Natural prehension in trials without haptic feedback but only when calibration is allowed.

Reach-to-grasp (prehension) movements are normally accurate, precise and stereotypical in movement pattern. These features disappear when haptic feedback is removed in 'virtual reality' systems or when participants pantomime prehension. [Goodale, M. A., Jakobsen, L. S., Keillor, J. M. (1994). Differences in the visual control of pantomimed and natural grasping movements. Neuropsychologia, 32, 1159-1178] suggested that pantomimed reaches are unnatural in form because the ventral rather than the dorsal stream mediates them. We tested whether calibration can prevent 'unnatural' prehension. Calibration refers to the use of an error (visual and/or kinaesthetic) signal to refine performance. We asked participants to reach-and-grasp in four conditions: (A) baseline; (B) reaching-to-grasp with haptic feedback (visual open-loop prehension to a physical object); (C) no feedback (visual-open-loop prehension to an object that could be seen but not felt); (D) a random mixture of (B) and (C). A 45 degrees mirror was used to display objects without any reduction in visual quality. The normal decrements in performance were observed in condition (C) but not in the identical trials randomly embedded with feedback trials in condition (D). These findings show that participants can produce normal visual-open-loop prehension in the absence of haptic feedback when calibration is allowed. Thus, dorsal stream processing can support pantomimed reaching when calibration is allowed.