Biomedical subjects
I L Bailey
Publications and source records attributed to I L Bailey.
Spatial adaptation to text on a video display terminal.
We employed vertical sinusoidal test gratings to search for spatial adaptation to lower-case text presented on a standard video display terminal. The parameters of the contrast sensitivity test were selected on the basis of waveform analysis of horizontal spatial luminance profiles of the text. We found that subjects exhibited a small (4-5 dB), but significant, frequency-specific spatial adaptation consistent with the frequency spectrum of the stimulus. The theoretical and practical significance of this finding is discussed.
Use of the Humphrey Lens Analyzer for off-axis measurements of spectacle lenses.
Automated focimeters can be used to make quick, precise measurements of off-axis power and prismatic effects corresponding to an eye rotating behind a spectacle lens. An automated focimeter, the Humphrey Lens Analyzer, was assessed in this regard. The Humphrey Lens Analyzer can be used to give a valid measure of off-axis power of lenses with low power, but not of lenses with moderate to higher power (greater than 3 D). For 3 D spherical lenses discrepancies of the order of 0.1 D occur at 30 degrees rotation, and 6 D spheres give discrepancies of 0.5 D at the same rotation. Small discrepancies were found for measurements of prism. The Humphrey Lens Analyzer was also used in a mode where the lens being tested is rotated about the center of curvature of its back surface. This is the mode often used to assess aberrations and prism of progressive-addition lenses. In this mode, the instrument provides reasonable accuracy in estimating off-axis power corresponding to eye rotation for lenses with low power, but not for lenses with moderate to higher power (greater than 3 D). However, it provides accurate values of the variation in off-axis surface power for low powered lenses with aspheric front surfaces. There were considerable systematic errors associated with the measurement of prism. A simple raytracing method was developed to predict the results of measurements with the Humphrey Lens Analyzer. Predictions of off-axis power were good when lenses were rotated about a position corresponding to the center-of-rotation of an eye, but were poorer when lenses were rotated about the center of curvature of their back surfaces. Predictions of primatic efforts were good in both situations. A method by which the Humphrey Lens Analyzer should provide an accurate measurement of off-axis powers corresponding to eye rotation behind a spectacle lens is described, but has not been tested.
Human electroretinogram responses to video displays, fluorescent lighting, and other high frequency sources.
Time-averaged human electroretinogram (ERG) responses were determined for several workplace visual stimuli which are temporally modulated at rates exceeding the perceptual critical fusion frequency (CFF). A clearly identifiable synchronous response was in evidence for a video display terminal (VDT) stimulus operating with a refresh rate as high as 76 Hz. A directly viewed fluorescent luminaire with controllable driving frequency elicited a synchronous response at rates as high as 145 Hz. In addition, an intense stimulus created by modulating the light from a slide projector produced responses at least as high as 162 Hz. The implications of these high-frequency responses are representing a potential basis for visual symptoms are discussed.
A new test for the evaluation of disability glare.
We describe a new glare test that uses low contrast letters and a variable surround glare source. The test is easy to administer clinically and all test targets, luminance levels, glare geometry, and viewing conditions are well controlled. We found the most useful index of disability glare to be the difference in visual acuity scores for the low contrast chart in the no-glare and high-glare conditions and refer to this as the disability glare index (DGI). We tested a group of normal subjects who were divided into younger (age 15 to 41 years) and older (age 50 to 82 years) groups. The mean DGI value for the older group (10.2 +/- 4.8) was significantly higher than that for the younger group (2.3 +/- 1.9). The DGI is a better discriminator between the two groups than either high or low contrast visual acuity. We attribute the significantly higher DGI values in the older group to increased intraocular light scatter. We find DGI is poorly correlated with high contrast visual acuity (r = 0.33). Our findings suggest that this test of disability glare is sensitive to relatively modest changes in the ocular media. It is a potentially useful tool in detection and assessment of subtle media disturbances and in monitoring changes in the ocular media over a period of time.
Visual performance at video display terminals--effects of screen color and illuminant type.
