Search PubMed⌕ Search

Biomedical subjects

T Gasser

Publications and source records attributed to T Gasser.

At least 199 records · Page 11Linked to original sources
PubMed1986

Does filtering and smoothing of average evoked potentials really pay? A statistical comparison.

Averaging of sweeps to obtain evoked potentials provides an unsatisfactory reduction of the background activity for a small number of stimuli. A posteriori Wiener filtering, time varying filtering, and smoothing of the average EP have been proposed to meet this problem. As to a posteriori Wiener filtering, a controversy regarding its merits has been going on for several years. The present paper gives a statistical comparison of the above methods, based on real data of two groups of subjects (flash evoked potentials in 41 subjects, pattern reversal evoked potentials in 9 subjects). It is shown that most of the improvement of the filtering approaches was due to an attenuation effect, without any improvement in smoothness of the potentials. The strength of the attenuation introduced by the filtering approaches depended on the specific underlying signal-to-noise ratio. This effect led to an artificially enhanced interindividual variability and could intraindividually lead to biased topographical distribution, when several electrodes are considered. The smoothing method did not show this undesired feature, but, when applying strong smoothing, this method also rendered sizable distortions of the potentials.

Adolescent↗
PubMed1985

[Continuous intragastric pH-metry].

Ambulatory intragastric pH-metry is a suitable method of assessing gastric acidity under conditions of normal daily life. A detailed profile of intragastric acidity is given which is a reproducible individual characteristic. There is a circadian rhythm of gastric acidity in which pH rises during the night and after meals. pH-metry was used to determine the effects of H2-antagonists and antacids, and the results are the basis for the clinical use of intragastric pH-metry.

Antacids↗
PubMed1985

Multidimensional analysis of the concentrations of 17 substances in the CSF of schizophrenics and controls.

The concentrations of 17 substances were determined in the cerebrospinal fluid (CSF) of 28 paranoid schizophrenic patients and 16 controls. Results were standardized and simultaneously evaluated through Multidimensional Scaling (MDS). The full data set can be considered as a cloud of points consisting of the 44 subjects in the 17-dimensional parameter space. MDS seeks a two-dimensional representation of this 17-dimensional cloud of points, while retaining as much as possible the distances between the subjects. The two-dimensional reduction of the 17 CSF parameters correctly separated 15 of 16 controls from the schizophrenic subjects. This indicates that a biological heterogeneity between schizophrenic and nonschizophrenic subjects can be detected by the simultaneous analysis of the CSF concentrations of substances related directly or indirectly to the neuronal activity in the brain.

Adult↗
PubMed1985

Test-retest reliability of spectral parameters of the EEG.

The topic of this paper is the intraindividual stability of the EEG at rest for repeated recordings with respect to two sets of spectral parameters. Variability arises due to changes in experimental conditions (such as vigilance) and also due to the inherent random elements of the EEG. The two sets of parameters considered are broad-band parameters and parameters characterizing rhythmic and 'diffuse' activity separately, derived from autoregressive fitting. In spite of some imprecision in the definition of the EEG at rest, satisfactory test-retest correlations were found. They proved to be quite homogeneous topographically, but not across frequency bands: delta power is less reliable and to some extent also beta activity, whereas the alpha bands perform well. The frequency of the alpha rhythm shows a good reliability and, given the difficulties involved with the concept, also the degree of synchronization. The power of rhythmic and of diffuse activity showed more modest retest correlations, probably to be attributed to methodological problems in determining these quantities. On the whole, the results confirm that the normal EEG can be treated as an intraindividually rather stable trait, that artifacts play a minor role in this respect and that 20 sec of activity are sufficient to reduce adequately the variability inherent in the EEG.

Adolescent↗
PubMed1985

The transfer of EOG activity into the EEG for eyes open and closed.

The transfer of EOG activity into the EEG is investigated for eyes open (EO) eyes closed (EC), relying on spontaneously produced EOG activity. Blinks were prominent with EO and eye movements with EC. A frequency domain approach is more appropriate compared to treating the EOG influence as constant over frequency (i.e., assuming that a fraction of the EOG amplitude is present at EEG derivations). A major problem is to take into account the coherent EEG activity present at the EOG derivations, and this holds true in particular for the higher frequency bands were EOG power is relatively low. Blinks and eye movements have different spectral patterns and are also transferred differently to the locations on the skull. Eye movements are transferred best at low frequencies. The gains of blinks peak in the theta band. Due to high random variability in the individual gain functions, sample average gain functions ('grand means') rather than individual ones were studied.

Adolescent↗
PubMed1985

Long-term ambulatory gastric pH monitoring: validation of a new method and effect of H2-antagonists.

