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D M Levi

Publications and source records attributed to D M Levi.

At least 127 records · Page 7Linked to original sources
PubMed1988

Evidence for nonlinear binocular interactions in human visual cortex.

Dichoptically presented uniform fields were sinusoidally modulated in luminance, with differing temporal frequencies between the eyes. This stimulus evokes a unique electrophysiological response termed "beats"; visual evoked potential (VEP) components reflecting nonlinear neural behavior and which could only arise from integrative binocular units. Individuals with strabismus or amblyopia, which accompany disrupted binocular function in the visual cortex manifest a severe reduction of this nonlinearity, suggesting an abnormality in the binocular cortical processes underlying this response.

Amblyopia↗
PubMed1988

The Glenn A. Fry award lecture: the "spatial grain" of the amblyopic visual system.

This paper reviews psychophysical evidence that the visual performance and, by inference, the underlying neural losses of strabismic and anisometropic amblyopes are fundamentally different. The data of amblyopes are considered in the light of recent models for normal spatial vision. It is argued that the spatial deficits which are found in anisometropic amblyopes can be understood largely in terms of reduced resolution and contrast sensitivity, as would be expected on the basis of early experience with a defocused image in one eye. In contrast, the spatial deficits found in strabismic amblyopes are more profound than can be predicted on the basis of either resolution, or contrast sensitivity, and may have their basis in a coarse cortical spatial sampling grain.

Amblyopia↗
PubMed1987

Positional uncertainty in peripheral and amblyopic vision.

Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.

Amblyopia↗
PubMed1987

Depth attraction and repulsion of disparate foveal stimuli.

Interaction in the domain of disparity can be either of the kind where the depth difference between adjacent targets is enhanced, as if the two targets repelled each other in depth, or it may be in the opposite direction, i.e. having the character of attraction. In the fovea, interaction between stimuli is of the latter kind if targets are separated by less than 2-8 min of arc, dependent on their positions and the observer; for further separations, repulsion is exhibited. When disparate neighbors induce a change in depth of a visual feature, only a portion of the effect can be ascribed to monocular localization shifts in the two monocular retinal images. Both attraction and repulsion can occur between targets of opposite contrast. Depth interaction measured by a psychophysical nulling method increases monotonically with disparity in the regions clearly governed by the repulsion and the attraction regimen; in the transition region, repulsion overtakes attraction when the disparity becomes larger. If the concept of "pooling" of disparity is invoked to account for the affinity of seen depth of closely-adjacent stimuli, the signals involved cannot be simply those of light weighted by disparity, but must be associated with individual features.

Depth Perception↗
PubMed1987

Dichoptic hyperacuity: the precision of nonius alignment.

The alignment of nonius targets can be judged with the same precision as the alignment of vernier targets for some target configurations. We measured dichoptic and monocular hyperacuity thresholds as a function of the separation between the target lines. At separations above a critical value (30-60 arcmin), monocular and dichoptic thresholds were identical and increased as a power function of line separation. At smaller separations, the dichoptic thresholds were 0.6-0.7 arcmin, independent of separation, and significantly higher than the comparable monocular thresholds. The dichoptic results can be modeled as the sum of two sources of noise: (1) intrinsic positional uncertainty, which depends on line separation and is common to both dichoptic and monocular judgments, and (2) disjunctive fluctuations in convergence. It seems likely that the mechanism that limits the precision of vernier acuity has its locus at a point in the visual pathways after the signals from the two eyes have been combined.

Form Perception↗
PubMed1987

Spatial-interval discrimination in the human fovea: what delimits the interval?

In this paper we describe four experiments concerned with spatial-interval discrimination. The first experiment demonstrates that perturbations in the internal light distribution of two bright bars alter the perceived separation of the bars without influencing the precision of spatial-interval discrimination. Specifically, alterations in the centroid of the luminance distribution of the bars were found to influence their perceived separation systematically. In the second experiment we investigated separation discrimination for a pair of bright lines, a pair of dark lines, and a pair of lines with opposite polarity (a bright-dark pair). For large interline separations, the thresholds for separation discrimination of the three stimuli were equivalent. However, for small separations (less than 4 arcmin) the thresholds for the opposite-polarity pair were significantly higher than those for either bright or dark lines. The third experiment showed that separation discrimination is also impaired when the direction-of-brightness change of adjacent edges is the same. In the fourth experiment, random luminance perturbations were introduced in the interval between two bright bars separated by 1.5 to 8 arcmin by the addition of a dim line within the interval. The effect of these perturbations was also to alter the perceived separation of the bars with no effect on the precision of spatial-interval discrimination. Increasing the intensity of the perturbating line (to equal that of the outer lines) elevated thresholds for small separations (less than 4 arcmin) to approximately 6 arcsec but had no effect on the threshold for larger separations. Taken together, these results suggest that large and small separations are processed differently and place strong constraints on models for spatial-interval discrimination.

Discrimination, Psychological↗
PubMed1987

Position sense of the peripheral retina.

