A neurological dissociation between shape from shading and shape from edges

Perceptual cognitive abilities in young athletes: A gender comparison

To achieve optimal performance in sports, it is essential to have strong perceptual cognitive abilities. Evidence suggests that athletes have superior perceptual abilities compared to nonathletes. However, gender differences in athletes’ perceptual cognitive abilities have not been previously reported. This project aims to evaluate perceptual cognitive abilities among male and female adolescents. To measure perceptual abilities, a 3-dimensional multiple-object tracking task was used. The findings confirm the superior perceptual cognitive abilities in young athletes relative to nonathletes. However, our results indicate differences in performance patterns between male and female athletes. These results demonstrate that sports engagement and perceptual cognitive abilities are strongly related during adolescence and that this relationship seems more prevalent in male athletes for this age group.

Perceived Contrast Explains Asymmetries in Visual-Search Tasks with Shaded Stimuli

Shaded stimuli have traditionally been used in the context of three-dimensional (3-D) shape perception. Many studies have shown a persistent asymmetry in that a circle filled with a shaded gradient that is dark at the top and bright at the bottom (top-dark circle) is much easier to locate among top-bright circles than in the opposite arrangement (a top-bright circle among top-dark circles). The immediate 3-D interpretation of top-dark and top-bright circles as hollows and protuberances, respectively, and the asymmetry just described have been explained in terms of 3-D percepts. The work described here challenges this view: the results of the first experiment show that top-dark circles are perceived as having 10% higher contrast than top-bright circles of the same physical contrast. Experiment 2 replicates classical visual-search experiments but adding a new condition where target and distractors were subjectively equated in contrast. For five of six subjects, the ubiquitous asymmetry disappears in this condition.

The shading cue in context

The shading cue is supposed to be a major factor in monocular stereopsis. However, the hypothesis is hardly corroborated by available data. For instance, the conventional stimulus used in perception research, which involves a circular disk with monotonic luminance gradient on a uniform surround, is theoretically 'explained' by any quadric surface, including spherical caps or cups (the conventional response categories), cylindrical ruts or ridges, and saddle surfaces. Whereas cylindrical ruts or ridges are reported when the outline is changed from circular to square, saddle surfaces are never reported. We introduce a method that allows us to differentiate between such possible responses. We report observations on a number of variations of the conventional stimulus, including variations of shape and quality of the boundary, and contexts that allow the observer to infer illumination direction. We find strong and expected influences of outline shape, but, perhaps surprisingly, we fail to find any influence of context, and only partial influence of outline quality. Moreover, we report appreciable differences within the generic population. We trace some of the idiosyncrasies (as compared to shape from shading algorithms) of the human observer to generic properties of the environment, in particular the fact that many objects are limited in size and elliptically convex over most of their boundaries.

Perceiving parts and shapes from concave surfaces

"A hole is nothing at all, but it can break your neck." In a similar fashion to the danger illustrated by this folk paradox, concave regions pose difficulties to theories of visual shape perception. We can readily identify their shapes, but according to principles of how observers determine part boundaries, concavities in a planar surface should have very different figural shapes from the ones that we perceive. In three experiments, we tested the hypothesis that observers perceive local image features differently in simulated 3-D concave and convex regions but use them to arrive at similar shape percepts. Stimuli were shape-from-shading images containing regions that appeared either concave or convex in depth, depending on their orientation in the picture plane. The results show that concavities did not benefit from the same global object-based attention or holistic shape encoding as convexities and that the participants relied on separable spatial dimensions to judge figural shape in concavities. Concavities may exploit a secondary process for shape perception that allows regions composed of perceptually independent features to ultimately be perceived as gestalts.

Smooth-shape assumption for perceiving shapes from shading

Humans can perceive three-dimensional shapes from shading, but reconstructing the original shape of an object from shading alone (luminance distribution) is mathematically impossible. Researchers have used different assumptions and reported that the human visual systems can resolve this difficulty. Here, we propose an assumption for perceiving shape from shading: that the object shape is assumed to be smooth rather than angular. In experiment 1, we investigated the effect of shape smoothness by manipulating the shading profile of the test region. In experiment 2, we further investigated the effect of shape smoothness by manipulating shapes of the regions bordering on the test region using binocular disparity. Each stimulus in our experiments is interpretable from shading as having either smooth or angular edges. Observers responded to the perceived shape while viewing the stimuli, and most tended to perceive smooth rather than angular edges. These results support the idea that the smooth-shape assumption is effective for perceiving shape from shading.

