Biomedical subjects

D M Levi

Publications and source records attributed to D M Levi.

At least 73 records · Page 4Linked to original sources

PubMed1997

Improvement in Vernier acuity in adults with amblyopia. Practice makes better.

PURPOSE: To determine the nature and limits of visual improvement through repetitive practice in human adults with naturally occurring amblyopia. METHODS: A key measure the authors used was a psychophysical estimate of Vernier acuity; persons with amblyopia have marked deficits in Vernier acuity that are highly correlated with their loss of Snellen acuity. The experiment consisted of three phases: pretraining measurements of Vernier acuity and a second task (either line-detection thresholds or Snellen acuity) in each eye with the lines at two orientations; a training phase in which observers repetitively trained on the Vernier task at a specific line orientation until each had completed 4000 to 5000 trials; and posttraining measurements (identical to those in the first phase). Two groups of amblyopic observers were tested: novice observers (n = 6), who had no experience in making psychophysical judgments with their amblyopic eyes, and experienced observers (n = 5), who had previous experience in making Vernier judgments with their amblyopic eyes (with the lines at a different orientation) using the signal-detection methodology. RESULTS: The authors found that strong and significant improvement in Vernier acuity occurs in the trained orientation in all observers. Learning was generally strongest at the trained orientation but may partially have been transferred to other orientations (n = 4). Significant learning was transferred partially to the other eye (at the trained orientation) in two observers with anisometropic amblyopia. Improvement in Vernier acuity did not transfer to an untrained detection task. In two observers, the improvement in Vernier acuity was accompanied by a commensurate improvement in Snellen acuity. CONCLUSIONS: Some adults with amblyopia retain a significant degree of neural plasticity. Although several observers (primarily novices) showed evidence of generalized learning, several amblyopic patients showed evidence for improvement that was orientation and task specific. In this latter group of observers, the improvement appeared to reflect alterations that were, at least in part, in early neural processes that were orientation specific and were localized beyond the site of convergence of the two eyes.

Adult↗

PubMed1996

Pattern perception at high velocities.

BACKGROUND: When objects are stationary, human pattern vision is exquisitely acute. A number of studies show, however, that Vernier acuity for lines is greatly impaired when the target velocity exceeds about 5 deg sec-1. The degradation of line Vernier acuity under image motion appears to be a consequence of a shift in the spatial scale of analysis to low spatial frequencies. If correct, this implies that Vernier acuity may not be subject to a strict velocity limit, and that with appropriate low spatial frequency stimuli, Vernier acuity might be preserved at high velocities. To test this notion, we measured Vernier acuity and contrast discrimination using low spatial frequency periodic gratings drifting over a wide range of velocities. RESULTS: Vernier acuity and contrast discrimination for low spatial frequency periodic gratings are both possible at velocities as high as 1000 deg sec-1. When both are specified in the same units (as Weber fractions), Vernier acuities are closely predicted by the observers' contrast discrimination thresholds. Our results suggest that Vernier acuity is subject to a spatiotemporal limit, rather than to a strict velocity limit. At temporal frequencies less than about 10 Hertz, Vernier acuity is independent of velocity, but is strongly dependent on stimulus contrast. At high temporal frequencies Vernier acuity is markedly degraded, and shows little dependence on contrast. CONCLUSIONS: Two mechanisms, which may have their neuronal counterparts early in the visual pathway, appear to limit the perception of moving targets at low and high temporal frequencies. Taken together with other recent work the present results suggest that the process of spatio-temporal interpolation in pattern analysis can operate at very high velocities.

Humans↗

PubMed1996

Neural plasticity in adults with amblyopia.

Amblyopia is a neuronal abnormality of vision that is often considered irreversible in adults. We found strong and significant improvement of Vernier acuity in human adults with naturally occurring amblyopia following practice. Learning was strongest at the trained orientation and did not transfer to an untrained task (detection), but it did transfer partially to the untrained eye (primarily at the trained orientation). We conclude that this perceptual learning reflects alterations in early neural processes that are localized beyond the site of convergence of the two eyes. Our results suggest a significant degree of plasticity in the visual system of adults with amblyopia.

Adult↗

PubMed1996

Position acuity with opposite-contrast polarity features: evidence for a nonlinear collector mechanism for position acuity?

