Better discrimination for illusory than for occluded perceptual completions

Abstract

We applied the thin-fat Kanizsa shape discrimination task invented by D. L. Ringach and R. Shapley (1996) to study perceptual completion by measuring whether the discrimination was more accurate for illusory than for occluded shapes. Differently from Ringach and Shapley, we tested naive observers with stereoscopic displays. Discrimination was consistently more accurate for illusory than for occluded shapes under a variety of stimulus conditions. However, the absolute performance was worse than Ringach and Shapley's experienced observers, who discriminated illusory and occluded shapes equally well. When our naive observers were trained, their performance approached that in Ringach and Shapley, and their performance difference diminished between the illusory and occluded. The more precise discrimination of the illusory shapes by untrained observers is consistent with the subjective impression that illusory contours appear clearer and positionally better defined. This makes sense from the perspective of Bayesian decision theory: the location of an illusory contour that is closer to an observer might be more important than an occluded contour, and hence obligatorily represented more precisely. We conclude the paper by discussing implications of our results on the current debate on mechanisms of perceptual completion (M. K. Albert, 2007; B. L. Anderson, 2007; P. J. Kellman, P. Garrigan, T. F. Shipley, & B. P. Keane, 2007).

Late, not early, stages of Kanizsa shape perception are compromised in schizophrenia

“…The trial presentation sequence (Figure 1B) was similar to earlier studies (Keane, Lu, et al, 2012; Ringach & Shapley, 1996; Zhou, et al, 2008) and consisted of a 1000 ms black screen, a 200 ms target presentation, a 50 ms uniform black screen, and 300 ms mask (to cap stimulus processing time). Another black screen would linger until a response, after which an auditory beep sounded for a correct answer.…”

“…In each half of the experiment, there were 64 practice trials and 84 non-practice trials, the latter half of which presented distractor lines. This number of practice trials—which was slightly smaller than what has been used in other studies (Keane, Lu, et al, 2012; Zhou, et al, 2008)—allowed subjects to get acclimated to the fast presentation times and slight orientation differences that they would be seeing for the rest of the experiment (see below). The first non-practice trial and the first distractor line trial in a block were excluded for the purposes of threshold estimation since such trials were more often missed by observers.…”

“…On each trial of our experiment, subjects discriminated the orientations of four sectored circles or pac-men (Gold, Murray, Bennett, & Sekuler, 2000; Gold & Shubel, 2006; Guttman & Kellman, 2004; Keane, et al, 2007; R. F. Murray, Sekuler, & Bennett, 2001; Pillow & Rubin, 2002; Ringach & Shapley, 1996; Zhou, Tjan, Zhou, & Liu, 2008). In the illusory condition, the sectored circles jointly formed a Kanizsa square, and subjects decided whether the elements formed a fat or thin shape (see Figure 1A).…”

“…(These two conditions have sometimes been described as differing by a geometric property, “relatability”, which governs when elements can and cannot connect via interpolation (Kellman & Shipley, 1991). ) Half of the trials involved distractor lines, which disrupt illusory contour formation and worsen illusory shape discrimination (Dillenburger & Roe, 2010; Keane, Lu, Papathomas, Silverstein, & Kellman, 2012, 2013; Ringach & Shapley, 1996; Zhou, et al, 2008). Task difficulty depended on pac-man rotational magnitude, with larger rotations making the alternatives easier to distinguish.…”

“…The underlying assumption was that the more that subjects fill-in illusory contours, the more that distractors would impair discrimination in the illusory relative to the fragmented condition. This second metric was chosen because others have shown that distractor lines near the edges of Kanizsa shapes worsen illusory shape perception, but exercise little, if any effect when illusory contours are blocked (Keane, Lu, et al, 2012, 2013; Ringach & Shapley, 1996; Zhou, et al, 2008). Reverse correlation and other studies have also revealed filling-in by examining behavioral or neural responses to line segments or luminance noise placed near filled-in boundaries (Dillenburger & Roe, 2010; Dresp & Bonnet, 1991; Dresp & Grossberg, 1997, 1999; Gold, et al, 2000; Gold & Shubel, 2006; Keane, et al, 2007; Keane, Lu, et al, 2013).…”