Psychological Science XX(X) 1–5 © The Author(s) 2009 Reprints and permission: http://www .sagepub.com/journalsPermissions.nav DOI: 10.1177/0956797609354735 http://pss.sagepub.com Causal Contraction: Spatial Binding in the Perception of Collision Events Marc J. Buehner and Gruffydd R. Humphreys Cardiff University Abstract Causality is a higher-level mental construct derived from low-level percepts such as contiguity in space–time. We show that low-level spatial perception is distorted by the presence of a causal connection, such that two objects appear closer in space when they are causally linked than when they are not. This finding parallels recent demonstrations of temporal causal binding and suggests that causality is at the root of a general ambiguity-resolution process operating on the human perceptual system. Keywords perceptual causality, binding, spatial perception, intentionality Received 4/23/09; Revision accepted 6/12/09 Research Report Time and space are modes by which we think and not condi- tions in which we live (Albert Einstein, as quoted in Forsee, 1963, p. 81) Einstein’s theory of relativity is not often evoked in psychol- ogy, despite decades of research demonstrating that human time perception is subject to situation-dependent biases (for overviews, see Eagleman, 2008; Fraisse, 1984), and despite textbooks full of visual illusions illustrating that space percep- tion is prone to context-induced distortions (e.g., Snowden, Thompson, & Troscianko, 2006). Perhaps one reason why Einstein’s conjectures are not prominent in psychology is that the analogy is imperfect. Whereas for Einstein, the relativity of time and space is conceptually related, visual and temporal illusions in psychology are not. Here we report a new visual illusion—a subjective contraction of space—that bears a 1:1 mapping to a recent, but now well-established, distortion of time perception. We demonstrate that objects forming a causal event sequence appear closer in space relative to objects involved in noncausal sequences. These results correspond perfectly to recent reports of causality-induced time contrac- tion (Buehner & Humphreys, 2009; Haggard, Clark, & Kalogeras, 2002; Humphreys & Buehner, in press) and suggest that the higher-level concept of causality has a pro- found influence on humans’ perception of both space and time. Causality-Induced Time Compression Temporal binding of causes to their effects (Eagleman & Hol- combe, 2002; Haggard et al., 2002) refers to simultaneous shifts in conscious awareness that result in instrumental actions being perceived as later compared with noninstrumental actions, and in target events resultant from instrumental actions being perceived as earlier relative to control events: Causes and effects mutually attract each other in subjective time. This effect is robust, has been demonstrated with a variety of meth- ods over a considerable interval range, and affects not only event awareness, but also perception of intervals as well as motor planning reliant on it (Buehner & Humphreys, 2009; Humphreys & Buehner, in press). The most comprehensive explanation of the phenomenon so far rests on a bidirectional Bayesian interpretation of Hume’s tenets of causality: Contin- gency and contiguity license causal inference; according to Bayes’ theorem, causally related events are therefore more likely than non-causally related events to be contiguous in space and time. Because temporal perception is inherently uncertain, systematic biases arise, leading to causality-induced compressions of subjective time (Eagleman & Holcombe, 2002). An alternative interpretation relies on forward models of motor command and hypothesizes the formation of associa- tions between instrumental actions and their goals (Haggard et al., 2002), limiting the phenomenon to intentional actions. Our own research has demonstrated that intentional action is not sufficient to produce temporal binding, and has confirmed that causality is the critical trigger (Buehner & Humphreys, 2009). Corresponding Author: Marc J. Buehner, School of Psychology, Cardiff University, Park Place, Cardiff CF10 3AT, Wales, United Kingdom E-mail: buehnerm@cardiff.ac.uk