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A Keil

Publications and source records attributed to A Keil.

At least 19 recordsLinked to original sources
PubMed2002

Comparison of data transformation procedures to enhance topographical accuracy in time-series analysis of the human EEG.

We describe a methodology to apply current source density (CSD) and minimum norm (MN) estimation as pre-processing tools for time-series analysis of single trial EEG data. The performance of these methods is compared for the case of wavelet time-frequency analysis of simulated gamma-band activity. A reasonable comparison of CSD and MN on the single trial level requires regularization such that the corresponding transformed data sets have similar signal-to-noise ratios (SNRs). For region-of-interest approaches, it should be possible to optimize the SNR for single estimates rather than for the whole distributed solution. An effective implementation of the MN method is described. Simulated data sets were created by modulating the strengths of a radial and a tangential test dipole with wavelets in the frequency range of the gamma band, superimposed with simulated spatially uncorrelated noise. The MN and CSD transformed data sets as well as the average reference (AR) representation were subjected to wavelet frequency-domain analysis, and power spectra were mapped for relevant frequency bands. For both CSD and MN, the influence of noise can be sufficiently suppressed by regularization to yield meaningful information, but only MN represents both radial and tangential dipole sources appropriately as single peaks. Therefore, when relating wavelet power spectrum topographies to their neuronal generators, MN should be preferred.

Artifacts↗
PubMed2002

Interleukin regulation of asymmetric antibody synthesized by isolated placental B cells.

PROBLEM: Protecting antibodies against trophoblast surface molecules were previously described. Here we analysed the synthesis of asymmetric IgG by placental B-lymphocytes. METHOD OF STUDY: B cells were isolated from human term placenta and cord blood, stimulated with anti-CD40 IgG and cocultured with transfected Fcgamma R-expressing mice Ltk-fibroblast. Interleukin-4, IL-6, IL-10, IL-11 and IL-13 were added to cultures for 14 days. Asymmetric IgG were assessed in culture supernatants by concanavalin A (Con A) fixation and enzyme-linked immunosorbent assay. RESULTS: When IL-6 was added to the cultures, the percentages of asymmetric IgG synthesized by placental B cells were: IL-6: 29 +/- 10; IL-6 + IL-10: 24 +/- 7; IL-4 + IL-10 + IL-6: 38 +/- 9. The last combination induced the highest increase in the asymmetric IgG synthesis as compared with control (19 +/- 10%, P < 0.05). Additionally, placental B cells synthesized more asymmetric IgG than umbilical cord blood B-lymphocytes (P = 0.0015). CONCLUSIONS: Isolated placental B-lymphocytes synthesized asymmetric IgG in response to Th2 interleukins, more notably IL-6 in combination with IL-4 and IL-10. The in vitro increase of protective asymmetric IgG synthesis in response to Th2-cytokines support the hypothesis that a local Th2-switch is beneficial for pregnancy outcome.

Animals↗
PubMed2001

Modulation of induced gamma band responses and phase synchrony in a paired associate learning task in the human EEG.

It has been proposed that associative learning is accomplished by the formation of cell assemblies and synchronous activity among the neurons of such an assembly. Induced gamma band responses (GBRs) and phase synchrony between electrode sites are discussed as a signature of activity within a cell assembly. To examine the activation of this network due to memory recall, a paired associate learning paradigm was used. EEG was analyzed in the frequency domain. Results showed a significant increase of induced GBRs at posterior and anterior electrode sites in the recall sequence of the learning condition. Furthermore, phase synchrony revealed a broad distribution pattern of phase synchrony between posterior and frontal electrode sites.

Adult↗
PubMed2001

Suppression of the auditory middle-latency response and evoked gamma-band response in a paired-click paradigm.

