Biomedical subjects

W D Thomson

Publications and source records attributed to W D Thomson.

9 recordsLinked to original sources

PubMed1999

A new approach to vision screening in schools.

PURPOSE: To determine the prevalence of visual "defects" among a sample of young schoolchildren and evaluate a new system for vision screening in schools. METHODS: A new system for vision screening in schools has been developed. The system is based on a computer program which may be run on a low specification personal computer. Information about the child's symptoms, history and family history is acquired by means of a parental questionnaire and entered into the program prior to the vision screening. Distance visual acuity and stereopsis are measured directly on the computer screen and colour vision is assessed using a reduced Ishihara test. The program carries out an "expert" analysis of the questionnaire data and the vision test results and determines the most likely diagnosis. Reports for parents, teachers, optometrists, doctors can be generated automatically and statistics relating to the overall screening program are available. An evaluation of a prototype version of the system was carried out on 245 schoolchildren aged between 5 and 8 years. RESULTS: Overall, 48 children failed the screening (excluding colour vision deficiencies), 32 of whom were unaware of any problem with their eyes. Comparison of the result of the vision screening with the outcome of a full eye examination gave a sensitivity of 93.8% and a specificity of 96.1%. CONCLUSION: A significant number of young school children have unsuspected remediable visual defects. The computer-based vision screener provides an efficient, sensitive and specific method for screening in schools.

Child↗

PubMed1998

Eye problems and visual display terminals--the facts and the fallacies.

Estimates of the prevalence of eye problems associated with using visual display terminals (VDTs) vary enormously depending on the sample tested and the research methods employed. However, most authors agree that eye problems are common among VDT users. In this paper, the relative contributions of the nature of VDT displays, workstation design, work-practices and optometric factors are discussed, drawing from the substantial body of literature which now exists on this subject.

Asthenopia↗

PubMed1997

The perception of flicker on raster-scanned displays.

Experiments have verified the observations of Corbett and White that people are more sensitive to flicker on raster-scanned displays (e.g., television and visual display terminals) if the field scan direction is changed from the conventional top-to-bottom direction to bottom to top, left to right, or right to left. When measured by means of the flicker fusion frequency (FFF), this effect is present only when normal eye movements are permitted. With fixation, the FFF is similar for all scan directions. The effect is demonstrated by the different frequencies at which a directional component is detected in the flicker (which we have called the rolling flicker threshold frequency, or RFTF); the RFTF is lower for top-to-bottom scanning. The effect does not appear to be associated with any asymmetry in eye movements or with eccentricity of the retinal image. The hypothesis that this difference is a consequence of adaptation to televisions and VDTs is supported by the observation that the effect's magnitude correlates with daily exposure time to televisions and VDTs and by a pilot study showing that the effect is reduced when one views a television scanned from bottom to top for 1 h.

Adult↗

PubMed1997

Recovery of spatial vision during dark adaptation in normal subjects.

We show, how sensitivity to sinusoidally-modulated gratings changes throughout the course of dark adaptation. The luminance required to resolve the bars of a variety of sinusoidal gratings of (0.6-14 c.p.d.) presented in the parafovea, was determined for 30 min following a full bleach. Data obtained for low and intermediate spatial frequencies (0.6 and 3.5 c.p.d.) was qualitatively similar to the classical dark adaptation function, showing discrete cone and rod phases. Threshold to a 14 c.p.d. grating fell initially, then after approximately 10 min, rose by between 0.40 and 1.17 log units. This effect was not apparent for the 8.3 c.p.d. grating, despite the fact that both spatial frequencies must have been detected by cones. The phenomenon has been attributed to rod-suppression of cone-mediated vision.

Dark Adaptation↗

PubMed1995

Effects of exercise on aspects of visual function.

Strenuous exercise has been reported to affect various aspects of visual and ocular function. In the first of two experiments, the effects of cycling, jogging and stair running on a range of visual functions were examined. None of these forms of exercise was found to have any significant effect on visual acuity, refractive error, dark focus, amplitude of accommodation or pupil size. Contrast sensitivity (CS), measured by an ascending method of limits (AML), was found to improve significantly after cycling and jogging but not after stair running. To investigate if the apparent improvement in CS was related to a change in 'sensitivity' or a change in decision criteria, CS was re-measured before and after cycling using both the AML and a 'criterion-free' method of constant stimuli (MOCS) procedure. Results obtained using the AML, confirmed the findings of the first experiment. However, results obtained using the MOCS procedure showed no significant change with exercise. This suggests that the improvement in CS found using the AML was due to a shift in subjects' decision criteria rather than a physiological change in 'sensitivity'. It is argued that this change in decision criteria may be related to the positive mood changes which have been found to occur after exercise. This study demonstrates the importance of differentiating between changes in 'sensitivity' and changes in decision criteria especially where mood or motivation (and hence decision criteria) may be confounded with the independent variable. This is particularly important when evaluating the effects of exercise on visual function or evaluating the efficacy of vision training.

