PubMed Health⌕ Search

Biomedical subjects

J Gilchrist

Publications and source records attributed to J Gilchrist.

At least 19 recordsLinked to original sources
PubMed1992

Binocular contrast summation and inhibition in amblyopia. The influence of the interocular difference on binocular contrast sensitivity.

The monocular contrast sensitivity loss in amblyopia is well documented. We investigated the influence of interocular sensitivity difference on binocular contrast sensitivity in amblyopia. Monocular and binocular contrast sensitivity functions of six amblyopes (three strabismic and three anisometropic) were measured. The monocular contrast sensitivity loss depended on the type of amblyope. Anisometropic amblyopes generally showed high frequency losses. Strabismic amblyopes showed losses at both low and high spatial frequencies. Binocular performance was assessed in terms of binocular ratios (binocular/non-amblyopic). A binocular ratio greater than 1 indicates binocular summation (binocular > monocular) while a ratio less than 1 shows binocular inhibition (binocular < monocular). In all subjects, the binocular ratio depended on the difference between the amblyopic and the non-amblyopic eye. Minimal interocular difference produced binocular summation, the magnitude of which decreased as the difference between the two eyes increased. Further increases in the monocular difference produced binocular inhibition. Anisometropic amblyopes showed a greater degree of binocular summation at low spatial frequencies compared to strabismic amblyopes. Both types of amblyopes showed binocular inhibition at high spatial frequencies. Clinical implications of binocular summation and inhibition in amblyopia are discussed.

Adolescent↗
PubMed1992

Dinucleotide repeat polymorphisms at the SCN4A locus suggest allelic heterogeneity of hyperkalemic periodic paralysis and paramyotonia congenita.

Two polymorphic dinucleotide repeats--one (dGdA)n and one (dGdT)n--have been identified at the SCN4A locus, encoding the alpha-subunit of the adult skeletal muscle sodium channel. When typed using PCR, the dinucleotide repeats display 4 and 10 alleles, respectively, with a predicted heterozygosity of .81 for the combined haplotype. We have applied these polymorphisms to the investigation of hyperkalemic periodic paralysis and paramyotonia congenita, distinct neuromuscular disorders both of which are thought to involve mutation at SCN4A. Our data confirm the genetic linkage of both disorders with SCN4A. Haplotype analysis also indicates the strong likelihood of allelic heterogeneity in both disorders.

Alleles↗
PubMed1992

QROC curves and kappa functions: new methods for evaluating the quality of clinical decisions.

The application of statistical methods for measuring the ability of clinical judgements and tests to discriminate between normal and diseased patients is increasing in optometry and ophthalmology. This paper presents two new methods for assessing the quality of such diagnostic decisions. Both are based on measurement of the kappa coefficient of association and offer theoretical and practical advantages over traditional methods. The paper first introduces the family of kappa coefficients with examples of their calculation and application. The use of weighted kappa values for measuring test sensitivity and specificity is explained, and it is shown how the results may be plotted in the form of the QROC curve which enables different aspects of test discriminability to be represented graphically. Kappa functions are then proposed as an alternative method for representing and predicting test performance, having the advantage of identifying optimal test criteria in addition to providing measures of discriminability. The clinical application of the new methods is illustrated using data for the detection of glaucoma from measurements of intraocular pressure.

Eye Diseases↗
PubMed1991

The importance of measuring binocular contrast sensitivity in unilateral cataract.

Monocular and binocular contrast sensitivities were measured in patients with uniocular cataract. The cataractous eye showed a greater monocular loss at higher spatial frequencies compared to lower spatial frequencies. Binocular contrast sensitivity depended on the contrast sensitivity differences between the two eyes. At low spatial frequencies, where the monocular sensitivity difference was minimal, binocular summation was obtained. As the sensitivity difference increased at higher spatial frequencies, the binocular contrast sensitivity decreased steadily until it reached a level below the sensitivity of the cataractous eye, demonstrating binocular inhibition. The clinical implications of binocular inhibition obtained with uniocular cataract are discussed.

Aged↗
PubMed1991

Loss of visual function associated with microalbuminuria in diabetes mellitus: a pilot study.

Visual function loss has been documented in diabetes mellitus in relation to flicker and contrast. However, no direct correlation between the degree of loss in sensitivity and the level of retinopathy has been established. It has been suggested that such non-invasive psychophysical procedures actually reflect metabolic disturbances within the diabetic retina. This study investigates the possibility of whether early nephropathy demonstrated by microalbuminuria, is an indicator of microangiopathy which may be a cause of retinal disturbance leading to a loss of visual function. The visual function of a group of diabetics showing microalbuminuria was studied. Contrast and flicker threshold were measured and the results compared with those obtained with an age-matched control diabetic group. The procedures used effectively separated the two groups and raises the issue of incorporating psychophysics in retinal screening programmes.

Albuminuria↗
PubMed1991

Comparison of non-stereo polaroids and slides in detection of diabetic retinopathy.

