Biomedical subjects

M K Yap

Publications and source records attributed to M K Yap.

10 recordsLinked to original sources

PubMed1997

Changes in ocular dimensions and refraction with accommodation.

The purpose of this study was to measure the changes in ocular dimensions with accommodation, with particular reference to the radius of curvature of the posterior surface of the crystalline lens. The increase in power of the eye with accommodation is considered to arise primarily from a decrease in the radius of curvature of the anterior surface of the lens, with the role of the posterior surface somewhat unclear. We measured the axial dimensions (A-Scan ultrasonography), corneal radius of curvature (keratometry), refractive error (auto-refractor) and radii of curvature of the lenticular surfaces (video phakometry) for 11 subjects, mean age 21.2 +/- 2.6 years, for five levels of ocular accommodation up to 8.00 D. At maximum accommodation the mean changes were a decrease in anterior chamber depth of 0.24 mm, an increase in lens thickness of 0.28 mm, a decrease in radius of curvature of the anterior surface of the lens of 4.95 mm and 1.34 mm for the posterior surface. The corresponding increase in power of the lenticular surfaces for an equivalent refractive index of 1.422 for the lens was 5.53 D and 3.10 D for the anterior and posterior surfaces respectively. No significant changes were recorded in axial length or vitreous chamber depth. We conclude that when crystalline lens power is calculated on the basis of an equivalent refractive index, changes in the posterior surface of the lens contribute around one third of the increase in lens power associated with 8.00 D of ocular accommodation.

Accommodation, Ocular↗

PubMed1996

Contrast and luminance as parameters defining the output of the VERIS topographical ERG.

The Visual Evoked Response Imaging System (VERIS I) topographical electroretinogram system (EDI Associates, San Francisco, CA) allows measurement of the response of large numbers of retinal areas simultaneously. This paper examines ERG responses derived with the VERIS system to changes in target contrast and to local reductions in luminance. There is a linear reduction of response with reductions of target contrast. Neutral density filters were placed over part of the display, to mimic the localised reductions in response which occur in glaucoma, age-related maculopathy or diabetic retinopathy. There is a definite reduction in response seen with a 0.4 ND filter, indicating that the system should have similar sensitivity in detection of retinal lesions to the commonly used visual field analysis systems.

Adult↗

PubMed1996

A new stereotest: the double two rod test.

A new stereoacuity test, the double two rod test, uses a two-alternative forced-choice (2AFC) paradigm. The subject's task is to state which pair of rods, the left or the right, contains depth information. The test gives repeatable results; comparison between the depth threshold measured binocularly and monocularly suggests that monocular cues, if present, provide only poor depth information, compared to the depth information arising from binocular disparity. As part of an investigation of stereoacuity and ageing, we tested stereoacuity as a function of viewing time and the number of comparisons made between the two pairs of rods. Stereoacuity was significantly reduced for viewing times less than 3 s while longer viewing times did not improve stereoacuity. The number of comparisons made between the two pairs of rods had no effect on stereoacuity.

Adult↗

PubMed1995

Differences in visual acuity between the eyes: determination of normal limits in a clinical population.

We determined the difference in visual acuity between the right and left eyes of patients and also determined the maximal level of acuity expected in a group of visually normal individuals. Visual acuity was measured in the right and left eyes of 72 subjects aged from 16 to 67 years using Bailey-Lovie type charts. The chart was read until fewer than three letters were read on a line and acuities were based on each letter correct contributing -0.02 to the overall score. For 12 of these subjects, visual acuity was measured a further four times. Average visual acuities for right and left eyes were -0.137 and -0.126log MAR, respectively. The signed visual acuity difference was normally distributed and had a standard deviation of 0.050log MAR. The mean visual acuity difference for the 12 subjects for whom the measures were repeated was 0.033log MAR (SD of the signed visual acuity difference: 0.049log MAR). We conclude that if a patient has a difference in visual acuity between the two eyes of more than 5 letters on a Bailey-Lovie style chart, further investigation is indicated.

Adolescent↗

PubMed1995

Ocular dimensions and refraction in Tibetan children.