Accommodation, Ocular↗

PubMed1995

A quantitative analysis of eye movements during the cover test--a preliminary report.

The aim of this study was to carry out quantitative analyses of eye movements during the cover test on a group (n = 57) of asymptomatic emmetropes. Eye movements were recorded during an automated cover test while subjects fixated a distance (3.4 m) and near (0.4 m) target. There was a significant difference between the amplitude of phoria measured after 2 s of occlusion compared to 10 s (P < 0.01). The mean phoria after 10 s of occlusion was 0.1 degree (eso) for distance fixation and 1.6 degrees (exo) for near fixation. The distribution was approximately normal for both distance and near fixation but the standard deviation was greater for near (2.0 degrees) than distance fixation (1.4 degrees). The pattern of eye movements during the recovery phase was more complex than is often assumed, in many cases consisting of a variety of saccadic and vergence movements involving both the covered and 'fixing' eye. The latency of the first recovery movement was significantly shorter for exophoric than esophoric deviations (P < 0.01) and the latency of some recovery movements were very short (< 150 ms).

Adolescent↗

PubMed1990

A new system for the measurement of ocular motility using a personal computer.

A new system (automated Hess screen, AHS) for the measurement of ocular motility using a personal computer is described. The subject is seated with his/her head restrained at a fixed distance from a colour monitor and views the screen through red/green goggles. A red target is displayed on the screen and the subject is instructed to move a green stimulus (using a joystick control) until the stimulus appears to be centred on the target. A button on the joystick is pressed and the computer registers the relative position of the target and stimulus. This procedure is repeated for a range of target locations (positions of gaze) and the entire routine is then repeated with the other eye fixing, by reversing the colour of the target and stimulus. The oculomotor fields may be displayed on the screen, printed, plotted or saved on a magnetic disc. A prototype system (based on an IBM AT with a 20 in monitor viewed from 25 cm) has been evaluated in a clinical environment on a sample of patients with a wide range of oculomotor problems, and compared with the electronic Hess screen (EHS). In general, the shapes of the oculomotor fields obtained using the two instruments were similar, but the AHS tended to measure a slightly converged displacement of the plots relative to the EHS and to record larger deviations, particularly in patients with incomitant fields. However, the test-retest reliability and the diagnostic value of the charts obtained were similar for both instruments. There was no significant difference between the durations of the examinations with the EHS and AHS. The AHS is a viable alternative to the EHS.

Adolescent↗

PubMed1987

Pupil response as an objective measure of visual acuity.

Pupillary responses to brief presentations of sinusoidal grating patterns were measured. The results show that the amplitude of the pupillary responses varies systematically with grating spatial frequency. Gratings of spatial frequencies in the range 1-5 c deg-1 elicited the maximum percentage change in pupil area, with higher and lower spatial frequency gratings producing smaller responses. The bandpass nature of the pupil response function was compared to contrast sensitivity functions obtained under the same viewing conditions. The effects of defocus and eccentric fixation on both contrast sensitivity and pupil response functions were qualitatively similar. Grating acuities calculated from contrast sensitivity data correlated well with similar data extracted from pupil response functions. The results suggest that automated pupillometry can provide an attractive means for objective measurement of visual acuity.

Fixation, Ocular↗

PubMed1984

An asymmetry in the optokinetic nystagmus response in normal individuals.

Optokinetic nystagmus (OKN) was induced by using a modified projector system to provide a moving pattern of irregular spots of light. A systematic electro-oculographic study of horizontal and vertical OKN for a series of stimulus velocities from 10 to 100 degrees s-1 was conducted on 10 healthy young subjects. It was found that the vertical responses, in particular to upward-moving stimuli, were less well sustained at velocities above 30 degrees s-1 than the horizontal responses. In view of these observations, care should be taken in the interpretation of OKN results clinically, and only when the response is absent in one direction or an extreme asymmetry is present should clinical significance be attached. This applies particularly for the vertical OKN responses. Clearly, the use of stimulus velocities below 40 degrees s-1 is advised and the standard suggested by Holm -Jensen and Peitersen [Acta oto-lar. 88, 110-116 (1968)] of a target moving at 20 degrees s-1 seems a reasonable choice.

Adult↗