EFFECTS OF SPATIAL CONFIGURATION AND NUMBER OF FIXATIONS ON KANIZSA TRIANGLE DETECTION

Purpose. Illusory figures, created by the visual system between visual izing real objects, are probably caused by processes designed to segre gate objects from background. Support ratio-that is, the ratio between the physically specified and total triangle side length-has been sugg ested to be the main spatial determinant for suprathreshold perception of a Kanizsa-type illusion. To test this scale invariance hypothesis at threshold, illusory figure perception was studied by determining th e effects of inducer size and distance at various exposure durations a nd fixation strategies on the frequency of seeing (FoS) an illusory Ka nizsa triangle. Methods. The effect of various support ratios was stud ied in the first experiment by varying the intercenter distance betwee n constant-size inducers viewed at various distances. In the second ex periment, the effects of various exposure durations and fixation strat egies were investigated; and the third experiment repeated the second one, with backward masking to control the processing time. In the four th experiment, the magnification of the stimulus configuration was var ied, with a support ratio that had yielded 100% FoS in the first exper iment, to study the range of scale invariance in illusory figure perce ption. Results. The support ratio was the main determinant for the per ception of an illusory figure at various inducer sizes, exposure durat ions, and masking conditions when fixation was steady; FoS always incr eased from 0% to 100% with the support ratio of 0.30 to 0.37. However, free viewing, with and without masking, resulted in 100% illusory fig ure perception at all support ratios tested. Furthermore, when fixatio n was steady and support ratio and exposure duration were held constan t, stimulus magnification reduced FoS from 100% to 0% at the smallest and largest stimulus sizes. Conclusions. The support ratio seems to be the main spatial determinant for illusory figure perception. However, scale invariance in Kanizsa triangle perception broke down in the sma llest and largest configurations, probably because of the limitations of visual acuity and spatial integration, respectively. Integration of information from several fixations enhances FoS at small support rati os, emphasizing the importance of the binding process between separate fixations for illusory figure perception.