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Biomedical subjects

G E Legge

Publications and source records attributed to G E Legge.

At least 19 recordsLinked to original sources
PubMed1992

Contrast sensitivity and reading through multifocal intraocular lenses.

Multifocal intraocular lenses are intended to increase depth of focus for patients with cataracts, but optical considerations predict reduced retinal-image contrast. We evaluated visual performance through multifocal intraocular lenses by measuring contrast sensitivity functions and reading speed for age-matched groups with multifocal and monofocal intraocular lenses and two normal control groups. Contrast sensitivity functions of the patients with multifocal lenses did not differ significantly for optical distances differing by 2.5 diopters, indicating substantial depth of focus. Normal and monofocal contrast sensitivity functions were nearly identical, and both were about a factor of two higher than multifocal contrast sensitivity functions. Patients with multifocal lenses showed deficits in reading speed only for low-contrast text (less than 30%) and small letters (0.2 degree and 1.0 degree).

Aging↗
PubMed1992

Psychophysics of reading. Clinical predictors of low-vision reading speed.

Clinicians need to estimate how well their low-vision patients will perform everyday visual tasks such as reading or driving. Typically, it is not practical to measure task performance directly or to administer a lengthy series of special tests. Recent laboratory research has suggested that some routine clinical data may be useful in predicting reading performance. The purpose of the present study was to determine whether a promising set of simple measures--Snellen acuity, status of the central fields and ocular media, diagnosis, and age--could be used in a clinical setting to predict reading speed. One hundred and forty one patients who entered the low-vision clinic of the Minneapolis Society for the Blind received thorough eye examinations and a test of reading speed. Snellen acuity accounted for only 10% of the variance in reading speeds overall, but played a more important role for subjects with central loss. Age was a better predictor than acuity. A diagnosis of age-related maculopathy predicted slower reading speed than other causes of central-field loss, but the difference was attributed to age. Media status (clear or cloudy) had no predictive value. Our set of clinical predictors accounted for only about 30% of the variance in low-vision reading speeds. While data from more detailed visual testing might improve prediction, nonvisual factors such as age probably also contribute to the variance. Rather than relying on predictions from visual testing, clinical assessment of low-vision reading may be accomplished most easily with a suitably designed reading test.

Adolescent↗
PubMed1991

Psychophysics of reading--X. Effects of age-related changes in vision.

This study examined the influence of age-related changes in vision on reading performance. Maximum reading speed was measured in groups of young (n = 16, mean age 21.6 years) and old (n = 14, mean age 68.3 years) subjects, all with acuities of 20/32 or better. A psychophysical procedure was used for measuring reading speed that has proven reliable and sensitive to visual factors in previous research. Data were collected for character sizes ranging from .15 degrees to 12 degrees. Research revealed that old subjects who were free of eye disease read as fast as the young subjects for character sizes ranging from .3 degrees to 1.0 degrees. This is the range in which reading speed is maximum for young subjects. Research also revealed that old subjects showed a deficit when reading text composed of very small or very large characters. Their speeds dropped to about 70% of the young adult speeds. These deficits may be due to age-related losses in visual contrast sensitivity.

Adult↗
PubMed1990

Psychophysics of reading. XI. Comparing color contrast and luminance contrast.

Text can be depicted by luminance contrast (i.e., differences in luminance between characters and background) or by color contrast (i.e., differences in chromaticity). We used a psychophysical method to measure the reading speeds of eight normal and ten low-vision subjects for text displayed on a color monitor. Reading speed was measured as a function of luminance contrast, color contrast (derived from mixtures of red and green), and combinations of the two. When color contrast is high, normal subjects can read as rapidly as with high luminance contrast (greater than 300 words/min). Curves of reading speed versus contrast have the same shape for the two forms of contrast and are superimposed when contrast is measured in multiples of a threshold value. When both color and luminance contrast are present, there is no sign of additive interaction, and performance is determined by the form of contrast yielding the highest reading rate. Our findings suggest that color contrast and luminance contrast are coded in similar ways in the visual system but that the neural signals used in letter recognition are carried by different pathways for color and luminance. We found no advantages of color contrast for low-vision reading. For text composed of 6 degrees characters, all low-vision subjects read better with luminance contrast than with color contrast.

Adult↗
PubMed1989

Stereopsis and contrast.

We have measured threshold disparity as a function of the spatial frequency (0.25-20 c/deg) and contrast (0.02-0.75) of sine-wave gratings. In forced-choice trials, subjects indicated whether a target grating had crossed or uncrossed disparity relative to a reference grating. Thresholds were lowest near 3 c/deg and rose in proportion to spatial period at lower frequencies. Above 3 c/deg, there were marked individual differences. Across the range of spatial frequencies, disparity sensitivity and contrast sensitivity were correlated (r = 0.84). Threshold disparity was inversely proportional to the square root of contrast. When the contrast seen by one eye was reduced producing unequal monocular contrasts, threshold disparity rose more than when the contrast seen by the two eyes was reduced by the same amount. Our results have implications for stereo models that use zero crossings, peaks and troughs, or centroids as matching primitives. These models can account for the decline in disparity sensitivity at low sapatial frequencies but only the peak model satisfactorily accounts for the effect of contrast. If the limiting sources of noise in the two eyes are highly correlated, the effect of unequal monocular contrast can be accounted for using a differential-amplifier principle.

