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Biomedical subjects

A H Clarke

Publications and source records attributed to A H Clarke.

16 recordsLinked to original sources
PubMed1991

Measuring three dimensions of eye movement in dynamic situations by means of videooculography.

A primary function of the vestibular system is the stabilisation of the eye during head movement. Consequently, evaluation of reflex eye movements represents an essential means to both clinical diagnosis and researching of the vestibular function. Movements in the eye can be resolved into three orthogonal components, i.e. horizontal, vertical and torsional. As an improvement on most current techniques, which provide only measurement of the horizontal and vertical components, videooculography (VOG) facilitates non-invasive measurement of all three of the defined components. To date, only the scleral coil technique, which involves the semi-invasive placement of coil rings onto the bulbi, yields a continuous measure of eye torsion. Employment of suitable solid-state devices permit the integration of a compact, high resolution video recording system. In the basic configuration, eye movements can be observed and simultaneously recorded for later analysis or documentation. The video images of the eye are obtained by means of a miniaturised CCD video sensor mounted on a light-occluding mask. Image processing of the acquired video images determines horizontal and vertical coordinates of eye position online. Ocular torsion, as reflected by the rotation of the natural iris, is measured for each video frame. The VOG algorithm has been implemented on a PC based workstation, which permits online observation, recording and evaluation of eye movements. In addition, the technique has found clinical application as a portable eye-movement observation and recording system, allowing bedside examination and recording of transient symptoms. Preliminary results from various studies, including the objective evaluation of positional nystagmus (BPPN), are presented.

Eye Movements↗
PubMed1988

Caloric stimulation during short episodes of microgravity.

Caloric testing was performed during parabolic flight at the NASA Reduced Gravity Facility in Houston, Texas. Six test subjects were stimulated with continuous unilateral air insufflation (25 degrees R), in a manner similar to the experiments performed in the extended weightlessness of orbital flight during the SL1 and D1 Spacelab missions. Nystagmus response was recorded by electro-oculography and eye video image. It was the purpose of the experiments to re-examine the apparent discrepancy between the disappearance of caloric nystagmus during short episodes of weightlessness and the finding that caloric responses can be elicited during periods of extended weightlessness. The present results agree with those of earlier experiments in that a prompt reduction of caloric nystagmus occurs on transition from hypergravity (1.8 G) to weightlessness. The time constant of nystagmus decay was estimated to be approximately 2-3s, a value which cannot be explained by cupular mechanics. A central gating mechanism involving the labyrinthine canal and otolithic afferents is proposed for the observed modulation of caloric nystagmus.

Caloric Tests↗
PubMed1987

Thermal stimulation of the vestibular labyrinth during orbital flight.

During the European Spacelab mission (SL1) in 1983, caloric testing was performed for the first time in long-term weightlessness. After 2 days into orbital flight an unequivocal caloric nystagmus was observed in both subjects tested which corresponded in both quality and intensity with that measured in one-g conditions on Earth. The subsequent D1 mission enabled the experiment to be repeated on further subjects and with improved measurement procedures. As with the SL1 findings, the observed caloric nystagmus response proved to be equivalent to that measured during baseline testing on Earth. Renewed consideration of peripheral and central mechanisms, which might be involved in the elicitation of the caloric response--both in one-g and zero-g environments--has led to the reopening of a number of associated issues. One important observation which has been addressed by various research groups concerns the influence of the labyrinth's orientation to the gravity vector on the caloric response. The present authors have examined a group of healthy subjects in various body positions in the sagittal plane. The interindividual variability in the response behavior was found to be high; indeed single cases were observed in which the nystagmus response did not invert from the supine to the prone positions. These findings are discussed together with earlier reports in the literature.

Caloric Tests↗
PubMed1986

European vestibular experiments on the Spacelab-1 mission: 3. Caloric nystagmus in microgravity.

Response to caloric stimulation was examined in two subjects in the weightless environment of orbital flight. Using air insufflation, a binaural temperature stimulus profile was performed twice on each subject during flight. In all but one test, which was carried out on the first mission day, a caloric nystagmus was registered. This zero-g nystagmus was similar to ground-based pre- and postflight responses with regard to its intensity, its temporal correlation with the stimulus profile (nystagmus was always directed towards the warmer ear) and the subjective sensations reported by the tested subjects. These findings demonstrate that mechanisms other than thermoconvection are involved in the elicitation of the caloric nystagmus response. At each stage of the temperature stimulus profile, linear acceleratory stimulation was presented in the form of oscillations in the X-axis. Some differences were found between one-g and zero-g conditions in the resultant modification of the nystagmus response.

Caloric Tests↗
PubMed1985

[Physiology of the caloric equilibrium reaction. Consequences from results of space experiments in Spacelab 1, December 1983].

