Though haptic feedback often relies on the mapping of information directly to a stimulus produced by tactile actuators, we can often leverage human sensory illusions to produce a richer tactile experience for users. One such illusion is the “Cutaneous Rabbit Illusion” (also known as sensory saltation):
The cutaneous rabbit illusion (also known as cutaneous saltation and sometimes the cutaneous rabbit effect or CRE) is a tactile illusion evoked by tapping two or more separate regions of the skin in rapid succession. The illusion is most readily evoked on regions of the body surface that have relatively poor spatial acuity, such as the forearm. A rapid sequence of taps delivered first near the wrist and then near the elbow creates the sensation of sequential taps hopping up the arm from the wrist towards the elbow, although no physical stimulus was applied between the two actual stimulus locations.
This illusion allows us to simulate the experience of movement across the skin using a smaller number of vibration motors – if we want to communicate direction or spatial relationships, sensory saltation allows us to quickly deliver the information by letting the human brain fill in the gaps.