Biomedical subjects

G E Legge

Publications and source records attributed to G E Legge.

At least 55 records · Page 3Linked to original sources

PubMed1987

Tolerance to visual defocus.

Low-resolution optical systems are more tolerant to defocus than are high-resolution systems. We wished to determine whether this principle applies to human vision. We used psychophysical methods to measure the effects of defocus in normal eyes under low-resolution conditions. Modulation transfer of sine-wave gratings was measured as a function of dioptric defocus at low and medium spatial frequencies. We defined the depth of focus at a given spatial frequency to be the dioptric range for which the modulation transfer exceeds 50% of its peak value. For dilated pupils, depth of focus increased from about 2.5 diopters (D) at 3.5 cycles/deg to about 17 D at 0.25 cycles/deg. From our results we predicted that tasks requiring only low spatial frequencies will be more tolerant to defocus than tasks requiring higher spatial frequencies. This prediction was confirmed in a letter-recognition experiment. The increasing tolerance to defocus at low spatial frequencies also implies that individuals with low acuity will be more tolerant to defocus than people with normal vision. We confirmed this prediction by measuring tolerance to defocus in 30 low-vision eyes.

Humans↗

PubMed1987

Contrast discrimination in peripheral vision.

Contrast discrimination provides a psychophysical method for studying contrast coding in vision. Our purpose was to compare properties of contrast discrimination in central and peripheral vision. We used forced-choice procedures to measure contrast-increment thresholds as a function of pedestal contrast. Our stimuli were 2-cycle/deg Gaussian-windowed sine-wave grating patches. They were centered at retinal loci ranging from 10 degrees nasal to 20 degrees temporal on the horizontal meridian. At each eccentricity, curves relating increment threshold to pedestal contrast had the same shape. When increment thresholds and pedestal contrasts were both normalized by the contrast thresholds at the retinal eccentricity in question, the curves became superimposed and fell along the same dipper-shaped contrast-discrimination function. We conclude that, after scaling by the local contrast sensitivity, properties of contrast discrimination are qualitatively and quantitatively similar from 0 degree to 20 degrees on the retina. These findings suggest that mechanisms of contrast coding are similar in central and peripheral vision.

Discrimination, Psychological↗

PubMed1987

Accommodation to stimuli in peripheral vision.

Can targets in peripheral vision elicit accommodation responses? We used a laser optometer to measure monocular steady-state accommodation for stimuli at retinal eccentricities ranging from 1 degree to 30 degrees. The optical distance from the eye to the stimulus was varied from 0 to -6 D by introducing lenses in front of the eye. The accommodative response was plotted as a function of optical distance to produce an accommodative stimulus-response function. The magnitude of accommodative response was defined as the difference between the maximum and minimum values of this function. The magnitude declined from 4 D at 1 degree to 1-2 D at 30 degrees eccentricity. The relation of the magnitude of accommodative response in peripheral vision to changes in acuity, contrast sensitivity, and depth of focus are considered. The role played by convergence accommodation is also discussed.

Accommodation, Ocular↗

PubMed1986

The importance of eye movements in the analysis of simple patterns.

How important are eye movements to visual pattern analysis? Previous findings indicate that at least one visual task (counting) is seriously impaired without them. We asked whether a comparable limitation applies to pattern recognition. Subjects were presented with pairs of randomly generated arrays composed of black and white pixels. The subjects indicated whether the arrays were identical or differed by one pixel. In one experiment, they were instructed to use normal eye movements, in another they were required to fixate on a point between the arrays. When eye movements were permitted, subjects' performance showed evidence for a search in which the discrepant pixel was eventually found, given adequate inspection time. When fixation was required, search was less efficient and the discrepant pixel was sometimes not found, despite prolonged inspection time. These results were independent of target size over a wide range. Our findings indicate that eye movements play a crucial role in pattern analysis that is not related to resolution.

Eye Movements↗

PubMed1986

Contrast sensitivity function as a screening test: a critique.

Currently, there is intense clinical, commercial, and academic interest in the potential value of the contrast sensitivity function (CSF) for detecting eye disease. This paper contains an evaluation of the CSF as a screening test. Questions are raised concerning its scoring, accuracy, reliability, and robustness in screening situations. We conclude that the CSF cannot be of much value in visual screening until these questions are answered.

Discrimination Learning↗

PubMed1986

Psychophysics of reading. IV. Wavelength effects in normal and low vision.

