Biomedical subjects
Publications and source records attributed to R J Jacobs.
Temporomandibular joint morphology and function can be evaluated by panoramic zonography. Thermoluminescent dosimetry was applied to evaluate the radiation dose to predetermined sites on a phantom eye, thyroid, pituitary, and parotid, and the dose distribution on the skin of the head and neck when the TMJ program of the Zonarc panoramic x-ray unit was used. Findings are discussed with reference to similar radiographic techniques.
The relation between refractive error and visual acuity has been measured by two very different methods. In one called "source methods," emmetropes or corrected ametropes view defocused stimuli presented on projection screens or photographs. In the type called "observer methods," focused stimuli are presented to the observers who are either uncorrected ametropes or emmetropes defocused by lenses placed (usually), in the spectacle plane. The study reported in this paper demonstrates for the first time that these two methods of defocusing retinal images and their effects on visual acuity can be correlated. Results show that the source method of producing defocus could be used interchangeably with the observer method in investigating the rates of change of visual acuity with defocus for young normal observers. The angular diameter of the defocused image of a point, the blur disc diameter in object space, allows the two methods to be compared. Although the results show that the two methods are highly correlated, they show that the source method gives a statistically but not clinically significant lower acuity. The results of both methods are used to derive an equation linking refractive error, visual acuity, and pupil diameter.
The defocus levels required for normal observers to notice the first perceptible blur of a clear test target (blur threshold) and the least perceptible change in the degree of blurriness of an already blurry target (threshold of perceived change in blur) were measured using both the source and observer methods. In the source method observers viewed defocused stimuli presented on a projection screen, whereas in the observer method focused stimuli were presented to observers who were defocused using lenses placed in the spectacle plane. Blur thresholds were found to be dependent on target size and when the Landolt ring targets were near threshold acuity size blur thresholds were as small as 0.10 D. For larger target sizes (0.6 log min arc or more above threshold acuity size) the blur thresholds remained relatively unchanged and were about 0.18 D. Thresholds of perceived change in blur were found to be independent of the initial defocus level. Measurements of the threshold of perceived change in blur were found to be 0.05 to 0.07 D, which is much smaller than the blur threshold values. Comparison of results from the two methods of producing defocus indicate that the source and observer methods can be used interchangeably. However, for the same angular blur disc diameter, the blur thresholds found with the source method were significantly lower than those found with the observer method.
We examined the effect of endogenous C1q on the sensitivity of the fluid-phase C1q binding assay (C1qBA) in detecting an immune complex (IC) model, heat-aggregated IgG (HAIgG), at concentrations of 10-10,000 micrograms/ml sample. Results in normal human serum (NHS) or plasma (NHP) were compared with those in heat-inactivated NHS (NHS/56) in which most endogenous C1q was depleted by heat denaturation. Higher HAIgG concentrations were required in NHP and NHS to produce the same 125I-C1q precipitation seen in NHS/56. This decreased sensitivity varied from 70% at low HAIgG concentrations to 0% at high concentrations, as predicted for a large pool of endogenous C1q, in equilibrium with 125I-C1q, but in excess of that which could bind to all but the highest concentrations of IC model. In serum depleted of functional C1q on an immunoadsorbant of HAIgG, the precipitation of radiolabeled HAIgG under C1qBA conditions was concentration dependent and generated a saturation curve, showing that only a fraction of IC are usually precipitated in this assay. HAIgG precipitation was enhanced 1.4-fold in NHS/56 (8 micrograms C1q/ml) and three-fold in NHS (67 micrograms C1q/ml) suggesting that IC size is increased by endogenous C1q. In dual label experiments using 131I-HAIgG, the precipitation of 125I-C1q in NHS/56 was directly proportional to IC model precipitation, but markedly discordant in NHP, showing the measurement of IC in heat-inactivated sera superior to that in native serum. A comparison of the C1q:HAIgG ratio in PEG precipitates with that in samples, indicated that equilibrium was established between C1q and IC model. Thus the precipitation of 125I-C1q in the C1qBA represents (1) the fraction of total C1q bound to IC, and (2) the fraction of IC precipitated by PEG.