A limited set of illuminants and phosphor colors are commonly used in video display terminal (VDT) working environments. This study attempts to identify any combinations of such conditions that influence performance on a visually demanding counting task. Experiments were performed to test whether the phosphor color, ambient lighting spectrum, or temporal frequency characteristics of the display and ambient lighting could alter performance. Under conditions where potentially contaminating variables such as reflectance level and screen glare patterns were equalized, no significant differences in performance were produced by the particular sets of stimulus conditions tested. The results suggest that displays and illuminant types that are in common use allow substantially equivalent visual performance.
Clinical grading and the effects of scaling.
In clinical practice, there has been a need to grade the magnitude or the severity of the functions and qualities that are assessed in the examination. It is popular to use a four-step grading scale to categorize the severity of clinical findings. The authors discuss clinical grading scales and their influence on the clinician's ability to detect change. These principles have been applied to grades or measures derived from either objective measuring instruments, subjective tests, or techniques in which the clinician makes subjective judgments. A hypothetical data set was used to show the problems associated with using grading scales that are too coarse. The authors presented a mathematic model that helps to estimate the benefits of using use of a finer scale. Data were presented from two separate studies, one on visual acuity measurement and the other on grading nuclear opacity, to show the advantages of using finer scales to enhance the sensitivity of clinical measurement. High levels of concordance between independent observations indicated that the grading scale was too coarse and that these scales needlessly reduced the clinician's ability to detect change in the parameter being assessed. For moderate sensitivity, the size of the scale increments should not exceed one standard deviation of the discrepancy so that the concordance of paired comparisons would not exceed 37%. For fine clinical sensitivity, the size of the scale increments should not exceed one third of the standard deviation of the discrepancy, in which case the concordance of paired comparisons would not exceed 13%. The theory and evidence presented here could prompt re-evaluations of common methods of clinical grading.
Face recognition in age-related maculopathy.
Patients with age-related maculopathy (ARM) complain frequently of difficulty with face recognition. The authors attempted to quantify the level of impairment by comparing face recognition with clinical tests of visual function, namely contrast sensitivity, grating acuity, letter-chart acuity, and word-reading acuity. For face recognition, we used 32 black-and-white photographs that had been cropped to remove the outline of hair so that identification was predominantly dependent on the facial features. The observer's distance from the screen on which the photographs were projected was varied. The angular size of the faces was indicated by the equivalent viewing distance (EVD). Four male and four female models were used, and for each model, there were four photographs with different facial expressions--happy, sad, angry, and afraid. For each photograph, the subject's task was to name the model and identify the facial expression. Threshold EVD (50%) was determined for correct identity recognition and expression recognition. For eight subjects all experimental procedures were repeated at a lower luminance level. For ARM subjects, increasing task complexity (grating/letters/words) substantially decreased resolution. Face-recognition abilities were most closely related to word-reading acuity when comparisons were made either across subjects or across luminances within subjects. Contrast sensitivity was associated poorly with face-recognition thresholds. In some subjects with more advanced ARM, identity recognition was substantially poorer than expression recognition.
Stand magnifiers: an evaluation of new optical aids from COIL.
This paper discusses stand magnifiers and the optical parameters that clinicians should know in order to understand the viewing requirements and the expected resolution for low vision patients. We report on measurements made of key optical parameters of a new series of stand magnifiers from Combined Optical Industries Limited (COIL) and comparison is made to the previous series of stand magnifiers from the same manufacturer. These results illustrate that manufacturers' specifications of optical parameters are generally inaccurate and misleading. We urge manufacturers to provide clinically relevant information about their optical products.
Task performance with base-in and base-out prism.
The effects of base-in (BI) and base-out (BO) prism upon performance of four occupational-like tasks were measured on a group of 10 subjects who had normal binocular vision. Performance on tasks which required depth perception was significantly poorer when subjects wore prism glasses with 8 and 12 delta BI and BO, whereas 4 delta BI and BO did not significantly affect performance. The performance decrements with prism (3.0 to 6.7%) were considerably less than those induced by denial of binocularity (20 to 30%) as measured in a previous study. Thus, vergence-inducing prism does impair task performance but not as much as does the denial of binocularity. The reading and counting tasks did not require critical depth judgement and were much less affected by the prism than the needle-threading and pointers-and-straws tasks.
