Visual illusions influence proceduralized sports performance.

Does the Ebbinghaus visual illusion really influence sports performances? Does the influence depend on the type of knowledge (procedural vs. declarative) that guides movement? To address these questions, we evaluated the knowledge hypothesis, a novel hypothesis according to which the more sports performance relies on procedural knowledge, the more it will be influenced by visual illusions. In the context of golf putting, we first used the high-error/low-error motor-learning technique (Experiment 1) or varied the number of practice trials (Experiment 2) to induce novice participants to rely more on procedural knowledge than on declarative knowledge (or vice versa). We then manipulated the perceived size of two golf holes by projecting a ring of small or large circles around them, which caused the holes to appear larger or smaller, respectively. This Ebbinghaus visual illusion had an influence on putting in both experiments. We also observed a pattern of findings consistent with the knowledge hypothesis: the procedural groups were moderately influenced by the illusion when putting, but the declarative groups were influenced only weakly, at best. Among the participants most sensitive to the illusion, the analyses confirmed a significantly stronger influence for the procedural group. Overall, these findings demonstrate that the effect of visual illusions on sports performance is a reliable phenomenon for proceduralized actions. The knowledge hypothesis represents an attractive way of reconciling earlier divergent findings.