Shape constancy from novel views

Prior experiments on shape constancy from novel views are inconclusive: Some show that shapes of objects can be recognized reliably from novel views, whereas others show just the opposite. Our analysis of prior results suggests that shape constancy from novel views is reliable when the object has properties that constrain its shape: The object has volumetric primitives, it has surfaces, it is symmetrical, it is composed of geons, its contours are planar, and its images provide useful topological information about its three-dimensional structure. To test the role of some of these constraints, we performed a set of experiments. Solid shapes (polyhedra) were shown on a computer monitor by means of kinetic depth effect. Experiment 1 showed that shape constancy can be reliably achieved when a polyhedron is represented by its contours (most of the constraints are present), but not when it is represented by vertices or by a polygonal line connecting the vertices in a random order (all the constraints are absent). Experiments 2 and 3 tested the role of individual constraints. Results of these experiments show that shape constancy from novel views is reliable when the object has planar contours and when the shapes of the contours together with topological information about the relations among the contours constrain the possible interpretations of the shape. Symmetry of the object and the topological stability of its image also contribute to shape constancy.     

A demonstration of shape constancy

In order to compare the recognition of objective (defined in three dimensions) and projective (on the frontal-parallel plane) shapes, subjects were asked to identify angles (either objective or projective) on random polygons that were displayed in complex and meaningful photographic slides in the frontal-parallel plane. The subjects’ judgments corresponded much more closely to the objective than to the projective shapes, almost independently of which of the two shapes they were asked to judge and of whether the slides were presented in a normal upright orientation or were rotated 180 deg. The results are incompatible with the strong form of the shape-slant hypothesis, which assumes that the primary perceptual information concerns the properties of the projective retinal image. The results indicate, instead, that a tacit conception of three-dimensional space is primary in the perceptual process.     

The influence of object size and surface shape on shape constancy from stereo

The failure of shape constancy from stereoscopic information is widely reported in the literature. In this study we investigate how shape constancy is influenced by the size of the object and by the shape of the object's surface. Participants performed a shape-judgment task on objects of five sizes with three different surface shapes. The shapes used were: a frontoparallel rectangle, a triangular ridge surface, and a cylindrical surface, all of which contained the same maximum depth information, but different variations in depth across the surface. The results showed that, generally, small objects appear stretched and large objects appear squashed along the depth dimension. We also found a larger variance in shape judgments for rectangular stimuli than for cylindrical and ridge-shaped stimuli, suggesting that, when performing shape judgments with cylindrical and ridge-shaped stimuli, observers rely on a higher-order shape representation.     

Binocular rivalry and visual awareness: the role of attention

When the right eye and the left eye view dissimilar scenes, the observer does not experience a stable superimposed percept of the images presented to the two eyes, but instead perceives an alternation between the images seen by each eye. A critical question confronting this robust and intriguing phenomenon of binocular rivalry is how the visual system selects the image to be perceived (dominant). The current main-stream literature emphasizes a bottom-up explanation in which the rivalry stimulus with the higher contour strength has the advantage, and becomes dominant in rivalry. Nevertheless, some workers in the past have favored an attention-selection explanation for binocular rivalry. We investigated the role of attention in binocular rivalry by employing novel psychophysical paradigms which capitalized on several established phenomena (e.g. the Cheshire Cat effect, attention cueing, pop-out effect). Our results revealed two major aspects of attention modulation in binocular rivalry. We found that a dominant image is less likely to be suppressed when voluntary attention is directed to it. This suggests the role of voluntary attention in retaining the dominant image in visual awareness. Second, a rivalry stimulus is more likely to become dominant if accompanied by a pop-out cue (in the same eye and proximity). Since a pop-out cue attracts involuntary attention to its location/eye, this result suggests that cue-mediated involuntary attention can promote the ability of a rivalry stimulus to reach visual awareness.     

The perception of rotated shapes: A process analysis of shape constancy

Ss made objective shape judgments of circular objects rotated in depth to provide a process analysis of shape constancy. The significant finding was that task difficulty, as reflected by proportion errors and correct reaction times; increased with increases in rotation from the frontal-parallel plane. This effect was located at the perceptual encoding stage of the shape judgment process. It was demonstrated that, in contrast to true shape information, the time to interpret slant and two-dimensional projected shape information was not critically dependent on degree of rotation. These results and a number of other additional observations demonstrate that the invariance hypothesis does not provide a sufficient account of shape constancy. Although projected shape and’ slant judgments can be made easily, perception of true shape involves encoding a figure-ground relationship by a process that does not rely exclusively on the discrete values of projected shape and slant.     

