Time-varying effects of a size-contrast illusion on grasping are not correlated with illusory perception.

Abstract

Visually-guided grasping movements tend to be less sensitive to size-contrast illusions than are size judgments. This finding has been taken as support for the existence of independent visual systems mediating action control and object perception. A number of studies have recently challenged the consensus view that visually-guided actions are refractory to various types of visual illusions. Some studies report equal effects of illusions for action and perception, whereas other studies report that illusion effects vary over the course of an unfolding action with relatively large early effects washing out towards the end of the response. Here we report a study of visually-guided grasping in the context of a size-contrast illusion with a sample of participants larger that that typically used in this research (n=42). Peak grip aperture was not affected by the illusion, and the sensitivity of grip aperture to the illusion did not vary systematically over the temporal course of the action. More importantly, time-varying illusion effects in grasping (i.e., illusion effects measured at various temporal points throughout the action) were not correlated with manual estimations of object size across participants. These results support a model in which the programming and control of actions proceeds independent of the perception of the target object's size. When present, flanker effects in action appear to be driven by mechanisms other than the perceptual mechanisms that underlie the visual size-contrast illusion.

Footnotes

 Funded in part by NSERC (DW).