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PubMed1994

Shape from shading. I: Surface curvature and orientation.

The human visual system makes effective use of shading alone in recovering the shape of objects. Pictures of sculptures are readily interpreted--a situation where shading provides virtually the sole cue to shape. However, shading has been considered a poor cue to depth in comparison with retinal disparity and kinetic cues. Curvature discrimination thresholds were measured with the use of a surface-alignment task for a range of surface curvatures from 0.16 cm-1 to 1.06 cm-1. Weber fractions were around 0.1, demonstrating considerable precision in this task. Weber fractions did not vary substantially as a function of surface curvature. Rotation of the light source around the line of sight had no effect on curvature discrimination but rotation towards the viewer increased discrimination thresholds. In contrast, slant discrimination declined with rotation of the light-source vector towards the viewpoint. When a band-limited random grey-level texture was mapped onto the sphere, curvature discrimination thresholds increased gradually as a function of texture contrast, even though texture and shading provided consistent cues to depth. Adding texture also increased slant discrimination thresholds, demonstrating that texture can act as a source of noise in shape-from-shading tasks. The psychophysical findings have been used to evaluate whether current algorithms for shape from shading in computer vision could serve as models of human three-dimensional shape analysis and to highlight low-level intramodular interactions between depth cues. It is demonstrated that, in the case of surfaces defined by shading, curvature descriptions are primary and do not depend upon the prior encoding of surface orientation, and Koenderink's local-shape index is suggested as an alternative intermediate representation of surface shape in the human visual system.

Attention↗
PubMed1994

Shape from shading. II. Geodesic bisection and alignment.

Pattern-acuity tasks have provided valuable information about the precision with which the visual system can make judgments about relative spatial position in two-dimensional images. However, outside the laboratory the visual system is habitually faced with the more difficult task of making positional judgments within a three-dimensional spatial environment. Thus our perceptual systems for representing surface shape also need to support the recovery of the location and disposition of features in a three-dimensional space. An investigation of the precision of three-dimensional position judgments in two spatial-judgment tasks, arc length bisection along geodesics and geodesic alignment, is reported. The spatial-judgment tasks were defined with reference to a sphere rendered by means of ray-casting techniques. The presence of shading and texture cues had no effect on discrimination thresholds in either task. Observers' constant errors were generally less than the just noticeable distance, demonstrating that the observers can perform these positional judgment tasks without substantial bias. It is argued that there is no explicit computation of arc length on the basis of shading and texture information and that surface-orientation information cannot be used as a reference in geodesic-alignment tasks. The results raise questions about the utility of a representation of surface orientation in the human visual system.

Attention↗
PubMed1994

Endogenous and exogenous control of visual selection.

Among the most fundamental issues of visual attention research is the extent to which visual selection is controlled by properties of the stimulus or by the intentions, goals, and beliefs of the observer. Before selective attention operates, preattentive processes perform some basic analyses segmenting the visual field into functional perceptual units. The crucial question is whether the allocation of attention to these perceptual units is under the endogenous control of the observer (intentions, goals, beliefs) or under the exogenous control of attention in tasks in which subjects search for a particular 'basic' feature (eg search for a unique colour, shape, or brightness). In the present review it is suggested that selectivity in these types of search tasks is dependent on the relative saliency of the stimulus attributes. It is concluded that the visual system automatically calculates differences in basic features (eg difference in shape, colour, or brightness) and that visual information occupying the position of the highest saliency across stimulus dimensions is exogenously passed on to the 'central presentation' that is responsible for further stimulus analysis. Alternative explanations of the present findings and tentative speculations resulting from the present approach are discussed.

Attention↗
PubMed1997

Integration of physical and semantic information in object processing.

Physiological studies report independent processing pathways for form and colour information. A more-complex picture on human subjects has previously been reported. A sequential matching task was used that was based on a physical property of an object and in which semantic relations between stimuli were manipulated. Performance was affected by semantic information when matching was based on a property of the form of an object (its orientation, shape, or size). Effects of semantic information were eliminated when matching was based on the colour of a local part of an object but were found again when subjects matched pictures on the basis of the percentage of a colour integrated across the shape boundary. The results suggest independent selection mechanisms in vision in which selection by local colour can be based on inhibition of the form-processing pathway whilst processing of the global configuration of the form of an object activates automatically the identification process.

Cognition↗
PubMed1999

The effects of eccentricity and vergence angle upon the relative tilt of corresponding vertical and horizontal meridia revealed using the minimum motion paradigm.

