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Pantomime-grasps entail a response to an area adjacent to (i.e., spatially dissociated pantomime-grasp), or previously occupied by (i.e., no-target pantomime-grasp) a target. Previous work has reported that pantomime-grasps differ kinematically from naturalistic grasps (i.e., grasping a physical target object) - a result taken to evince that pantomime-grasps are perception-based and mediated via relative visual information. However, such actions differ not only in terms of their visual properties, but also because the former precludes haptic feedback related to a target's absolute size. The current study provides four experiments examining whether experimenter-induced haptic feedback influences the information mediating spatially dissociated and no-target pantomime-grasps. Just-noticeable-difference scores were computed to determine whether grasps adhered to, or violated, the relative psychophysical properties of Weber's law. Spatially dissociated pantomime-grasps performed with haptic feedback adhered to Weber's law (Experiments 1-3), whereas their no-target pantomime-grasp counterparts violated the law (Experiment 4). Accordingly, we 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 not directly influenced by 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., (Copyright © 2017 Elsevier Inc. All rights reserved.)

The goal of the present investigation was to test the somatosensory processing model's (SPM) assertion that tactile actions and perceptions are mediated via egocentric and allocentric frames of reference, respectively (Dijkerman & de Hann's 2007: Behavioral and Brain Sciences). To accomplish that objective, Experiment 1 required that participants use their right hand to grasp and manually estimate differently sized objects placed on the forearm and palm of their left hand. Following each manual estimation trial, participants grasped the target object to equate tasks (i.e., grasping vs. manual estimation) for terminal tactile feedback. Notably, the different object locations (i.e., forearm and palm) were used to examine whether location-specific differences in mechanoreceptor density impacts the percept of object size (i.e., Weber's illusion). In addition, we computed just-noticeable-difference (JND) scores to determine whether grasping and manual estimations adhere to, or violate, the allocentric principles of Weber's law. Results for the grasping task elicited a null expression of Weber's illusion and JNDs for this task violated Weber's law. Results for the manual estimation task similarly exhibited a null expression of Weber's illusion; however, JNDs for the palm but not the forearm condition adhered to Weber's law. Experiment 2 showed that withdrawing terminal tactile feedback during forearm condition manual estimations resulted in responses that adhered to Weber's law. Thus, results provide some support for the SPM's contention that grasping and manual estimations are mediated via ego- and allocentric frames of reference, respectively. However, results further indicate that the dissociation is not complete and is, in part, influenced by the sensory consequences (i.e., terminal tactile feedback) associated with the response., (Copyright © 2014 Elsevier Inc. All rights reserved.)