Biomedical subjects

Srividhya Hariharan

Publications and source records attributed to Srividhya Hariharan.

4 recordsLinked to original sources

PubMed2005

"Crowding" in normal and amblyopic vision assessed with Gaussian and Gabor C's.

The purpose of this study was to investigate the extent and specificity of crowding in the normal fovea and periphery, and the central field of amblyopes, using "C"-like patterns. In the first experiment we measured the extent of crowding for C-patterns comprised of Gaussian patches, over a range of target sizes using a four-alternative forced-choice (up, down, left, right) method. We found that the extent of foveal crowding is proportional to target size. In contrast, in normal periphery and in the central field of amblyopes, crowding extends over large spatial distances and is not size dependent. Crowding for our stimuli occurred with both same-polarity and opposite polarity patches. To test whether the extended crowding in amblyopia resulted from a shift in the spatial scale of analysis, we measured crowding with band-limited C-patterns (comprised of Gabor patches) in a gap localization task (2-AFC). With band-limited stimuli, and a task that does not involve judging the orientation of the gap, the amblyopic eyes showed crowding over a longer distance than that of normal observers. We also tested the orientation specificity of crowding by varying the orientation of the flanks. In normal fovea, crowding is orientation specific: in amblyopia it is not. While crowding in normal fovea can be explained by simple pattern masking, crowding seen in normal periphery and amblyopes cannot. Instead we suggest that crowding in amblyopic and peripheral vision is a result of extended pooling at a stage following the stage of feature detection.

Amblyopia↗

PubMed2002

Suppressive and facilitatory spatial interactions in amblyopic vision.

Amblyopic vision is characterized by reduced spatial resolution, and inhibitory spatial interactions ("crowding") that extend over long distances. The present paper had three goals: (1) To ask whether the extensive crowding in amblyopic vision is a consequence of a shift in the spatial scale of analysis. To test this we measured the extent of crowding for targets that were limited in their spatial frequency content, over a large range of target sizes and spatial frequencies. (2) To ask whether crowding in amblyopic vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments our targets and flanks were comprised of Gabor features, thus allowing us to control the feature contrast, spatial frequency and orientation. (3) To examine the relationship between the suppressive and facilitatory interactions in amblyopic contrast detection and "crowding". Our results show that unlike the normal fovea [Levi, Klein, & Hariharan, Journal of Vision 2 (2002a) 140] crowding in amblyopia is neither scale invariant, nor is it attributable to simple contrast masking. Rather, our results suggest that suppressive spatial interactions in amblyopic vision extend over larger distances than in normal foveal vision, similar to peripheral vision of non-amblyopic observers [Levi, Hariharan, & Klein, Journal of Vision 2 (2002b) 167], for targets of the same size. Observers can easily detect the features that comprise our targets (Gabor patches) under conditions where crowding is strong. Thus, our speculation is that crowding occurs because the target and flanks are combined or pooled at a second stage that is coarse in the amblyopic visual system, following the stage of feature extraction. In amblyopic vision, this pooling takes place over a large spatial distance.

Adult↗

PubMed2002

Suppressive and facilitatory spatial interactions in foveal vision: foveal crowding is simple contrast masking.

Spatial interactions are a critical and ubiquitous feature of spatial vision. These interactions may be inhibitory (reducing sensitivity as occurs in crowding) or facilitatory (enhancing sensitivity). In this work, we had four goals. 1. To test the hypothesis that foveal crowding depends on target size by measuring the extent of crowding for novel targets that were limited in their spatial frequency content. We used a large range of target sizes and spatial frequencies. 2. To assess whether the critical spatial frequency model (Hess, Dakin, & Kapoor, 2000) provides a general model for foveal crowding. To test this model, we measured crowding for a direction-identification task that did not require judging the orientation of the gap. 3. To test the hypothesis that foveal crowding is simply contrast masking by remote flanks we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. 4. To assess the relationship between suppressive and facilitatory spatial interactions in foveal vision. Our results show that (1) foveal crowding is proportional to feature size over the more than 50-fold range of target sizes that we examined. Over this large range, foveal crowding is scale invariant. Our results also show it is the size of the envelope (SD) rather than the carrier (SF) that determines the extent of crowding in the fovea. 2. Crowding that occurs in the direction-identification task is quite similar to crowding where orientation information is available. Thus we conclude that the critical spatial frequency model does not provide a general explanation for foveal crowding. 3. Threshold elevation for crowding is similar to threshold elevation for masking as predicted by our test-pedestal model. Thus we conclude that foveal crowding is simple contrast masking. 4. Based on our comparison of threshold changes in crowding and masking, we conclude that in foveal vision, the suppressive spatial interactions due to nearby flanks are similar in the two tasks. However, the facilitatory interactions are quite different. In the crowding task, we find very little evidence for facilitation by flankers, whereas in the detection task, we find strong facilitation. We suggest that facilitation of detection by remote flanks may be, at least in part, a consequence of uncertainty reduction.

Contrast Sensitivity↗

PubMed2002

Suppressive and facilitatory spatial interactions in peripheral vision: peripheral crowding is neither size invariant nor simple contrast masking.

Peripheral vision is characterized by reduced spatial resolution and inhibitory spatial interactions that extend over long distances. This work had three goals. (1) We considered whether the extensive crowding in peripheral vision is a consequence of a shift in the spatial scale of analysis. To test this, using a large range of target sizes and spatial frequencies, we measured the extent of crowding for targets that were limited in their spatial frequency content. (2) We considered whether crowding in peripheral vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis, we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. (3) We examined the relationship between the suppressive and facilitatory interactions in peripheral contrast detection and crowding. Our results show that unlike the normal fovea (Levi, Klein, & Hariharan, 2002), peripheral crowding is not scale invariant nor is it attributable to simple contrast masking. Rather, our results suggest that inhibitory spatial interactions in peripheral crowding extend over larger distances than in the fovea for targets of the same size. In peripheral vision, the critical distance for crowding is approximately 0.1 times the target eccentricity. Observers can easily detect the features that compose our targets (Gabor patches) under conditions where crowding is strong. Thus, we speculate that peripheral crowding occurs because the target and flanks are combined or pooled at a second stage, following the stage of feature extraction. In peripheral vision, this pooling takes place over a large distance.

Contrast Sensitivity↗