Grouping and Occlusion in Perception and Art

In the first part of the lecture several meanings of the term “perceptual grouping” as it applies to contour and texture will be distinguished, especially grouping as segregation and grouping as cohesion. This will be expanded to a discussion of the segregation of surfaces in depth and the occlusion cues on which this is based, which also underlie the perception of “subjective contours”. Special attention will be given to the way pictorial cues interact with binocular stereopsis to influence perceived surface occlusion. Finally, the depiction of occlusion will be discussed as a major feature of art, with support for Kanizsa’s view that this is a perceptual not a conceptual achievement. As such I show that surface segregation can fail perceptually even when the conceptual intention is clear

Failure to detect changes in color for lines rotating in depth: the effects of grouping and type of color change

AbstractA new technique for measuring change detection was introduced in which contours rotating in depth around a vertical axis (in a computer display) could be altered in color as they passed through their point of minimum extension (the median plane) where a thin static vertical occluder hid the change. Sets of five or six contours were either strongly grouped (similar in length, orientation and spacing) or weakly grouped (of variable length, orientation and spacing). Changes consisted of one line changing to a new color or else two lines swapping colors. The measure was the proportion of missed changes. When subjects were not instructed to look for change almost no changes were reported although subjects were told beforehand that they would have to describe the configuration after viewing it. When subjects were instructed to look for changes, it was found that detection of color change was significantly better for strongly grouped lines. It is proposed that grouping, by reducing redundancy, also reduces attentional demands with respect to the properties on which it is based, making it easier to attend to and therefore detect changes in other properties. We found that it was much easier to detect the introduction of a new color than to detect a swap between two existing colors. It is hypothesized that swap-type changes were harder to detect because they required attention to a conjunction of position and color

Enhancing PhD Preparation through Shared Ideas across CSD Program

Shortages of PhD graduates to fill CSD faculty positions have been a concern for over 15 years. The ASHA AAB (collaborating with CAPCSD) completed interviews of Coordinators of almost all 76 CSD PhD Programs. Results and successful aspects of the programs will be highlighted to stimulate discussion among participants

Geometrical Features Underlying the Perception of Colinearity

The magnitude of the Poggendorff bias in perceived collinearity was measured with a 2AFC task and roving pedestal, and was found to be in the region of 6–8 deg, within the range of previous estimates. Further measurements dissected the bias into several components: (1) The small (∼1 deg) repulsion of the orientation of the pointer from the parallel, probably localized in the part of the line near the intersection (2) A small (1 deg) bias in the orientation of virtual lines crossing the gap between two parallels, towards the orientation of the parallels, or equivalently (4) An orthogonal bias in actively constructing a virtual line across the gap. We conclude that orientation repulsion by itself is an inadequate explanation of the Poggendorff effect, and that a full explanation must take account of the way in which observers construct virtual lines in visual space in order to carry out elementary geometrical tasks such as extrapolation