Biomedical subjects

D M Levi

Publications and source records attributed to D M Levi.

At least 55 records · Page 3Linked to original sources

PubMed1999

The effect of contour closure on shape perception.

We studied psychophysically whether 'contour closure' enhances the accuracy of shape perception. Stimulus configurations (presented on a blank background) always consisted of identical pattern elements, but the positions of the local elements were varied: the global contour shape either contained closure or not. In the first two stimulus conditions (Closure), the oriented pattern elements (Gabor patches) formed a 'closed' rectangular shape composed of either four long lines or four corners. In the third condition (No closure), the global shape was composed of the four corners, but they were outward oriented, and hence they did not form the outline of a closed contour. We measured the precision of shape perception using a discrimination task in which observers judged the aspect ratio of the outline shape i.e. whether the rectangular shape was tall or wide. We found that: (i) shape discrimination was better (more precise) for Closed contours than for Non-closed contours, i.e. the aspect ratio discrimination thresholds were lower for the Closed than Non-closed configurations. The improved performance could not be explained by differences in visibility of the local elements in the two conditions. (ii) For closed contours, shape discrimination was more precise when the local elements were aligned with the global shape, than when the local elements were orthogonal to it.

Form Perception↗

PubMed1998

Spatial uncertainty and sampling efficiency in amblyopic position acuity.

Spatial uncertainty and undersampling are two of the major hypotheses for the losses of amblyopic spatial vision. To test these two hypotheses, equivalent spatial uncertainty and spatial integration efficiency in spatial position judgments were quantified with a spatial perturbation paradigm. Specifically, three-line bisection thresholds were measured for the amblyopic eyes of two strabismic and two anisometropic amblyopes, and for normal controls. The horizontal stimulus lines comprised discrete dark dots distributed randomly around the mean line position according to a gaussian function. Line separation, the number of dots on each line (N), stimulus contrast (C), and the vertical standard deviation (sigma e) of the dot distribution were varied. An ideal observer analysis quantified the magnitude of equivalent spatial uncertainty (sigma s), the effective number of dots used (k), and spatial integration efficiency (k/N). At the optimal separation, equivalent spatial uncertainty (sigma s) is approximately ten-fold higher in both types of amblyopic visual systems than in control observers, even when stimulus visibility is accounted for. This apparent increase in sigma s is largely due to a shift in spatial scale of analysis in the amblyopic eye. Integration efficiency (k/N) increases in proportion to stimulus contrast or visibility (in units of detection threshold). Unlike sigma s, k/N is different between the two types of amblyopia. For the anisometropic observers, k/N is quantitatively similar to that of control observers. For the strabismic observers, on the other hand, k/N is reduced even after taking stimulus visibility into account. The decreased spatial integration efficiency in the strabismic visual system suggests that spatial undersampling may occur at a secondary stage of visual processing, beyond the detection stage.

Adult↗

PubMed1998

Stimulus uncertainty affects velocity discrimination.

Velocity discrimination thresholds were determined for 1 c/deg drifting gratings when uncertainty about the reference velocity was introduced by interleaving stimuli with different reference velocities from trial to trial. When drifting gratings with reference velocities spanning 4 octaves (1-16 deg/sec) were mixed randomly within a series of trials, the velocity discrimination threshold for a 4 deg/sec stimulus increased by more than a factor of 3. The threshold elevation decreased as the range of interleaved velocities was reduced from 4 to approx. 0.75 octaves, below which velocity interleaving had little effect. In contrast, when gratings that spanned a 4-octave range in spatial frequency were interleaved on successive trials, velocity discrimination for 4 deg/sec was essentially unaffected. Our results indicate that the psychophysical mechanisms underlying velocity discrimination are not spatial-frequency specific, but are turned to the velocity or speed of the stimulus.

Discrimination, Psychological↗

PubMed1998

Integration of local orientation in strabismic amblyopia.

In order to investigate the processes which integrate local orientation information in observers with strabismic amblyopia, we measured contrast thresholds for discriminating the global orientation of a pattern (3 "bars") comprised of Gabor patches. We found that in both eyes of amblyopic observers, as has been reported for normal observers [Saarinen, Levi & Shen (1997) Proceedings of the National Academy of Sciences USA, 94, 8267-8271], there is an approximately two-fold enhancement of contrast sensitivity when the global and local orientations are aligned (relative to mixed orientations), and a smaller enhancement when the global and local orientations are orthogonal. This orientation dependent enhancement occurs despite substantial losses of contrast sensitivity. These results suggest that the integration processes in the amblyopic eye that operate to enhance detection are essentially intact.

