The relation between cognitive-perceptual schizotypal traits and the Ebbinghaus size-illusion is mediated by judgment time

Abstract

In the Ebbinghaus illusion, a circle surrounded by smaller circles is perceived as larger than an identical one surrounded by larger circles. The illusion is reportedly weaker in individuals with (disorganized) schizophrenia or schizotypy than in controls, a finding that has been interpreted as evidence that both schizophrenia and schizotypy involve reduced contextual integration. In support of this view, we show that the Ebbinghaus illusion also decreases, in the general population, with cognitive-perceptual schizotypal traits (measured with both the cognitive-perceptual subscale of the Schizotypal Personality Questionnaire-Brief and the Magical Ideation scale). Our results were strong and separately replicable in different within-subjects and between-subjects conditions. However, a mediation analysis revealed that the reduction of the Ebbinghaus illusion was (statistically, hence without implying a causal relationship) entirely due to increased judgment time, i.e., the time subjects took to complete size comparisons. Judgment time increased with the strength of cognitive-perceptual schizotypal traits, but subjects with longer judgment times had smaller illusions regardless of these traits. We argue that there are at least two possible accounts of our results. Reduced contextual integration might be due to a reduced ability to integrate context, as previously suggested; alternatively, it could be due to a reduced tendency to integrate context-that is, to a detail-oriented processing style. We offer predictions for future research, testable with a deadline experiment that pits these two accounts against one another. Regardless of which account proves to be best, our results show that contextual integration decreases with cognitive-perceptual schizotypal traits, and that this relationship is mediated by judgment time. Future studies should thus consider either manipulating or measuring this time.

Keywords: Ebbinghaus illusion; contextual integration; magical ideation; schizophrenia; schizotypy.

Figure 1

Ebbinghaus illusion. The central disk on the left appears smaller than the physically identical one on the right. (Similar effects are obtained with circles instead of disks.)

Figure 2

Results. Ebbinghaus-illusion magnitude plotted against log-transformed judgment time. Subjects with an SPQB-CP score of 0 are represented in white; those with an SPQB-CP score larger than 0 are represented in black. The regression line is a fit to all data points regardless of SPQB-CP score (i.e., regardless of symbol color). Note that individuals with a positive SPQB-CP score tend to be less susceptible to the Ebbinghaus illusion, but to have a longer judgment time, and that regardless of SPQB-CP score, individuals with a longer judgment time tend to be less susceptible to the Ebbinghaus illusion.