Can't Touch This: Removing haptic feedback of the goal object during visually-guided grasping induces pantomime-like grasps

In a recent paper, Schenk (2012) argues that patient DF, who suffers from visual form agnosia following ventral-stream damage, uses haptic feedback to scale her grasps to the width of goal-objects to compensate for her profound visual deficit in perceiving their dimensions. Central to this claim is the fact that DF no longer scales her grip aperture in-flight to object width when the object is not actually grasped, despite directing her hand to the apparent position of the target viewed in a mirror. But it could also be the case that DF failed to scale because the absence of a felt object induced a shift in the nature of the task from real grasping towards something that resembles ‘pantomimed’ grasping, which is thought to be mediated by ventral-stream structures that are damaged in DF. Here we used Schenk’s mirror apparatus to see if grasping in the absence of a real object in normal observers resembles pantomimed grasping. Twenty participants were asked to grasp or pantomime-grasp cylinders of different sizes viewed either in the mirror or directly through a transparent pane of glass. The slopes relating maximum grip aperture to object size for grasps directed towards an absent object viewed in a mirror were similar to those of pantomimed movements directed away from objects either viewed in the mirror or directly through glass. Importantly, grip scaling to object size in all three of these conditions was quite different from that observed in real grasping. Additionally, RTs were longer in the three ‘pantomime’ tasks, consistent with the view that pantomiming recruits additional cognitive mechanisms. These results suggest that removing haptic feedback of the object, even when participants reach to the perceived location of the target, induces a switch from real-time motor control to a more cognitively-driven strategy. Meeting abstract presented at VSS 2013