Biomedical subjects

D M Levi

Publications and source records attributed to D M Levi.

At least 19 recordsLinked to original sources

PubMed1992

The role of local contrast in the visual deficits of humans with naturally occurring amblyopia.

We measured the positional acuity of amblyopic observers and their sensitivity to the local contrast information which provides the cue for the position judgement. Our results suggest that there exist fundamental differences in the neural losses in humans with strabismic and anisometropic amblyopia. The losses in positional acuity of anisometropic amblyopes may be accounted for on the basis of the reduced contrast sensitivity and increased neural pooling of the underlying visual filters; whereas strabismic amblyopes, like the normal periphery, show an extra loss, which may be accounted for on the basis of scrambling, or jitter in the topographic mapping of information from retina to cortex. Since neurons in the striate cortex of monkeys show precise positional coding, it would be of particular interest to examine the positional acuity and local contrast sensitivity in cortical neurons of monkeys with experimental amblyopia using the same stimuli to measure both.

Amblyopia↗

PubMed1992

"Weber's law" for position: the role of spatial frequency and contrast.

We used Gabor bars to measure the effects of spatial bandwidth, spatial scale, contrast and separation on three-line spatial interval discrimination (bisection). In the first experiment, we used stimuli that were well above threshold. Our results show that at all spatial scales, spatial interval discrimination (three-line bisection) thresholds are proportional to the separation of the Gabor patches (i.e. Weber's law) when the separation exceeds approximately 2.5 times the standard deviation (sigma) of the Gaussian envelope. The optimal threshold occurs when the separation is approx. 2-2.5 sigma, and for separations larger than the optimal, bisection thresholds are equal to a more or less constant Weber fraction (delta s/s) of approx. 0.02-0.04. These results are consistent with a number of previous studies. In the second experiment, we examined the effect of contrast. Our results show an interaction between separation and stimulus visibility. Reducing the stimulus contrast has a marked effect on spatial interval thresholds at small separations (e.g. separations less than about 3 sigma), and much less effect at larger separations. Thus, the Weber's law relationship appears to depend on the visibility of the stimuli, but does not depend on the spatial frequency or bandwidth of the stimuli. These results can be predicted by an ideal observer model of spatial interval discrimination.

Contrast Sensitivity↗

PubMed1992

The two-dimensional shape of spatial interaction zones in the parafovea.

The spatial analysis of a target may be strongly degraded by the simultaneous presentation of nearby pattern elements. The present study investigated the shape and extent of the region of interaction as a function of retinal location. The stimuli consisted of 3 collinear [symbol: see text] s which were randomly oriented up ([symbol: see text]) or down ([symbol: see text]). The task was to discriminate the orientation of the middle [symbol: see text]. The retinal locations studied were at 0, 2.5, 5 and 10 degrees, on the lower vertical meridian and on the nasal halves of both the horizontal and the 45 degrees diagonal visual field meridians. The extent of the interaction region was defined as the separation between the midpoint of two adjacent [symbol: see text] s that resulted in 75% correct discrimination. The shape of the interaction region was determined by using several orientations (horizontal, vertical, left diagonal and right diagonal) for the virtual line joining the 3. [symbol: see text] s. Our results show that the size of the interaction regions varies linearly with eccentricity as does the size of a just resolved individual [symbol: see text]. However, the size of the interaction region varies much more rapidly than does the resolution threshold for an individual [symbol: see text]. The spatial interaction zones appear to be elongated radially, so that they have an elliptical shape. The size of the major axis is about 2-3 times the size of the minor axis. The major axis is along the meridian through the central visual field (i.e. it is oriented radially) while the minor axis is oriented tangentially (i.e. isoeccentrically).

Adult↗

PubMed1992

Spatial localization without visual references.

