The effect of illusory size on force production when grasping objects

Milner and Goodale (1995) have proposed that visuomotor and perceptual proc esses are mediated by discrete visual systems that reflect the functional i ndependence of action and perception. The visuomotor system is proposed to be insensitive re pictorial illusions of object size, whereas the perceptua l system is reliably "tricked" by such figures. Brenner and Smeets (1996) a nd Jackson and Shaw (2000) demonstrated that grasp preshaping, but not gras ping force, is immune to the Ponzo visual illusion, suggesting that not all visuomotor processes operate independently of the perceptual system. The p resent study investigated the effect of illusory object size on prehension kinematics and grasping dynamics (i.e., grip force and load force) as well as perceptual judgements of object size. Unlike previous investigations, ob ject mass was held constant independent of changes in size. The Ponzo figur e reliably affected perceptual estimates of object size, but this effect wa s restricted to one form of the illusion. Some aspects of the prehension mo vement were sensitive to veridical but not illusory object size (peak grip aperture:, peak grip force, peak vertical wrist acceleration), whereas othe r movement parameters demonstrated illusory size effects (movement time, pe ak wrist velocity). Still other move ment parameters were not sensitive to veridical or illusory object size (peak load force). Together the data sugg est that certain prehension components are immune to pictorial illusions of object size, whereas others are not. Complex interactions between the perc eptual and visuomotor systems appear to underlie the anticipatory scaling o f grasping forces in prehension.