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Biomedical subjects

B Gillam

Publications and source records attributed to B Gillam.

17 recordsLinked to original sources
PubMed1992

Perspective, orientation disparity, and anisotropy in stereoscopic slant perception.

Stereoscopic depth estimates are not predictable from the geometry of point disparities. The configural properties of surfaces (surface contours) may play an important role in determining, for example, slant responses to a disparity gradient, and the marked anisotropy in favour of slant around a horizontal axis. It has been argued that variation in slant magnitude are attributable to the degree of perspective conflict present and that anisotropy is attributable to orientation disparity, which varies with the axis of slant. Three experiments were conducted in which configural properties were varied to try and tease apart the respective roles of orientation disparity and conflicting perspective in determining stereoscopic slant perception and slant axis anisotropy. The results could not be accounted for by the magnitude of the orientation disparities present. Conflicting perspective cues appeared to play a role but only for slant around a vertical axis. It was concluded that there are important configural effects in stereopsis attributable neither to orientation disparity nor to perspective.

Anisotropy↗
PubMed1991

Orientation disparity, deformation, and stereoscopic slant perception.

Koenderink and van Doorn's theory, that the basis of stereoscopic slant perception is the deformation component of the disparity, field, was tested for slant around a horizontal axis, which produces images with a vertical ramp of horizontal disparity (horizontal shear) characterised by a global orientation disparity at the vertical meridian. The disparity field in this case can be parsed into two components, deformation and curl, which each contribute half of the orientation disparity. This case was compared with similar random-dot stimuli in which the deformation component was doubled and the curl component eliminated or vice versa. All three types of stimuli had identical orientation disparity at the vertical meridian. A condition in which there was no such orientation disparity, but deformation was present, was also included. It was found that perceived slant was not related to the deformation present, as Koenderink and van Doorn's theory would predict, but was predictable from the orientation disparity at the vertical meridian per se.

Depth Perception↗
PubMed1991

Motion capture by a frame: global or local processing?

The possibility that frames serve to capture lines within them so that they appear on a coplanar surface was investigated, using coherence in direction of rotary motion (Gillam, 1972) as a quantitative measure of the coplanarity of frame and internal lines. It was found that perceptual coherence between a pair of lines was greatly increased by surrounding them with a frame, if the frame was perspectivally consistent with the lines. A second experiment showed that this grouping can be attributed to a global effect of the frame and cannot be accounted for by local grouping of the internal lines with components of the frame.

Form Perception↗
PubMed1988

Postfusional latency in stereoscopic slant perception and the primitives of stereopsis.

Random dot stereograms of slanted surfaces were constructed, each representing one or two slanted surfaces in different relative arrangements and with different axes. Latency to fusion and from fusion to stereoscopic resolution was measured for each stimulus. It was found that latency to fusion was always very brief but that latency to stereoscopic resolution varied markedly, depending upon the orientation and arrangement of the stereoscopic surfaces. A gradient of discontinuities at a surface boundary produced an instant slant response for that surface, whereas a gradient of absolute disparities across the surface did not, except under conditions where vertical declination (a form of orientation disparity) was present. We conclude that stereopsis is not based on the primitives used in matching the images for fusion and that it is, at least initially, a response to disparity discontinuities which play no role in the fusion process. We also conclude that vertical declination is responded to globally as a slant around a horizontal axis but that other forms of orientation disparity are ineffective. The evidence from our experiments does not support the existence of a stereoscopic ability to respond globally to differences in magnification (or spatial frequency). It is suggested that stereoscopic perception of slant around a vertical axis is slow because it results from the integration of local processes.

Attention↗
PubMed1988

The role of monocular regions in stereoscopic displays.

Random-dot stereograms of an object standing out from a background always contain a monocular region at the side of the foreground object. This is equivalent to the monocularly occluded part of the background in the real-life viewing of one object in front of another. The role of these monocular regions in the stereoscopic process has not been investigated previously, although it is generally assumed that they are a source of difficulty in stereoscopic resolution because of the unmatchable texture within them. The basis of the present study was a prediction that the presence of texture within these regions would facilitate rather than retard stereoscopic processing. This prediction follows from a hypothesis that stereoscopic processing is initially located at disparity discontinuities. Unmatched regions are only found at such discontinuities, and could serve to locate them.

Attention↗
PubMed1984

Aggregation and unit formation in the perception of moving collinear lines.

The degree to which collinear lines are treated as a unit in resolving rotary motion in depth was investigated with the use of parallel projection to make direction of motion ambiguous. The proportion of time that the collinear lines appeared to rotate in the same direction was used as an index of their perceptual coherence. When the gap between the lines was small, there was strong grouping of the lines with respect to direction of motion as well as appearance of rigidity for the configuration. For larger gaps the grouping for direction of motion was maintained, but the lines appeared to have different axes of rotation leading to an appearance of nonrigidity. It is concluded that line elements can be aggregated for the resolution of certain properties without constituting a unit in any general sense.

Depth Perception↗
PubMed1981

Separation relative to length determines the organization of two lines into a unit.

The probability that two lines will form a perceptual unit, in the sense of reversing together under conditions of depth ambiguity, decreases as their separation is increased. In these studies, the critical separation for perceptual grouping is shown to be neither the retinal nor distal separation, but the ratio of separation to line length.

Depth Perception↗
PubMed1976

Grouping of multiple ambiguous contours: towards an understanding of surface perception.

Coplanar lines of different orientations rotating in depth and viewed in parallel projection may appear to rotate in different directions and reverse independently of each other, or they may appear coplanar and all reverse together. The frequency of the latter "grouped" appearance for lines with an orientation range of 30 degrees was a function of the number of lines in a given space. Grouping was greater for lines converging to an implicit vanishing point and for lines with collinear edges regardless of their orientations. Line length, and by inference, interline separation also had effect of the nature of the response.

Analysis of Variance↗