Position acuity was measured over a wide range of eccentricities, from 3 min to 10 deg in the horizontal meridian, by using both a three-dot bisection task and a three-dot vernier task. A foveal fixation dot appeared for 1 sec before an outer pair of test dots was flashed for 200 msec. Bisection and vernier tasks were used to measure position acuity in the radial and tangential directions, respectively. The vernier data were well fitted by a straight line on linear axes of offset threshold versus eccentricity. The bisection data, on the other hand, were poorly fitted by a single straight line. However, a double-line fit worked very well. The line segment at large eccentricities (greater than 0.5 deg) had an x intercept of about 0.6 deg, in good agreement with previous estimates based on cortical magnification and on hyperacuity in the presence of flanks. These results imply that three-dot vernier thresholds are set by a single orientation mechanism at all eccentricities and that three-dot bisection thresholds are set by a pair of mechanisms. For eccentricities less than 15 min, thresholds are in good agreement with calculations based on spatial-frequency filters. For larger eccentricities, the bisection thresholds agree with scaled anatomical modules that are presumed to exist in the human visual cortex. The thresholds for position acuity in the tangential direction are as low as 0.005 times the eccentricity. In the radial direction, thresholds are poorer, implying that additional cortical factors may further constrain performance.

Humans↗
PubMed1987

Peripheral hyperacuity: three-dot bisection scales to a single factor from 0 to 10 degrees.

Performance of three-dot bisection was determined as a function of the feature separation at eccentricities of 0-10 deg along the inferior vertical field meridian by using dot stimuli scaled in size to compensate for eccentricity. For these briefly flashed dot stimuli, the entire function of three-dot bisection acuity against dot separation worsens away from the fovea with a single scaling factor that is compatible with the change of the cortical magnification factor in area V1. When the presentation duration of the stimulus was lengthened from 150 msec to 1 sec, the improvement was much greater in the fovea than at 10-deg eccentricity for closely separated stimuli. We attribute this difference to a luminance cue (detection of a brightness change) that is present for a long stimulus duration at small separations in the fovea, but not in the periphery.

Fovea Centralis↗
PubMed1987

Peripheral hyperacuity: isoeccentric bisection is better than radial bisection.

Performance of three-dot bisection was determined as a function of orientation for a variety of feature separations and field meridians at eccentricities of 0-10 deg for two observers. The dot stimuli and separations were scaled in size to compensate for eccentricity. The precision of three-dot bisection was found to depend on the direction of test-feature offset. In the fovea, horizontal and vertical bisections were better than oblique bisections, while at eccentricities of 5-20 deg, isoeccentric (on a tangent to a circle of a given eccentricity) bisection was better than radial bisection. The direction of offset was more important than the orientation of the stimulus. Large separations showed a stronger effect than small separations. The anisotropy of bisection appears different from the meridional effect for resolution and is unlikely to be simply related to a local anisotropy of the cortical magnification factor.

Discrimination, Psychological↗
PubMed1985

Selectivity of the evoked potential for vernier offset.

Hyperacuity thresholds of a few arc seconds can be achieved psychophysically for a variety of spatial localization tasks. The present experiments show that evoked potentials can be elicited in response to the introduction of vernier offsets, but not by the introduction of other cues to hyperacuity such as bisection or relative pattern motion, although each of these cues is equally salient psychophysically. Moreover, vernier acuity measurements and the evoked potentials elicited in response to vernier offsets are strongly degraded by the introduction of flanking stimuli 2-4 min from the vernier target. This suggests that the hyperacuity VEP is a cortical correlate of a very specific type of hyperacuity, that produced by vernier offsets (colinearity failure).

Adult↗
PubMed1985

Vernier acuity, crowding and cortical magnification.

When a vernier target is flanked by optimally positioned lines, foveal vernier discrimination is strongly degraded (Westheimer and Hauske, 1975). We confirmed this observation (Experiment I) and have mapped out a 2 dimensional "perceptive field" for crowding in the fovea using a 2 dot target (Experiment II). Crowding was also measured in peripheral vision, using either small flanking dots as masks (Experiment III), or using repetitive vernier gratings (Experiment IV). The results showed that when scaled in proportion to recent estimates of the cortical magnification factor, vernier acuity is as good in the periphery as it is centrally. Both centrally and peripherally, there appears to be a psychophysical processing module which we term a "perceptive hypercolumn". At all eccentricities vernier thresholds were found to be approximately 1/40 of the size of a perceptive hypercolumn and were elevated if interfering contours are present in the same (or adjacent) hypercolumns.

Form Perception↗
PubMed1985

Vernier acuity, crowding and amblyopia.

When a vernier target is flanked by a pair of optimally positioned flanks, offset discrimination is strongly degraded. Spatial interference with vernier acuity was studied in each eye of observers with unilateral amblyopia associated with strabismus, anisometropia or both, and were compared to the functions obtained in the normal periphery (Levi et al., 1985). The results showed that: (1) For both strabismic and anisometropic amblyopes, as in normal central and peripheral vision, the extent of spatial interference was proportional to the unflanked vernier threshold. (2) For anisometropic amblyopes, grating and vernier acuity are affected similarly. (3) For strabismic amblyopes, like the normal periphery, vernier and grating acuity are decoupled, with vernier falling off faster than grating acuity. (4) The preferred eyes of strabismic but not anisometropic amblyopes have poorer vernier acuity than the normal controls. A conceptual framework for amblyopia based upon spatial filtering and spatial sampling is discussed.

