Biomedical subjects

Manish Singh

Publications and source records attributed to Manish Singh.

At least 19 recordsLinked to original sources

PubMed2006

Bayesian estimation of the shape skeleton.

Skeletal representations of shape have attracted enormous interest ever since their introduction by Blum [Blum H (1973) J Theor Biol 38:205-287], because of their potential to provide a compact, but meaningful, shape representation, suitable for both neural modeling and computational applications. But effective computation of the shape skeleton remains a notorious unsolved problem; existing approaches are extremely sensitive to noise and give counterintuitive results with simple shapes. In conventional approaches, the skeleton is defined by a geometric construction and computed by a deterministic procedure. We introduce a Bayesian probabilistic approach, in which a shape is assumed to have "grown" from a skeleton by a stochastic generative process. Bayesian estimation is used to identify the skeleton most likely to have produced the shape, i.e., that best "explains" it, called the maximum a posteriori skeleton. Even with natural shapes with substantial contour noise, this approach provides a robust skeletal representation whose branches correspond to the natural parts of the shape.

Algorithms↗

PubMed2006

Perceived orientation of complex shape reflects graded part decomposition.

Although the orientation of line segments and simple shapes is a well-studied area of vision, little is known about geometric factors that influence perceived orientation of complex multipart shapes. The study of these factors is of interest because it allows for an insight into the basic problem of how local geometric attributes are integrated perceptually into a global shape representation. We examined the perceived orientation of two-part shapes using an adjustment method and a 2AFC task. In particular, we investigated the influence of the perceptual salience, or distinctiveness, of a part--as defined by the turning angles at its boundaries--and its area relative to the main "base" part. In contrast to previous results on simple shapes, our results exhibited large and systematic deviations of perceived orientation from the principal axis of the shape. For shapes with sharp part boundaries, perceived global orientation deviated maximally from the principal axis and was approximated by the axis of the main base part of the shape. With weakening part boundaries, the perceived orientation gradually approached the principal axis of the entire shape, reflecting that both parts were taken into account in estimating orientation. The results are consistent with a differentially weighted principal-axis computation in which the attached part is given systematically lower weighting with increasing turning angles at the part boundaries. They thus allow a quantitative characterization of part salience in terms of part independence: Turning angles at a part's boundaries determine the extent to which its influence is perceptually separable from the rest of the shape. We suggest that Robust Statistics may provide a useful framework for quantifying the influence of part segmentation on visual estimation.

Adult↗

PubMed2006

Combining achromatic and chromatic cues to transparency.

We investigated how achromatic and chromatic cues interact to produce transparency. Observers were shown six-region stimulus displays similar to those used by R. Kasrai and F. A. A. Kingdom (2001) and made adjustments of the color and luminance attributes of one of the filter regions to achieve the best percept of transparency. The dependent measure of primary interest was setting reliability, the reciprocal of setting variance. We wished to determine whether the combination of chromatic and achromatic information leads to enhanced reliability of perceived transparency. In Experiment 1, we measured reliability for achromatic, L, superimposed luminance with color, L + C, and superimposed luminance with polarity-reversing color, L + iC. We found that observers' reliability was highest for the L + C condition, consistent with effective cue combination. In a second experiment, we compared setting reliability for L, L + C, and a new chromatic-only condition C. In the L + C condition, observers were asked to make separate and iterative settings of luminance and color to achieve the best percept of transparency. We compared their settings in L with the luminance settings in L + C and their settings in C with their color settings in L + C. Color adjustments were more reliable when accompanied by luminance information but not vice versa. In Experiment 3, we manipulated the transmittance of the achromatic and chromatic filters separately and investigated how this influences the settings made for each attribute. No systematic influence of filter transmittance on the settings made for perceived transparency was found.

Color↗

PubMed2006

Surface geometry influences the shape of illusory contours.

