Search PubMed⌕ Search

Biomedical subjects

A Kok

Publications and source records attributed to A Kok.

At least 73 records · Page 4Linked to original sources
PubMed1992

Removal of ocular artifacts from the REM sleep EEG.

The present report concerns the first study in which electrooculographic (EOG) contamination of electroencephalographic (EEG) recordings in rapid eye movement (REM) sleep is systematically investigated. Contamination of REM sleep EEG recordings in six subjects was evaluated in the frequency domain. REM-active and REM-quiet series were obtained for each subject. Transfer coefficients and power spectra of EOG and EEG indicated that (a) increases in transfer coefficients beyond 4.5 Hz are brought about by residual EEG in the EOG, and (b) EOG-EEG contamination in the delta band is most pronounced in frontal, intermediate in central and negligible in occipital leads. It was found that correction of the REM-active series resulted in significant (c) reductions in power, (d) increases in interhemispheric coherences and (e) reductions in degree of lateral asymmetry. These effects were largest for frontal leads, but still marked for central ones. The results are discussed in the light of previous findings concerning models of hemispheric functioning during REM sleep.

Adult↗
PubMed1991

The effect of acute and chronic protein loading on urinary pepsinogen A excretion.

In 8 healthy male volunteers, urinary excretion (UE) and fractional clearance (FC) of pepsinogen A (PGA), beta 2-microglobulin (beta 2-m) and albumin were measured after 6 days high protein diet (HPD; 2.0 g/kg/day) and compared to values obtained after 6 days low protein diet (LPD; 0.5 g/kg/day). In addition, the effect of an acute protein load (APL; 500 g beef) on these variables were measured. Both chronic and acute protein loading induced a rise in glomerular filtration rate (GFR) of about 10% together with a parallel rise in effective renal plasma flow. UE PGA and FC PGA increased both after HPD (UE PGA 1,707 +/- 1,106 ng/min; FC PGA 23 +/- 12%) as compared to LPD (UE PGA 1,200 +/- 987 ng/min, p less than 0.01; FC PGA 18 +/- 12%, p less than 0.05), and after APL (UE PGA 2,276 +/- 1,389 ng/min; FC PGA 26 +/- 16%) as compared to baseline (UE PGA 1,418 +/- 965 ng/min, p less than 0.02; FC PGA 21 +/- 12%, p less than 0.05). UE and FC of beta 2-m and albumin were not affected by protein loading. As PGA is nearly freely filtered, it is concluded that the increase in fractional PGA clearance reflects a decrease in fractional tubular PGA reabsorption. Our results suggest that an increase in fractional protein clearance after protein loading is not necessarily due to an impaired glomerular permselectivity but represents a decreased fractional tubular reabsorption as a result of a GFR-mediated increase in filtered load without a concomitant increase in tubular reabsorption.

Adult↗
PubMed1990

Internal and external control: a two-factor model of amplitude change of event-related potentials.

The present paper critically examines the contributions of Event-Related Potential (ERP) measures in mental chronometry research. It is argued that amplitude variations in ERP components may provide valuable information regarding the intensity and timing of information processes, and that these amplitude changes are related to energetical rather than to computational processes. It is also suggested that amplitude variations of ERP components in visual discrimination and selective attention tasks are caused by two different processing modes, denoted as external and internal control that are associated with different neural structures. It is further assumed that these two control systems converge upon thalamic neurons that regulate the sensory input to cortex, and that the direction of sustained ERP amplitude changes reflects which system is dominant. Recent ERP studies have shown that the effects of task variables related to motor control are manifested in a surprisingly early phase of the ERP waveform, and that these effects overlap in time with the effects of task variables related to input control. These findings suggest that at least in visual discrimination and selective attention tasks external and internal modes of processing may be activated in parallel.

Arousal↗
PubMed1989

Stimulus probability and motor response in young and old adults: an ERP study.

