Figure 6.

Illustration of motion quartets (visual and tactile). When a subject is presented with two successive visual images (‘Visual loop’) with two black dots moving from one configuration (on one diagonal) to another (on the inverse diagonal), the subject may perceive either a horizontal or a vertical displacement of the two black dots, and switch from one percept to the other (‘Percept’). The same bistability illusion may be obtained with tactile stimuli (‘Tactile loop’), using motion touch zones on the thumb.