Physical aspects of softness perception

Softness can be physically expressed in different ways, for example as a stiffness value (spring constant), compliance value (the inverse of stiffness), or Young’s modulus. The relation between these quantities is discussed, as well as the psychophysical techniques for characterising the perception of these quantities. For perception of softness, multiple cues are available: visual cues include the deformation of the material around the fingers, while haptic cues include the ratio between applied force and resulting displacement of the material, and the force distribution over the contact area. Furthermore, to register these haptic cues, both cutaneous and kinaesthetic information can be used. The role of these different cues and types of information is discussed and also the interaction between the hand and the deformable material.