A cross-sectional study of 404 Tibetan children (212 males, 192 females) aged 6 to 16 years was conducted in the Bouda region of Kathmandu, Napal in April, 1992. Examination procedures included retinoscopy (1% cyclopentolate HCl), keratometry A-scan ultrasonography, and video ophthalmophakometry. The mean refractive error was +1.11 D (SD: 0.56 D) at age 6 years decreasing to +0.63 D (SD: 0.34 D) at age 16 years with a prevalence of myopia in this group of 3.9%. Most children examined had low refractive errors, with 95.5% having errors in the range -0.50 to +1.50 D. Crystalline lens power decreased by 2.59 D, with an associated increase in its anterior radius of curvature of 1.98 mm and 0.49 mm in its posterior radius of curvature over the age range studied. Vitreous chamber depth increased by 0.69 mm, but no significant changes were recorded in anterior chamber depth, lens thickness, or corneal curvature. We conclude that the balance between the decrease in crystalline lens power and the increase in vitreous length is the major factor in maintaining the tendency to emmetropia in these children.

Adolescent↗

PubMed1995

Effects of exposure to simulated altitudes on visual fields, contrast sensitivity, and dazzle recovery.

Previous studies have shown that visual performance may be impaired at altitudes above 12,000 ft when supplementary oxygen is not used. What is not known is whether there may be subtle changes in visual function at altitudes to which airline cabins are usually pressurized. In this study, we measured spatial and temporal contrast sensitivity, central visual field thresholds, and macular photostress recovery time in 12 male subjects at sea level and at simulated altitudes of 7000 ft and 12,000 ft. Our results showed insignificant changes in spatial and temporal contrast sensitivity, central visual field thresholds and dazzle recovery time with respect to simulated altitudes.

Adult↗

PubMed1994

The repeatability of the noninvasive blue field entoptic phenomenon method for measuring macular capillary blood flow.

The speed of macular capillary blood flow can be estimated by using the blue field entoptic phenomenon. The technique uses a matching task in which the subject matches what is seen entoptically with a computer simulation of the phenomenon. In this study, we tested the repeatability of the leukocyte speed results obtained using this technique. Fourteen young subjects were tested on five different occasions. Group mean leukocyte speed did not vary significantly from one recording session to the next. However, readings for individuals varied substantially from one recording session to the next. This variability limits the usefulness of this technique for screening or monitoring macular blood flow in individuals.

Adult↗

PubMed1994

Age and the van Herick ratio in a Chinese population.

We derived estimates of the van Herick ratio or grading in a retrospective study using a Chinese clinical population. We found that the van Herick ratio decreased with age and increased with higher degrees of myopia. A lower van Herick ratio was found to be associated with higher intraocular pressure. There are significantly more patients with lower van Herick ratios or grades in our population than in populations described by others.

Adolescent↗

PubMed1993

Decrease in stereoacuity in the seventh decade of life.

We measured stereoacuity in 41 subjects with normal Snellen acuity in each eye, and normal ocular health. Patients were measured in each of the age ranges 21-28, 41-49, 51-59 and 60-70 years. Stereoacuity was reduced from about 16 sec arc for the three younger groups to about 27 sec arc for the older subjects. Since stereopsis has a cortical neural substrate, these data suggest that there may be cortical changes affecting the interaction of information from the two eyes with advancing age, but whether these changes result from loss of contrast sensitivity, instability of ocular alignment or binocular stress, all of which have been reported in elderly subjects, is not known.

Adult↗

PubMed1993

Mechanical manipulation of the lids and tear break-up time measurements in Hong Kong Chinese.

We measured tear break-up time (TBUT) after lid-rubbing in a single masked experiment, and found that TBUT was unaltered. A further single masked experiment in which non-invasive tear break-up time (NITBUT) was measured after the lids were raised as if to apply fluorescein also showed no effect of this interference with the lids. An experiment in which fluorescein was applied either with the lid raised or not raised during fluorescein application, again with the experimenter masked with respect to treatment, also produced no significant difference in TBUT. When we measured TBUT with the lid raised so that it was not in contact with the cornea, there was no difference in TBUT for the group between the 'lifted' and 'non-lifted' eye; some subjects showed substantially shorter TBUT values, while others showed substantially longer TBUT values. These findings are surprising in view of the concerns expressed in the literature regarding the sensitivity of the tear film to mechanical interference, and suggest that the tear film is more resistant to disruption than has previously been thought.

Adult↗