This study was designed to assess the effects of symmetry and plane of presentation on the determination of the perceptual center of flat figures. Experiment 1 demonstrates the existence of effects in improving center determination, both in the number of sides of the shape and in rotational and reflective symmetry (confounded in the experiment). Experiment 2 shows that the presentation plane has no effect on center determination. In Experiment 3, we divide the effects of the two symmetry types, showing that rotational symmetry alone is as effective as the presence of both symmetry types--that is, the presence of symmetry axes is not very useful in finding perceived centers.

An attempt is made to develop a general model of perceptual-centre determination. The model describes the possible 'perceptual operations' involved in determining the perceptual centre of flat figures. The specific steps in the model are: (i) determining the orientation and size of the figure; (ii) determining the 'base' of the figure and its 'top-bottom axis'. It is suggested that anisotropy along the main axes of the figures determines a displacement of the perceptual centre toward the phenomenal top of the shape. A quantitative measure for the displacement of the perceptual centre from the area barycentre of the figure is introduced.

In 4 experiments it was found that global frameworks and local distinctive figural characteristics influenced the perception of shape and of pointing. In Experiments 1 and 2, Ss were asked to mark the center of the middle figure in arrays of 3 aligned figures (either triangles or squares). Displacements of the center indicated a perceived deformation of the shapes. In Experiment 3, Ss were asked to adjust the height of triangles in arrays similar to those in Experiment 1. Height adjustments made along the axis of apparent pointing resulted in extents that were shorter than the height necessary to form equilateral triangles. In Experiment 4, stimuli were isosceles triangles in which the apparent distortion had been nulled; however, Ss continued to perceive them as pointing in a direction consistent with their alignment, and hence shape distortion cannot be the cause of pointing.

An analysis is presented of a phenomenological model of illusory contours. The model is based on amodal completion as the primary factor giving rise to the illusory figure. In the experiment, conducted by the method of paired comparisons, the same parameter was manipulated in two series of equivalent configurations. The first series yielded examples of amodal completion, the second examples of illusory figures. Three groups of subjects evaluated the magnitude of completion, the brightness contrast of the illusory figure, and the contour clarity of the illusory figure. A control experiment was conducted, which demonstrated that in these configurations amodal completion and amodal continuation behave in the same way. Line displacement did not influence the brightness or the contour clarity of the illusory figures, though it influenced the magnitude of amodal completion. These results are in agreement with the energetic model developed by Sambin.

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