When does the Titchener Circles illusion exert an effect on grasping? Two- and three-dimensional targets

This study used the Titchener Circles illusion to investigate the functional dissociation of the dorsal visuomotor and ventral perceptual systems. In order to investigate the visual requirements for an action to be driven by the dorsal stream, two-dimensional (2D) and three-dimensional (3D) targets were compared. Thirteen subjects made visual open loop manual estimations or grasping actions towards 2D and 3D versions of the illusion. No illusion effect was found for immediate grasping, but the illusion did influence manual estimation, irrespective of whether the target was 2D or 3D. It is suggested that the underlying representations used to drive grasping actions towards 2D targets are fundamentally the same as those used to drive natural grasping actions. While stereoscopically specified depth may be important to the dorsal visuomotor system, it does not appear to be necessary in order for an action to be based on dorsal representations. It is suggested that an action must be goal directed in order for that action to be driven by dorsal stream processes. (C) 2003 Elsevier Science Ltd. All rights reserved.