Three-month-olds’ sensitivity to orientation cues in the three-dimensional depth plane

Three-month-olds are sensitive to orientation changes of line drawings when they have a three-dimensional (3-D) interpretation and when the changes are defined by both 3-D depth and two-dimensional (2-D) picture plane cues [Bhatt, R. S., & Bertin, E. (2001). Pictorial cues and three-dimensional information processing in early infancy. Journal of Experimental Child Psychology, 80, 315-332]. In the current study, we examined whether 3-month-olds are sensitive to pictorial line junction cues that signal orientation changes solely in the 3-D depth plane. The results revealed that infants discriminated a misoriented elongated cube in an array when the stimuli contained both shading and lines (Experiment 2) but not when only lines depicted the elongated cubes (Experiment 1). Testing with comparable 2-D images revealed that, even in the presence of shading information, detection of orientation changes is specific to images that have a 3-D interpretation. Together, the results suggest that 3-month-olds are sensitive to pictorial line junction cues that signal orientation changes in the 3-D depth plane to adults provided that shading information is available and the images have a 3-D interpretation.

Ventral occipital lesions impair object recognition but not object-directed grasping: an fMRI study

D.F., a patient with severe visual form agnosia, has been the subject of extensive research during the past decade. The fact that she could process visual input accurately for the purposes of guiding action despite being unable to perform visual discriminations on the same visual input inspired a novel interpretation of the functions of the two main cortical visual pathways or 'streams'. Within this theoretical context, the authors proposed that D.F. had suffered severe bilateral damage to her occipitotemporal visual system (the 'ventral stream'), while retaining the use of her occipitoparietal visual system (the 'dorsal stream'). The present paper reports a direct test of this idea, which was initially derived from purely behavioural data, before the advent of modern functional neuroimaging. We used functional MRI to examine activation in her ventral and dorsal streams during object recognition and object-directed grasping tasks. We found that D.F. showed no difference in activation when presented with line drawings of common objects compared with scrambled line drawings in the lateral occipital cortex (LO) of the ventral stream, an area that responded differentially to these stimuli in healthy individuals. Moreover, high-resolution anatomical MRI showed that her lesion corresponded bilaterally with the location of LO in healthy participants. The lack of activation with line drawings in D.F. mirrors her poor performance in identifying the objects depicted in the drawings. With coloured and greyscale pictures, stimuli that she can identify more often, D.F. did show some ventral-stream activation. These activations were, however, more widely distributed than those seen in control participants and did not include LO. In contrast to the absent or abnormal activation observed during these perceptual tasks, D.F. showed robust activation in the expected dorsal stream regions during object grasping, despite considerable atrophy in some regions of the parietal lobes. In particular, an area in the anterior intraparietal sulcus was activated more for grasping an object than for just reaching to that object, for both D.F. and controls. In conclusion, we have been able to confirm directly that D.F.'s visual form agnosia is associated with extensive damage to the ventral stream, and that her spared visuomotor skills are associated with visual processing in the dorsal stream.

Cortical areas related to attention to 3D surface structures based on shading: an fMRI study

The aim of the present study was to determine which cortical areas are activated in relation to attention to a three-dimensional (3D) structure of a surface based on shading. Cortical activities were examined using functional magnetic resonance imaging while subjects discriminated whether the central part of the surface protruded or was recessed based on shading without any binocular disparity cues. Relatively broad cortical areas including both dorsal and ventral visual pathways were recruited when shading was used as a crucial cue for the perception of the 3D structure of a surface. In these cortical areas, however, the right intraparietal area was shown to be commonly activated in all subjects and in all sessions by multisubject conjunction analysis. These results strongly suggest that the intraparietal area plays an important role in perception of the 3D structure of a surface, even when based only on monocular depth cues without binocular disparity cues.

