Illusions in action: consequences of inconsistent processing of spatial attributes

The Brentano illusion influences goal-directed movements of the left and right hand to the same extent

“…Gonzalez et al (2006) found a larger illusion eVect on the peak grip aperture during precision grasping movements performed with the left hand, regardless of the subject's handedness. Milner and Goodale (2006) have argued that this Wnding challenges the idea of the presence of dissociation between position and extent, rather than between perception and action, as an explanation for the illusion eVects on various tasks Brenner 1999, 2001;Smeets et al 2002). As argued in the introduction, the results of Gonzalez et al (2006) can easily be explained by the dissociation between position and extent.…”

Attenuating the haptic horizontal—vertical curvature illusion

“…However, the methods used are not all that different from those used in other research for the study of illusions (Amazeen & DaSilva, 2005) and have advantages over the use of a fingers posture for size estimates (e.g., Heller et al, 1997;Heller, Joyner, & Dan-Fodio, 1993;Smeets, Brenner, de Grave, & Cuijpers, 2002). The rulers were in different spatial locations than the extents being judged, and conceivably, this could prompt systematic errors.…”

Grasping reveals visual misjudgements of shape

“…This raises the question whether the effects of pictorial illusions are fundamentally different from visual deformations of shape. We do not think so: to explain the effects of the orientation illusion, it is sufficient to assume that the planned grip orientation and grip locations do not have to be mutually consistent (Smeets et al 2002).…”

“…Neurological evidence that perceiving shapes veridically is not crucial for grasping them correctly has been taken as a support for grasping having access to different, veridical, visual information about the object (Goodale et al 1994;Goodale and Milner 1992). Since different aspects of the visual information do not have to be mutually consistent (Smeets et al 2002), any difference between performance on two tasks could be explained by assuming that different aspects of the visual information are used for the two tasks. For example, if grasping is accomplished by moving the fingertips to suitable locations on the target's surface then the distance between these points is irrelevant so that an illusion of size should have no effect (Smeets and Brenner 1999).…”

“…They concluded that different sources of information are used for movement planning and for on-line control. However, if the illusion affects the perceived orientation of the target's surface, but not the perceived locations at which to grasp the surface, the illusion's influence will automatically diminish as the hand approaches the target (Smeets et al 2002).…”