Absolute cues support delayed tactile grasping only when the target object is touched


The somatosensory processing model (SPM) asserts that ventral and dorsal cortical pathways support tactile perceptions (relative cues) and actions (absolute cues), respectively.  In accordance to SPM, our previous work showed that peak grip aperture (PGA) associated with grasping a tactile object presented on the palm of the hand was refractory to Weber’s law, whereas manual estimations elicited apertures that adhered to the law’s psychophysical principles.  In other words, results support the SPM’s contention of dissociable tactile processing streams. In the present investigation, we sought to determine whether introducing a delay between target presentation and response initiation influences the tactile metrics supporting grasping control.  Further, we were interested in determining whether subsequent tactile feedback from grasping the object influences the processing stream supporting grasping control.  Therefore, we placed differently sized objects on participants’ (N=15) left palm for a 4,000ms tactile preview and subsequently asked participants to either grasp the object with their right hand immediately after the preview (i.e., closed-loop) or complete a pantomime-grasp once the object had been removed for a 2,000ms delay (i.e., memory-guided).  In a third condition, following the pantomime grasp the object was repositioned on participants’ palm for them to physically grasp and receive terminal tactile feedback related to object size (i.e., memory-guided with feedback).  We computed just-noticeable difference (JND) scores at the time of PGA.  Results showed a null scaling of JNDs to object size in the closed-loop and memory-guided with feedback conditions, whereas a linear scaling of JNDs to object size was observed in the memory-guided task.  These findings suggest that memory-guided grasping is mediated via relative and memory-based target representations supported by the ventral processing stream.  In turn, results for the memory-guided with feedback condition demonstrate that the inclusion of terminal feedback supports the use of absolute tactile cues specified by the dorsal streams.

Acknowledgments: Natural Sciences and Engineering Research Council