Cavanagh and Leclerc shape-from-shadow pictures: do line versions fail because of the polarity of the regions or the contour?

Shape-from-shadow perception fails when the contour bordering a shadowed area is reduced to a black line, and the shadow area becomes white. It might be that the polarity of the shadowed and illuminated areas has to be from dark on the shadowed side to light on the illuminated side for successful perception. Or it may be that the line, which has two contours, has one too many for shape-from-shadow processing. Alternatively, the problem might be that one of the contours of the line is incorrectly polarised. To test these explanations, three shape-from-shadow figures were prepared, each depicting the same referent--an elderly person. All three figures had two correctly polarised areas. One figure had a correctly polarised contour at the border between the areas. One had two correctly polarised contours. The other had one correctly polarised contour and one incorrectly polarised contour. The referent of the figure with one incorrectly polarised contour was the one difficult to make out. The result has implications for several theories, including an account of a demonstration by Hering involving penumbra.

Cortical regions involved in perceiving object shape

The studies described here use functional magnetic resonance imaging to test whether common or distinct cognitive and/or neural mechanisms are involved in extracting object structure from the different image cues defining an object's shape, such as contours, shading, and monocular depth cues. We found overlapping activations in the lateral and ventral occipital cortex [known as the lateral occipital complex (LOC)] for objects defined by different visual cues (e.g., grayscale photographs and line drawings) when each was compared with its own scrambled-object control. In a second experiment we found a reduced response when objects were repeated, independent of whether they appeared in the same or a different format (i.e., grayscale images vs line drawings). A third experiment showed that activation in the LOC was no stronger for three-dimensional shapes defined by contours or monocular depth cues, such as occlusion, than for two-dimensional shapes, suggesting that these regions are not selectively involved in processing three-dimensional shape information. These results suggest that common regions in the LOC are involved in extracting and/or representing information about object structure from different image cues.

Orientation tuning of shape from shading

Four experiments were performed to assess the effect of different orientations and direction of lighting on the visual processing of shaded or bipartite disks. In the first two experiments, observers were presented with nine different shading orientations from 0 degree to 180 degrees. Targets were detected in a rapid and parallel fashion for shaded disks when the orientation of the shading gradient was not horizontal (90 degrees) or oriented at 67.5 degrees. Search asymmetries favoring the detection of "pock" targets over "ball" targets were found for all orientations. The search rates for bipartite disks were similar to the shaded disks at 0 degree, 22.5 degrees, and 90 degrees but not for intermediate orientations, and no search asymmetries were found. These differences suggest that shaded displays and bipartite displays are processed by different underlying mechanisms. The third experiment showed that the direction of the light source (left or right) had no influence on search asymmetries around the 90 degrees point. Shading gradient orientation affected magnitude estimates of depth in the fourth experiment. These experiments show that the visual system's "assumption" of overhead lighting is broadly tuned.

Probing unconscious visual processing with the McCollough effect

The McCollough effect, an orientation-contingent color aftereffect, has been known for over 30 years and, like other aftereffects, has been taken as a means of probing the brain's operations psychophysically. In this paper, we review psychophysical, neuropsychological, and neuroimaging studies of the McCollough effect. Much of the evidence suggests that the McCollough effect depends on neural mechanisms that are located early in the cortical visual pathways, probably in V1. We also review evidence showing that the aftereffect can be induced without conscious perception of the induction patterns. Based on these two lines of evidence, it is argued that our conscious visual experience of the world arises in the cortical visual system beyond V1.

The objects of action and perception

Two major functions of the visual system are discussed and contrasted. One function of vision is the creation of an internal model or percept of the external world. Most research in object perception has concentrated on this aspect of vision. Vision also guides the control of object-directed action. In the latter case, vision directs our actions with respect to the world by transforming visual inputs into appropriate motor outputs. We argue that separate, but interactive, visual systems have evolved for the perception of objects on the one hand and the control of actions directed at those objects on the other. This 'duplex' approach to high-level vision suggests that Marrian or 'reconstructive' approaches and Gibsonian or 'purposive-animate-behaviorist' approaches need not be seen as mutually exclusive, but rather as complementary in their emphases on different aspects of visual function.

