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Biomedical subjects

K A Stevens

Publications and source records attributed to K A Stevens.

36 records · Page 2Linked to original sources
PubMed1989

Differential contributions of circular and elongated spatial filters to the Café Wall illusion.

The Café Wall illusion is induced in part by the spatial filtering properties of circular-symmetric receptive fields at the retina, and in part by cortical processes, presumably that have elongated receptive fields. Two experiments are reported that address these two contributions. The first shows a reversal effect that is consistent with brightness-induction and related theories. The second shows extinction of the effect in a manner predicted by recent observations of gating nonlinearity in cortical cells.

Corpus Striatum↗
PubMed1989

Binocular depth from surfaces versus volumes.

Subjects were asked to compare the relative depths of two binocular targets embedded in different random dot stereogram backgrounds. The disparities of the background points were either randomized, corresponding to a scattering of points within a volume, or arranged according to a sawtooth (triangle-wave) disparity profile (i.e., a set of slanted planar surfaces separated by sharp depth discontinuities). When the targets were embedded in the random volume, their depths were perceived in accordance with their relative disparities. But when the target points were embedded in the sawtooth surfaces their depths were systematically misperceived in a manner predicted by the incorrect depth interpretation of the background points. Rather than seeing a sawtooth pattern, the background points resembled a staircase in depth, and the targets, which appeared embedded in different steps, were misjudged in depth accordingly. The effect suggests a distinction between the depth processing of isolated binocular features and those associated with continuous surfaces.

Attention↗
PubMed1989

The analogy between stereo depth and brightness.

Apparent depth in stereograms exhibits various simultaneous-contrast and induction effects analogous to those reported in the luminance domain. This behavior suggests that stereo depth, like brightness, is reconstructed, ie recovered from higher-order spatial derivatives or differences of the original signal. The extent to which depth is analogous to brightness is examined. There are similarities in terms of contrast effects but dissimilarities in terms of the lateral inhibition effects traditionally attributed to underlying spatial-differentiation operators.

Color Perception↗
PubMed1988

Integrating stereopsis with monocular interpretations of planar surfaces.

Experiments are reported that involved spatial judgments of planar surfaces that had contradictory stereo and monocular information. Tasks included comparing the relative depths of two points on the depicted surface and judging the surface's apparent spatial orientation. It was found that for planar surfaces the 3D perception was dominated by the monocular interpretation, despite the strongly contradictory stereo information. We propose that stereo information is effectively integrated only where the surface exhibits curvature features or edge discontinuities, i.e. where the second spatial derivatives of disparity are nonzero. Planar surfaces induce constant gradients of disparity and are thus effectively featureless to stereopsis. Further observations are reported regarding nonplanar surfaces, where contradictory monocular information can still be effectively rivalrous with that suggested stereoscopically.

Depth Perception↗
PubMed1988

The concave cusp as a determiner of figure-ground.

The tendency to interpret as figure, relative to background, those regions that are lighter, smaller, and, especially, more convex is well known. Wherever convex opaque objects abut or partially occlude one another in an image, the points of contact between the silhouettes form concave cusps, each indicating the local assignment of figure versus ground across the contour segments. It is proposed that this local geometric feature is a preattentive determiner of figure-ground perception and that it contributes to the previously observed tendency for convexity preference. Evidence is presented that figure-ground assignment can be determined solely on the basis of the concave cusp feature, and that the salience of the cusp derives from local geometry and not from adjacent contour convexity.

Attention↗
PubMed1987

Probing depth in monocular images.

It is generally expected that depth (distance) is the internal representational primitive that corresponds to much of the perception of 3D. We tested this assumption in monocular surface stimuli that are devoid of distance information (due to orthographic projection and the chosen surface shape, with perspective projection used as a control) and yet are vividly three-dimensional. Slant judgments were found to be in close correspondence with the actual geometric slant of the stimuli; the spatial orientation of the surfaces was perceived accurately. The apparent depth in these stimuli was then tested by superimposing a stereo depth probe over the monocular surface. In both the perspective and orthographic projection the gradient of perceived depth, measured by matching the apparent depth of the stereo probe with that of the monocular surface at a series of locations, was substantial. The experiments demonstrate that in orthographic projection the visual system can compute from local surface orientation a depth quantity that is commensurate with the relative depth derived from stereo disparity. The depth data suggests that, at least in the near field, the zero value for relative depth lies at the same absolute depth as the stereo horopter (locus of zero stereo disparity). Relative to this zero value, the depth-from-slant computation seems to provide an estimate of distance information that is independent of the absolute distance to the surface.