The effects of responding hand and stimulus probability were investigated in young and old subjects in an RT task in which both rare and frequent stimuli required a response. It was found that the effect of stimulus probability was less pronounced in old subjects than in young, and that the latency of P3 was longer in the elderly, although their RTs were not different from young subjects. ERPs for right hand responses were larger than for left hand responses; this difference was discernible already in the P2 and N2 peaks of the ERP. A tentative explanation is offered for these large and unexpected hand differences. An interpretation in terms of an age-related decrease in resources is proposed for the increased P3 latency and the decreased probability effect on P3 amplitude in old subjects.

Adolescent↗
PubMed1989

Identification of mycobacterial antigens for "ELISA" serology in the diagnosis of leprosy and tuberculosis.

Using an immunoblotting assay (ImBA), several immuno-crossreactive antigenic components (ImCRAC-myc) have been identified in the whole sonicates of M. bovis-BCG, and M. tuberculosis (Mtb) and M. leprae (ML) whereby the sera of 100% lepromatous leprosy (L-Lep) reacted to 29/33 KD doublet and that of 100% tuberculoid leprosy (T-Lep) reacted to 64 KD bands. The antigens upon purification from Mtb Sonicates were used in a direct ELISA to measure antibody isotypes in the sera from L-Lep, T-Lep, healthy Lep. contacts (Lep. c), normal Dutch controls (N) and tuberculosis (TB) patients. A significantly high IgG titre to the doublet 29/33 KD and to 64 KD were observed among L-Lep and T-Lep patients respectively in comparison to sera from other groups of individuals. In certain cases of L-Lep patients, raised IgM titre to either or both to 29/33 KD doublet and 64 KD were also found. On the other hand, consistantly but significant high IgA-antibody titre to cell wall (CW), cytosol (cyt) and P90 fractions of Mtb distinguished clearly the TB patients from Lep groups, normals (NN) and Lep-c. It appeared that such antibody reactivity of TB sera might be directed to the groups of 58-60, 38-40, 18-20 and 14 KD antigens of mycobacteria e.g. Mtb. On the basis of the present observations we conclude that the measurement of class specific antibody response to the panel of these antigens could diagnose differentially between Lep, TB and NN/Lep-c among the population at large in an endemic area.

Antibodies, Bacterial↗
PubMed1988

The development of lateral event-related potentials (ERPs) related to word naming: a four year longitudinal study.

Event-related potentials (ERPs) were recorded from the left and right temporal and parietal sites during a word naming task. Subjects were a group of children that were followed over four consecutive years starting at Kindergarten. ERP waveforms contained a sequence of positive and negative components (N150, P240, N360, N530 and SW). All components, except N150, showed changes in amplitude as a function of age, whereas SW, N360 and N150 also changed in hemispheric distribution. In addition, a relationship was found between reading performance and ERP amplitudes over the right parietal hemisphere in young children, and over the left temporal hemisphere in older children. Proficient readers showed larger (more negative) parietal N530 amplitudes than less proficient readers, especially when stimuli were degraded words. The results are discussed in terms of age-related changes in right and left hemisphere functions involved in early and advanced stages of reading, that might possibly be related to visual word recognition.

Aging↗
PubMed1988

Overlap between P300 and movement-related-potentials: a response to Verleger.

In reply to Verleger's commentary, I first clarify what the term "MRP overlap" implies; by means of a simple additive simulation it is shown that overlap between a stable P300 and a MRP that moves in time with reaction-time, may cause two types of artefacts--amplitude reduction and double peaking of P300. Following Verleger's suggestion, it is then argued that the effects of stimulus-degradation on P300 amplitude in our experiments was, at least in part, due to overlap with negative components of the MRP (in the go-intact condition), and with positive CNV return (in the nogo-intact condition). It is concluded that MRP overlap is not only an artefact but also a useful index of motor-preparation in reaction tasks, and that more refined methods should be developed in future research for accurate estimation of MRP overlap in stimulus-locked ERP waveforms.