Perceiving visually presented objects: recognition, awareness, and modularity

Object perception may involve seeing, recognition, preparation of actions, and emotional responses--functions that human brain imaging and neuropsychology suggest are localized separately. Perhaps because of this specialization, object perception is remarkably rapid and efficient. Representations of componential structure and interpolation from view-dependent images both play a part in object recognition. Unattended objects may be implicitly registered, but recent experiments suggest that attention is required to bind features, to represent three-dimensional structure, and to mediate awareness.

The brain as a symbol-processing machine

The knowledge accumulated about the biochemistry of the synapsis in the last decades completely changes the notion of brain processing founded exclusively over an electrical mechanism, toward that supported by a complex chemical message exchange occurring both locally, at the synaptic site, as well as at other localities, depending on the solubility of the involved chemical substances in the extracellular compartment. These biochemical transactions support a rich symbolic processing of the information both encoded by the genes and provided by actual data collected from the surrounding environment, by means of either special molecular or cellular receptor systems. In this processing, molecules play the role of symbols and chemical affinity shared by them specifies the syntax for symbol manipulation in order to process and to produce chemical messages. In this context, neurons are conceived as message-exchanging agents. Chemical strings are produced and stored at defined places, and ionic currents are used to speed up message delivery. Synaptic transactions can no longer be assumed to correspond to a simple process of propagating numbers powered by a factor measuring the presynaptic capacity to influence the postsynaptic electrical activity, but they must be modeled by more powerful formal tools supporting both numerical and symbolic calculations. It is proposed here that formal language theory is the adequate mathematical tool to handle such symbolic processing. The purpose of the present review is therefore: (a) to discuss the relevant and recent literature about trophic factors, signal transduction mechanisms, neuromodulators and neurotransmitters in order (b) to point out the common features of these correlated processes; and (c) to show how they may be organized into a formal model supported by the theory of fuzzy formal languages (d) to model the brain as a distributed intelligent problem solver.

Vision without knowledge

A brain-damaged patient (D.F.) with visual form agnosia is described and discussed. D.F. has a profound inability to recognize objects, places and people, in large part because of her inability to make perceptual discriminations of size, shape or orientation, despite having good visual acuity. Yet she is able to perform skilled actions that depend on that very same size, shape and orientation information that is missing from her perceptual awareness. It is suggested that her intact vision can best be understood within the framework of a dual processing model, according to which there are two cortical processing streams operating on different coding principles, for perception and for action, respectively. These may be expected to have different degrees of dependence on top-down information. One possibility is that D.F.'s lack of explicit awareness of the visual cues that guide her behaviour may result from her having to rely on a processing system which is not knowledge-based in a broad sense. Conversely, it may be that the perceptual system can provide conscious awareness of its products in normal individuals by virtue of the fact that it does interact with a stored base of visual knowledge.

Differences in perceived shape from shading correlate with activity in early visual areas

The perception of shape from shading depends on the orientation of the shading gradient [1] [2] [3] [4]. Displays composed of elements with vertically oriented shading gradients of opposite polarity produce a strong and stable percept of 'concave' and 'convex' elements. If the shading gradients are rotated 90 degrees , the depth percept is reduced and appears much more ambiguous. Results from psychophysical [1] [2] [3] [4] [5] [6], neuropsychological [7] and computational studies [8] [9] suggest that the perception of shape from shading engages specific mechanisms in early cortical visual areas. In a three-dimensional functional magnetic resonance imaging (fMRI) study at 1.5 Tesla using a three-dimensional, interleaved-echoplanar imaging technique and a surface radio frequency (RF) coil placed under the visual cortex, we investigated the activity in these early visual areas associated with viewing shape from shading displays at two different orientations. We found significantly greater activation in area V1 and neighbouring low-level visual areas of cortex when subjects viewed displays that led to weak and unstable depth percepts than when they viewed displays that led to strong and stable depth percepts.