A caloric nystagmus was observed during weightlessness in orbital flight. This cannot be reconciled with the classical convection theory of the caloric mechanism. Alternative hypotheses are discussed. It appears that a direct thermal effect on the canal afferents could only play a small part. It is suggested that a stronger effect is produced by the temperature-mediated volume change in the horizontal semicircular canal, the asymmetric stimulation of the canal ring leading to volume displacement towards the cupula. On earth, the convective torque produced by redistribution of endolymph mass may be considered as a secondary effect, comparable to a stimulus producing a nystagmus threshold response. Modulation of the caloric and rotatory nystagmus by body position and the role of the otoliths are also discussed.

Caloric Tests↗
PubMed1976

A mathematical approach to epidemic control.

A mathematical model of an influenza epidemic which occurred in 1961 is suggested. The mathematics imply conclusions on the practical control of similar outbreaks. This is a technique applicable to one general practice.

Disease Outbreaks↗
PubMed1975

Radioimmunoassay of progesterone, 17-hydroxyprogesterone, estradiol-17beta and prostaglandin F in human corpus luteum.

Radioimmunoassay procedures have been adapted for the assay of progesterone, 17-hydroxyprogesterone, estradiol-17beta, and prostaglandin F in human corpus luteum. The method utilises a single homogenisation and extraction of the tissue followed by fractionation of the steroids on alumina, and separation of the prostaglandins of the F series from the E and A series on silica gel, prior to radioimmunoassay. An attempt has been made to validate the method for the progestins by comparison with results after fractionation of the progestins on Sephadex LH-20, for estradiol-17beta by comparison with values obtained with competitive protein-binding, and for prostaglandin F by comparison with values after additional purification. The results showed that peak concentrations of the three steroids in corpora lutea from women during the luteal phase of the menstrual cycle were comparable to those found in corpora lutea from women in early pregnancy. However, in six out of fourteen corpora lutea from non-pregnant women, prostaglandin F levels were higher than those found in corpora lutea from seven women in early pregnancy, i.e. 13-46 ng/g compared with 1-7 ng/g. Of the above six corpora lutea, four were on days 23-25 of the cycle, at a time when luteolysis would be commencing. The results in this paper support the conclusion that the corpus luteum is a major site of synthesis of the three steroids examined, although the site of synthesis of prostaglandin F is still equivocal.

Adult↗
PubMedDate not supplied

The caloric vestibular reaction in space. Physiological considerations.

Caloric stimulus testing was performed as part of the vestibular research program during the European Spacelab 1 mission in Nov/Dec 1983. Contrary to prediction according to the classical endolymph flow theory originally forwarded by Bárány, caloric nystagmus was elicited in both tested astronauts. The intensity of the response was found comparable to that measured on earth. The theoretical consequences of these findings are discussed and possible mechanisms are considered. The direct volume displacement hypothesis is favoured as the primary effect responsible for the observed vestibulo-ocular nystagmus. Estimated differential pressure conditions resulting in the endolymph canal support this hypothesis and are in agreement with the observed response intensity. It is further speculated that interaction in the central vestibular system between canal and otolith signals be responsible for the well-known body position modulation of the observed nystagmus.

Biomechanical Phenomena↗
PubMedDate not supplied

Body position and caloric nystagmus response.

The observation that the caloric nystagmus response is dependent on body position has been repeated in several studies during the course of this century. For many, this position-dependent modulation of the caloric response has been interpreted as evidence in favour of the thermoconvection theory as originally proposed by Bárány. However, the adequacy of this theory has been put into question by recent observations of caloric nystagmus during weightlessness in orbital flight. These zero-g findings clearly demonstrate that any hypothesis based on thermoconvection alone must prove insufficient as a description of the caloric nystagmus response. In the light of these recent findings, it has also become necessary to reconsider the influence of body position on caloric nystagmus intensity and the physiological mechanisms involved. Caloric testing was performed with a group of 30 healthy test subjects. Each person was tested in eight different body positions in the sagittal plane. Caloric nystagmus response was registered by means of horizontal and vertical EOG. The observed modification of the caloric response (SPV) by assumed body position is discussed with reference to associated reports in the literature.

Adult↗
PubMedDate not supplied

The anti-motion sickness mechanism of ginger. A comparative study with placebo and dimenhydrinate.

A controlled, double-blind study was carried out to determine whether nystagmus response to optokinetic or vestibular stimuli might be altered by some agent contained in powdered ginger root (Zingiber officinale). For comparative purposes, the test subjects were examined after medication with ginger root, placebo and with dimenhydrinate. Eye movements were recorded using standard ENG equipment and evaluation was performed by automatic nystagmus analysis. It could be demonstrated that the effect of ginger root did not differ from that found at baseline, or with placebo, i.e. it had no influence on the experimentally induced nystagmus. Dimenhydrinate, on the other hand, was found to cause a reduction in the nystagmus response to caloric, rotatory and optokinetic stimuli. From the present study it can be concluded that neither the vestibular nor the oculomotor system, both of which are of decisive importance in the occurrence of motion sickness, are influenced by ginger. A CNS mechanism, which is characteristic of the conventional anti-motion sickness drugs, can thus be excluded as regards ginger root. It is more likely that any reduction of motion-sickness symptoms derives from the influence of the ginger root agents on the gastric system.

Adult↗