Adult↗

PubMed1998

Rectification nonlinearity in cortical end-stopped perceptive fields.

End-stopped perceptive fields associated with line targets were demonstrated previously with length and width Westheimer functions. In this study we investigated rectifying non-linearity in these perceptive fields to examine whether they directly reflect the organization of cortical receptive fields. Specifically, we reversed the polarity of parts of the background field associated with a specific perceptive field sub-region and examined threshold changes in corresponding length or width Westheimer functions. Results showed full-wave rectification in end-stopping and half-wave rectification in center summation and flank-inhibition preceding linear summation in end-stopped perceptive fields. Half-wave rectification in center summation and surround-inhibition preceding linear summation was also found in circular perceptive fields associated with spot targets. These results are inconsistent with direct links between perceptive fields and cortical receptive fields. Rather they suggest that these perceptive fields are likely the second-order fields formed by pooled non-linearly rectified outputs from cortical receptive fields.

Adult↗

PubMed1998

Selective attention to specific location cues: the peak and center of a patch are equally accessible as location cues.

Asymmetric patterns have several spatially distinct cues for spatial localization. These cues include the peak of the luminance distribution, the centroid of the contrast distribution, zero-crossings in the second derivative of the luminance profile, and the midpoint of the visible area. If these cues are represented as primitives in the visual system, the observer should be able to access them at will. To examine whether observers can selectively attend to particular cues, we measured perceived alignment for an asymmetric pattern with two distinct instructions: "align the peak", and "align the center". We found that observers could align the patterns in accord with the instructions with identical precision, suggesting that the peak and the center cues were equally accessible by the observer. We conclude that multiple localization cues are represented in and can be selectively accessed by the visual system.

Computer Graphics↗

PubMed1998

Naso-temporal asymmetry of spatial interactions in strabismic amblyopia.

PURPOSE: Naso-temporal asymmetries of visual acuity and contrast sensitivity have been reported in strabismic amblyopia and attributed to asymmetries of interocular suppression. In this study, we investigated the naso-temporal asymmetry of cortical spatial interactions in two strabismic amblyopes (one esotrope and one exotrope). METHODS: Length and width Westheimer functions were measured on both amblyopes at the 10 deg retinal eccentricity of both nasal and temporal visual fields. RESULTS: Spatial interactions in the two amblyopic eyes were more degraded in the temporal visual fields than in the nasal visual fields. A comparison with results from the preferred eyes suggested that this asymmetry was caused mainly by a loss of spatial interactions in the temporal visual fields of amblyopic eyes, with those in the nasal visual fields being normal. CONCLUSION: Our results suggest that intracortical connections underlying cortical spatial interactions might have been degraded by amblyopia. This degradation exists not only in the areas of the strabismic visual cortex responding to foveal stimuli but also in those responding to stimuli presented in the temporal visual fields.

Adult↗

PubMed1997

Feature integration in pattern perception.

The human visual system is able to effortlessly integrate local features to form our rich perception of patterns, despite the fact that visual information is discretely sampled by the retina and cortex. By using a novel perturbation technique, we show that the mechanisms by which features are integrated into coherent percepts are scale-invariant and nonlinear (phase and contrast polarity independent). They appear to operate by assigning position labels or "place tags" to each feature. Specifically, in the first series of experiments, we show that the positional tolerance of these place tags in foveal, and peripheral vision is about half the separation of the features, suggesting that the neural mechanisms that bind features into forms are quite robust to topographical jitter. In the second series of experiment, we asked how many stimulus samples are required for pattern identification by human and ideal observers. In human foveal vision, only about half the features are needed for reliable pattern interpolation. In this regard, human vision is quite efficient (ratio of ideal to real approximately 0.75). Peripheral vision, on the other hand is rather inefficient, requiring more features, suggesting that the stimulus may be relatively underrepresented at the stage of feature integration.

Contrast Sensitivity↗

PubMed1997

Integration of local pattern elements into a global shape in human vision.