When the corresponding horizontal meridia of the two eyes are aligned, the corresponding vertical meridia are tilted outwards in a temporal direction, a phenomenon first described by Helmholtz. However, it is not known if this effect is confined to the principal meridia or whether the same relationship exists between corresponding horizontal and corresponding vertical meridia at eccentric retinal locations. We sought to address this issue by exploiting the technique of Nakayama (1977 Proceedings of the Society of Photo-Optical Instrument Engineers 120 2-9) in which the positions of alternating dichoptic images that produce minimal apparent motion were used to measure the relative tilt of corresponding meridia at a range of eccentricities up to +/- 16 deg away from the fovea. Stimuli were composed of dichoptic images, one containing a blank field and the other a pair of dots, which alternated at a rate of 0.63 Hz and the relative tilt (binocular orientation difference) between the pairs of dots presented to the two eyes was varied between +/- 11 degrees. Nonius lines were used to maintain vergence angle, which was varied between 28 cm and infinity. Subjects judged which pair of alternating images produced the smallest amount of apparent motion (position change). It was found that at all eccentricities examined the corresponding horizontal meridia were generally aligned but the corresponding vertical meridia were consistently offset (extorted) by about +/- 2 degrees. The tilts of corresponding principal meridia were typically unaffected when vergence angle was varied, indicating that little or no cyclovergence accompanied changes in horizontal vergence. The results suggest that the binocular correspondence system appears to be mapped by a horizontal shear distortion that extends to retinal locations at least as far as 16 deg away from the foveae. The invariant extortion of corresponding principal vertical meridia with vergence state is consistent with previous suggestions that the empirical vertical horopter becomes progressively inclined with respect to the vertical as viewing distance increases.

Depth Perception↗
PubMed2001

Of holes and wholes: the perception of surrounded regions.

Fully enclosed regions in a two-dimensional image can often be perceived either as an object in front of a surface or as a hole through a surface. Several experiments were conducted to determine what factors affect perception of holes versus objects. Three types of factors were tested and found to influence this outcome. First, depth factors directly indicate that the enclosed region lies behind its surrounding surface. Second, grouping factors relate the enclosed region to an outer region that is generally perceived as a continuation of the surface seen through the hole. Finally, figural factors indicate whether the enclosed region is to be perceived as figure or ground. Relations among these factors and their implications for perceptual organization are discussed.

Analysis of Variance↗
PubMed2006

Who owns the contour of a visual hole?

Holes are useful in the study of shape, contour curvature, and border ownership. Several authors have suggested that holes have figural or quasi-figural status. I discuss three criteria to test the evidence that holes behave more like figures than like ground: (i) holes perceived as such; (ii) similar performance for holes and figures; (iii) different performance for holes and other ground regions. Using these criteria, I review the literature and conclude that holes do not have figural status in relation to border ownership. I also argue that holes are ideal stimuli to study figure-ground organisation.

Contrast Sensitivity↗
PubMed1996

The development of a dummy head for 3-D audiovisual recording for transmitting telepresence.

Manufacturing systems that employ modern technology often deprive workers of their humanity. One such example is seen in teleconferencing between remote plants. Here, it is difficult for participants to make eye-to-eye contact, a major obstacle in the communication of the participants; intentions and emotions. Indeed participants often complain that the facial expressions or emotions of the other side are difficult to perceive, that it is difficult to carry on a friendly, warm conversation, or that it is difficult to convey one's intentions to the other. They thus tend to become irritated during these conferences, and at times make erroneous judgements. In such situations, it can be said that the system users are being deprived of their humanity. This occurs because a machine or computer, when placed between two people communicating, interferes with a natural communication. What measures are there then to achieve a natural communication in such a setting, with a machine or computer between those communicating? These methods are classified here. In this study, a system for communicating 'presence' was employed to achieve a natural communication. The prototype system is call a 'dummy head for 3-D audiovisual recording'. Using this dummy head, dialogue between operators, remote control, as well as the recording of a skilled worker's use of his hands were carried out, and the results were evaluated. From these results, the authors report on how the prototype system, when placed between humans, helps to achieve a natural communication. Finally, some examples are given to show the human need for achieving a natural communication in such interface situations.

Communication↗
PubMed1981

The role of visual and haptic cues in children's discrimination learning.

Three experiments using kindergarten boys and girls (Ns = 126, 84, and 72, respectively) tested the hypothesis that adding haptic to visual information facilitates discrimination of three-dimensional objects on more difficult (three-choice) problems, but not on easier (two-choice) problems. Kindergartners were given a two-or three-choice discrimination problem under one of four conditions of cue availability: visual cues, haptic cues, and visual plus haptic cues with stimuli touched or not touched before choosing. Addition of haptic cues did not improve performance either on two-choice problems (two or three experiments), as predicted, or on three-choice problems (all three experiments), contrary to predictions.

Child, Preschool↗
PubMed1985

Color association performances of dyslexic and normal children.

The performances of normal and dyslexic children were evaluated on a test of nonverbal color association. All performances by developmental dyslexics were well within normal limits. In addition, all normal children age 6 and above performed at normal adult levels in color association.

Adolescent↗
PubMed1990

Toward a cognitive neuropsychology of awareness: implicit knowledge and anosognosia.

Although a systematic cognitive neuropsychology of awareness has not yet emerged, a number of phenomena reported in the literature provide an empirical basis for developing it. The present discussion focusses on two such phenomena: implicit knowledge, which refers to knowledge that is expressed in task performance unintentionally and with little phenomenal awareness; and anosognosia, which refers to unawareness of neuropsychological deficits. Two types of theoretical accounts of these phenomena are discussed. A first order theoretical account entails postulating damage to, or disconnection of, a system or process that generates awareness across multiple domains. A second-order account does not postulate disruption of a cross-domain awareness mechanism, but instead appeals to difficulties in gaining access to particular kinds of domain-specific information that are associated with aware expressions of knowledge in individual domains. Instances of first- and second-order accounts are illustrated with examples from studies of memory and amnesia. The relation between implicit knowledge and anosognosia is also discussed.

Agnosia↗