To explain the veridical percept of the spatial ordering of objects and the generation of eye movements to peripheral targets, Lotze (1885 Microcosmos. Edinburgh: T. & T. Clark) proposed that there is a position label (local sign) for each retinal element. To estimate the precision of local sign information, we measured absolute localization thresholds at various eccentricities in the nasal visual field, in the complete absence of visual references. To eliminate perception of the visual surround, observers viewed a large display screen through a neutral density filter (2.0 log unit) in a dark room. The fixation target was extinguished at various times (interstimulus intervals or ISIs) prior to the onset of the test stimulus. In general, our results show that localization thresholds are proportional to the target eccentricity at all ISIs. At each eccentricity, localization thresholds are elevated after the extinction of the visual reference compared to thresholds when the reference is present. However, relative to the referenced threshold, unreferenced thresholds are elevated by a greater proportion at smaller eccentricities than at larger eccentricities. Our threshold vs ISI data can be adequately modeled on the basis of an intrinsic positional uncertainty, which increases with eccentricity, and additive and multiplicative sources of noise. The additive noise appears to reflect primarily the increasing scatter in eye position when the fixation target is extinguished. Our model's estimate of intrinsic positional uncertainty in the isoeccentric direction appears to reflect primarily the intrinsic positional uncertainty of the peripheral retina (the local sign), being very similar to cumulative cone position uncertainty and to the spacing between ON-P beta ganglion cells. In the isoeccentric direction, the estimated precision of the local sign mechanism across eccentricities is slightly better than the precision of saccadic endpoints, suggesting that noise in the motor system must also contribute to the scatter of saccadic endpoints in the isoeccentric direction. Interestingly, in the radial direction, we find a surprising similarity in our observers' positional uncertainty and the precision of saccadic endpoints.

Fovea Centralis↗

PubMed1992

Topography of the evoked potential to spatial localization cues.

Visual tasks that are perceptually diverse might be expected to elicit unique evoked-potential waveforms that exhibit differing topographic maps. To investigate this possibility, multichannel visual-evoked potentials (VEPs) were recorded in response to several dot spatial localization stimuli that are physically similar yet produce different percepts (vernier offsets, steroscopic disparity, bisection, orientation, and relative displacement) to determine if the unique percepts arising from these stimuli reflect the activation of different cortical neural populations. The resulting evoked potentials were all similar in waveform, although the stereoscopic VEPs were relatively delayed. Topographic maps of the evoked-potential activity to each stimulus revealed a late major component with two independent foci: one 7 or more centimeters above the inion lateral to the midline, and the other at least 6 cm lateral to OZ. The scalp localization of both peaks was independent of both the position of the stimulus in the visual field and the particular stimulus cue presented. An asymmetric response to pattern appearance vs. disappearance indicated strong pattern specificity for each stimulus type except unreferenced motion. The timing of the VEP responses and relative insensitivity to retinal locus of stimulation suggest the involvement of higher cortical areas. The two map foci might be interpreted as activation of inferotemporal and parietal cortices whose roles are thought to be visual object interpretation and spatial attention and localization, respectively.

Evoked Potentials, Visual↗

PubMed1992

The perceived strength of illusory contours.

Illusory contours are not well understood, partially because a lack of physical substance complicates their specification via physical standards. One solution is to gauge illusory contours with respect to luminance-defined contours, which are easily quantified physically. Accordingly, we chose a metric (perceived contrast) that expresses illusory contour strength in terms of the physical contrast of luminance-defined contours. Using this metric, adult observers adjusted the contrast of a luminance-defined contour until it matched the perceived contrast of an illusory contour. Illusory contour length, inducer size, and inducer contrast all influenced illusory contour strength. The results are adequately explained via low-level visual processes. It appears that matching paradigms can be beneficial in quantitative studies of illusory contours.

Adult↗

PubMed1991

Spatial-interval discrimination in two-dimensions.

This study is concerned with the precision of spatial-interval discrimination when the stimulus is varied in two-dimensions rather than one-dimension. For the two-dimensional task, observers were required to judge the centrality of a dot within a circle. Similar measurements were made in a one-dimensional task, i.e. the test dot was displaced only along one meridian, and the results were compared. This study shows that thresholds for the two-dimensional task are approx. square root of 2 times the threshold of the one-dimensional task when the observer's task is to simply detect an offset in both one- and two-dimensional conditions. However, thresholds are about a factor of two higher for the two-dimensional task if the observer is required to label the direction of offset.

Discrimination, Psychological↗

PubMed1991

Binocular summation in vernier acuity.

Monocular and binocular abutting line vernier acuities were measured as a function of contrast. Over a range of contrasts from near the line-detection threshold to approximately 20 times threshold, binocular vernier thresholds are lower (better) than monocular thresholds by approximately 50-60%, similar to the binocular improvement found for the detection of both a thin line and a dipole. At higher contrasts the binocular advantage diminishes, apparently as a result of saturation.

Contrast Sensitivity↗

PubMed1990

Equivalent intrinsic blur in spatial vision.