Vernier acuity for opposite-contrast polarity stimuli clearly poses problems for local contrast models of relative position processing. In Expt 1 we show that vernier thresholds for abutting, or closely separated features of opposite-contrast polarity, are degraded across a wide range of stimulus strengths and configurations; but for widely separated stimuli they are more or less independent of contrast polarity (confirming and extending previous work). In Expts 2 and 3 we use a one-dimensional spatial noise masking paradigm to investigate to what extent the same mechanisms masked by this noise contribute to the relative position processing of same and opposite polarity stimuli. The orientation tuning functions determined using this paradigm are quite different for same and opposite polarity targets, for both line vernier acuity, and closely spaced two-dot alignment. However, for widely separated targets (24 min arc or more), they are similar. Over a range of separations from 3 to 30 min arc, for same and opposite polarity dots, masking is strongest at a spatial frequency of about 10 c/deg. Our results are consistent with the notion that signals from early (and relatively high spatial frequency) linear filters are collected in a second-stage nonlinear mechanism, which collates information along an orientation trajectory. We suggest that different properties of the mechanisms at each level of processing, can constrain positional acuity at small and large separations.

Contrast Sensitivity↗

PubMed1996

Intrinsic uncertainty and integration efficiency in bisection acuity.

A spatial perturbation paradigm was used to determine equivalent intrinsic uncertainty and spatial integration efficiency in bisection. Specifically, three-line bisection thresholds were measured in the fovea of four normal observers with stimulus lines comprised of discrete dark dots distributed randomly around the mean line position according to a Gaussian function. The standard deviation of the Gaussian distribution (sigma e), the number (N), and the strength (C) of the dots as well as line separation were varied. Bisection thresholds were modeled by an ideal integrator, from which the magnitude of equivalent internal uncertainty (sigma i), the equivalent effective number of dots (k), and equivalent integration efficiency (k/N) were quantified. At the 2 min arc separation, sigma i decreases (down to a few sec arc) as N and/or C increases. The effects of both N and C can be accounted for by the stimulus visibility (V, in multiples of detection threshold). At the 16 min arc separation, sigma i is independent of N, C, or V, and is about 1 min arc. The two different forms of sigma i indicate that bisection judgments are limited by at least two separate sources of limiting noise, consistent with the hypothesis of two separate mechanisms (i.e. spatial filters and local signs). A visibility dependent sigma i at the 2 min arc separation can be explained on the basis of contrast sensitive spatial filter mechanisms. A fixed sigma i at the 16 min arc separation indicates a genuine positional uncertainty, consistent with local-sign mechanisms. Interestingly, equivalent integration efficiency (k/N) is very similar at the two line separations. k/N is critically dependent on, and proportional to C, indicating a common limitation in a detection mechanism.

Depth Perception↗

PubMed1996

Angle judgement: is the whole the sum of its parts?

This study concerns whether the discrimination of a geometric angle depends on the orientations of its bounding lines or on angle size. In Experiment 1, thresholds for angle discrimination were measured in three observers for angles ranging from 15 to 180 deg, oriented either vertically or obliquely. Angle discrimination thresholds were found to depend primarily on angle size for most of the range of angles (angle-dependent, or Weber's law regime). However, in a small region near 90 deg (orientation-dependent regime) angle discrimination depends on the orientations of the bounding lines. When our data in the angle-dependent regime were fitted with a power function, the exponents were close to or < 0.5, suggesting that a step-increment approach was used to calculate angle. In Experiment 2, orientation discrimination thresholds for lines corresponding to the bounding lines of the vertically and obliquely oriented 15, 90 and 165 deg angles were measured. Confirming previous studies, a strong meridional anisotropy in line orientation discrimination was found for all three observers. The orientation discrimination thresholds were then used to predict the discrimination thresholds for the corresponding angles based on a simple statistical model. The predicted angle discrimination thresholds were worse than those measured empirically except for the titled 90 deg angles. This result indicates that angle discrimination thresholds are not limited by the same noise as orientation discrimination for most angles except for the tilted 90 deg angle, where the limiting factor may be the precision in determining the orientations of the bounding lines. In Experiment 3, we show that angle discrimination is quite robust to small amounts of orientation jitter, suggesting that angle judgments are made at a level beyond the early filter representation.

Differential Threshold↗

PubMed1996

Limitations on position coding imposed by undersampling and univariance.