Geometric and neural models of illusory-contour (IC) synthesis currently use only local contour geometry to derive the shape of ICs. Work on the visual representation of shape, by contrast, points to the importance of both contour and surface geometry. We investigated the influence of surface-based geometric factors on IC shape. The local geometry of inducing-contour pairs was equated in stereoscopic IC displays, and the shape of the enclosed surface was varied by manipulating sign of curvature, cross-axial shape width, and medial-axis geometry. IC shapes were measured using a parametric shape-adjustment task (Experiment 1) and a dot-adjustment task (Experiment 2). Both methods revealed large influences of surface geometry. ICs enclosing locally concave regions were perceived to be systematically more angular than those enclosing locally convex regions. Importantly, the influence of sign of curvature was modulated significantly by shape width and medial-axis geometry: IC shape difference between convex and concave inducers was greater for narrow shapes than wider ones, and greater for shapes with straight axis and symmetric contours (diamond versus bowtie), than those with curved axis and parallel contours (bent tubes). Even at the level of illusory "contours," there is a contribution of region-based geometry which is sensitive to nonlocal shape properties involving medial geometry and part decomposition. Models of IC synthesis must incorporate the role of nonlocal region-based geometric factors in a way that parallels their role in organizing visual shape representation more generally.

Depth Perception↗

PubMed2006

The perceived transmittance of inhomogeneous surfaces and media.

A series of experiments was performed to determine how the visual system computes the transmittance of inhomogeneous surfaces and media. Previous work (Anderson, B. L. (1999) Stereoscopic surface perception. Neuron, 26, 919-928; Anderson, B. L. (2003) The role of occlusion in the perception of depth, lightness, and opacity. Psychological Review, 110, 762-784) has suggested that the visual system employs a transmittance anchoring principle in determining when transparency is perceived. This principle states that the visual system interprets the highest contrast region along contours and surfaces as a region in plain view and uses this anchor as a reference point for transparency computations. In particular, recent work has shown that the transmittance of homogeneous transparent surfaces is well described by a ratio of contrasts model (Singh, M., & Anderson, B. L. (2002). Toward a perceptual theory of transparency. Psychological Review, 109, 492-519). In this model, the transmittance of a transparent surface is determined by the contrast of a transparent image region normalized by the contrast of the region in plain view. Here, a series of experiments is reported that assesses this model for inhomogeneous transparent surfaces that vary in both space and time. The results of these experiments reveal that transmittance anchoring has both a spatial and temporal component, and that the perceived transmittance of transparent surfaces is well described by a ratio of perceived contrasts model.

Contrast Sensitivity↗

PubMed2006

The role of part structure in the perceptual localization of a shape.

The process of object localization may be accomplished with respect to a particular reference location, such as the center of gravity, COG (eg Vishwanath and Kowler, 2003 Vision Research 43 1637-1653). Here, we investigated how part structure affects an object's reference location. The reference location was evaluated with a measure of the illusory displacement of an internal target element embedded within a larger object (Morgan et al, 1990 Vision Research 30 1793-1810). To examine whether the reference location is different for shapes with part structure, two shapes were tested: circle (small and large; no part structure) and bell (shape with two parts, one larger than the other). Results were examined with respect to two predictions: either the location of an object is based on its shape as a whole, disregarding part structure (ie a single, overall COG), or the parts are processed separately (different COGs). With the circles, the results showed a systematic illusory displacement of the internal target toward the COG. With the bell, the illusion was significantly weaker than with both circles--even though the main part of the bell had the same size as the small circle, and its horizontal axis had the same extent as the large circle. Moreover, the distance judgments for the bell were consistent with a (weaker) reference point being located at the COG of the larger part, rather than at the COG of the entire bell. These results show that the part structure of a shape plays a role in the representation of its location, and that for complex shapes the perceived location of an embedded element depends more on the parts within which it is embedded, rather than on the whole shape.

Distance Perception↗

PubMed2005

Photometric determinants of perceived transparency.

