Tying Finger Pad Skin Deformation to Tactile Perception of Object Compliance at Various Time Scales

We regularly touch soft, compliant fruits and tissues. To help us discriminate them, we rely upon cues embedded in spatial and temporal deformation of finger pad skin. However, we do not yet understand, in touching objects of various compliance, how such patterns evolve over time, and thereby drive perception. Prior efforts have focused upon the analysis of finger displacement, reaction force, and 2-D estimates of terminal contact area. However, characterizing the deformation of the skin surface, induced upon contact with compliant surfaces, requires 3-D empirical measurements over a short time-scale, due to its complex nonlinear elasticity and geometry. Herein we couple the empirical measurement of skin deformation – across compliance, indentation depth, indentation rate, and time duration – with human perceptual experiments. In particular, we develop a 3-D stereo imaging technique and metrics for quantifying skin deformation to move 2-D estimates of terminal contact area to 3-D spatiotemporal changes in penetration depth, surface curvature, and force. We observe a complementary interplay between and evolvement of these cues over about a 0.3 – 0.6 msec duration at which the stimuli become discriminable, with distinctions between compliances less or more stiff than the skin. We examined the compliance discriminability across slow and fast indentation rates and concluded that the detection rate is higher at slower indentation velocity. Additionally, the minimum time required for differentiating compliance can also be determined psychophysically and biomechanically. These observations of the skin’s deformation may guide the design and control of haptic actuation. Moreover, using our metrics, people can potentially model the skin mechanics of the fingertip and link them to the afferent responses transduced by tactile stimuli.