Contrast Sensitivity↗
PubMed1989

Psychophysics of reading. VI--The role of contrast in low vision.

The effect of contrast on reading performance was measured in 19 low-vision observers with a wide range of visual disorders and degrees of vision loss. The observers read text composed of 6 deg letters, ranging in contrast from 0.96 down to contrast threshold for reading. Reading performance was characterized by two parameters: peak reading rate is the reading rate at maximum contrast, and critical contrast is the contrast at which reading rate drops to half its maximum value. Peak reading rates were lower in observers with central field loss than in observers with intact central vision. In 16 of 19 cases, critical contrasts were higher for low-vision observers than for normal observers (averaging 3.9 times higher), indicating a decreased tolerance to contrast reduction. Values of critical contrast were closely linked to contrast sensitivity for letters (r = 0.87), but did not vary systematically with type of vision loss. Five observers read white-on-black text faster than black-on-white at both high and low contrasts. Four of the five had cloudy ocular media. We attribute this contrast polarity effect to abnormal light scatter in eyes with cloudy media. We examined the hypothesis that our low-vision observers' deviation from normal performance could be characterized (1) by a contrast scaling factor representing an attenuation of effective contrast and (2) that this scale factor could be identified with reduced contrast sensitivity. Such a description provided a good account for subjects with cloudy ocular media, where contrast attenuation results from intraocular light scatter. It provided a first order, but incomplete account for subjects with field loss where contrast attenuation is related to contrast sensitivity losses due to neural factors.

Adult↗
PubMed1989

Psychophysics of reading. VIII. The Minnesota Low-Vision Reading Test.

This is the eighth in a series of papers dealing with the role of vision in reading. In previous papers, we have evaluated the effects of stimulus and subject variables on reading rate using a drifting-text procedure. In this paper, we describe a new test of reading rate that uses static text, called the Minnesota Low-Vision Reading Test (MNread). It is microcomputer-based, and more easily set up and administered than the drifting-text procedure. It is of potential value as a standardized psychophysical test of reading and should be useful in research, clinical, and educational applications. Some types of low-vision aids rely on drifting text and others on static text. Is reading performance different for these two modes of text presentation? We measured reading rate as a function of angular character size for normal and low-vision subjects with drifting and static text. Although reading rates were highly correlated for the two modes of text presentation, normal subjects usually read static text more rapidly. The reverse was true for low-vision subjects; their reading rates for drifting text were slightly higher (average 15%) than for static text.

Adult↗
PubMed1989

Efficiency of graphical perception.

The term graphical perception refers to the part played by visual perception in analyzing graphs. Computer graphics have stimulated interest in the perceptual pros and cons of different formats for displaying data. One way of evaluating the effectiveness of a display is to measure the efficiency (as defined by signal-detection theory) with which an observer extracts information from the graph. We measured observers' efficiencies in detecting differences in the means or variances of pairs of data sets sampled from Gaussian distributions. Sample size ranged from 1 to 20 for viewing times of 0.3 or 1 sec. The samples were displayed in three formats: numerical tables, scatterplots, and luminance-coded displays. Efficiency was highest for the scatterplots (approximately equal to 60% for both means and variances) and was only weakly dependent on sample size and exposure time. The pattern of results suggests parallel perceptual computation in which a constant proportion of the available information is used. Efficiency was lowest for the numerical tables and depended more strongly on sample size and viewing time. The results suggest serial processing in which a fixed amount of the available information is processed in a given time.

Attention↗
PubMed1987

Psychophysics of reading--V. The role of contrast in normal vision.

How does contrast affect reading rate? What is the role of contrast sensitivity? We measured reading rate as a function of the contrast and character size of text for subjects with normal vision. Reading rates were highest (about 350 words/min) for letters ranging in size from 0.25 degree to 2 degrees. Within this range, reading was very tolerant to contrast reduction--for 1 degree letters, reading rate decreased by less than a factor of two for a tenfold reduction in contrast. The results were very similar for white-on-black and black-on-white text. Reading rate declined more rapidly for very small (less than 0.25 degree) and very large (greater than 2 degrees) letters. People with low vision usually require large characters to read, so high contrast is particularly important for them. Taking 35 words/min to be a threshold for reading, we constructed a contrast-sensitivity function (CSF) for reading. We were able to relate the shape of this CSF to the shape of sine-wave grating CSFs.

Humans↗
PubMed1987

Contrast discrimination in noise.

Even the highest contrast sensitivities that humans can achieve for the detection of targets on uniform fields fall far short of ideal values. Recent theoretical formulations have attributed departures from ideal performance to two factors--the existence of internal noise within the observer and suboptimal stimulus information sampling by the observer. It has been postulated that the contributions of these two factors can be evaluated separately by measuring contrast-detection thresholds as a function of the level of externally added visual noise. We wished to determine whether a similar analysis could be applied to contrast discrimination and whether variation of the increment threshold with pedestal contrast is due to changes in internal noise or sampling efficiency. We measured contrast-increment thresholds as a function of noise spectral density for near-threshold and suprathreshold pedestal contrasts. The experiments were conducted separately for static and dynamic noise. Our findings indicate that the same formulation can be applied to contrast discrimination and that changes in the estimated values of internal noise, rather than changes in sampling efficiency, play the major role in determining properties of contrast discrimination. Implications for models of contrast coding in vision are discussed.

Humans↗
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↗