Photometric constraints for the perception of transparency were investigated using stereoscopic textured displays. A contrast discontinuity divided the textured displays into two lateral halves, with one (reference) half fixed. Observers adjusted the luminance range within the other (test) half in order to perform two tasks: (i) indicate the highest luminance range for which the test side is perceived to be transparent, and (ii) indicate the lowest luminance range for which the test side is seen as being in plain view. Settings were obtained for multiple values of test mean luminance, in order to map out the perceptual locus of transition between transparency and non-transparency. The results revealed a systematic violation of Metelli's magnitude constraint in predicting the percept of transparency. Observer settings were approximated instead by a constraint based on perceived contrast (which matched Michelson contrast for the textures used). The results also revealed large asymmetries between darkening and lightening transparency. When the test was darker than the reference, settings were highly consistent across observers and closely followed the Michelson-contrast prediction. When the test was lighter, however, there was greater variability across observers, with two observers exhibiting shifts toward Metelli's magnitude constraint. Moreover, each observer's setting reliability was significantly worse for lightening transparency than darkening transparency. These results suggest that (polarity-preserving) darkening serves as an additional cue to perceptual transparency.

Contrast Sensitivity↗

PubMed2005

What change detection tells us about the visual representation of shape.

Many recent findings suggest that human observers are surprisingly "blind" to changes in visual displays, failing to notice when substantial scene elements are added, subtracted, or altered in successive presentations of the scene. But observers are far more sensitive to certain visual changes than others, and we suggest that which types of changes enjoy differential sensitivity can reveal a great deal about the underlying visual representations. In this study, we investigate how the human visual system represents the shape of objects by demonstrating a previously unknown influence on detection of changes in shape: the sign of contour curvature. We show that subjects are substantially more sensitive to changes in concave regions of a shape's contour than to changes in convex regions, even when these changes do not alter the number or location of parts. Further, we show that this effect is modulated by figure-ground assignment, so that changes to the same physical contour are more or less detectable, depending on the contour's perceived figural status, which determines whether the change falls in a concave or convex region. The results demonstrate a heightened sensitivity for changes at concavities that is not reducible to a sensitivity to changes in gross part structure.

Form Perception↗

PubMed2005

Visual extrapolation of contour geometry.

Computing the shapes of object boundaries from fragmentary image contours poses a formidable problem for the visual system. We investigated the extrapolation of contour shape by human vision. Measurements of extrapolation position and orientation were taken at six distances from the point of occlusion, thereby yielding a detailed representation of the extrapolated contours. Analyses of these measurements revealed that: (i) extrapolation curvature increases linearly with the curvature of the inducing contour, although there is individual bias in the slope; (ii) the precision with which an extrapolated contour is represented is roughly constant, in angular terms, with increasing distance from the point of occlusion; (iii) there is a substantial cost of curvature, in that the overall precision of an extrapolated contour decreases systematically with curvature; (iv) the shapes of visually extrapolated contours are characterized by a nonlinear decrease in curvature, asymptoting to zero; and (v) this decaying pattern of curvature is explained by a Bayesian model in which, with increasing distance from the point of occlusion, the prior tendency to minimize curvature gradually dominates the likelihood tendency to minimize variation in curvature.

Bayes Theorem↗

PubMed2005

Information along contours and object boundaries.

F. Attneave (1954) famously suggested that information along visual contours is concentrated in regions of high magnitude of curvature, rather than being distributed uniformly along the contour. Here the authors give a formal derivation of this claim, yielding an exact expression for information, in C. Shannon's (1948) sense, as a function of contour curvature. Moreover, they extend Attneave's claim to incorporate the role of sign of curvature, not just magnitude of curvature. In particular, the authors show that for closed contours, such as object boundaries, segments of negative curvature (i.e., concave segments) literally carry greater information than do corresponding regions of positive curvature (i.e., convex segments). The psychological validity of this informational analysis is supported by a host of empirical findings demonstrating the asymmetric way in which the visual system treats regions of positive and negative curvature.