A new ambulatory monitoring system was evaluated for long-term measurements of gastric acidity. A close correlation was observed between values indicated by the pH electrode of the system and the pH of simultaneously aspirated gastric juice, suggesting that the electrode signaled the pH of the gastric fluid content. When the pH electrode was passed via an endoscope, and its bulb was placed against the corpus mucosa, a higher acidity was recorded as compared with gastric juice. To test whether the electrodes measured mucosal pH during ordinary test conditions, the readings of pH probes with mechanically shielded bulbs that did not come into direct contact with the mucosa were compared with those of nonshielded probes in identical positions. Similar results were observed, supporting the hypothesis that nonshielded probes measured the pH of gastric contents rather than that of the mucosa. The importance of a standardized electrode position and a fixed meal schedule was demonstrated in simultaneous recordings of antral and fundic pH. Under fasting conditions, acidity was similar in both regions. After ingestion of a meal, gastric contents were more alkaline in the fundus than in the antrum. A wide range of 24-h acidity (19-83 mmol/L) was detected in 25 healthy subjects. The day-to-day reproducibility of the method as revealed in two consecutive 24-h tests was good. The effect of cimetidine and ranitidine on gastric acidity was evaluated in 9 subjects in a double-blind, double-dummy trial. Mean 24-h H+ activity was 37.4 +/- 4.6 mmol/L under placebo medication. It was lower with cimetidine, two doses of 400 mg (23.8 +/- 4.0); cimetidine, four doses of 400 mg (10.2 +/- 3.0); ranitidine, two doses of 150 mg (10.3 +/- 3.6), and two doses of 300 mg (10.0 +/- 3.5), respectively. In conclusion, ambulatory long-term pH monitoring is a suitable method to assess the physiologic pattern of gastric acidity and the effect of antisecretory drugs.

Adolescent↗
PubMed1985

Uridylate-trapping sugar analogs in combination with inhibitors of uridylate synthesis de novo and 5-fluorouridine.

The combination of 3 types of antipyrimidines was studied in AS-30D hepatoma cells in suspension culture and in the rat in vivo. Cellular UTP and CTP pools can be depleted most effectively by combining an inhibitor of de novo UMP synthesis with sugar analogs diverting UMP to UDP-sugar analogs. The following UMP-trapping sugar analogs were employed: D-galactosamine, D-galactosone, D-glucosone, and D-glucosamine. These D-galactose and D-glucose analogs intensified the depletion of UTP and CTP pools induced by the following inhibitors of de novo UMP synthesis: acivicin, PALA, lapachol, pyrazofurin, and 6-azauridine. The sugar analogs, in the absence of inhibitors of the de novo pathway, enhanced the rate of de novo UMP synthesis several-fold, as indicated by incorporation of 14CO2 into intermediates and products of the pathway and by the expansion of the acid-soluble uracil nucleotide pool. Reduction of UTP and CTP contents to less than 5 and 15% of control, respectively, by D-galactosamine and PALA resulted in a decrease of the rate of RNA synthesis to 19% of control as calculated from the changes in specific activities of [14C]CTP and of [14C]cytidine in RNA after labeling with [14C]uridine. Hepatoma cells released uridine and cytidine into the extracellular fluid. This release was reduced to one third in UTP-deficient cells, indicating that pyrimidine nucleoside excretion is regulated by pyrimidine nucleotide levels, possibly by UTP and CTP regulation of uridine kinase. Determination of the rates of de novo pyrimidine synthesis, of the formation of RNA pyrimidines, and of pyrimidine nucleoside excretion indicates that de novo synthesis provides only about 67% of the pyrimidines required for the consuming processes. The difference, as well as the dilution of labeled pyrimidine nucleotide pools under conditions of a blocked de novo pathway, suggests a considerable salvage of pyrimidine nucleosides derived from RNA. This salvage of pyrimidines may be intracellular and/or by an excretion and re-uptake process. Depletion of UTP and CTP pools, induced in hepatoma cells by D-galactosamine and 6-azauridine, leads to growth inhibition in suspension culture; this inhibition becomes irreversible in an increasing percentage of cells, killing all cells after 20 hr of UTP deficiency. The enhanced uptake of 5-fluorouridine by UTP-deficient cells was associated with an increase of FUMP incorporation into RNA up to 4-fold and with stronger inhibition of cell growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Potentiation of antimetabolite action by uridylate trapping.

