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Biomedical subjects

G E Legge

Publications and source records attributed to G E Legge.

At least 37 records · Page 2Linked to original sources
PubMed1995

Human efficiency for recognizing 3-D objects in luminance noise.

The purpose of this study was to establish how efficiently humans use visual information to recognize simple 3-D objects. The stimuli were computer-rendered images of four simple 3-D objects--wedge, cone, cylinder, and pyramid--each rendered from 8 randomly chosen viewing positions as shaded objects, line drawings, or silhouettes. The objects were presented in static, 2-D Gaussian luminance noise. The observer's task was to indicate which of the four objects had been presented. We obtained human contrast thresholds for recognition, and compared these to an ideal observer's thresholds to obtain efficiencies. In two auxiliary experiments, we measured efficiencies for object detection and letter recognition. Our results showed that human object-recognition efficiency is low (3-8%) when compared to efficiencies reported for some other visual-information processing tasks. The low efficiency means that human recognition performance is limited primarily by factors intrinsic to the observer rather than the information content of the stimuli. We found three factors that play a large role in accounting for low object-recognition efficiency: stimulus size, spatial uncertainty, and detection efficiency. Four other factors play a smaller role in limiting object-recognition efficiency: observers' internal noise, stimulus rendering condition, stimulus familiarity, and categorization across views.

Contrast Sensitivity↗
PubMed1995

Human efficiency for recognizing and detecting low-pass filtered objects.

Recently, Tjan, Braje, Legge and Kersten [(1995) Vision Research, 35, 3053-3069] found that human efficiency for object recognition was less than 10%, indicating that humans fail to use much of the information available to an ideal observer. We examine two explanations for these low efficiencies: (1) humans are inefficient in using high spatial-frequency information; and (2) humans are inefficient in detecting image samples. We tested the first possibility by measuring human efficiency for recognizing low-pass filtered objects, rendered as line drawings and silhouettes, in luminance noise. Efficiency did not improve when high frequencies were removed, and the first explanation was rejected. We tested the second explanation by comparing efficiencies for object detection and recognition. Recognition efficiency was higher than detection efficiency for silhouettes but not line drawings, showing that detection efficiency does not place a ceiling on recognition efficiency. The results indicate that human vision is designed to extract image features, such as contours, that enhance recognition. A computer simulation suggests that this can occur if the observer views the world through a band-pass spatial-frequency channel.

Algorithms↗
PubMed1995

The 'uniqueness constraint' and binocular masking.

In stereo-matching algorithms, the 'uniqueness constraint' requires that a feature in one stereo half-image be matched to, at most, one similar feature in the other half-image. Experiments are reported in which binocular contrast thresholds and depth-discrimination judgments have been used to determine whether the human stereo system makes unique matches. A single high-spatial-frequency target in the left eye was paired stereoscopically with two identical targets, presented near retinal correspondence (+/- 3.5 min of disparity), in the right eye. Contrast-increment thresholds were measured for each of the targets in the right eye, and it was found that the target in the left eye masked both. Indeed, the amount of binocular masking for each member of the double target nearly equaled the masking observed when only a single target was presented to the right eye. Depth judgments confirmed that the target in the left eye had been matched to both targets in the right eye. It is concluded that uniqueness is not an absolute constraint on human stereo matching.

Algorithms↗
PubMed1995

Pictorial depth cues: a new slant.

Pictorial depth cues such as perspective projection, aspect ratio, and texture gradients can specify mathematically the slant of a planar surface. We performed experiments to measure the accuracy of human perception of surface slant from these cues. We calculated the perceived slant from judgments of the relative lengths of a pair of orthogonal lines embedded in the surface. Our results indicate that slant judgments are accurate to within 3 deg. This level of accuracy was achieved whether the cues were luminance differences or equiluminous color differences. We found no evidence of the recession to the frontal plane that has been reported by Gibson [J.J. Gibson, The Perception of the Visual World (Houghton Mifflin, Boston, Mass., 1950]) and others. We did find evidence suggesting that subjects do not make accurate depth estimates of disconnected surfaces. This may be the source of the discrepancy between our measures and those of Gibson and others. This research, combined with previous findings, supports a model of perception that involves at least two and possibly more representations of space: one local veridical representation of surface orientation derived primarily from pictorial cues and another global representation of observer-centered distance derived primarily from binocular disparity and motion parallax.

Depth Perception↗
PubMed1994

Stereo matching precedes dichoptic masking.

Stereo matching can intervene to prevent dichoptic masking. In a dichoptic masking paradigm we measured the contrast threshold for a bar target, presented to one eye, as a function of the contrast of an identical masking bar, presented at retinal correspondence in the other eye. Confirming previous studies of dichoptic masking with sinusoidal gratings, the test bar thresholds rose proportionally with increasing masking contrast. This threshold elevation was almost nullified when an extra bar was presented to the eye seeing the test stimulus. Release from masking occurred when the disparity between the masking bar and extra bar was < 20 min arc over a range of contrast levels (8-45%), and for bars containing either broad spatial frequency spectra or bars with only high spatial frequencies (peak = 12 c/deg). The latter result rules out an explanation for the release from masking based on contrast discrimination in low spatial frequency channels. The extra bar was effective in releasing the test bar from masking as long as the extra bar's contrast was greater than about one-fifth the contrast of the mask, a result that suggests that there is a contrast threshold for stereo matching. We interpret our findings to indicate that a stage of stereo matching occurs prior to the neural site limiting dichoptic contrast discrimination.

Contrast Sensitivity↗
PubMed1993

Color improves object recognition in normal and low vision.

Does color improve object recognition? If so, is the improvement greater for images with low spatial resolution in which there is less shape information? Do people with low visual acuity benefit more from color? Three experiments measured reaction time (RT) and accuracy for naming food objects displayed in 4 types of images: gray scale or color, and high or low spatial resolution (produced by blur). Normally sighted Ss had faster RTs with color, but the improvement was not significantly greater for images with low spatial resolution. Low vision subjects were also faster with color, but the difference did not depend significantly on acuity. In 2 additional experiments, it was found that the faster RTs for color stimuli were related to objects' prototypicality but not to their color diagnosticity. It was concluded that color does improve object recognition, and the mechanism is probably sensory rather than cognitive in origin.

Adult↗
PubMed1993

Multifocal intraocular lenses and glare.

In a previous paper, we reported finding deficits in the contrast sensitivity functions of patients with diffractive multifocal intraocular lenses (IOL's). The results were consistent with optical measurements of the modulation transfer function (MTF) of the IOL. When this MTF is treated as a linear spatial frequency filter, it predicts the existence of a glare effect; contrast threshold for the recognition of target letters should be elevated by a bright, adjacent stimulus. We tested this prediction by measuring contrast thresholds for recognizing 0.2 degrees Sloan letters on a background luminance of 11.2 cd/m2. The letters were presented inside bright (300 cd/m2) annular rings with inner diameters ranging from 0.42 to 1.22 degrees. Thresholds were measured for seven multifocal subjects, age-matched groups of monofocal subjects and phakic-control subjects, and a young group. Multifocal subjects exhibited a greater glare effect than monofocal subjects, and they in turn exhibited a greater effect than phakic-control subjects. The observed glare effect for multifocal subjects was about twice that expected from the spatial filtering property of the multifocal IOL.

Adult↗
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↗