Taking aim at the Müller-Lyer goalkeeper illusion: an illusion bias in action that originates from the target not being optically specified.

Van der Kamp and Masters (2008) reported that goalkeeper postures that mimic the Müller-Lyer (1889) illusion affect the location of handball penalty throws. In four experiments, we aimed to verify that the effects on throwing are consistent with an illusory bias (Experiments 1 and 2), and to examine how these observations can be understood in the context of Milner and Goodale's (1995, 2008) two-visual systems model (Experiments 3 and 4). Experiments 1 and 2 confirmed that the goalkeeper Müller-Lyer posture may indeed induce an illusory bias in throwing, implying that allocentric information is used in far-aiming action tasks. Experiment 3 demonstrated that the bias was not related to a participant's throwing skill. Experiment 4 suggested that an absence of visual information to instantaneously specify target location may have induced use of context-dependent allocentric information, causing the throwing bias. The results are discussed in the context of recent debates about the roles of the two-visual systems in perception and action. It is suggested that the two systems are first and foremost perceptual systems that serve the pickup of different sources of information.