Does the color of text influence its legibility? There are reasons why it may do so for specific groups of low-vision observers. We used psychophysical methods to measure the effects of wavelength on the reading performance of four normal observers, two dichromats, and twenty-five low-vision observers. Reading rates were measured for text scanned across the face of a television (TV) monitor. We compared performance under four luminance-matched conditions in which sets of neutral-density and Wratten color filters were placed in front of the TV screen--blue (lambda max = 430 nm), green (lambda max = 550 nm), red (lambda max = 650 nm), and gray. Under photopic conditions, the reading rates of normal subjects were independent of wavelength, with the exception of characters near the acuity limit. At lower luminances, wavelength effects could be explained by the shift from photopic to scotopic vision. It was hypothesized that light scatter or absorption in eyes with cloudy ocular media would result in depressed performance in the blue. Only one of seven subjects demonstrated this effect, which we traced to wavelength-specific absorption. Observers with advanced photoreceptor disorders tended to read blue text faster than red text. This could not be explained on the basis of photopic spectral sensitivities alone. Finally, the presence of central or peripheral field loss was not predictive of wavelength-specific effects in reading. On the whole, wavelength only occasionally plays a significant role in reading. When it does, performance tends to be depressed either in the red or the blue and to be nearly optimal for green or gray.

Adult↗

PubMed1985

Psychophysics of reading--I. Normal vision.

This paper is about the visual requirements for reading with normal vision. It is the first in a series devoted to the psychophysics of reading with normal and low vision. We have measured reading rates for text scanned across the face of a TV monitor while varying parameters that are important in current theories of pattern vision. Our results provide estimates of the stimulus parameters required for optimal reading of scanned text. We have found that maximum reading rates are achieved for characters subtending 0.3 degree to 2 degrees. Contrast polarity (black-on-white vs white-on-black text) has no effect. Reading rate increases with field size, but only up to 4 characters, independent of character size. When text is low-pass spatial-frequency filtered, reading rate increases with bandwidth, but only up to two cycles/character, independent of character size. When text is matrix sampled, reading rate increases with sample density, but only up to a critical sample density which depends on character size. The critical sample density increases from about 4 X 4 samples/character for 0.1 degree characters to more than 20 X 20 samples/character for 24 degrees characters. We suggest that one spatial-frequency channel suffices for reading.

Humans↗

PubMed1985

Psychophysics of reading--II. Low vision.

Very little is known about the effects of visual impairment on reading. We used psychophysical methods to study reading by 16 low-vision observers. Reading rates were measured for text scanned across the face of a TV monitor while varying parameters that are likely to be important in low vision: angular character size, number of characters in the field, number of dots composing each character, contrast polarity (white-on-black vs black-on-white text), and character spacing. Despite diverse pathologies and degrees of vision loss in our sample, several major generalizations emerged. There is a wide variation in peak reading rates among low-vision observers, but 64% of the variance can be accounted for by two major distinctions: intact central fields vs central-field loss and cloudy vs clear ocular media. Peak reading rates for observers with central-field loss were very low (median 25 words/minute), while peak reading rates for observers with intact central fields were at least 90 words/minute (median 130 words/minute). Most low-vision readers require magnification to obtain characters of optimal size. Sloan M acuity was a better predictor of optimal character size than Snellen acuity, accounting for 72% of the variance. Low-vision reading is similar to normal reading in several respects. For example, both show the same dependence on the number of characters in the field. Our results provide estimates of the best reading performance to be expected from low-vision observers with characteristic forms of vision loss, and the stimulus parameters necessary for optimal performance. These results will be useful in the development of clinical tests of low vision, and in the design of low-vision reading aids.

Adolescent↗

PubMed1985

Psychophysics of reading. III. A fiberscope low-vision reading aid.

Existing reading aids providing magnification in excess of 6X (24 diopters) are awkward to use, expensive, or not portable. The authors have developed a new type of reading aid, a low-resolution fiberscope, that provides magnification up to 40X (160 diopters), and is easy to use, inexpensive, and portable. The fiberscope consists of an objective-lens assembly, a flexible bundle of optical fibers, and an eyepiece that together transmit an image from the page to the reader's eye. Complete construction details are included. The design is based on the authors' previous findings that reading requires a field of only four characters and a resolution of only 2 cycles/character. After only half an hour of training, a diverse group of low-vision readers read with the fiberscope at rates between 12 and 95 words/minute. Low-vision readers with central field loss had a median reading rate of only 18 words/min, whereas those with intact central fields had a median reading rate of 67 words/min, not far short of the median rate of 73 words/min for normal readers using the fiberscope. This is consistent with the authors' previous findings that low-vision readers with central field loss read much more slowly than those with intact central fields, even when optimal magnification is provided. The authors conclude that the fiberscope may be very useful as an inexpensive portable reading aid for people requiring high magnification.

