Weber's law in tactile grasping and manual estimation: feedback-dependent evidence for functionally distinct processing streams.

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.