Fundus photography plays an important role in detection and assessment of diabetic retinopathy. This study compared the detectability of diabetic retinopathy lesions on Polaroid prints and Ektachrome slides obtained with a non-mydriatic camera. The number of lesions detected with Ektachrome slides was higher compared to Polaroids. The method of viewing the Ektachrome slides was also shown to be important. Slides when projected onto a white screen revealed a higher number of lesions compared to the same slide viewed through a macroscope. If a non-mydriatic camera is used to screen diabetic retinopathy, judgement from a projected Ectachrome slide is recommended.

Adult↗
PubMed1990

Binocular inhibition: psychophysical and electrophysiological evidence.

Binocular summation, defined as an increase in the binocular response compared with the monocular, occurs when the sensitivities of the two eyes are equal. We investigated the psychophysical and electrophysiological binocular response to a difference in monocular retinal illuminance. Different levels of unequal monocular sensitivities were induced by means of neutral density filters placed in front of one eye. Both studies produced similar results. In the absence of filters, maximum binocular summation was produced. With increasing difference in monocular illuminance, the binocular response decreased steadily until it reached a level below the monocular. The clinical implications of binocular inhibition, a perceptual phenomenon similar to Fechner's Paradox, are discussed.

Adult↗
PubMed1990

The effect of monocular defocus on binocular contrast sensitivity.

The monocular contrast sensitivity loss with defocus is well known. We measured binocular contrast sensitivity of sinewave gratings of 6 c/deg in the presence of different levels of monocular defocus. In the absence of defocus, the binocular sensitivity was about 42% higher than monocular. With increasing monocular defocus, the binocular sensitivity decreased steadily until it reached a level below the monocular, showing binocular inhibition. The clinical implications of binocular inhibition with monocular defocus are discussed.

Contrast Sensitivity↗
PubMed1990

Binocular contrast sensitivity with monocular glare disability.

Glare disability plays an important part in the investigation and assessment of cataract. We investigated the effect of monocular glare disability on binocular contrast sensitivity. The magnitude of binocular summation was measured at different degrees of monocular glare disability. In the absence of glare, maximum binocular summation was shown. With increasing glare disability, the degree of binocular summation decreased steadily until binocular inhibition was produced. Binocular inhibition is defined as a reduction in binocular sensitivity to reach a level below the monocular. The clinical implications of binocular inhibition with monocular glare disability are discussed.

Adult↗
PubMed1989

The subjective assessment of cataract.

The medical treatment of cataract now appears to be a distinct possibility. A number of anti-cataract formulations are being clinically tested, and more clinical trials are being planned. To obtain a true assessment of a drug's efficacy, a battery of tests are needed which can accurately assess cataract progress. A clinical trial of the proposed anti-cataract drug Bendalina is being conducted. The six subjective methods used to assess cataract progression in the trial are presented. These are refractive error, LogMAR visual acuity, contrast sensitivity, glare sensitivity, retinal visual acuity and displacement threshold hyperacuity. The reasons for using each technique and their method of measurement is explained.

Aged↗
PubMed1989

Contrast sensitivity and glare sensitivity changes with three types of cataract morphology: are these techniques necessary in a clinical evaluation of cataract?

LogMAR visual acuity, contrast sensitivity and glare sensitivity measurements were made on 39 eyes of 18 cataractous subjects and compared against normative data. Only cataracts of one of the main three morphological cataract types were used--cortical, nuclear and posterior subcapsular. Results indicate that contrast sensitivity decline with cataract is an intermediate and high spatial frequency loss. For nuclear and cortical cataracts with a LogMAR visual acuity of less than 0.5 (Snellen equivalent better than 6/18), there was no loss of contrast sensitivity at the lowest spatial frequency (1 c/deg). For posterior subcapsular cataracts, low spatial frequency contrast sensitivity loss did occur but was unrelated to visual acuity. Glare sensitivity increased for all cataract types. This was related to visual acuity for both cortical and nuclear cataracts but was not for the posterior subcapsular type. It was concluded that contrast and glare sensitivity measurements are a useful part of the assessment of visual function in patients with posterior subcapsular cataract.

Aged↗
PubMed1989

The effect of spatial frequency on binocular contrast inhibition.

Previous work has shown that binocular contrast summation, obtained with equal monocular sensitivities, remains constant over a range of spatial frequencies. We measured binocular contrast detection with a log 1.00 ND filter placed in front of one eye. For all eight subjects, the binocular contrast detection decreased to a level below that of the monocular detection, demonstrating a contrast analogy to Fechner's paradox. The degree of binocular inhibition, like summation, remained constant across the range of spatial frequencies.

Adult↗
PubMed1989

The accommodative contribution to binocular vergence eye movements.

Forced vergence fixation disparity curves were generated for both with and without an accommodative stimulus (accommodative closed and open loop conditions respectively) for fifteen asymptomatic subjects. All subjects exhibited an exophoric shift in fixation disparity for the accommodative open loop condition at the forced convergence side of the curve. No corresponding bias was found at the forced divergence side of the curve. The findings are discussed in terms of the results of previous studies and the existing models of vergence eye movements.

Accommodation, Ocular↗
PubMed1975

Operative biopsy of the pancreas.

A review has been undertaken of the 47 patients who have had open pancreatic biopsy performed at the Royal Free Hospital between 1967 and 1972. It is concluded that not only is the procedure safe when using the technique described but also it is a valuable and accurate diagnostic procedure.

Aged↗