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Biomedical subjects

A H Clarke

Publications and source records attributed to A H Clarke.

At least 19 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↗