The effect of a pictorial illusion on closed-loop and open-loop prehension

Abstract

It has been proposed that movements to visible and remembered targets are sensitive to qualitatively different types of visual information. When the target is continuously visible, prehensile movements are thought to reflect veridical object size, whereas memory-dependent prehension is sensitive to the perceived size of the object. This hypothesis was explored by assessing the influence of illusory target width on prehension kinematics in three visual conditions: closed-loop (CL; full vision during the response), open-loop brief-delay (OL; visual occlusion coincident with the movement initiation cue) and open-loop 3-s delay (OL3; visual occlusion 3 s prior to movement initiation). To modulate illusory target width, objects were placed on backgrounds consisting of three forms of the Müller-Lyer (ML) figure. Peak grip aperture was sensitive to the ML figure in the OL and OL3, but not CL conditions, suggesting that perceptual information is used to modulate this grasping parameter when the movement is programmed and executed on the basis of visual memory. Peak-aperture velocity was affected by the ML illusion in all three visual conditions, suggesting that perceived object size might be important for modulating this aspect of prehension, independent of memory requirements. The different sensitivity of grip aperture and aperture velocity to illusory target width in the CL condition suggests that grasp preshaping might reflect multiple visuomotor processes. The results of this study are consistent with the tenets of the two-stream model of visual processing.

Dynamic effects of the Ebbinghaus illusion in grasping: Support for a planning/control model of action

“…Although visual feedback may seem to be the most intuitive candidate for the correction of illusion effects on line, the present study suggests that its role under the present circumstances may be relatively minor. However, in other studies using the Müller-Lyer illusion, the role of continuousvision of the effectors and targets has been more evident (Gentilucci et al, 1996;Glover & Dixon, 2001c;Westwood et al, 2000), and a complete understanding of the relative importance of continuous vision in the on-line correction of illusion effects on action requires further research.…”

“…However, other results are more consistent with the planning/control model than with the perception/action model. For example, it has been found that illusion effects on action are smaller when visual feedback is available to participants, as compared with when it is not available (Gentilucci et al, 1996;Glover & Dixon, 2001c;Westwood, Heath, & Roy, 2000). This supports the notion, inherent in the planning/control model, that on-line control processes are responsible for the small illusion effects on actions.…”

Probabilistic information on object weight shapes force dynamics in a grip-lift task

“…Such precaution was not necessary in our experiment, where the object was a cuboidal block attached to a lever. Veridical and illusory size cues (pictorial illusions of object size) influence both prehensile movements and manipulative forces (MonWilliams and Murray 2000; Westwood et al 2000a). The peak GF is higher for larger objects than for smaller objects of the same weight (Westwood et al 2000b).…”

Opposite haptic and visual induction effects observed with Titchener’s ⊥

“…3 Because of insufficient equipment, we could not comply with argument (2). Instructing subjects to close their eyes before starting to reach (as done by Vishton et al, 1999) is likely to result in delayed responding, and hence may introduce a memory confound; see Westwood, Heath, and Roy (2000) for estimates of relevant reaction times (RTs). Instead, we introduce a new paradigm that can be considered an alternative to the standard obstruction-of-view technique (cf.…”