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line "resolution"; (ii) line detection; and (iii) spatial interval discrimination, in both central and peripheral vision. The results of our experiments can be summarized as follows. (i) 2-Line "resolution": thresholds for pairs of unblurred, low contrast, stimuli are approx. 0.5 min arc in the fovea. When the stimulus blur is small, it has little effect upon 2-line "resolution"; however, when the stimulus blur, sigma, exceeds 0.5 min, thresholds are degraded. We operationally define this transition point as the equivalent intrinsic blur or Bi. When the standard deviation of the stimulus blur, sigma, is greater than Bi, then the "resolution" threshold is approximately equal to sigma. Both the unblurred "resolution" threshold, and the equivalent intrinsic blur, Bi, vary with eccentricity in a manner consistent with the variation of cone separation within the central 10 deg. When the stimulus blur exceeds the equivalent intrinsic blur, "resolution" in the periphery is the same as in the fovea. (ii) Line detection: when the standard deviation of the stimulus blur, sigma, is less than Bi, then the line detection threshold is approximately inversely proportional to sigma (it is approximately TdBi/sigma) i.e. it obeys Ricco's law. When the standard deviation of the stimulus blur, sigma, is greater than Bi, then the "resolution" threshold is approximately equal to sigma and the detection threshold is approximately a fixed contrast (to be referred to as Td). According to (i) and (ii), the equivalent intrinsic blur, Bi, plays a dual role in determining both the "resolution" threshold and the detection threshold, Bi corresponds to the "Ricco's diameter" for spatial summation in a detection task, and it also corresponds to the "resolution" threshold for thin lines. This connection between detection and "resolution" is somewhat surprising. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, the thresholds represent a "hyperacuity" (i.e. they are smaller than the "resolution" threshold). For unblurred lines, the optimal separation is approximately 2-3 times the "resolution" limit at all eccentricities, so the optimal separation varies with eccentricity at the same rate as the equivalent intrinsic blur, Bi. However, the optimal spatial interval threshold falls off with eccentricity about 3-4 times more rapidly, consistent with the rate of decline of other position acuity tasks.(ABSTRACT TRUNCATED AT 400 WORDS)

Fovea Centralis↗

PubMed1990

Equivalent intrinsic blur in amblyopia.

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line resolution; (ii) line detection; and (iii) spatial interval discrimination, in observers with amblyopia due to anisometropia, strabismus, or both. The results of our experiments can be summarized as follows. (i) 2-Line resolution: in normal foveal vision, thresholds for unblurred stimuli are approx. 0.5 min arc in the fovea. When the standard deviation (sigma) of the stimulus blur is less than 0.5 min, it has little effect upon 2-line resolution; however, thresholds are degraded when the stimulus blur, sigma, exceeds 0.5 min. We operationally define this transition point, as the equivalent intrinsic blur, or Bi. When the stimulus blur, sigma, is greater than Bi, then the resolution threshold is approximately equal to sigma. In all of the amblyopic eyes, 2-line resolution thresholds for unblurred stimuli were elevated, and the equivalent intrinsic blur was much larger. When the stimulus blur exceeds the equivalent intrinsic blur, resolution thresholds were similar in amblyopic and nonamblyopic eyes. (ii) Line detection: in both normal and amblyopic eyes, when the stimulus blur, sigma, is less than Bi, then the line detection threshold is approximately inversely proportional to sigma; i.e. (it obeys Ricco's law). When sigma is greater than Bi, the equivalent intrinsic blur, then the detection threshold is approximately a fixed contrast. All of the amblyopic eyes showed markedly elevated thresholds for detecting thin lines, but normal or near normal thresholds for detecting very blurred lines. Consequently, Ricco's diameter is larger in amblyopic than in normal eyes. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds represent a "hyperacuity" (i.e. they are smaller than the resolution threshold). For unblurred lines, the optimal separation is approx. 2-3 times Bi. In the normal fovea, and in the amblyopic eyes of anisometropic amblyopes the optimal spatial interval discrimination threshold is about one-fifth of the resolution threshold (i.e. a hyperacuity); and over a wide range of separations, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds about one-third of the separation between the lines as long as the contrast is sufficiently high. In contrast, in strabismic amblyopes, like the normal periphery, the optimal spatial interval discrimination thresholds are worse (higher) than would be expected based upon the resolution limit of the strabismic amblyopic eye.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1990

The role of separation and eccentricity in encoding position.

