Although the softness of an object can truly be perceived only by touching, the role of cutaneous mechanoreceptors in coding softness is unknown. Perceived softness is a subjective measure of an object's compliance - the latter defined as the ratio of the amount of deformation of the object to the applied force. The skin is also deformed in response to applied forces when the object is contacted directly or indirectly through an intermediary object such as a tool held in the hand. Psychophysical measurements of the magnitude of softness will be obtained when silicone rubber objects of differing compliance and shape are brought in direct contact with the fingerpad, thereby producing both spatial and temporal cues, under conditions of active or passive touch (with or without proprioception) or indirect contact via a tool (a stylus) thereby producing temporal cues alone. Recorded force traces measured on the stylus will be played back and faithfully reproduced on the stylus via a piexoelectric actuatoor coupled to a torque motor. This device provides control over the tactile signals generated on a tool during active or passive probing of """"""""virtual objects"""""""" of differing softness. The same compliant objects (or virtual-object signals from the instrumented stylus) will be applied to the monkey fingerpad and evoked responses recorded in a spatially distributed population of slowly adapting (SA) and rapidly adapting (RA and Pacinian) cutaneous mechanoreceptive peripheral nerve fibers. We hypothesize that there is a spatiotemporal neural code for the softness of objects with deformable surfaces in direct contact with the skin contact which most likely is the rate of changing shape of the spatially distributed discharge rates of the SA population. Softness sensed through a tool may have both a temporal neural code, in the rate of change in discharge rates, and a spatial neural code in the relative activation of Pacinians, RAs and SAs, in response to the rate of the rising phase of the applied force. The proposed studies will provide magnitude scaling functions and their tactile neural codes for the perception of softness and novel information ont he fundamental role of tactile signals in the use of tools for the perception and manipulation of objects.
| Friedman, Robert M; Hester, Kim D; Green, Barry G et al. (2008) Magnitude estimation of softness. Exp Brain Res 191:133-42 |
| LaMotte, R H (2000) Softness discrimination with a tool. J Neurophysiol 83:1777-86 |
