Grasping a 2D object: terminal haptic feedback supports an absolute visuo-haptic calibration

Grasping a three-dimensional (3D) object results in the specification of motor output via absolute size information. In contrast, the impoverished visual cues (e.g., binocular and vergence) associated with grasping a two-dimensional (2D) object are reported to render aperture formation via an object's perceptual and relative visual features. It is, however, important to recognize that 3D and 2D grasping differ not only in terms of their visual properties, but also because the latter does not entail the provision of haptic feedback. As such, the present work examined whether haptic feedback influences the nature of the information supporting 2D grasping. Participants grasped differently sized 3D objects (i.e., 3D task) and completed a 'traditional' 2D grasping task to line drawings without receiving haptic feedback (i.e., 2DH- task). As well, we included a separate condition using the same objects as the 2DH- task; however, the experimenter placed a 3D object (i.e., one corresponding to the size of the 2D object) between the thumb and forefinger of participants' grasping limb once they completed their response (i.e., 2DH+ task). Thus, the 2DH+ task provided haptic feedback related to absolute object size. Notably, we computed just-noticeable-difference (JND) scores to determine whether the different tasks adhered to, or violated, the relative psychophysical principles of Weber's law. JNDs for the 2DH- task adhered to Weber's law, whereas 3D and 2DH+ tasks violated the law. Thus, results evince that 2DH- and 2DH+ tasks are specified via relative and absolute object size information, respectively. Accordingly, we propose that haptic feedback supports an absolute visuo-haptic calibration and contend that our results highlight the importance of multi-sensory cue integration in goal-directed grasping.