The effect of Muller-Lyer illusions on far aiming tasks

Abstract

Milner and Goodale (1995) proposed two neuro-anatomical separate visual systems: the dorsal system is associated with the implicit control of action whereas the ventral system is involved in perception. The latter operates both implicitly and explicitly. By and large the behavioral evidence for this distinction stems from relatively simple laboratory tasks such as pointing and grasping. Recently, we have advocated the generalization of the two-visual systems model to more complex sport tasks (Van der Kamp, Rivas, Van Doorn & Savelsbergh, 2007). In this paper, we will present data on perception and action in far aiming tasks like shuffle boarding, golf putting and the free throw in handball. From the perspective of the two-visual system model, effects of visual illusions flag the contribution of the consciously accessible ventral system. Participants are instructed to throw, putt or shuffle a projectile to a target that is embedded in one of different variants of the Müller-Lyer illusion. For instance, when a goalkeeper raises her or his arms diagonally she or he assumes a posture that resembles a ‘fins-out’ Müller-Lyer configuration. We show that by assuming this posture, the goalkeeper looks bigger and report the effects of this illusion on the accuracy of the handball free throw. We also ask whether the illusion bias is dependent on skill level.