If you look at a waterfall for about 30 seconds, and then shift your gaze to a nearby stationary object, such as a rock or a tree, that object will seem to drift slowly upwards. This well known optical illusion demonstrates a phenomenon called the motion after-effect, which is thought to occur as a result of adaptation – the brain compensates for movement in one direction, causing us to momentarily perceive a stationary objects to be moving in the other.

Although illusory motion can also be induced in the sense of touch, the brain is thought to process visual and tactile motion separately. But now researchers from MIT have found that not only can moving visual stimuli induce a tactile motion after-effect, but also that moving tactile stimuli can induce a visual motion after-effect. The findings, which are published in Current Biology, show that the senses of vision and touch are closely linked, and that each can influence the other.

IF a rapid series of taps are applied first to your wrist and then to your elbow, you will experience a perceptual illusion, in which phantom sensations are felt along the skin connecting the two points that were actually touched. This feels as if a tiny rabbit is hopping along your skin from the wrist to the elbow, and is therefore referred to as the “cutaneous rabbit”. The illusion indicates that our perceptions of sensory inputs do not enter conscious awareness until after the integration of events occuring within a certain time window, and that the sensory events taking place at a certain point can be influenced by future events.

A group of Japanese researchers now shows that this illusion is not just confined to the body. In a new study published today in the Journal of Neuroscience, they report that the cutaneous rabbit can easily be induced to “hop out” of the body, so that the illusory sensations are perceived to originate not from the body itself, but from external objects that interact with it.