When grasping a physical object, the sensorimotor system is able to specify grip aperture via absolute sensory information. In contrast, grasping to a location previously occupied by (no-target pantomime-grasp) or adjacent to (spatially dissociated pantomime-grasp) an object results in the specification of grip aperture via relative sensory information. It is important to recognize that grasping a physical object and pantomime-grasping differ not only in terms of their spatial properties but also with respect to the availability of haptic feedback. Thus, the objective of this dissertation was to investigate how terminal haptic feedback influences the underlying mechanisms that support goal-directed grasping in visual- and tactile-based settings.
In Chapter Two I sought to determine whether absolute haptic feedback influences tactile-based cues supporting grasps performed to the location previously occupied by an object. Results demonstrated that when haptic feedback was presented at the end of the response absolute haptic signals were incorporated in grasp production. Such a finding indicates that haptic feedback supports the absolute calibration between a tactile defined object and the required motor output. In Chapter Three I examined whether haptic feedback influences the information supporting visually guided no-target pantomime-grasps in a manner similar to tactile-guided grasping. Results showed that haptic sensory signals support no-target pantomime-grasping when provided at the end of the response. Accordingly, my findings demonstrated that a visuo-haptic calibration supports the absolute specification of object size and highlights the role of multisensory integration in no-target pantomime-grasping. Importantly, however, Chapter Four demonstrated that a priori knowledge of haptic feedback is necessary to support the aforementioned calibration process. In Chapter Five I demonstrates that, unlike no-target pantomime-grasps, spatially dissociated pantomime-grasps precluded a visuo-haptic calibration. Accordingly, I propose that the top-down demands of decoupling stimulus-response relations in spatially dissociated pantomime-grasping renders aperture shaping via a visual percept that is immutable to the integration of haptic feedback. In turn, the decreased top-down demands of no-target pantomime-grasps allows haptic feedback to serve as a reliable sensory resource supporting an absolute visuo-haptic calibration
