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At least 523 records · Page 29Linked to original sources
PubMed1988

Perception of shape from shading.

The human visual system can rapidly and accurately derive the three-dimensional orientation of surfaces by using variations in image intensity alone. This ability to perceive shape from shading is one of the most important yet poorly understood aspects of human vision. Here we present several findings which may help reveal computational mechanisms underlying this ability. First, we find that perception of shape from shading is a global operation which assumes that there is only one light source illuminating the entire visual image. This implies that if two identical objects are viewed simultaneously and illuminated from different angles, then we would be able to perceive three-dimensional shape accurately in only one of them at a time. Second, three-dimensional shapes that are defined exclusively by shading can provide tokens for the perception of apparent motion, suggesting that the motion mechanism is remarkably versatile in the kinds of inputs it can use. Lastly, the occluding edges which delineate an object from its background can also powerfully influence the perception of three-dimensional shape from shading.

Depth Perception↗
PubMed1991

Vertical disparities and perception of three-dimensional shape.

The information about depth and three-dimensional shape available from the horizontal component of the stereo disparity field requires interpretation in conjunction with information about egocentric viewing distance (D). A novel computational approach for estimating D was proposed by Mayhew and Longuet-Higgins, who demonstrated that the horizontal gradient of vertical disparities uniquely specifies the viewing distance. We have now used random dot stereograms in a shape judgement task to show that changes in vertical disparities have no effect on perceived three-dimensional shape. Changes in ocular convergence do alter perceived shape, suggesting substantial changes in the subjects' scaling of horizontal disparities. We conclude that vertical disparities are not used to scale disparities for viewing distance, and that extraretinal signals must be considered when analysing human three-dimensional shape perception.

Depth Perception↗
PubMed2002

Visual information process in Williams syndrome: intact motion detection accompanied by typical visuospatial dysfunctions.

It has been suggested that visuospatial cognitive disabilities seen in children with Williams syndrome (WMS) are related to a dysfunction of the dorsal stream in the visual information analysis system. We investigated whether visual motion detection is also impaired in WMS because it is one of the main functions of the dorsal stream. Using various psychophysical examinations and magnetoencephalography, we studied a child with WMS who had the typical features of the syndrome. We found profound impairments in the visuospatial cognitions, as previously reported in WMS. In contrast, he had normal ability for the direction discrimination of coherent motion on a background of randomly moving dots, and he perceived apparent motion as do normal children. Furthermore, the latencies of both responses to the coherent and incoherent motions as measured by magnetoencephalography were within the mean +/- 2 SD among normal adults and the estimated origins were near the human homologue of V5/MT (visual area 5/middle temporal area). The results indicate that the visuospatial cognitive deficits in WMS can occur without impairment of the visual motion detection. We consider that the deficits are caused by a restricted dysfunction of the neural groups for position and three-dimensional form perceptions in the dorsal stream of the visual system, though other possibilities are not excluded.

Brain Mapping↗
PubMed2001

Peer review of classroom teaching: an interim report.

This paper provides interim findings from the implementation of peer review of classroom teaching in a large UK University School of Nursing, Midwifery and Health Visiting. A total of 258 classroom teaching sessions, involving 129 teaching staff (n=129) were included over a period of 7 months. Teachers had two teaching sessions reviewed by a colleague. One reviewer was self-nominated and the Dean of School allocated the other. The paper is in two parts. Part one includes a short literature review followed by a descriptive account of how peer review was implemented. Part two focuses upon individual experiences from a small scale study with data obtained from 14 teachers (n=14) who took part in qualitative interviews based on their experiences of being both reviewer and reviewee. The larger study gathered data from several sources. This paper presents the initial findings from interview data with nurse teachers. Their perceptions form the basis of a discussion, which raises issues related to the experience of both implementing and being involved in, peer review of classroom teaching. Four themes emerged from the interview transcriptions; peer review was initially perceived as a threat; the experience of peer review was a positive one; teachers felt that being a reviewer was an interesting and worthwhile experience and perceived peer review positively when good relations existed between them and their reviewer.

Attitude↗
PubMed1975

Monocular and binocular rivalry between contours.

The temporal characteristics of binocular and monocular rivalry between orthogonal gratings of the same or complementary colours were investigated. Rivalry was measured in terms of the dominance of either grating or the visibility of composites comprised of parts of both gratings. The total duration for which either grating was dominant was significantly longer in binocular rivalry between gratings of complementary colours. A comparison of binocular and monocular rivalry indicated considerable phenomenal differences between them. Dominance in binocular rivalry corresponds to the visibility of one grating alone; this occurs rarely in monocular rivalry, which is characterized by fluctuations in the distinctiveness of the gratings. The changes in distinctiveness are influenced by colour in a similar manner to that in binocular rivalry, and the frequencies of fluctuations are higher for gratings of complementary colours.

Color Perception↗