When two clicks are presented within 500 ms and the clicks are separated by several seconds, a typical finding is a suppression of the amplitude of the P50 component of the middle-latency auditory-evoked response. In the present study, we investigated whether only the P50 or also the earlier components Po, Na, Pa and Nb, and the exogenous components N100 and P200 exhibit an amplitude suppression to the second click. In addition, we studied the suppression behaviour of the auditory-evoked 40-Hz gamma-band response in the time and frequency domain. We found a significant amplitude suppression to the second click for all components of the auditory-evoked potential following Po, which was most pronounced at electrode Cz. When testing the successive peaks and troughs of the evoked 40-Hz gamma-band response in the time domain, we found a significant amplitude suppression for peaks and troughs with the same latency and polarity as the middle-latency components following Po, which was most pronounced at electrodes Fz and Cz. Consequently, the amplitude of the 40-Hz evoked gamma-band response in the frequency domain paralleled the findings of the time domain, with a significant amplitude suppression to the second tone, which was most pronounced at electrodes Fz and Cz. Results are discussed with reference to the early sensory-gating hypothesis.

Acoustic Stimulation↗
PubMed2001

Functional correlates of macroscopic high-frequency brain activity in the human visual system.

The present article reviews empirical findings of large-scale gamma oscillations in the human brain, in the context of their functional correlates. Evidence supporting the fact that high-frequency neuronal oscillations are involved in several aspects of visual processing is presented, with a focus on bottom-up and top-down visual feature processing, selective attention, and emotional evaluation. This evidence suggests that visual processing involves the integrated activity of wide spread neuronal assemblies that can be studied with respect to time course and topography, employing frequency-domain analyses. Possible mechanisms underlying these phenomena are considered. Furthermore, the effects of attention and motivation, as well as characteristics of experimental paradigms are discussed as determinants of reliability and validity of measures of high-frequency oscillations.

Animals↗
PubMed2001

Human large-scale oscillatory brain activity during an operant shaping procedure.

The present study aimed at examining the oscillatory brain-electric correlates of human operant learning using high-density electroencephalography (EEG). Induced gamma-band activity (GBA) was studied using a fixed-interval reinforcement schedule with a variable limited hold period, which was decreased depending on response accuracy. Thus, participants' behavior was shaped during the course of the learning session. After each response, numbers indicating the money value of that response served as reinforcing stimuli. Random reinforcement and self-paced button pressing without reinforcement were added as control conditions. GBA around 40 Hz was enhanced at posterior electrodes in response to visual feedback stimuli during shaping and random reward compared to the self-paced pressing condition where no visual feedback was provided. Furthermore, shaping was associated with a pronounced left frontal lower gamma (20-30 Hz) increase in response to feedback stimuli, whereas this pattern was not observed in the random reinforcement and self-paced pressing conditions. The present findings are in line with the notion that macroscopic high-frequency dynamics of neuronal cell assemblies may be regarded as a mechanism involved in learning and memory formation.

Adult↗
PubMed2001

Effects of emotional arousal in the cerebral hemispheres: a study of oscillatory brain activity and event-related potentials.

OBJECTIVE: The present study aimed at examining the time course and topography of oscillatory brain activity and event-related potentials (ERPs) in response to laterally presented affective pictures. METHODS: Electroencephalography was recorded from 129 electrodes in 10 healthy university students during presentation of pictures from the international affective picture system. Frequency measures and ERPs were obtained for pleasant, neutral, and unpleasant pictures. RESULTS: In accordance with previous reports, a modulation of the late positive ERP wave at parietal recording sites was found as a function of emotional arousal. Early mid gamma band activity (GBA; 30-45 Hz) at 80 ms post-stimulus was enhanced in response to aversive stimuli only, whereas the higher GBA (46-65 Hz) at 500 ms showed an enhancement of arousing, compared to neutral pictures. ERP and late gamma effects showed a pronounced right-hemisphere preponderance, but differed in terms of topographical distribution. CONCLUSIONS: Late gamma activity may represent a correlate of widespread cortical networks processing different aspects of emotionally arousing visual objects. In contrast, differences between affective categories in early gamma activity might reflect fast detection of aversive stimulus features.

Adult↗
PubMed2000

Modulation of induced gamma band activity in the human EEG by attention and visual information processing.

