Abstract
Two ways of theorizing may be distinguished in the explanation of the concentric circular illusion. The first tries to explain the illusion in terms of spatial interaction of the two contours forming the pattern, while the second in terms of the “figure-ground” relationship. The purpose of this article is to point out some faults of the first and to present some experimental evidence supporting the second point of view.
In Experiment Ia, the distances from the observer of the three stimulus fields, i.e., the field outside the outer circle, that between the inner and the outer circles, and that inside the inner circle, were systematically varied, while the two circular contours were always presented on a plane at fixed distances from the observer (see Figs. 1 and 2). Thus, with the “figure-ground” or the “object” character of the whole pattern was modified, with the spatial relationship of the two contours being kept constant. The results showed that the amount of the illusion, measured with the usual method of limits, is not solely a function of the distance between the two contours (or the size ratio of the two circles) but depends on the so-called “object” character of the pattern. It was also shown that this was not due to the effect of accommodation or textural differences (Experiment Ib, c, d).
In Experiment IIa, a modified method of magnitude estimation was introduced, and the “largeness” of the control circle was, estimated. The results suggested that the observer's judgement of “largeness” in many of the experiment of the concentric circular illusion must have been made in terms of “area” bounded by the contours, not in terms of the “distance between the two contours” or of “displacement of a contour”. The results of Experiment IIb, using the square comparison stimuli and the method of limits, supported this “area” concept again.
