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

It has been proposed that movements to visible and remembered targets are s ensitive to qualitatively different types of visual information. When the t arget 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 v isual conditions: closed-loop (CL; full vision during the response), open-l oop brief-delay (OL; visual occlusion coincident with the movement initiati on cue) and open-loop 3-s delay (OL3; visual occlusion 3 s prior to movemen t initiation). To modulate illusory target width, objects were placed on ba ckgrounds consisting of three forms of the Muller-Lyer (ML) figure. Peak gr ip aperture was sensitive to the ML figure in the OL and OL3, but not CL co nditions, suggesting that perceptual information is used to modulate this g rasping parameter when the movement is programmed and executed on the basis of visual memory. Peak-aperture velocity was affected by the ML illusion i n all three visual conditions, suggesting that perceived object size might be important for modulating this aspect of prehension, independent of memor y requirements. The different sensitivity of grip aperture and aperture vel ocity to illusory target width in the CL condition suggests that grasp pres haping might reflect multiple visuomotor processes. The results of this stu dy are consistent with the tenets of the two-stream model of visual process ing.