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The authors investigated whether the early or later stages of closed-loop (CL) and open-loop (OL) grasping movements were differentially influenced by the Müller-Lyer (ML) illusion. Participants (N = 21) reached out and grasped small (5 cm) and large (7 cm) objects embedded within fins-in and fins-out ML configurations. Grasping time (GT) was normalized, and absolute grip aperture (GA) as well as scaled illusion effects were computed at 20%, 40%, 60%, and 80% of GT. The results indicated that CL trials were refractory to the illusory array (i.e., from 20% to 80% of GT), whereas OL trials were influenced by the ML figure during that same time. Those findings suggest that CL trials were supported by unitary and metrical visual information, whereas OL trials were entirely supported by perception-based visual information.

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The goal of the present investigation was to determine the time-course by which the motor system might resolve the context-dependent effects of a visual illusion [i.e., the Müller-Lyer (ML) figure]. Specifically, we asked participants to scale their grip aperture (GA) to the perceived size of an object embedded within a ML figure in advance of closed-loop (CL) and open-loop (OL) grasping movements. As a result, premovement GA was biased in a direction consistent with the perceptual effects of the illusion. We reasoned that such a manipulation might provide a novel opportunity to determine whether the motor system is able to resolve a biased GA immediately following the onset of a response [i.e., in accord with the perception/action model (PAM); Milner and Goodale 1995, The visual brain in action, Oxford University Press], or gradually as the action unfolds [i.e., in accord with the planning/control model (PCM); Glover and Dixon 2002, Percept Psychophys 64:266-278]. It was found that biasing GA in advance of movement resulted in a reliable effect of the ML figure throughout CL and OL trials (i.e., up to 80% of grasping time). Although the present findings appear contrary to the theoretical tenets of the PAM and the PCM, it is proposed that biasing GA in advance of movement leads to offline visual processing and a feedforward mode of grasping control, thus accounting for the illusion-induced effect throughout the grasping response.

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