Grasping adjustments to haptic, visual and visuo-haptic object perturbations are contingent on the sensory modality.

Haptics provides plentiful information about the size and position of a handheld object. However, it is still unknown how haptics contributes to action correction if a sudden perturbation causes a change in the configuration of the handheld object. In this study, we asked participants to reach and grasp an object with their right hand while the size of the object held in the left hand was occasionally perturbed during the movement. We compared the corrective responses to perturbations in three different sensory conditions: Visual (participants had full vision of the object), Haptic (object size was sensed by the left hand and vision was prevented) and Visuo-Haptic (both visual and haptic information were available throughout the movement). We found that haptic inputs evoked faster contralateral corrections than visual inputs, although actions in Haptic and Visual conditions were similar in movement velocity and grip aperture at the moment of perturbation. Strikingly, the corrective responses in the Visuo-Haptic condition were as fast as those found in the Haptic condition, a result that is contrary to that predicted by simple summation of unisensory signals. These results suggest the existence of a haptomotor reflex that can trigger automatic and efficient grasping corrections of the contralateral hand that are faster than those initiated by the well known visuomotor reflex.