Effects of form rotation on the speed of classification: the development of shape constancy

Ss classified a test form as matching or not matching one of a set of memorized forms relative to which the test forms were rotated up to 90 deg. “Match” reaction times increased monotonically with both number of forms memorized and degrees of rotation. Disappearance of the rotation effect after practice was attributed to a change from considering rotational information during comparison to comparing rotation invariant features. The change in strategy is considered an indicant of the development of shape constancy. Time taken to memorize the patterns increased linearly with the size of the memory set, justifying interpretation of the RT effects in terms of comparison time differences rather than differences in memorial specification of the patterns.     

Routes to object constancy: Implications from neurological impairments of object constancy

Previous studies have established the existence of neurological impairments of object constancy: the ability to recognize that an object has the same structure across changes in its retinal projection. Five case studies of brain-damaged patients with deficits in achieving object constancy are reported. To test object constancy, patients discriminated two photographs of a target object, taken from different views, from a photograph of a visually similar distractor object. Four patients showed impaired matching only when the principal axis of the target object in one photograph was foreshortened. The fifth patient showed impaired matching only when the saliency of the target object's primary distinctive feature was reduced. This double dissociation suggests that normally there may be two independent means of achieving object constancy: one by processing an object's local distinctive features, the other by describing the object's structure relative to the frame of its principal axis. Neurological damage can selectively impair either process. Further, this impairment can be independent of deficits in processing visual form, since two patients with a selective deficit in the foreshortened matching task showed relatively normal form discrimination. The patient dependent on local distinctive feature information showed a deficit in size discrimination. It is suggested that this patient fails to utilize global properties of form. This failure may underlie both his impairment in achieving object constancy and in processing certain dimensions of form.     

Generalization to novel views from view combination

Four experiments examined whether or not exposure to two views (A and B) of a novel object improves generalization to a third view (C) through view combination on tasks that required symmetry or recognition memory decisions. The results of Experiment 1 indicated that exposure to either View A or View B alone produced little or no generalization to View C on either task. The results of Experiment 2 indicated that exposure to both View A and View B did improve generalization to View C, but only for symmetrical objects. Experiment 3 replicated this generalization advantage for symmetrical but not asymmetrical objects, when objects were well learned at study. The results of Experiment 4 showed that Views A and B did not have to be presented consecutively to facilitate responses to View C. Together, the pattern of results suggests that generalization to novel views does occur through view combination of temporally separated views, but it is more likely to be observed with symmetrical objects.     

The contingent a priori and the publicity of a priori knowledge

Kripke maintains that one who stipulatively introduces the term ‘one meter’ as a rigid designator for the length of a certain stick s at time t is in a position to know a priori that if s exists at t then the length of s at t is one meter. Some (e.g., Soames 2003) have objected to this alleged instance of the contingent a priori on the grounds that the stipulator's knowledge would have to be based in part on substantive metalinguistic knowledge. I examine Soames's argument for the a posteriority of the relevant metalinguistic knowledge, and I argue that its main premise is false.     

Speed constancy as a function of size constancy

If perceived velocity were to be a function of phenomenal extent traversed per unit time, speed constancy would be derivable from size constancy. Experiments were performed to test this possibility in which visible frames of reference were eliminated. With cues to distance, judgments ofspeed approached constancy; without such cues, judgments of speed were in terms of rate ofimage displacement.     

Importance of relative width differences and instructions on shape constancy performance

Adult subjects were instructed to select either the wider physical or the wider perspective shape of two rotated rectangles. With equal differences between the physical and perspective widths, responses were governed by instructions and by the magnitude of the differences between widths of the forms. However, with unequal differences between physical and perspective widths of the two rectangles, responses were governed primarily by the magnitude of the differences. Regardless of instructions, responses were to the dimension with the greater difference between widths. The results suggest that performance in the shape constancy experiment is a function of relative dimensional discriminability, as defined by instructions, and relative differences of widths between forms presented for comparison.     

Actuality and the a priori

Book Information Nietzsche's Middle Period. Nietzsche's Middle Period Ruth Abbey New York Oxford University Press 2000 xvii + 208 Hardback £33.50 By Ruth Abbey. Oxford University Press. New York. Pp. xvii + 208. Hardback:£33.50,