Position judgements, which are exquisitely precise in the fovea, are markedly degraded in the periphery. In a recent article [Hess & Field (1993) Vision Research, 33, 2663-2670] argue that the poor representation of positional information in peripheral vision is a consequence of uncalibrated spatial disorder of cortical connections rather than due to undersampling of the retinal image. Specifically, Hess and Field argued that if positional uncertainty is due to undersampling, then because of univariance, there should be an associated contrast uncertainty. In this report we show that the univariance model is limited in its generality since: (1) the Hess and Field data in which contrast and position discrimination are decoupled do not preclude undersampling with large univariant filters: (2) aliasing can decouple position from contrast; (3) undersampling or noise at a second stage of processing can lead to selective losses of position information without any degradation of contrast information.

Contrast Sensitivity↗

PubMed1996

Spatial properties of filters underlying vernier acuity revealed by masking: evidence for collator mechanisms.

Models of vernier acuity based on the differential response of oriented filters receive support from the finding that vernier threshold elevation peaks for grating mask orientations which are slightly different from the orientation of the vernier bars. We replicate this effect using long, abutting vernier bars, and masks which possess gaps up to 22.5' wide (Experiments 1 and 3); a surprising result considering that vernier acuity improves little for bars longer than 10'. To account for this we suggest the involvement of elongated mechanisms (referred to as collators or collectors) that "integrate" responses of numerous smaller filters along the axis of their common orientation. The collator model explains patterns of threshold elevation obtained with a variety of mask-vernier configurations. In particular, the model predicts that masks located midway between separated vernier bars will interfere with integrative processes occurring over the entire region encompassing both bars (Experiment 2). In confirmation of this prediction we find that centrally placed masks produce significant orientation-specific threshold elevation. In suggesting a contribution to vernier acuity from integrative mechanisms, our results, along with others, emphasize the importance of global processes in vernier acuity.

Filtration↗

PubMed1996

Vernier in motion: what accounts for the threshold elevation?

Vernier acuity is susceptible to degradation by image motion. The purpose of this study was to determine to what extent vernier thresholds are elevated in the presence of image motion because of reduced stimulus visibility, due to contrast smearing, or to a shift in the spatial scale of analysis. To test the visibility hypothesis, we measured vernier thresholds as a function of stimulus velocity (0-6 deg/sec), for various levels of stimulus visibility, each normalized to the detection threshold at the respective velocity. Contrary to the prediction of the visibility hypothesis, vernier thresholds worsen as the velocity increases, even when the stimuli are equally visible. To test the shift in spatial scale hypothesis, we determined spatial frequency tuning functions for vernier discrimination and line detection tasks, using a masking paradigm. We measured vernier and line detection thresholds as a function of spatial frequency of a sine-wave mask (0.5-32 c/deg), and for stimulus and mask velocities ranging from 0 to 4 deg/sec. Peak masking for both vernier discrimination and line detection, which indicates the most sensitive band of spatial frequencies for each task, shifts systematically toward lower spatial frequencies as the velocity increases. The progressive increase in spatial scale largely accounts for the worsening of vernier thresholds for moving stimuli. Differences between peak masking for vernier discrimination and line detection were found at 0 and 1 deg/sec, suggesting that different mechanisms mediate the two tasks, at least at low velocities. The masking results are consistent with previous findings that directionally selective motion detectors mediate detection of moving stimuli, but suggest that these detectors do not analyze vernier offsets. We conclude that the elevation of vernier threshold for a moving stimulus is accounted for primarily by a shift of sensitivity to mechanisms of lower spatial frequency, and not by decreased stimulus visibility.

Contrast Sensitivity↗

PubMed1996

Localization of a peripheral patch: the role of blur and spatial frequency.

Peripheral vision serves to direct our attention and fixation to objects of interest. This requires that the visual system be capable of accurately localizing peripherally presented targets having different spatial structures. The question we address is "to what extent does stimulus spatial structure influence the precision of peripheral localization?" To address this issue, we measured the precision of spatial localization (with reference to a foveal target) for a single Gaussian or Gabor patch briefly presented in the periphery. For both stimuli, we find that when the standard deviation of the stimulus envelope (SD) is less than 1/5 the stimulus eccentricity, localization thresholds are independent of SD and are approximately 1/50 of eccentricity. For larger values of SD, localization thresholds increase linearly with increasing SD, and are approximately 1/5 of SD. The results hold over a range of eccentricities (from 2.5 to 10 deg) and stimulus contrasts (from near detection threshold to 80%). In addition, for Gabor patches, the results are independent of frequency, phase and orientation of the carrier.

