Identifying 3-D spatiotemporal skin deformation cues evoked in interacting with compliant elastic surfaces.

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 drive perception. Using a 3-D stereo imaging technique in passive touch, we develop metrics for quantifying skin deformation, across compliance, displacement, and time. The metrics map 2-D estimates of terminal contact area to 3-D metrics that represent spatial and temporal changes in penetration depth, surface curvature, and force. To do this, clouds of thousands of 3-D points are reduced in dimensionality into stacks of ellipses, to be more readily comparable between participants and trials. To evaluate the robustness of the derived 3-D metrics, human subjects experiments are performed with stimulus pairs varying in compliance and discriminability. The results indicate that metrics such as penetration depth and surface curvature can distinguish compliances earlier, at less displacement. Observed also are distinct modes of skin deformation, for contact with stiffer objects, versus softer objects that approach the skin's compliance. These observations of the skin's deformation may guide the design and control of haptic actuation.