The finding that the Ebbinghaus/Titchener illusion deceives perception but not grasping is usually seen as strong evidence for Goodale and Milner’s (1992) notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. We argue that these results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size esti-mate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.

The planning/control model of action assumes that grasping is sensitive to the context of an object only in early stages of the movement (planning), but not in later stages (control). In consequence, the effects of context–induced illusions (as the Ebbinghaus/Titchener illusion) should decrement during a grasping movement. Here, we tested this claim by reanalysing a large data set (N=26) on grasping in the Ebbinghaus illusion. Contrary to the predictions of the planning/control model, we found that the effects of the illusion did not decrease over time. Instead, the illusion effects stayed remarkably constant.

We investigaCC whether theorigina Roelofs e#ectae the induced Roelofs e#ectae cactS by the saS shift in perceived straeiv arae Subjects were presented witha tahSz withina frai in complete dapleteS Taple a fral could both be shifted to the left or right of objectivestrativ arat OnsepaflIC triaflI subjectsgaj verba estimasS atim the position of either thetaSxH or thefraxP The eccentricity of thefraL wa underestimaPfl (theorigina Roelofs e#ect). However, the perceived position of the taeSx did not follow this misjudgement of the eccentricity of thefraz (the induced Roelofs e#ectwa not present). Thus, it is unlikelytha both e#ectshac a common origin in misjudging egocentricstraent arae # 2002 Elsevier Science Ltd. All rights reserved.

We examined whether a pictorial depth illusion influences the manual pursuit of a moving dot to the same extent that it influences the dot's apparent displacement. Fourteen subjects performed two tasks. In one case, they used their unseen hand to track a dot that moved on an elliptical path. In the other, they first watched the dot move on the same path, and then set an ellipse to match the shape of the dot's path. The illusion influenced the two tasks to the same extent, suggesting that the visual information processing is the same for the two tasks.

We investigated whether humans use prior knowledge of the geometry of faces in visually guided reaching. When viewing the inside of a mask of a face, the mask is often perceived as being a normal (convex) face, instead of the veridical, hollow (concave) shape. In this "hollow-face illusion, " prior knowledge of the shape of faces dominates perception, even when in conflict with information from binocular disparity. Computer images of normal and hollow faces were presented, such that depth information from binocular disparity was consistent or in conflict with prior knowledge of the geometry. Participants reached to touch either the nose or cheek of the faces or gave verbal estimates of the corresponding distances. We found that reaching to touch was dominated by prior knowledge of face geometry. However, hollow faces were estimated to be flatter than normal faces. This suggests that the visual system combines binocular disparity and prior assumptions, rather than completely discounting one or the other. When comparing the magnitude of the hollow-face illusion in reaching and verbal tasks, we found that the flattening effect of the illusion was similar for verbal and reaching tasks.

Approche psychophysique des dissociations perception-action Effet de la détection de distracteurs au seuil sur l’atteinte de cibles visuelles

Abstract—We examined whether a pictorial depth illusion influences the manual pursuit of a moving dot to the same extent that it influences the dot’s apparent displacement. Fourteen subjects performed two tasks. In one case, they used their unseen hand to track a dot that moved on an elliptical path. In the other, they first watched the dot move on the same path, and then set an ellipse to match the shape of the dot’s path. The illusion influenced the two tasks to the same extent, suggesting that the visual information processing is the same for the two tasks. Exploring whether a visual illusion has the same influence on a motor task as on its perceptual counterpart has become a standard way to test for common information processing for perception and action. Some studies have reported dissociations, with different patterns of results for perceptual-judgment and motor tasks. Such findings have been regarded as support for the separate processing of visual information for perception and action (e.g., Goodale & Milner, 1992; Loomis, Da Silva, Fujita, & Fukusima, 1992). However, visual illusions frequently involve incongruous effects on related visual attributes (Gillam, 1998). The reported dissociations can therefore also be interpreted as indicating that different visual attributes were used to perform the different tasks (Smeets & Brenner, 2001). In support of such an interpretation, it can be noted that similar dissociations are found even when both tasks involve perceptual judgments or both involve actions (Smeets & Brenner, 1995; Vishton, Rea, Cutting, & Nunez, 1999). Thus, whenever actions resist an illusion that distorts perception, it is necessary to check whether the motor response is being driven by the same visual attributes as the perceptual judgment. (See,

Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, or to redistribute to lists, requires prior specific permission and/or a fee.

control does not eliminate motion-induced illusory displacement.