Here we present a series of four studies aimed to investigate the link between induced gamma band activity in the human EEG and visual information processing. We demonstrated and validated the modulation of spectral gamma band power by spatial selective visual attention. When subjects attended to a certain stimulus, spectral power was increased as compared to when the same stimulus was ignored. In addition, we showed a shift in spectral gamma band power increase to the contralateral hemisphere when subjects shifted their attention to one visual hemifield. The following study investigated induced gamma band activity and the perception of a Gestalt. Ambiguous rotating figures were used to operationalize the law of good figure (gute Gestalt). We found increased gamma band power at posterior electrode sites when subjects perceived an object. In the last experiment we demonstrated a differential hemispheric gamma band activation when subjects were confronted with emotional pictures. Results of the present experiments in combination with other studies presented in this volume are supportive for the notion that induced gamma band activity in the human EEG is closely related to visual information processing and attentional perceptual mechanisms.

Attention↗
PubMed2000

Nitric oxide inhibits growth of glomerular mesangial cells: role of the transcription factor EGR-1.

UNLABELLED: Nitric oxide inhibits growth of glomerular mesangial cells: Role of the transcription factor Egr-1. BACKGROUND: In previous studies, we found a close link of early growth response gene-1 (Egr-1) expression to mesangial cell (MC) proliferation. Antiproliferative agents inhibited mitogen-induced Egr-1 expression. Here we investigated the effect of S-nitrosoglutathione (GSNO) on the proliferation of MCs, specifically asking how GSNO regulates the transcription factor Egr-1, which we have previously shown to be critical for the induction of MC mitogenesis. METHODS: The proliferation of MCs was measured by thymidine incorporation and cell counting. Egr-1 mRNA and protein levels were detected by Northern and Western blots. Electrophoretic mobility shift assays (EMSAs) and chloramphenicol acetyltransferase (CAT) assays were performed to test whether GSNO modulates DNA binding and transcriptional activation of Egr-1. RESULTS: GSNO strongly inhibited serum-induced MC proliferation (-84% at 1 mmol/L). A mild inhibition of serum-induced Egr-1 mRNA was observed at GSNO concentrations from 50 to 200 micromol/L, whereas mRNA levels increased again at concentrations above 500 micromol/L. This increased mRNA expression, however, was not translated into Egr-1 protein. Instead, Egr-1 protein induction was inhibited (-40%). EMSAs indicated that GSNO inhibited specific binding of Egr-1 to its DNA consensus sequence. Moreover, transcriptional activation by Egr-1 in CAT assays using a reporter plasmid bearing three Egr-1 binding sites was strongly suppressed by GSNO. CONCLUSIONS: Our data identify GSNO as a potent inhibitor of MC growth with potential beneficial effects in proliferative glomerular diseases. This antimitogenic property is mediated at least in part by inhibitory effects of GSNO on Egr-1 protein levels and by reducing the ability of Egr-1 to activate transcription by impairing its DNA binding activity.

Animals↗
PubMed2000

Expectations and quality of life of cancer patients undergoing radiotherapy.

Expectations, real or false, affect the way patients respond to their illnesses. We assessed therapy-related expectations in relation to global quality of life in 55 cancer patients before and after radiotherapy. Factor analysis indicated that therapy-related expectations come into three broad categories--pain/emotional control, healing and tumour/symptom control. 35 patients expected 'healing' even though curative treatment was intended in only 19 and all patients had been fully informed. The expectation of healing was associated with high quality of life, and the same was true of perception of healing after radiotherapy. In the group as a whole, quality of life was little altered by radiotherapy, but it became substantially worse in those patients who had expected healing but perceived that this had failed, even though physician-assessed Karnofsky status did not change. These findings indicate that the expectation of healing, in cancer patients, is a component of a good global quality of life, whereas more limited expectations (pain control, tumour control) relate to lower quality of life. Patients' expectations deserve further study as a novel approach to improving care.

Analysis of Variance↗
PubMed1999

Human gamma band activity and perception of a gestalt.

