Grasping Visual Illusions: No Evidence for a Dissociation Between Perception and Action

Abstract

Neuropsychological studies prompted the theory that the primate visual system might be organized into two parallel pathways, one for conscious perception and one for guiding action. Supporting evidence in healthy subjects seemed to come from a dissociation in visual illusions: In previous studies, the Ebbinghaus (or Titchener) illusion deceived perceptual judgments of size, but only marginally influenced the size estimates used in grasping. Contrary to those results, the findings from the present study show that there is no difference in the sizes of the perceptual and grasp illusions if the perceptual and grasping tasks are appropriately matched. We show that the differences found previously can be accounted for by a hitherto unknown, nonadditive effect in the illusion. We conclude that the illusion does not provide evidence for the existence of two distinct pathways for perception and action in the visual system.

Words’ Origin of an Affective State, but not Valence, Shape the Reaction Latencies in a Word-Sign Choosing Ambiguous Task

“…With ambiguous stimuli, for example, one visual figure or image can be interpreted in two different ways, depending on presuppositions (e.g. multistable perception in The Necker cube; Eagleman 2001) or prior knowledge (Franz et al 2000). Not only can visual illusions be treated as instances of ambiguity, but also information uncertainty can lead to ambiguity elicitation.…”

Processing Spatial Layout by Perception and Sensorimotor Interaction

“…But there is a confound in the study; the perceptual measure summated the small-among-large circle illusion with the largeamong-small illusion, while grasping at a circle involved only one illusion or the other. Franz, Gegenfurtner, Bülthoff, and Fahle (2000) pointed out that the two illusions sum nonlinearly and failed to replicate the contrast between perception and grasp when the two illusions were separated. A recent summary of the dissociation, and the controversy, is given in Goodale and Westwood (2004), with the conclusion that the two visual systems, while distinct, are not as independent as had once been assumed.…”

10 years of illusions.

“…The finding that rotating the positions of the flankers influences the orientation of the hand when grasping also suggests that the flankers are regarded as obstacles (de Grave et al, 2005). The possibility that flankers act as obstacles can be used to defend the claim of Aglioti et al (1995) from the direct criticism of Franz and colleagues (Franz et al, 2000). However, this possibility leads to the unfortunate conclusion that peak grip aperture is not a reliable measure for determining how visual information is processed for grasping.…”