The spatial extent of the cortical filters selective for different spatial frequencies and orientations is limited. We studied psychophysically how information from the local filters is integrated into global pattern shapes, i.e., whether performance in the identification of a global pattern consisting of small, locally oriented Gabor elements depends on the orientations of those elements. The observer was presented with an E-like stimulus pattern shape comprised of oriented Gabor patches on a blank background, and the performance measure was the threshold contrast for identifying the orientation of the E pattern (four possible rotated orientations). The results showed that contrast thresholds were significantly lower when the local elements all shared the same orientation (e.g., all horizontal) compared with the condition in which the elements had mixed orientations (both horizontal and vertical). The enhancement effect due to uniform local orientations can be explained by two factors: One is local facilitatory interactions between the orientation selective filters, and the other is second-order information integration across the filters.

Humans↗

PubMed1997

Vernier acuity with non-simultaneous targets: the cortical magnification factor estimated by psychophysics.

The eccentricity at which peripheral thresholds double their foveal value (E2) may relate to the visual system's anatomical organization. Using a variety of experimental approaches, previous estimates of E2 for vernier acuity have ranged from less than 0.1 deg to greater than 15.0 deg. This broad range of values seems to challenge the usefulness of E2 for determining visual topography. We explain that the varying contributions from at least two different regimes, spatial filter and local sign, may explain the broad range of E2 values found previously. We attempt to limit responses to the local sign regime, where it may be possible to determine the psychophysical analog to the gradient of the cortical spatial grain. In our experiments we measure how vernier task performance falls off with eccentricity. We hypothesize that if the vernier features are adequately separated in time, they will fall outside of the spatial filter's temporal integration span and the local sign regime would then predominate for precise positional processing. Using an interstimulus interval ranging from 20 to 200 msec between the two vernier features, we estimate that vernier thresholds in the local sign regime double at about 0.8 +/- 0.2 deg eccentricity, which is similar to anatomical estimates of the eccentricity at which the linear spacing of human cortical units doubles.

Contrast Sensitivity↗

PubMed1997

Vernier acuity with plaid masks: the role of oriented filters in vernier acuity.

Superimposition of oriented grating masks on vernier targets results in bimodal patterns of vernier threshold elevation, with peaks occurring on either side of vernier target orientation. These bimodal masking effects suggest a contribution to vernier acuity from spatial filters tuned to orientations on either side of the target. We report similar bimodal threshold elevation with plaid masks composed of symmetrically oriented pairs of gratings. Since filters oriented to either side of the vernier stimulus will be affected similarly by plaid masks, it is unlikely that threshold elevation reflects disruption of relative filter activity that is used to code for change in target orientation. Instead, the results support the proposition that misalignments are detected on the basis of differential (i.e. absolute rather than relative) activity of spatial filters. Our plaid-mask data also rule out the possibility that: (i) "off-channel" looking; or (ii) detection of orientation shifts (e.g. tilt illusions), underlie bimodal masking effects. The finding that weak bimodal threshold elevation occurs with dot targets separated by 40 min arc further suggests that the mechanisms involved in detecting misalignments over large regions [possibly collator/collector-type mechanisms] also do so via analysis of their differential activity.

Contrast Sensitivity↗

PubMed1997

The influence of adaptation on perceived visual location.

We demonstrate a marked effect of prior adaptation upon the perceived position of subsequently presented stimuli using both first-order (luminance-defined) and second-order (texture-defined) stimuli. The effect of varying the contrast of the adapting and test stimuli depends only upon the ratio of adapting/test contrast. Adaptation effects for the two types of stimuli differ in terms of interocular transfer and rate of decay. Whilst adapting and testing with the same type of stimulus (first- or second-order) produces large shifts in perceived position, little or no crossover effect was found. The data are accounted for by a model in which the centroid of the linear combination of after-image and test stimulus is extracted.

Adaptation, Ocular↗

PubMed1997

Cortical end-stopped perceptive fields: evidence from dichoptic and amblyopic studies.

Psychophysical length and width spatial interactions associated with a line target were measured in normal observers dichoptically and in observers with naturally acquired amblyopia to investigate the neural locus of end-stopped perceptive fields. Results show (1) interocular transfer of psychophysical end-stopping, flank-inhibition, and length and width summation; and (2) severe, but significantly different, loss of end-stopping and flank-inhibition in the central visual fields of amblyopic eyes. Together, these results suggest a cortical basis for end-stopped perceptive fields, and that psychophysical end-stopping and flank-inhibition are a consequence of distinct cortical inhibition. The damaging effects of amblyopia on end-stopping and flank-inhibition are weaker and less different from each other under transient conditions. Our results provide further evidence supporting the suggestion that end-stopped perceptive fields are the psychophysical analogs of cortical end-stopped receptive fields.

