Perceptual organization is thought to involve an analysis of both textural discontinuities and perceptual grouping. In earlier work, we found that textural discontinuities were detected normally even when visual attention was engaged elsewhere. Here we report how perceptual grouping is affected when visual attention is engaged by a concurrent visual task. To elicit perceptual grouping, we used the Gestalt demonstrations of grouping on the basis of proximity and of similarity. Four tasks were investigated, some requiring the observer to discriminate between horizontal and vertical grouping, and some requiring the observer to merely detect the presence or absence of grouping. Visual attention was engaged at the center of the display by a form identification task. The detection of a textural discontinuity served as a control task. Concurrent form identification conflicted with all four grouping tasks, resulting in a significant reduction of grouping performance in each case. No performance reduction was observed when either form identification or grouping discrimination was combined with the detection of a textural discontinuity. These results suggest that perceptual grouping and form identification compete for visual attention, whereas the detection of a textural discontinuity does not.

A model for perceptual grouping based on measurements of spatial correlations is proposed and tested. Gestalt-like grouping experiments were performed to study and quantify the effect of element similarity (shape, luminance) and proximity. Observers reported the horizontal or vertical organization of stimuli with proximity and similarity providing conflicting grouping cues. Proximity grouping was found to be perceived much faster than similarity grouping. However, with increasing processing time, similarity was found to dominate grouping. The experimental results can be accounted for by assuming a process that compares horizontal and vertical intensity autocorrelations. The model suggests that correlations are measured across a limited spatial range, and that this range increases with processing time.

Using this simple method and the function provided, the eroded area of the anterior part of the glenoid in anterior glenohumeral instability can be calculated preoperatively using a 3D CT reconstruction of the glenoid with "subtraction" of the humeral head, obviating the need for sophisticated software to obtain this critical information for preoperative decision making.

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A translating homogeneous edge viewed through an aperture is an ambiguous stimulus, while a translating edge discontinuity is unambiguous. Under what conditions does the visual system use unambiguous velocity estimates to interpret ambiguous velocity estimates? We considered a translating rectangle visible through a set of stationary apertures. One aperture displayed a rectangle edge while the other apertures displayed corners. Observers reported the direction in which the edge appeared to translate. The results suggest that collinearity and terminator proximity determine whether the unambiguous corner velocity was used to interpret the ambiguous edge velocity. These results suggest some of the ways in which the visual system controls the integration of velocity estimates across image space.