Arousal↗
PubMed1988

Learning where to look: electrophysiological and behavioral indices of visual search in young and old subjects.

The present investigation explores the way young and elderly subjects use regularities in target location in a visual display to guide search for targets. Although both young and old subjects show efficient use of search strategies, slight but reliable differences in reaction times suggest decreased ability in the elderly to use complex cues. Event-related potentials were very different for the young and the old. In the young, P3 amplitudes were larger on trials where the rule that governed the location of the target became evident; this was interpreted as an effect of memory updating. Enhanced positive Slow Wave amplitude indicated uncertainty in random search conditions. Elderly subjects' P3 and SW, however, seemed unrelated to behavioral performance, and they showed a large negative Slow Wave at central and parietal sites to randomly located targets. The latter finding was tentatively interpreted as a sign of increased effort in the elderly to allocate attention in visual space. This pattern of behavioral and ERP results suggests that age-related differences in search tasks can be understood in terms of changes in the strategy of allocating visual attention.

Adolescent↗
PubMed1986

Hemispheric distribution of ERP components and word naming in preschool children.

Event-related potentials (ERPs) were recorded at the left and right temporal and parietal sites during a word-naming task in 61 children ranging in age from 5.1 to 6.2 years. Principal component analysis (PCA) was performed on the average ERP waveforms. Analyses of the factor scores revealed that the P240, N380, and SW components were asymmetrically distributed over the hemispheres. Fast reading acquisition was associated with smaller N380 and larger SW activity than slow reading acquisition. Multiple regression analyses indicated that only right hemispheric ERP components were significantly associated with rate of reading acquisition. These findings are discussed in terms of right hemispheric involvement in early reading acquisition.

Brain Mapping↗
PubMed1986

Effects of degradation of visual stimulation on components of the event-related potential (ERP) in go/nogo reaction tasks.

In the present experiments, event-related potentials (ERPs) were measured to randomly presented letters, requiring either a right-hand button-press response (go), or requiring no response (nogo). In both go and nogo conditions, the letters could be intact or degraded. ERPs time-locked with voluntary right hand button-presses were measured in a separate condition. Nogo stimuli elicited larger negative components (N400, N480) at the frontal electrode sites, and larger late-positive components (P600, P640) at the central electrode sites, than go stimuli. Additionally, intact stimuli elicited larger late-positive components and smaller Slow Wave (SW) components then degraded stimuli, but these amplitude-differences were seen more clearly in the nogo than in the go conditions. These results are discussed in relation to cognitive and motor aspects of go and nogo stimuli, and the possibility of contamination of ERP components with movement-related potentials.

Adolescent↗
PubMed1986

Effects of haemodilution with fluorocarbons or dextran on oxygen tensions in the acutely ischaemic myocardium.

The effect of haemodilution with dextran or with oxygen transporting fluorocarbons (Fluosol-DA 20%) on myocardial oxygen tension (PmO2) during experimental myocardial ischaemia was studied in pigs. Polarographic oxygen microelectrodes were introduced 3 mm to the left ventricle wall and the distal one-third of the left anterior descending coronary artery (LAD) was occluded. Anaesthesia was maintained with 0.5% halothane in oxygen to ensure maximal oxygen content of the fluorocarbons. The animals were divided into three groups of five animals each. In group I no treatment was given and in groups II and III, after bleeding 20 ml kg-1, haemodilution was performed with Fluosol-DA 20% or 5% dextran respectively. Occlusion of the LAD caused no significant changes in cardiovascular vascular variables and only in group III (dextran) were significant effects of haemodilution observed. LAD occlusion caused highly significant decreases in PmO2. After haemodilution, PmO2 in the Fluosol group II (in contrast to groups I and III) was no longer significantly different from pre-occlusion values. After 5 h of occlusion, mean PmO2 in this group had returned to 92.2% of the pre-occlusion values, whereas in groups I and III it was 27.8% and 33.7%, respectively.

Animals↗