Optics of progressive addition lenses.
The optical characteristics of the major progressive addition lenses were measured using an automated lensometer with a specially designed lens holder to simulate eye rotation. Measurements were made every 3 degrees (about 1.5 mm) and graphs of isospherical equivalent lines and isocylinder lines were developed. Generally the near zone of these lenses is narrower and lower than in bifocal or trifocal lenses. Distinct differences exist between the various progressive lenses. The width of the near zone, rate of power progression, amount of unwanted cylinder (level with the distance center), and clarity of the distance zone are compared for the various lenses. The optical measurements demonstrate an apparent trade-off between the size of the cylinder-free area of the lens and the amount of the cylinder.
Readability of computer display print enlarged for low vision.
A letter counting task was presented on a Macintosh computer screen using two different versions of 24 point Times Roman print. One version, called "grainy," had 12 matrix units per font height and the other, called "smooth," had 24. A mixed group of low vision subjects and a normally sighted group had speed and accuracy measured as they performed the test task from two different viewing distances. For both population groups, the remote test distances were arranged individually so that the letters subtended an angular size scarcely larger than threshold. The close distances were fairly representative of practical working distances. It was shown that, for both groups of subjects, smooth letters allowed faster performance for the closer working distances only. Smoothing the letters helped accuracy of performance at the far distances, and at the close distances, smoothing improved the accuracy for the low vision group but not for the normals. The implications are discussed.
Binocular vs. monocular task performance.
Functional advantages of binocularity were investigated by having 13 subjects perform a group of occupational-type tasks under monocular and binocular conditions. Significant binocular advantages ranging from 29.5% (pointers in straws) to 3.7% (reading speed) were measured. Tasks with many disparity cues showed the greatest binocular advantage. This shows that patients with normal binocular vision use binocular cues, most likely stereopsis, to enhance performance. In a second experiment, three subjects with normal binocular vision underwent monocular occlusion for 5 days to investigate whether monocular skills improved to compensate for the loss of binocular vision. During that period binocular performance was consistently better than monocular performance, and both monocular and binocular performance improved, even though the subjects were only gaining monocular experience. Although the 5 day occlusion period does not simulate the long-term denial of normal binocularity that strabismics or monocular patients experience, it shows that binocular superiority remains after short-term loss of binocularity.
Bioptic telescopes.
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New method for determining the magnifying power of telescopes.
A new method of measuring the power of optical telescopes is described. This method makes use of the vergence amplification that occurs when the light incident on the objective lens at a telescope is divergent or convergent. The relation between the vergence incident on the objective and vergence emergent from the eyepiece depends on the magnifying power and the length of the telescope. The method is most simple to apply to short-length, low-powered Galilean tesescopes, such as those used as low vision aids, in sports glasses, and in telescopic loupes. With such telescopes, holding a lens of known power against the objective and measuring the back vertex power of the lens-telescope combination using the lensometer and then measuring the telescope length allows the determination of the magnifying power of the telescope. The method can be adapted to determine the magnification rating of near-vision telescopic loupes.
New design principles for visual acuity letter charts.
This paper intoduces new principles for the design and use of letter charts for the measurement of visual acuity. It is advocated that the test task should be essentially the same at each size level on the chart. Such standardization of the test task requires the use of letters of equal legibility, the same number of letters on each row, and uniform between-letter and between-row spacing. It is also advocated that, combined with the test task standardization, there should be a logarithmic progression of letter size. Charts incorporating these design features have been made. These charts facilitate the use of nonstandard testing distances which might be used when there is low visual acuity, when examination room layout prevents testing at the standard distance, or when it is necessary to validate visual acuity scores or detect malingering. Adjusting the visual acuity score according to the chosen testing distance is simplified by the use of logarithmic scaling.
Corneal changes from hydrophilic contact lenses.
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Analyzing contact lens induced changes of the corneal curvature.
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