Functional connectivity between the MTL seed and the visual association cortex increased with load, but activity within the MTL and the PFC correlated with fewer voxels, suggesting that more specific neural networks were engaged. These data indicate that WM for multiple items depends on an increased strength of top-down control of activity within the IT cortex by the MTL.

Research paper thumbnail of Medial Temporal Theta/Alpha Power Enhancement Precedes Successful Memory Encoding: Evidence Based on Intracranial EEG

Journal of Neuroscience, 2011

Not only poststimulus, but also prestimulus neural activity has been shown to be predictive for l... more Not only poststimulus, but also prestimulus neural activity has been shown to be predictive for later successful memory encoding. However, it is still not clear which medial temporal lobe processes precede effective memory formation. Here, our aim was to investigate whether such prestimulus markers for successful memory encoding can be specified based on intracranial recordings directly from the hippocampus and rhinal cortex. For this purpose, we analyzed subsequent memory effects during a continuous word recognition paradigm in 31 presurgical epilepsy patients. We found that rhinal and hippocampal theta and successive alpha power enhancement before word presentation predicted successful memory encoding. Previous studies suggest that stimulus-triggered hippocampal theta activity is particularly related to memory retrieval and activation of a mnemonic context, whereas the alpha rhythm reflects inhibitory top-down control of task processing and executive functioning. In line with these suggestions, we propose that the observed medial temporal theta and alpha power increases before stimulus presentation reflect activation of contextual information and inhibitory top-down control processes preparing for stimulus-triggered memory processing.

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

Research paper thumbnail of The Interaction of Rhinal Cortex and Hippocampus in Human Declarative Memory Formation

Reviews in the Neurosciences, 2002

Human declarative memory formation crucially depends on processes within the medial temporal lobe... more Human declarative memory formation crucially depends on processes within the medial temporal lobe (MTL). These processes can be monitored in real-time by recordings from depth electrodes implanted in the MTL of patients with epilepsy who undergo presurgical evaluation. In our studies, patients performed a word memorization task during depth EEG recording. Afterwards, the difference between event-related potentials (ERPs) corresponding to subsequently remembered versus forgotten words was analyzed. These kind of studies revealed that successful memory encoding is characterized by an early process generated by the rhinal cortex within 300 ms following stimulus onset. This rhinal process precedes a hippocampal process, which starts about 200 ms later. Further investigation revealed that the rhinal process seems to be a correlate of semantic preprocessing which supports memory formation, whereas the hippocampal process appears to be a correlate of an exclusively mnemonic operation. These studies yielded only indirect evidence for an interaction of rhinal cortex and hippocampus. Direct evidence for a memory related cooperation between both structures, however, has been found in a study analyzing so called gamma activity, EEG oscillations of around 40 Hz. This investigation showed that successful as opposed to unsuccessful memory formation is accompanied by an initial enhancement of rhinal-hippocampal phase synchronization, which is followed by a later desynchronization. Present knowledge about the function of phase synchronized gamma activity suggests that this phase coupling and decoupling initiates and later terminates communication between the two MTL structures. Phase synchronized rhinal-hippocampal gamma activity may, moreover, accomplish Hebbian synaptic modifications and thus provide an initial step of declarative memory formation on the synaptic level.