Neuronal oscillations in the gamma band (above 30 Hz) have been proposed to be a possible mechanism for the visual representation of objects. The present study examined the topography of gamma band spectral power and event-related potentials in human EEG associated with perceptual switching effected by rotating ambiguous (bistable) figures. Eleven healthy human subjects were presented two rotating bistable figures: first, a face figure that allowed perception of a sad or happy face depending on orientation and therefore caused a perceptual switch at defined points in time when rotated, and, second, a modified version of the Rubin vase, allowing perception as a vase or two faces whereby the switch was orientation-independent. Nonrotating figures served as further control stimuli. EEG was recorded using a high-density array with 128 electrodes. We found a negative event-related potential associated with the switching of the sad-happy figure, which was most pronounced at central prefrontal sites. Gamma band activity (GBA) was enhanced at occipital electrode sites in the rotating bistable figures compared with the standing stimuli, being maximal at vertical stimulus orientations that allowed an easy recognition of the sad and happy face or the vase-faces, respectively. At anterior electrodes, GBA showed a complementary pattern, being maximal when stimuli were oriented horizontally. The findings support the notion that formation of a visual percept may involve oscillations in a distributed neuronal assembly.

Adult↗
PubMed1999

Processing of affective pictures modulates right-hemispheric gamma band EEG activity.

The present study was designed to test differential hemispheric activation induced by emotional stimuli in the gamma band range (30-90 Hz). Subjects viewed slides with differing emotional content (from the International Affective Picture System). A significant valence by hemisphere interaction emerged in the gamma band from 30-50 Hz. Other bands, including alpha and beta, did not show such an interaction. Previous hypotheses suggested that the left hemisphere is more involved in positive affective processing as compared to the right hemisphere, while the latter dominates during negative emotions. Contrary to this expectation, the 30-50 Hz band showed relatively more power for negative valence over the left temporal region as compared to the right and a laterality shift towards the right hemisphere for positive valence. In addition, emotional processing enhanced gamma band power at right frontal electrodes regardless of the particular valence as compared to processing neutral pictures. The extended distribution of specific activity in the gamma band may be the signature of cell assemblies with members in limbic, temporal and frontal neocortical structures that differ in spatial distribution depending on the particular type of emotional processing.

Adult↗
PubMed1999

Selective visual-spatial attention alters induced gamma band responses in the human EEG.

OBJECTIVES: The present study was designed to investigate the attentional modulation of gamma band responses in a visual spatial attention task using a 128-channel-EEG-montage. METHODS: Colored rectangles were presented on a screen. After 500 ms an arrow indicated whether subjects had to shift their attention to the left or right half of the screen to detect target stimuli. During the task, either the attended half of the screen rotated horizontally while the unattended part remained motionless, or vice versa. RESULTS: When subjects attended the rotating stimulus, we found significantly higher power in a specific gamma band from 35-51 Hz on parieto-occipital electrode sites contralateral to the stimulation side. In addition, after the onset of the arrow which indicated what side subjects should direct their attention to, the 35-51 Hz response shifted from a broad posterior distribution to an increase of power at parieto-occipital sites contralateral to the to-be-attended side. Furthermore, the rotating stimulus elicited higher gamma band power as compared to the standing stimulus at electrode locations, which may be related to the activity of underlying cortical structures specialized for motion processing. CONCLUSIONS: The present results replicate important parts of previous findings of enhanced gamma power when a moving stimulus was attended.

Adult↗
PubMed1999

Regulation of CD95 expression and CD95-mediated cell death by interferon-gamma in acute lymphoblastic leukemia with chromosomal translocation t(4;11).