Uridylate-trapping analogs of D-galactose or D-glucose divert the uridylate moiety of UDPglucose and/or UTP to UDP-sugar analogs that accumulate while the pools of UTP and of related pyrimidine nucleotides are depleted. The uridylate-trapping action of sugar analogs is determined by the enzyme pattern of the target tissue. D-Galactose analogs are preferentially metabolized by hepatoma cells and hepatocytes. TA3-mammary tumor cells are susceptible to the action of D-glucosamine and other D-glucose analogs. A high rate of de novo pyrimidine synthesis and/or an active salvage of extracellular uridine compensate for the uridylate-trapping action of sugar analogs and prevent depletion of UTP pools. Accordingly, synergistic actions are induced by combining sugar analogs, such as D-galactosamine or D-glucosamine, with inhibitors of de novo pyrimidine synthesis, such as lapachol or 6-azauridine. Uridylate-trapping by D-galactosamine, acting on hepatocytes, shifts the balance between uridine consumption and uridine release by the liver and results in a fall of uridine and cytidine concentrations in blood plasma (20). Cultured hepatocytes produce uridine and cytidine. Combination of 5-fluorouridine with sugar analogs results in the formation of fluorinated UDP-sugar analogs in hepatoma cells or in mammary tumor cells. Formation of FUDP-sugar analogs transiently removes intracellular FUTP, but FUDP can be released subsequently in glycosyltransferase reactions (22). Pretreatment of hepatoma cells or of TA3-mammary tumor cells with an uridylate-trapping sugar analog in combination with an inhibitor of de novo pyrimidine synthesis enhances the uptake of 5-fluorouridine, its incorporation into RNA, and its growth inhibitory effect. The chemotherapeutic action of 5-fluorouridine in rats and mice, carrying the AS-30D and the TA3 ascites tumor, respectively, is significantly improved by pretreatment with an amino sugar together with 6-azauridine.

Amino Sugars↗
PubMed1985

Short term changes of event related potentials during concept learning.

In a concept learning task, the time course of P3 amplitudes following the stimuli to be categorized was investigated. That time course did not coincide with the learning curve: it was not until the 4th to 6th trial after learning that P3 amplitudes grew larger. Some implications of this effect are discussed.

Adolescent↗
PubMed1984

Variability of single visual evoked potentials evaluated by two new statistical tests.

In this paper the trial-to-trial variability of brain responses (as opposed to the non-event-related background variability) in flash-evoked potential experiments is investigated by means of two new statistical tests, proposed by M ocks et al. (1984). In two groups of normal children, a considerable number of inhomogeneous responders could be detected, and evidence was found that two different modes of response variability are independently present in the data. The paper also gives a precise description of the computing algorithm and an empirical validation of the applicability to EP data of the two tests.

Adolescent↗
PubMed1984

How to select epochs of the EEG at rest for quantitative analysis.

Before performing a quantitative analysis of the EEG at rest, it has to be specified how many seconds are to be used later on, and a criterion has to be set down according to which data are selected for further analysis. In this paper it is investigated which duration is appropriate for an analysis of background, non-transient activity. Furthermore, different criteria for automatically selecting such epochs for quantitative analysis are compared with respect to broad-band parameters of a sample of 42 children. A length of 20 sec turned out to be sufficient, since using 40 sec or 60 sec did not improve the statistical variability. Moreover, a criterion based on minimizing EOG power from 1.5 to 7.5 Hz was mildly superior to other criteria considered. The particular choice of epoch was, however, less crucial than anticipated.

Adolescent↗
PubMed1984

EEG-responsiveness to eye opening and closing in mildly retarded children compared to a control group.

Changes in the ongoing EEG when repeatedly closing and opening the eyes are quantified and compared for a group of mildly retarded children and a matched control group. The most prominent changes occur for the alpha-rhythm at posterior derivations. Blocking of the alpha is faster than its restriction, but there is no group difference in this respect. Many of the amplitude changes are also quite similar for the two groups. The differences found are associated with a lowered arousal by the experimental group.

Adolescent↗
PubMed1983

Correlating EEG and IQ: a new look at an old problem using computerized EEG parameters.

Explorations of the relationship between EEG activity and scores of tests of intelligence have a long history. Results have been rather discrepant and several explanations can be offered for this. Almost all previous studies relied on parameters derived from a visual rather than a computerized analysis of the EEG. Here broad band spectral parameters were mainly used, and in addition the frequency of the dominant peak (at Pz, O1 and O2) between 6.5 and 14.0 Hz. All EEG parameters were standardized for age, as were tests of intelligence. Correlations of EEG with IQ were computed separately for a group of normal and a group of mildly retarded children. As postulated in advance, correlations were substantially higher for the retarded groups. Even more remarkable than the size of the correlations found was their pattern, associated with an interesting interpretation: the sign of the larger correlations was such that children developmentally more advanced in their EEG parameters had on the average higher IQ scores. Moreover, correlations were large for those frequency bands, and to a lesser degree for those derivations where we found a high developmental relevance in a previous investigation for the age range considered. Some disappointing or even discrepant results in earlier studies can be explained in the light of the results given here.

Brain↗