Humans↗

PubMed2004

Lightness constancy through transparency: internal consistency in layered surface representations.

Asymmetric lightness matching was employed to measure how the visual system assigns lightness to surface patches seen through partially-transmissive surfaces. Observers adjusted the luminance of a comparison patch seen through transparency, in order to match the lightness of a standard patch seen in plain view. Plots of matched-to-standard luminance were linear, and their slopes were consistent with Metelli's alpha. A control experiment confirmed that these matches were indeed transparency based. Consistent with recent results, however, when observers directly matched the transmittance of transparent surfaces, their matches deviated strongly and systematically from Metelli's alpha. Although the two sets of results appear to be contradictory, formal analysis reveals a deeper mutual consistency in the representation of the two layers. A ratio-of-contrasts model is shown to explain both the success of Metelli's model in predicting lightness through transparency, and its failure to predict perceived transmittance--and hence is seen to play the primary role in perceptual transparency.

Contrast Sensitivity↗

PubMed2004

Modal and amodal completion generate different shapes.

Mechanisms of contour completion are critical for computing visual surface structure in the face of occlusion. Theories of visual completion posit that mechanisms of contour interpolation operate independently of whether the completion is modal or amodal--thereby generating identical shapes in the two cases. This identity hypothesis was tested in two experiments using a configuration of two overlapping objects and a modified Kanizsa configuration. Participants adjusted the shape of a comparison display in order to match the shape of perceived interpolated contours in a standard completion display. Results revealed large and systematic shape differences between modal and amodal contours in both configurations. Participants perceived amodal (i.e., partly occluded) contours to be systematically more angular--that is, closer to a corner--than corresponding modal (i.e., illusory) contours. The results falsify the identity hypothesis in its current form: Corresponding modal and amodal contours can have different shapes, and, therefore, mechanisms of contour interpolation cannot be independent of completion type.

Depth Perception↗

PubMed2004

Efficient oxidizing methods for the synthesis of oxandrolone intermediates.

Mild, efficient and eco-friendly oxidation of 17alpha-methylandrostan-3beta-17beta-diol (1) has been studied with three different reagents viz. pentavalent iodine reagent 2-iodoxy benzoic acid (IBX) in DMSO at 65 degrees C, sodium hypochlorite and H2O2/Na2WO4 under phase transfer conditions to give 17beta-hydroxy-17alpha-methylandrostan-3-one (mestanolone 2), a drug intermediate as oxidized product. The H2O2/Na2WO4/PTC gave mestanolone in high yield and purity whereas sodium hypochlorite/PTC system yielded some chlorinated material along with the mestanolone. However, 1 with 2.5 equivalent of IBX gave 17beta-hydroxy-17alpha-methyl-Delta1-androsten-3-one (3) under the similar reaction conditions in good yield and single step reaction.

Androstane-3,17-diol↗

PubMed2004

Lumboperitoneal shunts: review of 409 cases.

BACKGROUND AND AIMS: A prospective study was carried out to evaluate the lumboperitoneal shunt procedure. MATERIAL AND METHODS: Four hundred and nine patients having communicating hydrocephalus were selected for the procedure during a 10-year period from March 1992 to February 2002. The average follow-up was 45.34 months. RESULTS: Tubercular meningitis (TBM)-related hydrocephalus was detected in 285 patients. Forty per cent of the patients were less than 15 years of age. Glasgow Coma Scale (GCS) of less than 8 was seen in 40% patients and 14.9% patients were in GCS 13-15. At the time of discharge 56.7% patients improved in their GCS to 13 -15 and 14.9% were in GCS 8 or less. The overall mortality was 5.13% and shunt-related mortality was seen in 2% patients. Shunt malfunction requiring revision was seen in 32 patients (7.8%) and the total number of shunt revisions was 44 (11%). Shunt infection was noted in 3.4% patients. CSF leak at the lumbar end occurred in 12 patients. Four patients required conversion of LP shunt to VP shunt. CONCLUSIONS: Lumboperitoneal shunt is an effective shunting procedure in communicating hydrocephalus.