We measured two- and three-line spatial interval and alignment thresholds for a wide range of separations and eccentricities. In order to avoid confounding the role of separation and eccentricity, the test lines were presented on iso-eccentric arcs, with radii between 0.625 and 10 deg. The iso-eccentric paradigm allows separation to be varied over a large range while holding eccentricity constant. Our main finding is that for all tasks, at all eccentricities, Weber's law fails at large separations. Our results are consistent with the hypothesis that distance judgements are limited by at least two factors: (1) when the separation of the iso-eccentric test lines is small with respect to the eccentricity position thresholds are proportional to the separation of the features, and show little dependence on eccentricity. This is Weber's law for position, and the threshold is approx. 0.03-0.05 times the stimulus separation; (2) when the separation of the iso-eccentric test lines is comparable in size to the eccentricity, position thresholds are proportional to the target eccentricity, and are essentially independent of separation. In this "eccentricity regime", position discrimination thresholds are = ECC * k where ECC is the stimulus eccentricity in degrees, and k is a fraction of the eccentricity (approximately 0.01-0.03). The observer uses whichever mechanism is more sensitive to the stimulus. Under the conditions of our experiments, the observers perform a Weber computation when the separation, S, between the pair of iso-eccentric test lines is less than about 0.5 * ECC, and apply a "cortical ruler" when S is greater than about 0.5 * ECC. If the angle subtended by the fixation point and the pair of iso-eccentric test lines is considered, then for both the two-line and the three-line tasks, when the angle is less than approx. 30 deg, thresholds are proportional to separation, and when it is between 30 and 180 deg, thresholds are proportional to eccentricity.

Discrimination, Psychological↗

PubMed1990

Spatial interval discrimination with blurred lines: black and white are separate but not equal at multiple spatial scales.

We used Gaussian blurred lines of same- and opposite-polarity to measure the effects of blur on 3-line spatial interval discrimination (bisection). The results of our experiments can be summarized as follows. Spatial interval discrimination (3-line bisection) thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds for same-polarity lines represent a "hyperacuity" as small as 2 sec arc. For same-polarity Gaussian blurred lines, over a wide range of the blur standard deviations (sigma), the optimal threshold occurs when the separation is approx. 2 sigma, and the optimal threshold is about 0.02 sigma, or a Weber fraction (delta s/s) of 0.01. For opposite-polarity lines, under conditions where same-polarity stimuli yield the best thresholds (at a separation approximately 2 sigma), spatial interval thresholds are an order of magnitude worse than that for same-polarity lines, suggesting that the localization of stimili of opposite-polarity is much worse than that of same-polarity stimuli over a wide range of spatial scales. At large separations, greater than about 5 sigma, spatial interval discrimination thresholds are more or less independent of both contrast and polarity. While hyperacuity is generally thought of in terms of the tiny spatial thresholds which are obtained at small separations with stimuli comprised of thin lines, the present results, and those of others, suggest that for same-polarity stimuli, hyperacuity thresholds are a general property of the visual system, occurring at many spatial scales. The present results also suggest that the poor localization of opposite-polarity lines occurs at multiple spatial scales, when the line separation is less than about five times the stimulus spread. We consider several models which can account for particular features of our data.

Contrast Sensitivity↗

PubMed1990

The imprecision of stereopsis.

In comparison to lateral judgments of distance, stereoscopic judgments are not precise. Although stereoacuity thresholds for targets presented in the fixation plane can equal the best thresholds for the monocular hyperacuities, i.e. a few sec arc, the increment thresholds for disparity are substantially larger than the increment thresholds for lateral separation (width). We measured the minimum detectable change in the three-dimensional distance separating two features, one presented in the fixation plane, and the other some distance in front of it, i.e. with a significant standing disparity between the two features. For briefly-presented targets (150 msec), the Weber fraction for disparity was 10-20% over the range from 1 to 20 min arc, while the Weber fraction for width was only 2-3% under comparable conditions. The disparity thresholds were substantially improved for a longer duration target (1000 msec), but they were still a factor of two worse than the monocular width thresholds. In a related experiment, the vernier acuity for a standard vernier target was profoundly degraded by pairing the offset upper line presented to one eye with a disparate line in the other eye; the vernier threshold was elevated for disparities ranging from 3 to 30 min arc. This finding shows that the more precise monocular signals are actively suppressed in fused or partially-fused stereoscopic images.

