Grasping visual illusions: Consistent data and no dissociation

Abstract

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 estimate 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.

Visual stability

“…In contrast, pointing to the target remains veridical [13]. While this finding is consistent with the claim that spatial stability is predominantly a feature of the motor system [9], such perception-action dissociations are always problematic to interpret (for reviews, see [11,14]). …”

Effect of visual and haptic feedback on grasping movements

“…However, even when haptic feedback was available throughout the block, the MGA was subject to the same depth constancy failure observed during Pretraining and Posttraining blocks. This second compelling result concerning the similarity between perceptual biases and biases observed for reach-to-grasp actions endorses the hypothesis that processing of visual information for both perceptual judgments and visually guided actions is indicative of similar and interrelated underlying mechanisms (Foster et al 2011;Franz et al 2000Franz et al , 2009Franz and Gegenfurtner 2008;Schenk 2010Schenk , 2012. In particular, the present findings would indicate that depth scaling of binocular disparities is the same for perception and action, a result incompatible with the dual visual systems theory (Goodale et al 1991;Goodale and Milner 1992).…”

No Dissociation between Perception and Action in Patient DF When Haptic Feedback is Withdrawn

“…In this model, the ventral system serves object and scene recognition and the dorsal system uses vision to guide actions (Milner and Goodale, 2006). But in recent years, many of the model's claims were challenged (Schenk and McIntosh, 2010;Schenk et al, 2011), most notably, its claim that action is immune to perceptual illusions (Smeets and Brenner, 2006;Franz and Gegenfurtner, 2008), its claim that the dorsal stream does not provide observer-invariant visual information (Konen and Kastner, 2008) and does not make a contribution to perception (Schenk, 2006), the assertion that visual form agnosia and optic ataxia constitute a proper double-dissociation (Pisella et al, 2006), and the assumption that memory-based action is not processed by the dorsal stream (Himmelbach and Karnath, 2005;Himmelbach et al, 2009). In response to this critique, Goodale and colleagues noticed that critics fail to take the evidence from their patient, DF, into account and argued that one cannot dispute the two-visual stream hypothesis unless an alternative account for DF's surprisingly good visuomotor behavior is provided (Milner and Goodale, 2008, Goodale and Milner, 2010, Westwood and Goodale, 2011.…”