Stereoscopic depth but not shape perception from second-order stimuli.
(25/1967)
Depth can be seen using either linear (first-order) or non-linear (second-order) stereo micropatterns when, in the latter, contrast envelopes contain the disparity information. We examined whether a second-order mechanism can contribute to the perception of 3-D surface shape. Using a variety of different stimulus types, we show that for each, shape is easy to see with linear stimuli. Over a wide range of parameters however, none of our observers perceived shape, however faintly, from the non-linear stimuli. To explore why these elements failed, we simplified our stimulus to a step-edge in depth and measured performance while varying the number of elements. We show how performance declined when more than two non-linear elements were used. We discuss reasons for the limitation found for non-matching elements, including a dissociation for stereopsis between seeing surface shape and depth. (+info)
Temporal constraints on the grouping of contour segments into spatially extended objects.
(26/1967)
The speed of contour integration was investigated in a task that can be solved by grouping contour segments into elongated curves. Subjects had to detect a continuous curve, which could be intersected by one or two other curves. At locations where these curves came in close proximity, the assignment of contour segments to the different curves could be based on collinearity. Reaction times exhibited a strong dependence on (1) the presence of intersections among curves; and (2) the context provided by the stimulus set from which individual stimuli were selected. Reaction times were shortest when grouping of contour segments depended on information at a single location in the visual field. In this condition, responses to stimuli containing an intersection were faster than responses to stimuli that did not. When responses were determined by information at spatially separate locations, responses were delayed, and every intersection increased the reaction time considerably. This result contrasts with earlier investigations which have suggested that contour integration on the basis of collinearity is performed pre-attentively but is in accordance with studies on curve tracing. We propose that the assignment of contour segments to equally coherent curves, a process which may be called figure-figure segregation, is a function of object-based attention. Moreover, the protracted reaction times for some of the stimuli indicate that spread of attention within an object costs time. This implies that object recognition is not always as fast as is sometimes assumed. (+info)
Texture filling-in and texture segregation revealed by transient masking.
(27/1967)
When a texture pattern was briefly presented followed by a small annular mask, it was found that the central area of the texture was strongly suppressed within the mask. Analogous to filling-in of brightness in a uniform luminance area (Paradiso, M. A. & Nakayama, K. (1991) Vision Research, 31, 1221-1236), this phenomenon demonstrates filling-in of texture; the texture area was unperceived because filling-in of the texture area was interrupted by the contour in the mask. However, odd local features within the texture, which were assumed to pop out, were selectively perceived while other features were suppressed within the mask. These results suggest that: (1) rapid pattern segregation occurs before and/or separately from texture filling-in, and that (2) filling-in is initiated at boundaries between surfaces rather than at luminance gradients. (+info)
A self-organizing neural system for learning to recognize textured scenes.
(28/1967)
A self-organizing ARTEX model is developed to categorize and classify textured image regions. ARTEX specializes the FACADE model of how the visual cortex sees, and the ART model of how temporal and prefrontal cortices interact with the hippocampal system to learn visual recognition categories and their names. FACADE processing generates a vector of boundary and surface properties, notably texture and brightness properties, by utilizing multi-scale filtering, competition, and diffusive filling-in. Its context-sensitive local measures of textured scenes can be used to recognize scenic properties that gradually change across space, as well as abrupt texture boundaries. ART incrementally learns recognition categories that classify FACADE output vectors, class names of these categories, and their probabilities. Top-down expectations within ART encode learned prototypes that pay attention to expected visual features. When novel visual information creates a poor match with the best existing category prototype, a memory search selects a new category with which classify the novel data. ARTEX is compared with psychophysical data, and is bench marked on classification of natural textures and synthetic aperture radar images. It outperforms state-of-the-art systems that use rule-based, backpropagation, and K-nearest neighbor classifiers. (+info)
Measuring perceived 3D shape at multiple spatial scales.
(29/1967)
We present and test a novel multiscale representation of perceived 3D surface orientation: the orientation path. Using a multiscale probe, we measure perceived surface orientation at multiple spatial scales; linking the measurements for a given surface location yields that location's orientation path. The multiscale data obtained show that observers consistently see different surface orientations at different spatial scales. We demonstrate that such multiscale data can reveal multiscale differences between observers' percepts of a stimulus and the stimulus geometry. We also demonstrate the use of the orientation path in evaluating the multiscale effects of adding a depth cue to a 3D display. (+info)
Spatial sensitization of increments and decrements: a border-contrast process and a net-excitation process.
(30/1967)
We investigated the spatially local factors that adjust the sensitivity of the human visual system within a small patch of visual space. A very small adapting field was varied in diameter to map out the strength and extent of the spatially local processes that adjust sensitivity for both increments and decrements. The results demonstrated antagonistic center/surround adaptation regions with a decremental test probe comparable to those demonstrated previously for incremental probes (Westheimer, G., 1965. Spatial interaction in the human retina during scotopic vision, Journal of Physiology 81, 812-894; Westheimer, G., 1967. Spatial interaction in human cone vision, Journal of Physiology 190, 139-154) implying comparable antagonistic regions in the ON and OFF channels. In addition to spatial interactions based on light adaptation, we report a weaker effect that is based on the location of a border (luminance edge) and is governed by the contrast of this edge. Finally, we show that these effects are elicited by both highly localized edges (1' ring pairs) and radial lines (Ehrenstein figure) as well. We conclude that both a border-contrast mechanism and a net-excitation mechanism govern the spatially local adaptation of the visual system and that this view fits well with the behavior of single units reported previously. (+info)
Vernier judgments in the absence of regular shape information.
(31/1967)
Vernier acuity is a form of hyperacuity in which the threshold offset between a test object and a reference object is smaller than the size of a foveal cone. Because the test and the reference objects usually have regular shapes (e.g. rectangular, triangular or circular), relatively few studies have addressed the role of shape information in determining hyperacuity thresholds. In this study, we investigated the effect of shape information on hyperacuity performance using targets of irregular shape with different skew and symmetry properties. Vernier thresholds smaller than 10 arc-sec were obtained for closely spaced asymmetric irregular-shape targets. Thresholds for dots and asymmetric irregular shapes increased with increase in center-to-center gap between the targets. Unlike dots, the thresholds for asymmetric irregular shapes also increased with target area. Although the thresholds for asymmetric irregular shapes were higher than those for dots, thresholds for symmetric irregular shapes were similar. Target skew below a certain level had a negligible effect on Vernier thresholds for asymmetric shapes. Our results suggest the existence of feature-independent neural circuitry that can support hyperacuity thresholds and are consistent with the use of the centroid as a primitive for relative localization. (+info)
Macaque inferior temporal neurons are selective for disparity-defined three-dimensional shapes.
(32/1967)
Real-world objects are three-dimensional (3D). Yet, it is unknown whether the neurons of the inferior temporal cortex, which is critical for object recognition, are selective for the 3D shape of objects. We tested for such selectivity by comparing responses to stereo-defined curved 3D shapes derived from identical pairs of monocular images. More than one-third of macaque inferior temporal neurons were selective for 3D shape. In the vast majority of those neurons, this selectivity depended on the global binocular disparity gradient and not on the local disparity. Thus, inferior temporal cortex processes not only two-dimensional but also 3D shape information. (+info)