Points and Endpoints: A Size/Spacing Constraint for Dot Grouping

Abstract

One-dimensional arrangements of dots immediately group into contours. It is reported that, when these contours participate in certain larger arrangements, there is an abrupt point at which the percept changes as a function of dot spacing (or density along the contour). Closely spaced arrangements give rise to subjective effects involving apparent brightness and depth, whereas sparsely spaced ones do not. The effects are most clear in configurations that involve endpoints and possible occlusions. For these configurations, densely dotted contours are perceptually equivalent to solid ones, but sparse ones are not. This change in percept occurs abruptly and consistently at a dot to space ratio of 1:5, when the dot density is normalized by dot size, and this point is called the size/spacing constraint. It holds only for dots of the order of 1 min visual angle in diameter when small to modest contrast values are used. The subjective effects are not present for dotted contours (or even for solid ones) that are smaller (less than 0.5 min), and differ for contours that are larger (greater than 10 min). To demonstrate the significance of size/spacing constraints for early vision, a framework for grouping consisting of processes at many different levels is outlined, and the requirements for the earliest one (orientation selection) are sketched in greater detail. The size/spacing constraint follows directly from one of these requirements--receptive field structure--and seems to indicate a switch from early orientation-selection processes to later ones.

Psychophysical evidence for low-level processing of illusory contours and surfaces in the kanizsa square

“…The fact that threshold elevations are considerably lower or even absent when the target is located on the illusory harder shows that elevations recorded on either side of it cannot be explained by stray light. This result is compatible with phenomenal and psychophysical observations which suggest that the mechanisms underlying illusory contour perception are essentially determined by the collinearity and proximity of parts and not by their luminance (Prazdny, 1983;Zucker, Stevens & Sander, 1983;Zucker & Davis, 1988). The determining influence of collinearity on the activity of orientation-selective cells has been demonstrated in recent electrophysiological studies (cf.…”

Visual Perception in Japanese Rock Garden Design

“…For example, the formation of boundary contours, and hence image segmentation, is affected by the ratio of visual cue size to the spatial distances between them. Zucker and Davis (1988) found a salient change in contour perception at a cue size: empty space ratio of about 1:5. More sparsely sampled contours fail to generate various classical illusions associated with the reconstruction of incomplete boundary contours.…”

Glass Patterns: Some Contrast Effects Re-Evaluated

“…If the correlated elements comprising a Glass pattern are of opposite contrast polarity then, it is claimed, orientation structure cannot be perceived (Glass and Switkes 1976;Prazdny 1986;Zucker and Davis 1988;Zucker et al 1983). However, Kovacs and Julesz (1992) presented psychophysical data showing that, for dense Glass patterns at least, perceived flow is not destroyed but rotated through 90°.…”