Humans↗

PubMed1996

Two-dot alignment across the physiological blind spot.

Three competing hypotheses have been proposed for the cortical representation of the blind spot. These are: (i) the regions surrounding the blind spot maintain their spatial values; (ii) the opposite sides of the blind spot are represented adjacently at the cortex, so that the blind spot is "sewn-up"; and (iii) the blind spot is sewn-up with compensation occurring in the immediate surround of the blind spot, so that spatial values are distorted only in the immediate surround of the blind spot. To distinguish between these hypotheses we used a two-dot alignment task, with the two dots straddling the blind spot at varying dot separations. Thresholds in the two-dot alignment task are limited by the cortical separation of the two dots. When thresholds for alignment across the blind spot are compared with thresholds over intact retina at the same eccentricity, the three hypotheses predict: (i) no change in thresholds; (ii) a lowering of thresholds; and (iii) a lowering of thresholds but only at separations slightly greater than the diameter of the blind spot. Thresholds across the blind spot were closely similar to thresholds across intact retina. The results do not support a sewing-up (with or without compensation) of the blind spot. Rather, our results are consistent with a preservation of spatial values around the blind spot.

Fixation, Ocular↗

PubMed1996

Meridional anisotropy in the discrimination of parallel and perpendicular lines--effect of body tilt.

It is well documented that orientation discrimination is poorer for stimuli oriented obliquely than for those that are vertical or horizontal. Buchanan-Smith and Heeley recently reported that in the absence of a spatial reference this anisotropy follows gravitational rather than retinal coordinates, suggesting a high-level basis for the anisotropy in unreferenced orientation discrimination tasks. In the present study, unlike the previous one, the effects of body tilt on orientation discrimination have been examined in the presence of explicit simultaneous spatial references. The thresholds for discrimination of two parallel or two perpendicular lines were estimated for the retinally principal and oblique orientations, with the body either erect or tilted 45 degrees with respect to gravity. In agreement with previous studies, meridional anisotropy for both parallelism and perpendicularity discrimination was found when observers were seated upright. When the observer's body was tilted, the anisotropy for the parallelism task was mapped to retinal and not to gravitational coordinates after compensating for countertorsion. Initially, the anisotropy for the perpendicularity task was not mapped to retinal coordinates, but after extensive practice for both the erect and the tilted body conditions it eventually followed retinal coordinates. The results reported here suggest that contrary to orientation discrimination without a spatial reference, the ultimate limits for both parallelism and perpendicularity discriminations are located at orientation-sensitive cortical neurons. However, the effect of perceptual learning in the perpendicularity task suggests that the internal frame of reference (gravity cues and body axis) also plays an important role.

Analysis of Variance↗

PubMed1996

Ricco's diameter for line detection increases with stimulus velocity.

The purpose of this study was to determine whether Ricco's diameter, the spatial extent within which sensitivity demonstrates a perfect reciprocity between contrast and area, enlarges as the stimulus velocity increases. Detection thresholds were measured for a single line of length 10 arcmin as a function of linewidth that varied between 0.31 and 21.7 arcmin and for velocity ranging from 0 to 6 deg/s. We fitted the detection threshold versus linewidth data with two power functions of slope 0 and 1 and defined the intersection of these two functions as Ricco's diameter. For an increase in velocity from 0 to 6 deg/s, Ricco's diameter increases in dimension by approximately a factor of 4. Similar results were obtained when Ricco's diameter was estimated by comparing detection threshold of a thin line to that of an edge. The increase in Ricco's diameter with stimulus velocity suggests that the spatial-frequency mechanism that mediates line detection shifts progressively toward lower spatial frequencies for faster moving stimuli.

Contrast Sensitivity↗

PubMed1996

Evaluation of minor penetrating duodenal injuries.

