3. Vibrotactile Sensation and Softness Perception

Soft or deformable objects, be they rubber ducks or running shoe inserts, are rarely thought of as sources of mechanical vibrations. For similar reasons, it is often overlooked that material softness can be communicated through the vibrotactile sensory channel—that is, through the subset of the haptic perceptual system that is sensitive to mechanical vibration. In this chapter we review current knowledge about the relation between vibrotactile sensation and softness perception. It is possible to distinguish between two main types of softness perception—one pertaining to the surface material qualities of a palpated object and the other linked to volumetric compliance. This information can be obtained through four types of interactions, which will be analysed separately: direct skin contact, indirect skin contact, transient contact, frictional sliding. We review contemporary research on softness perception in these four scenarios. This research has shed light on the perceptual salience of vibrotactile stimuli and on the action-phase dependence of vibrotactile cues for softness. We also highlight the importance of the physiological and mechanical aspects of the interactions for softness perception. In most cases, vibrotactile cues have a comparatively weaker influence on perception than the cues described in other chapters produced by directly manipulating a compliant object with deformable surfaces. Nonetheless, vibrations lead to an appreciable change on perceived compliance that can be exploited in addition to other cues or when such cues are not available.