The regulatory effects of IFNgamma on CD95 expression and CD95-mediated cell death were investigated in three high-risk pro-B acute lymphoblastic leukemia (ALL) lines that carry the chromosomal translocation t(4;11)(q21;q23). These leukemias are characteristically refractory to conventional chemotherapeutic treatments operating through the induction of apoptosis. However, the mechanisms leading to increased cell survival and resistance to cell death in these leukemias are largely unknown. Interferon-gamma (IFNgamma), a potent inhibitor of hematopoiesis, acts in part by upregulating CD95 and sensitizing cells to CD95-induced apoptosis. The t(4;11) lines SEM, RS4;11, and MV4;11 expressed low levels of CD95, but were completely resistant to CD95-mediated death. Addition of IFNgamma markedly upregulated CD95 expression in SEM (8-9-fold), RS4;11 (2-3-fold), and MV4;11 (2-3-fold) lines. However, after treatment with IFNgamma, only an 11% increase in sensitivity to CD95-mediated cell death was observed in SEM cells, whereas RS4;11 and MV4;11 cells remained resistant. Cycloheximide, but not actinomycin D or brefeldin A, increased CD95-specific cell death only in IFNgamma-treated RS4;11 cells by approximately 12%. Abundant levels of Bcl-2 and Bcl-XL, known to inhibit CD95-signaling in some cells, were present suggesting a possible role for both molecules in the resistance to CD95-mediated cell death. Resistance of the leukemic blasts to CD95-mediated cell death and the failure of IFNgamma to substantially sensitize the CD95-signaling pathway may contribute to the highly malignant phenotype of pro-B ALL with translocation t(4;11).

Antigens, Surface↗
PubMed1999

TGF-beta1 in liver fibrosis: an inducible transgenic mouse model to study liver fibrogenesis.

Transforming growth factor-beta1 (TGF-beta1) is a powerful stimulus for collagen formation in vitro. To determine the in vivo effects of TGF-beta1 on liver fibrogenesis, we generated transgenic mice overexpressing a fusion gene [C-reactive protein (CRP)/TGF-beta1] consisting of the cDNA coding for an activated form of TGF-beta1 under the control of the regulatory elements of the inducible human CRP gene promoter. Two transgenic lines were generated with liver-specific overexpression of mature TGF-beta1. After induction of the acute phase response (15 h) with lipopolysaccharide (100 microgram ip), plasma TGF-beta1 levels reached >600 ng/ml in transgenic animals, which is >100 times above normal plasma levels. Basal plasma levels of uninduced transgenic animals were about two to five times above normal. As a consequence of hepatic TGF-beta1 expression, we could demonstrate marked transient upregulation of procollagen I and procollagen III mRNA in the liver 15 h after the peak of TGF-beta1 expression. Liver histology after repeated induction of transgene expression showed an activation of hepatic stellate cells in both transgenic lines. The fibrotic process was characterized by perisinusoidal deposition of collagen in a linear pattern. This transgenic mouse model gives in vivo evidence for the important role of TGF-beta1 in stellate cell activation and liver fibrogenesis. Due to the ability to control the level of TGF-beta1 expression, this model allows the study of the regulation and kinetics of collagen synthesis and fibrolysis as well as the degree of reversibility of liver fibrosis. The CRP/TGF-beta1 transgenic mouse model may finally serve as a model for the testing of antifibrogenic agents.

Animals↗
PubMed1998

Dynamical aspects of motor and perceptual processes in schizophrenic patients and healthy controls.

The present study examined the temporal stationarity of the performance of 16 schizophrenic patients and 16 controls matched for age and sex in a bimanual coordination task and a perceptual task. In the motor task, rhythmic finger oscillations (alternating activity of homologue muscle groups) at increasing speed levels resulted in two measures, the preferred oscillation frequency and the critical frequency at which phase transitions (change towards simultaneous activity of homologue muscle groups) occurred. A measure of local dimensional complexity (pointwise D2 or PD2), which is a measure of non-linear dynamics, was determined for the acceleration profiles of the subjects' movements. Schizophrenics exhibited less stable movement dynamics than controls in horizontal finger cycling, indicated by a lower ratio critical/preferred frequency (critical ratio) and by higher means and standard deviations of the pointwise D2. In vertical cycling, the critical ratio did not differentiate between groups, while PD2 means and standard deviations did. Groups also differed specifically in perception of two ambiguous figures (Schroeder stairs and Rubin vase). Schizophrenics showed significantly higher reversal rates for the Rubin vase and a differential perceptive in comparison to controls in the perception of the Schroeder stairs. Measures of perceptual and motor stability were unrelated, which suggests that perceptual and motor processes are not influenced by a common underlying mechanism.

Adult↗