Journal of Experimental Psychology: Human Perception and Performance 1978, Vol. 4, No. 1, 101-111 Perceptual Processes That May Create Stick Figures and Balance Josef Psotka University of Waterloo, Waterloo, Canada A series of experiments looked at the pattern of dot placements created when people individually placed a single dot inside empty outline figures. The superimposed results were found to compare better with the predictions of Blum's grassfire model than with a size-constancy process. The pattern of superimposed dots conformed to the stick figure of the outlined contours. A perceptual model based on spatial harmonic analysis of complex visual scenes is offered as an explanation for the pattern of results. Gestalt psychologists emphasize the im- portance of structural interactions within perceptual figures to create new, different, and influential global properties of the con- figuration. They also suggest that these structural properties arising from physio- logical processes resemble characteristics of dynamic fields (Koehler, 1972). Evidence in support of the existence of cortical, iso- morphic field processes has come from fig- ural aftereffects (Gibson, 1933; Koehler & Wallach, 1944), illusions (Sickles, 1942; Yokose & Ichikawa, 1953), psychophysio- logical investigations (Barrett, 1969), and work on pattern recognition (Motokawa, 1950; Yokose, 1970). The field-theoretic basis of this work has led to the prediction of emergent structures -forming inside outlined figures. McCulloch (1965, p. 314) argued that field processes could sweep through the cortex and run through a group of contractions and dilata- tions of a figure. If the figure was a square, the corners would tend to fill in the di- agonals, which would dominate the trans- This research was supported by National In- stitute of Mental Health Grant MH 14229 to W. R. Garner at Yale University, Grant 037-7137 from the University of Waterloo to the author, and National Research Council of Canada Grant A0294. Requests for reprints should be sent to Joseph Psotka, Department of Psychology, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1. form. This process would also allow for the perception of size constancy. For a rec- tangle, it would produce a set of concentric rectangles of the same shape inside the perceived rectangle, again producing a trans- form dominated by the diagonals, as in- dicated in Figure 1. The diagonals are not marked in this figure, but emerge as cog- nitive contours connecting the corners. A somewhat different theoretical field process has been proposed by Blum (1973). By devising a geometry based on the point and disk, he was able to describe configura- tions in terms of symmetric axes: the locus of the center of the largest disks that fit in- side a figure. For tutorial purposes, Blum has provided an engaging physical model of the process based on grassfires. If we take any outlined figure and simultaneously light a fire all along its edges, the fire will burn away from the edge and quench itself at all points along the symmetry axes of the form. These quench points will fall along the bi- sector of any angle on the outline of the figure if the interior angle is less than 180 degrees. The quench points will also begin at the interior focus of any arc on the out- line. This system's predictions for the form of interior structures are easy to work out for any outline figure. The interior struc- tures look like skeletons or stick figures. The symmetry axis, or stick figure, of a circle is just the center of the circle; of a triangle, the bisectors of the angles; of a Copyright 1978 by the American Psychological Association, Inc. All rights of reproduction in any form reserved. 101