Glasgow Coma Scale↗

PubMed2003

Detection of change in shape: an advantage for concavities.

Shape representation was studied using a change detection task. Observers viewed two individual shapes in succession, either identical or one a slightly altered version of the other, and reported whether they detected a change. We found a dramatic advantage for concave compared to convex changes of equal magnitude. Observers were more accurate when a concavity along the contour was introduced, or removed, compared to a convexity. This result sheds light on the underlying representation of visual shape, and in particular the central role played by part-boundaries. Moreover, this finding shows how change detection methodology can serve as a useful tool in studying the specific form of visual representations.

Discrimination, Psychological↗

PubMed2002

The interpolation of object and surface structure.

One of the main theoretical challenges of vision science is to explain how the visual system interpolates missing structure. Two forms of visual completion have been distinguished on the basis of the phenomenological states that they induce. Modal completion refers to the formation of visible surfaces and/or contours in image regions where these properties are not specified locally. Amodal completion refers to the perceived unity of objects that are partially obscured by occluding surfaces. Although these two forms of completion elicit very different phenomenological states, it has been argued that a common mechanism underlies modal and amodal boundary and surface interpolation (the "identity hypothesis"; Kellman & Shipley, 1991; Kellman, 2001). Here, we provide new data, demonstrations, and theoretical principles that challenge this view. We show that modal boundary and surface completion processes exhibit a strong dependence on the prevailing luminance relationships of a scene, whereas amodal completion processes do not. We also demonstrate that the shape of interpolated contours can change when a figure undergoes a transition from a modal to an amodal appearance, in direct contrast to the identity hypothesis. We argue that these and previous results demonstrate that modal and amodal completion do not result from a common interpolation mechanism.

Depth Perception↗

PubMed2002

Toward a perceptual theory of transparency.

Theories of perceptual transparency have typically been developed within the context of a physical model that generates the percept of transparency (F. Metelli's episcotister model, 1974b). Here 2 fundamental questions are investigated: (a) When does the visual system initiate the percept of one surface seen through another? (b) How does it assign surface properties to a transparent layer? Results reveal systematic deviations from the predictions of Metelli's model, both for initiating image decomposition into multiple surfaces and for assigning surface attributes. Specifically, results demonstrate that the visual system uses Michelson contrast as a critical image variable to initiate percepts of transparency and to assign transmittance to transparent surfaces. Findings are discussed in relation to previous theories of transparency, lightness, brightness, and contrast-contrast.

Cues↗

PubMed2002

Perceptual assignment of opacity to translucent surfaces: the role of image blur.

In constructing the percept of transparency, the visual system must decompose the light intensity at each image location into two components one for the partially transmissivc surface, the other for the underlying surface seen through it. Theories of perceptual transparency have typically assumed that this decomposition is defined quantitatively in terms of the inverse of some physical model (typically, Metelli's 'episcotister model'). In previous work, we demonstrated that the visual system uses Michelson contrast as a critical image variable in assigning transmittance to transparent surfaces not luminance differences as predicted by Metelli's model [F Metelli, 1974 Scientific American 230(4) 90 98]. In this paper, we study the contribution of another variable in determining perceived transmittance, namely, the image blur introduced by the light-scattering properties of translucent surfaces and materials. Experiment 1 demonstrates that increasing the degree of blur in the region of transparency leads to a lowering in perceived transmittance, even if Michelson contrast remains constant in this region. Experiment 2 tests how this addition of blur affects apparent contrast in the absence of perceived transparency. The results demonstrate that, although introducing blur leads to a lowering in apparent contrast, the magnitude of this decrease is relatively small, and not sufficient to explain the decrease in perceived transmittance observed in experiment 1. The visual system thus takes the presence of blur in the region of transparency as an additional image cue in assigning transmittance to partially transmissive surfaces.

Contrast Sensitivity↗