Adult↗
PubMed1985

Hyperacuity thresholds of 1 sec: theoretical predictions and empirical validation.

Bisection thresholds were measured as a function of the separation of the lines. For separations of less than 1.5 min, the addition of flanking lines facilitates bisection so that thresholds of less than 1 sec for discriminating the direction of offset could be reliably obtained. For larger separations an interval could be bisected to an accuracy of 1 part in 60. Experiments varying the length, luminance, and overlap of the lines suggest that different cues are used in these two regimes. A dual space-size analysis is presented that can account for these bisection thresholds over a wide range of experimental conditions. This quantitative analysis produces viewprints of the stimuli (analogous to the voiceprint of audition). Each viewprint shows the output of many spatial filters of different positions and sizes. A new filter shape is introduced that has advantages for modeling the visual system. The sensitivity of each filter is fixed by the contrast-response function. The analysis further shows that the limiting factors in spatial hyperacuity are both the contrast-response function and the spatial grain.

Discrimination, Psychological↗
PubMed1985

The relationship between binocular rivalry and strabismic suppression.

Increment-threshold spectral sensitivity functions were determined for normal observers during binocular rivalry and for esotropic observers during strabismic suppression and under viewing conditions that normally induce binocular rivalry. Depending on the spatial and temporal properties of the test stimulus, the normal observers exhibited a wavelength-specific loss in sensitivity during the suppression phase of rivalry, which suggests that binocular rivalry differentially attenuates the sensitivity of the chromatic mechanisms relative to the luminance mechanisms. In contrast, regardless of the test stimulus dimensions, the esotropic observers did not manifest a wavelength-specific loss in sensitivity either during strabismic suppression or under conditions that normally induce binocular rivalry. The different patterns of suppression shown by the normal and esotropic subjects suggest that strabismic observers do not demonstrate normal binocular rivalry, and that strabismic suppression and normal binocular rivalry suppression are mediated by different neural mechanisms.

Adult↗
PubMed1984

Central and peripheral contrast sensitivity in amblyopia with varying field size.

The purpose of the present study was to establish the extent of the amblyopic deficit at 0 degree, 10 degrees, and 20 degrees retinal eccentricity using contrast sensitivity measures for sine wave gratings subtending various field sizes. Contrast sensitivity functions were shown to vary with stimulus field size, retinal location and degree of amblyopia. Foveally, peak contrast sensitivity of amblyopic eyes increased markedly with increasing field size, to reach the same magnitude as that of the contralateral non-amblyopic eyes for large fields. Peripherally, peak contrast sensitivity remained lower in the amblyopic eyes for all field sizes examined. High spatial frequency cut-offs were reduced both centrally and peripherally with all field sizes in the amblyopic eyes. The effect of increasing field size on the cut-off acuity was found to be smaller than the effect on peak contrast sensitivity. The most significant conclusions drawn from the results are that: (1) The amblyopic deficit in terms of both peak contrast sensitivity and acuity is not restricted to the central foveal region; and (2) amblyopic eyes benefit to a greater extent from increased stimulus field size than non-amblyopic eyes in terms of peak contrast sensitivity. A model is proposed which suggests that the results obtained are due to fewer and/or less sensitive cortical neurons being driven by the amblyopic eye in humans with naturally occurring amblyopia.

Adult↗
PubMed1984

Detection and discrimination of the direction of motion in central and peripheral vision of normal and amblyopic observers.

This paper describes the "motion" properties of the amblyopic fovea and compares them to the normal periphery. Specifically, thresholds for detection of the displacement of a grating pattern, and for discrimination of displacement direction were measured. The main findings of these experiments were: in the central vision of both normal and amblyopic observers, unreferenced displacement are detected with an accuracy equal to the observer's grating acuity; in the normal periphery, unreferenced motion thresholds fall off at a slower rate than does grating acuity; in amblyopic eyes, displacement thresholds are most elevated centrally; the addition of an abutting reference improves detection of motion for the normal fovea and in anisometropic amblyopes, but elevates motion thresholds in both the normal periphery and in the fovea of amblyopes with strabismus. The adequacy of the normal periphery as a model for the central vision of amblyopes is discussed.

Adult↗
PubMed1984

Spatial and velocity tuning of processes underlying induced motion.

A nulling procedure was used to quantify the velocity and spatial frequency tuning of induced motion for sinusoidal gratings. For each spatial frequency of test and inducing gratings, there was a range of low velocities which resulted in strong induction, with a gain of close to 1. For low spatial frequencies induction occurred at higher velocities than was the case for high spatial frequencies. Induced motion shows bandpass spatial frequency tuning, with a bandwidth of about two octaves at half-height. Induced motion appears to be mediated by spatial channels with a low pass temporal characteristic. To a first approximation, induced motion appears to be a product of velocity and spatial frequency.

Form Perception↗