Penetrating duodenal injuries can present a confusing picture for the surgeon. A variety of treatment modalities exist including primary repair alone, primary repair with pyloric exclusion, duodenal resection, duodenal diverticulization, and the Whipple procedure. We reviewed 40 consecutive penetrating duodenal injuries in order to determine factors that led to complications of duodenal injuries, and to determine the most appropriate way to treat these injuries. Fourteen patients had combined pancreaticoduodenal injuries, five of whom developed pancreatitis. None of the patients without a combined pancreaticoduodenal injury developed pancreatitis (P < 0.05). Sixteen patients with minor duodenal injuries were treated by primary repair alone. In this group, five had combined pancreaticoduodenal injuries. Two of these five patients developed a suture line dehiscence and leak (P < 0.05). None of the 11 patients without a combined pancreaticoduodenal injury that were treated by primary repair alone developed a leak. Three patients with combined intermediate pancreaticoduodenal injuries were treated by primary repair with pyloric exclusion. None of these three patients developed a suture line dehiscence or leak. We conclude that combined pancreaticoduodenal injuries are more likely to develop pancreatitis, and that minor or intermediate combined pancreaticoduodenal injuries are more likely to develop a suture line dehiscence and leak. We recommend that the treatment of duodenal injuries be based on severity, location, and the presence of associated injuries. Minor injuries can be treated by primary repair alone, adding pyloric exclusion if there is a concomitant pancreatic injury.

Adolescent↗

PubMed1996

Visibility of motion in infant vernier displays, using adult subjects.

Motion is frequently incorporated in stimuli used for psychophysical testing of vernier acuity in infants and young children. In such stimuli, detection of the vernier offset is necessary in order to perceive the motion. Research described in this report tested whether the perception of a vernier offset is sufficient to signal the stimulus motion in adults. We measured how motion detectability changed as a function of vernier offset for two adult subjects, using a stimulus similar to that employed by other authors to measure vernier acuity in infants and children. Motion visibility varied with offset size, achieving a detectability of motion (d') of 0.95 (comparable to two-alternative forced-choice thresholds) at stimulus offsets of 16-19 s arc. In comparison to the motion, the stimulus offset itself was much easier to see, being detectable on 95-100% of trials with the smallest offset, 6.6 s arc. This distinction, between the visibility of motion and the visibility of the vernier offset itself, should be considered when interpreting vernier results using such displays, especially in infants and children for whom motion may be the attractive cue.

Adult↗

PubMed1995

Perceptual learning in vernier acuity: what is learned?

It has been suggested that the improvement of vernier acuity in the course of practice reflects "fine tuning" of the visual mechanisms underlying vernier acuity. Masking studies suggest that an important source of information by which the visual system may accomplish fine vernier acuity is the activity in orientation tuned channels. Therefore, we investigated whether improvement in vernier acuity after training was accompanied by systematic changes in the orientation tuning characteristics of vernier acuity (as revealed by simultaneous spatial noise masking). The results show large interindividual variation in learning vernier acuity. However, they reveal a close correspondence between the improvement in vernier acuity and the narrowing of the orientation tuning function. Thus, the results provide some support for the notion of narrowing of the orientation characteristics of vernier acuity in the course of learning.

Humans↗

PubMed1995

Perceptual learning in parafoveal vision.

The present study tests the effects of practice on parafoveal vernier and resolution acuity. By measuring task specificity, transfer of training to other retinal locations in the trained eye and transfer of training to the untrained eye, we directly address whether improvement on these tasks is the result of changes in the underlying physiological processes or simply the development of new cognitive strategies. We found that: (1) significant learning can occur for both vernier and resolution acuity in many (but not all) individuals; (2) there were significant individual differences in the degree and time-course of learning: (3) learning transfers to the untrained task; and (4) learning transfers to the other eye particularly when the visual pathway leads to the trained hemisphere. These results suggest that both physiological and cognitive processes contribute to the improvement seen after repetitive practice on these visual tasks.

Adolescent↗

PubMed1995

On the filling in of the visual blind spot: some rules of thumb.

In monocular viewing there is a region in the peripheral visual field that is blind owing to the absence of photoreceptors at the site where the optic nerve exits the eye. This region, like certain other blind spots, nonetheless appears filled in. Several novel demonstrations of filling in at the blind spot have recently been reported. Here the implications of many of these effects are critically reevaluated. Specifically, it is argued that many blind-spot phenomena taken to support early filling in (eg pop out and alteration in apparent motion) are actually consistent with the thesis that the visual blind spot is treated by early perceptual processing as a region of reduced or absent information. In support of this, it is shown that many perceptual effects observed in blindspot completion are similar in detail to the amodally perceived completion of partly occluded objects viewed somewhat peripherally. The goals were to point out striking similarities between blind-spot completion and the amodal completion of occluded parts of surfaces, and to provide a common theoretical framework for understanding these phenomena in the context of surface segregation and perceptual interpolation.

Humans↗