Adult↗

PubMed1997

Moving vernier in amblyopic and peripheral vision: greater tolerance to motion blur.

The purpose of this study was to examine the hypothesis that higher stimulus velocities could be tolerated in amblyopic and normal peripheral vision. The basis for this hypothesis is that a shift in the spatial scale of processing appears to account for the degradation in vernier acuity for moving stimuli in normal vision, and, to a large degree for the degradation in vernier acuity for stationary stimuli in amblyopic and peripheral vision. Vernier thresholds were determined using a pair of long abutting lines, for velocities ranging between 0 and 8 deg/sec. Comparisons were made between non-amblyopic and amblyopic eyes in two amblyopic observers, and between central and peripheral (5 and 10 deg) vision in two normal observers. We analyzed our threshold vs velocity data using an equivalent noise analysis, and defined the knee of the function, the point at which vernier threshold is elevated by a factor of square root of 2, as the "critical velocity" beyond which image motion degrades vernier acuity. Critical velocities were found to be higher in amblyopic than in nonamblyopic eyes; and higher in peripheral than central vision. Our results are consistent with the predictions from the shift in spatial scale notion--that higher velocity of image motion can be tolerated because of the shift in sensitivity toward lower spatial-frequency filter mechanisms in amblyopic and normal peripheral vision.

Adult↗

PubMed1997

Cortical components of the Westheimer function.

The Westheimer function in human cone vision was measured in normal observers under dichoptic conditions and in observers with naturally acquired amblyopia. Results show interocular transfer of both desensitization and sensitization under either "sustained" or "transient" stimulus conditions if binocular rivalry is eliminated. The spatial sensitization branches of the amblyopic functions are considerably broadened as compared with those of the non-amblyopic function. Our results are consistent with cortical components for the Westheimer function which probably reflect the behavior of cortical spatial filters.

Adult↗

PubMed1997

Spatial facilitation predicted with end-stopped spatial filters.

We examined the role of putative end-stopped spatial filters in determining spatial facilitation associated with a line target flanked by square inducers. Results obtained in normal and amblyopic observers were well predicted by end-stopping and other receptive field features of end-stopped spatial filters revealed in a modified Westheimer paradigm. The role of target-inducer collinearity, the effects of inducer polarity, and facilitation associated with non-orientational circular targets, were also studied. Our results suggest that spatial facilitation results from antagonism surrounding spatial filter centers, with end-stopping playing a prominent role.

Adult↗

PubMed1997

Development of Vernier acuity in childhood.

PURPOSE: To measure Vernier acuity and resolution development after 3 years of age. METHODS: Observers were 39 children with normal vision (aged 3 to 12 years), 10 adult observers with normal vision (aged 19 to 24 years), and 7 adults with amblyopia. Vernier acuity and resolution were measured using uncrowded static stimuli and a 3AFC psychophysical paradigm. Curve fitting was used to estimate A2, the age at which thresholds are twice asymptotic levels. RESULTS: Vernier acuity was hyperacute (i.e., finger than predicted from foveal cone size or spacing) in 3- to 4-year-old observers, but developed later (A2 = 5.6 +/- 1.5 years) than resolution acuity (A2 = 2.2 +/- 0.9 years). CONCLUSIONS: Children's Vernier thresholds are poorer than would be predicted solely from their decreased foveal photon capture. Therefore cortical immaturity may play a role in children's relative position acuity deficit. R/V ratios (resolution/Vernier thresholds) for the youngest age group are similar to those for adult nonstrabismic amblyopes, but better than for strabismic amblyopes.

Adult↗

PubMed1997

End stopping and length tuning in psychophysical spatial filters.

Psychophysical spatial filters or channels are usually modeled after simple-cell receptive fields, although many cortical cells are end stopped and length tuned. Using psychophysical masking, we demonstrate an analog to receptive field end stopping and length tuning in psychophysical spatial filters tuned to a wide range of spatial frequencies. Specifically, masking is maximal when the mask is approximately 5-6 arcmin longer than the target but is reduced when the mask exceeds this length, consistent with the properties of end-stopped cells. The strength and the extent of psychophysical end stopping appear to be determined by filter's spatial-frequency tuning, but length tuning varies with target length. The latter implies that spatial filters tuned to the same spatial frequency could have different length tuning and that there is no fixed length-to-width ratio of the filter size. Phase effects suggest linear length summation but nonlinear psychophysical end stopping which suggest that both first- and second-order visual processing is involved in end-stopped spatial filters.

Adult↗