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Biomedical subjects

A Kok

Publications and source records attributed to A Kok.

At least 91 records · Page 5Linked to original sources
PubMed1985

Effects of visual-field and matching instruction on event-related potentials and reaction time.

Vertical letter pairs were presented randomly in the left and right visual hemifields in a physical identity match and name identity match condition. The reaction times showed a right visual field superiority for name matches, and a left visual field superiority for physical matches. Event-related potentials to letter pairs showed a sequence of three waves: a negative wave (N2, around 270 msec), a positive wave (P3, around 500 msec), and a broad positive slow wave (SW, around 600-700 msec), respectively. P3 and SW amplitudes were consistently larger at the left hemisphere than at the right hemisphere, regardless of the field of stimulation. At both hemispheres, N2 waves were always larger to stimuli presented in the visual field contralateral to a hemisphere than stimuli presented in the visual field ipsilateral to a hemisphere. The positive waves (P3, SW) showed the opposite pattern: smaller amplitudes to stimuli that were presented contralaterally than stimuli that were presented ipsilaterally to a given hemisphere. These results were attributed to a shift in sustained negativity on the directly stimulated hemisphere, relative to the indirectly stimulated hemisphere, reflecting either sensory at attentional processes in the posterior cerebral hemispheres.

Adult↗
PubMed1984

Mechanism of neuroadenolysis of the pituitary for cancer pain control.

Several theories have been advanced to explain how neuroadenolysis of the pituitary (NALP) relieves cancer pain. Interference with hormonal regulation, interruption of pain pathways and a compensatory overproduction of brain endorphins have been proposed. The purpose of the present experimental study is to determine whether neuronal activity of the pituitary gland, as related to the primary somatosensory cortex (PSC), may be involved in the pain perception pathway influenced by NALP, using EEG and tooth pulp evoked potentials (TPEPs). Pituitary activity showed a high voltage slow activity which slowed after alcohol injection into the sella turcica. When naloxone was injected after NALP, a rhythmic high voltage activity appeared in the pituitary gland. After NALP, the TPEP recorded from the PSC disappeared, while NALP induced an increase in the amplitude of the TPEP in the pituitary gland. An injection of naloxone severely decreased this response in the pituitary gland, in contrast to changes in the PSC where the original response reappeared after an injection of naloxone. Our hypothesis is that an increase of TPEPs (hyperactivity) in the pituitary gland is produced after alcohol wounding (wounding effect), leading to a decrease of pain response in the sensory cortex (decrease of TPEPs). This wound effect may be influenced by endorphins, because naloxone, a specific antagonist of opiate receptors, reversed the changes in TPEPs in both places.

Analgesia↗
PubMed1984

Fluorocarbons and the microcirculation.

A short discussion of oxygen transport and delivery by fluorocarbon containing blood substitutes is presented. Those physical properties of the preparations that would tend to cause improvement in oxygenation in areas of damaged or impeded microcirculation are also discussed. A number of studies carried out at the Erasmus University, Rotterdam have demonstrated certain important characteristics of fluorocarbons with respect to tissue oxygenation. In the first instance, it has been demonstrated that extracorporal perfusion with Fluosol-DA 20% can maintain oxygenation and viability of isolated extremities for periods of up to 48 hours. Secondly it has been shown that the preparation can indeed penetrate compromised capillary beds, its low viscosity enabling it to travel along collateral perfusion pathways and to provide oxygenation of areas of ischaemic hypoxia. The low particle of the emulsion may, in combination with the low viscosity of the product, contribute to the re-establishment of perfusion in areas of circulatory arrest. The clinical implications of the above findings are numerous and wide spread. The importance of Fluosol in maintaining and improving micro-circulatory oxygenation should not be underestimated.

Animals↗
PubMed1980

The effect of repetition of infrequent familiar and unfamiliar visual patterns on components of the event-related brain potential.

This experiment examined changes in the waveforms of the event-related brain potential (ERP) during repeated presentations of infrequent-familiar and infrequent-unfamiliar visual patterns. The EEG waveforms were averaged separately for each presentation of the two types of stimuli across different stimulus blocks. Principal components analysis and baseline-to-peak measurement identified three ERP components, a P3, P4 and a slow wave of diminishing positivity at anterior, central and posterior scalp sites. Unfamiliar stimuli evoked components of greater positivity and P3 waves of longer latencies than familiar stimuli. Repetition of these stimuli was associated with a progressive diminution of the amplitude of the three components and a shortening of P3 latency. For the slow wave the decline in amplitude was more pronounced during repetition of unfamiliar than familiar stimuli. These effects are interpreted as signs of alterations in cognitive processes that play a role during orienting and perceptual learning.

Adolescent↗
PubMed1980

Components of the event-related potential following degraded and undegraded visual stimuli.

Event-related potentials were recorded in response to visual stimuli in two different reaction tasks in which subjects were instructed to react immediately to the stimuli, or to delay their response for a 2 sec period, respectively. There were four types of stimuli: frequent-degraded, frequent-undegraded, infrequent-degraded and infrequent undegraded letters. In all conditions the stimuli evoked complex waveforms which comprised a frontal-negative wave (N400) and a late positive wave that reached a maximum amplitude on the parietal scalp site (P500). In addition, a slow positive wave with a central-parietal scalp distribution was found in the waveforms that were associated with the delayed reaction task. A principal components analysis of the waveforms yielded two major components: an early composite component that peaked around 400 msec, and a late component that became maximally active towards the end of the recording epoch. The scores of the earlier component were more negative (or less positive) and the scores of the late component were more positive when infrequent or degraded stimuli were presented, in comparison with frequent or undegraded stimuli.

Adult↗
PubMed1978

The effect of warning stimulus novelty on the P300 and components of the contingent negative variation.

This experiment evaluated the effects of stimulus and response probability on the P300 and components of the contingent negative variation (CNV) in three different reaction tasks. In the first task the warning stimulus (S1) indicated that the imperative stimulus (S2) would require either a left or a right button press response; in the two other tasks the S1 indicated that S2 would either require a button press response or not. The a priori probability of S1 and the response to S2 were independently varied. The results show that a novel S1 significantly increased the amplitude and latency of the P300 and early CNV component: the late CNV component, however, increased significantly when a motor response to S2 was required. Early and late CNV components also differed in midline distribution. It is concluded that the early CNV can be viewed as the slow wave part of the event-related potential to S1, while the late CNV is an event preceding shift that strongly relates to the motor requirements of S2. It is finally suggested that both the P300 and early CNV reflect an orienting response to S1.

Acoustic Stimulation↗