Depth Perception↗

PubMed1989

Peripheral positional acuity: retinal and cortical constraints on 2-dot separation discrimination under photopic and scotopic conditions.

The precision of discriminating the separation of two dots was measured as a function of separation for eccentricities of 0-10 deg under photopic and scotopic conditions. At each eccentricity, the 2-dot separation discrimination thresholds showed a V-shaped dependence on separation. For separations less than approximately twice the resolution threshold, performance deteriorated from photopic to scotopic conditions and appeared to be limited by ganglion cell receptive field size or spacing. For separations smaller than 10% of the effective eccentricity (eccentricity + 0.6 deg), the photopic 2-dot separation discrimination thresholds were significantly better than 3-dot bisection thresholds previously measured under similar experimental conditions, supporting the hypothesis that 3-dot bisection suffers from spatial interference for these separations. Interestingly, under scotopic conditions, 2-dot separation discrimination thresholds were better than resolution for a range of separations at each eccentricity, implying that cone input was not necessary for hyperacuity performance. 2-dot separation discrimination thresholds for large separations were little changed from photopic to scotopic luminance conditions.

Humans↗

PubMed1989

Binocular beats: psychophysical studies of binocular interaction in normal and stereoblind humans.

We describe a psychophysical method for assessing binocular integration using dichoptically-presented uniform fields. By temporally modulating uniform field luminances at different frequencies between the eyes, a rhythmic beat is produced--a visual percept characterized by undulations in luminance at a frequency equal to the arithmetic difference between the two monocular stimulus frequencies. Using a signal detection paradigm, we studied the beat as a function of modulation depth in normal and binocularly deficient subjects. Normal subjects easily detected the beat, even at low modulation depths, while stereoblind subjects (with stereoacuity worse than 2000 sec arc) failed to detect beats at any modulation depth or with any combination of stimulus frequencies tested. The beat provides evidence for the confluence of monocular signals into binocular integrating mechanisms. Our results therefore suggest that the status of functioning binocular mechanisms is related to the detectability of the beat. This uniform-field stimulus does not require accurate accommodation, fixation, vergence or high spatial resolution, thus making this technique particularly attractive for the study of binocular interaction in developing infants and in binocularly deficient adults.

Adult↗

PubMed1989

Both separation and eccentricity can limit precise position judgements: a reply to Morgan and Watt.

Weber's law is ubiquitous in position judgements. In a variety of position acuity tasks, the position threshold is proportional to the separation of the reference features. Recently, we (Klein & Levi, 1987; Levi, Klein & Yap, 1988) suggested that two sensory processes may serve to limit position acuity, and thus contribute to Weber's law for position. One is the target separation, and the other is the target eccentricity. In order to test this idea, we pitted separation against eccentricity by measuring spatial interval discrimination thresholds on an iso-eccentric arc (Levi et al., 1988). Over a 5-fold range of separations, we found that thresholds were independent of separation, and concluded that at large separations, eccentricity can limit precise position judgements. In the preceding article, Morgan and Watt (1989) have questioned this conclusion, and have shown that the effects of eccentricity are small in an arc length discrimination task. In the present article, we: (i) address the objections raised by Morgan and Watt; (ii) show that our data and 2-mechanism model are consistent with many previous studies; and (iii) show that Morgan and Watt's task is inherently difficult, so that rather than tapping the sensory limits imposed by the target eccentricity, performance on the arc length task is constrained by the cognitive demands of the task, or by the difficulty of reconstruction. In contrast, the measurement of chord length (spatial interval discrimination on an iso-eccentric arc) can be simply done by calculating the distance between the endpoints. Thus, at large separations, thresholds for the chord length judgements are much lower than those of Morgan and Watt, and are proportional to the eccentricity of the targets.

Humans↗

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↗

PubMed1979

Humans deprived of normal binocular vision have binocular interactions tuned to size and orientation.

A suprathreshold grating presented to one eye elevated the threshold for the discrimination of gratings similar in size and orientation presented to the fellow eye. The magnitude and stimulus specificity of these binocular interactions in human observers with normal bioncular vision were similar to those in observers deprived of normal binocular visual experience; however, the latter showed a failure of binocular summation at threshold or subthreshold contract levels. Whereas strabismus or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections seem not to have been disrupted.

Amblyopia↗