Adult↗

PubMed1984

Binocular contrast summation--I. Detection and discrimination.

Binocular summation was evaluated for contrast detection and discrimination. Monocular and binocular forced-choice psychometric functions were measured for the detection of 0.5-c/deg sine-wave gratings presented alone (simple detection), or superimposed on identical background gratings (discrimination). The dependence of detectability d' on signal contrast C could be described by: d' = (C/C')n. C' is threshold contrast, and n is an index of the steepness of the psychometric function. n was near 2 for simple detection, near 1 for discrimination, and was approximately the same for monocular and binocular viewing. Monocular thresholds were about 1.5 times binocular thresholds for detection, but the ratio dropped for suprathreshold discrimination. These results reveal a dependence of binocular summation on background contrast. For simple detection, binocular detectabilities were at least twice monocular detectabilities . For contrast discrimination, the amount of binocular summation decreased. For a 25%-contrast background, there was little or no binocular summation. It is concluded that binocular contrast summation decreases as background contrast rises.

Adult↗

PubMed1984

Binocular contrast summation--II. Quadratic summation.

Quadratic summation is presented as a rule that describes binocular contrast summation. The rule asserts that for left-eye and right-eye contrasts CL and CR, there is an effective binocular contrast C given by the formula: (formula; see text) Pairs of left-eye and right-eye stimuli that produce equal values of C are equivalent. Quadratic summation is applied to the results of experiments in which stimuli presented to the two eyes differ only in contrast. It provides a good, first-order account of binocular summation in contrast detection, contrast discrimination, dichoptic masking, contrast matching and reaction time studies. A binocular energy-detector model is presented as a basis for quadratic summation.

Form Perception↗

PubMed1984

Symmetry and constancy in the perception of negative and positive luminance contrast.

The perception of suprathreshold luminance contrast was investigated by forced-choice psychophysical procedures that were designed to define contrast equivalence relations. Observers compared the perceived contrast of rectangular bars that were presented for 500 msec at 3.9 deg on opposite sides of the fovea. The results show a nearly symmetrical relation between the perception of negative and positive contrast that is largely invariant over four decades of background luminance. Thus, for any fixed background luminance, equal absolute contrasts evoke approximately equal perceived contrasts. Symmetry also held with variations in the width, the eccentricity, and the focus of the bars. Symmetry was investigated further by determining equivalent contrast relations for negative contrasts as a function of background luminance and by contrast scaling. These results show evidence for nearly perfect contrast constancy for targets of low to moderate contrast and departures form constancy for high-contrast targets. These new findings on negative contrast, symmetry, and contrast constancy are discussed in relation to underlying mechanisms for contrast perception and classic experiments on brightness and lightness constancy.

Dark Adaptation↗

PubMed1983

Light and dark bars; contrast discrimination.

Contrast increment thresholds were measured for light and dark bars as a function of the base contrast of the bars. The bars were superimposed on a uniform field of 340 cd/m2. They had either rectangular or Gaussian luminance profiles, varied in width from 0.1 degree to 10 degrees, and in duration from 10 to 200 msec. For the 200-msec presentations, the resulting contrast discrimination functions all had approximately the same shape when contrast was defined as (Lmax - Lmin)/(Lmax + Lmin), and closely resembled corresponding results for sine-wave gratings. The similarity in shape of contrast discrimination functions for light and dark bars is attributed to a retinal nonlinear intensity transformation. The 10 msec contrast discrimination functions differed from the 200-msec functions in ways that can be explained by differences in temporal integration.

Darkness↗

PubMed1983

Convergence accommodation.

Steady-state accommodation responses were measured in both eyes as a function of vergence angle and direction of lateral gaze. The measurements were made with a binocular laser optometer. Small speckle patterns were used as fusional stimuli in an otherwise dark field. These patterns have the advantage of providing no blur stimulus to accommodation. Convergence accommodation for vergence angles ranging from 0 to 25 deg was measured for lateral-gaze angles of +32, -32, and 0 deg. The average accommodation of the two eyes was linearly related to vergence angle over the observer's accommodation range but was independent of the angle of lateral gaze. The mean convergence accommodation/convergence ratio for three subjects, in diopters per meter-angle, was 0.9. Our measurements of convergence accommodation using laser-speckle targets are in good agreement with previous studies that used small pupils. Accommodation responses for binocular viewing of letters of a Snellen chart were also measured. When luminance was reduced, night myopia was observed. No similar effect was found for convergence accommodation. Accommodation to a dim target corresponded closely to the convergence accommodation.

Accommodation, Ocular↗

PubMed1981