Depth Perception↗
PubMed1984

On gradients and texture "gradients".

Texture gradients, systematic variations in image texture due to perspective projection of surface texture, provide information about the shape and disposition of surfaces relative to a viewer. Although this three-dimensional information is usually described in terms of mathematical relationships involving gradients of image properties (Cutting & Millard, 1984), the perceptual processes that derive this information need not necessarily measure or register these gradients explicitly (Stevens, 1981). It is difficult to conclude, therefore, that the perception of, say, flatness or curvature derives from processes that capitalize on these gradient relationships, per se. The difficulty becomes most apparent when attempting to formalize the perceptual computations and the spatial information that is made explicit.

Discrimination Learning↗
PubMed1983

Multiple endocrine neoplasia, type I. Association with marfanoid habitus, optic atrophy, and other abnormalities.

A patient had a parathyroid adenoma and prolactin-secreting pituitary tumor, suggestive of the multiple endocrine neoplasia (MEN) I syndrome. The presence of a marfanoid habitus--found more typically in MEN III syndrome--as well as mitral valve prolapse, mental retardation, and bilateral optic atrophy suggests a new variant of the MEN syndrome, possibly representing widespread dysplasia of endocrine and other tissues.

Adenoma↗
PubMed1983

Slant-tilt: the visual encoding of surface orientation.

A specific form for the internal representation of local surface orientation is proposed, which is similar to Gibson's (1950) "amount and direction of slant". Slant amount is usually quantified by the angle sigma between the surface normal and the line of sight (0 degrees less than or equal to sigma less than or equal to 90 degrees). Slant direction corresponds to the direction of the gradient of distance from the viewer to the surface, and may be defined by the image direction tau to which the surface normal would project (0 degrees less than or equal to tau less than or equal to 360 degrees). Since the direction of slant is specified by the tilt of the projected surface normal, it is referred to as surface tilt (Stevens, 1979; Marr, 1982). The two degrees of freedom of orientation are therefore quantified by slant, an angle measured perpendicular to the image plane, and tilt, an angle measured in the image plane. The slant-tilt form provides several computational advantages relative to some other proposals and is consistent with various psychological phenomena. Slant might be encoded by various means, e.g. by the cosine of the angle, by the tangent, or linearly by the angle itself. Experimental results are reported that suggest that slant is encoded by an internal parameter that varies linearly with slant angle, with resolution of roughly one part in 100. Thus we propose that surface orientation is encoded in human vision by two quantities, one varying linearly with slant angle, the other varying linearly with tilt angle.

Differential Threshold↗
PubMed1983

Evidence relating subjective contours and interpretations involving interposition.

Subjective contours, according to one theory, outline surfaces that are apparently interposed between the viewer and background (because of the disruption of background figures, sudden termination of lines, and other interposition 'cues') but are not explicitly outlined by intensity discontinuities. This theory predicts that if the cues are not interpreted as evidence of interposition, no intervening surface need be postulated, hence no subjective contours would be seen. This prediction, however, is difficult to test because observers normally interpret the cues as interposition evidence and normally see the subjective contours. Tests are reported on a patient with visual agnosia who is unable to make the usual interposition interpretations and unable to see subjective contours, but has normal ability to interpret standard visual illusions, stereograms, and in particular, stereogram versions of the standard subjective contour figures, which elicit to him strong subjective edges in depth (corresponding to the subjective contours viewed in the monocular versions of the figures.

Aged↗
PubMed1981

The information content of texture gradients.

Texture gradients are systematic variations in projected surface texture. It is expected that image variables such as texture density or size carry information about the surface orientation and distance. This article reexamines the information content of texture gradients and reviews the relevant psychophysics. Slant is shown to be difficult to infer from a texture gradient, but tilt (the direction of slant) is easily and reliably determined. Regarding distance, a new texture measure is introduced that has a simple geometric definition, and from which distance can be readily computed. Variables affecting the precision and the accuracy of the distance computation are discussed.

Humans↗
PubMedDate not supplied

Nursing diagnoses in wellness childbearing settings.

Although most nurses recognize the benefits of nursing diagnoses, difficulties surrounding clinical application of the NANDA system have often occurred. Many nurses feel that the NANDA system focuses mainly on illness or problem diagnoses, which makes application to healthy processes and areas such as childbearing more difficult. Examples of the flexibility of the NANDA system for stating diagnoses involving healthy processes as well as illness/problem diagnoses for postpartum clients are cited to illustrate that with simple modifications and in combination with a conceptual framework, this system can be used to formulate individualized diagnoses in childbearing settings.

Child Behavior Disorders↗