A set of near Bailey-Lovie logMAR letter charts that varied in contrast from 0.40 dB (C = 0.95) to 58.0 dB (C = 0.001) were used to measure the middle and high spatial frequency range of the contrast sensitivity function (CSF) of 15 subjects with age-related maculopathy (ARM) and 15 age-matched normal subjects. The letter charts were shown to have good test-retest reliability. Compared with measures made using CRT generated square wave gratings of variable contrast and spatial frequency they were also shown to provide a valid measure of CSF in the mid to high spatial frequency range. The 20.0 dB letter chart alone was shown to be a good screening device for macular disease. The letter charts do not provide a measure of the peak of the CSF and a supplementary test of contrast sensitivity is needed to quantify contrast sensitivity at a low spatial frequency. Three measures are proposed as necessary to document foveal visual capability of patients with macular disease: distance logMAR visual acuity to measure high spatial frequency resolution, visual acuity with letter charts of 20 dB contrast to assess mid spatial frequency resolution; and contrast sensitivity for the detection of an edge to estimate contrast sensitivity for larger objects.
Sets of edge and square wave grating photographic plates of varying contrasts were used to measure the mid to low spatial frequency range of the contrast sensitivity function (CSF) of 20 subjects with evident ocular disease and 20 age-matched normal subjects. Both plate tests were shown to have good test-retest reliability and to correlate well with electronic cathode ray tube (CRT) measures of CSF. The edge test, when administered in 2-dB steps of ascending contrast, has optimum sensitivity of 0.70 and specificity of 0.84 for detecting the patient with ocular disease with a fail criterion of less than 38 dB. A measure of edge contrast sensitivity was also shown to be a good predictor of the peak of the CSF, which is shown to be largely independent of the visual acuity of the subject. The 2 c/deg and 4 c/deg plates did not provide more information about the visual difficulties of subjects than the edge test. We advocate the use of an edge test as a simple clinical measure of low spatial frequency contrast sensitivity that provides information about visual dysfunction not provided by a measure of visual acuity.
There are two principal methods of simulating refractive errors. Either the retinal image can be defocused by an optical system, usually a positive lens, placed in front of an observer's eye (observer method), or the source of the retinal image can be defocused as it is projected onto a screen or photograph (source method). There are significant differences between the two methods, differences that make it difficult to compare results. However, the source method, which is the more artificial, seems to be superior for a number of reasons. The results of these two methods can be compared using a common or interchangeable parameter for specifying the level of defocus. A convenient parameter is the size of the defocused image of a point, measured either in image space (linear or angular diameter on the retina) or in object space (angular diameter of the blur disc projected back into object space), with the angular diameter measured from the respective nodal point of the eye. Methods of measuring the angular blur-disc diameter for both methods are discussed and the validity of the formula omega = D delta L, is investigated, where omega is the angular diameter of the blur disc, D is the observer's pupil diameter, and delta L is the dioptric defocus.
Whilst most optometrists today are aware of the need for a practical method of measuring the contrast sensitivity function (CSF), cost and length of testing have usually precluded this measurement. As the Arden Grating Test is a screening test for normal CSF, and since many patients with low vision cannot see any of its gratings, there is a need for a test with an extended contrast range which is calibrated to allow quantitative measurement of contrast thresholds in the clinic. This paper discusses the problems associated with the use of the photographic process to manufacture such a test. A series of stimuli covering a wide range of contrasts were manufactured photographically and evaluated photometrically and psychophysically to validate the photographic process. The results show that the photographic techniques can be controlled sufficiently to enable the manufacture of stimuli of the required contrasts and that, at low contrasts where calibration is inaccurate or impossible, the contrasts produced are linearly related to exposure. The results also show that the illumination and observation conditions need to be accurately controlled for the plate contrast to be unaffected by veiling glare.
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Among other things, airport control towers should be sited so that controllers can readily detect whether an aircraft cleared for takeoff has commenced its takeoff run. The detection of movement is not well enough understood to enable confident prediction that a particular site for a tower will enable commencement of takeoff run to be easily observed. A field study was undertaken to establish detection times for commencement of takeoff run by groups of trained and trainee air traffic controllers and untrained observers. It was found that the mean response of observers occurs when the aircraft is displaced about 5' of arc, a value essentially independent of observer experience, observation distance, aircraft velocity, and the clarity with which the aircraft can be seen. Binoculars reduce the mean response time and response variability, although not as much as might be expected. Domains within which control towers can be located to enable detection of takeoff runs within 2 s and 4 s are defined.
This paper examines the contribution of the macular region to visual function. Several test targets were used to measure visual performance within 10 deg of the center of the fovea for normal observers and for an observer with a uniocular central 6-deg scotoma. We conclude that the nature of the test target is more important in the measurement of parafoveal than central vision and that the macular region contributes significantly to the visibility of objects as large as 2.5 deg.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.