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.
The cutaneous rabbit illusion was first described in 1972 by psychologists Frank Geldard and Carl Sherrick, who were then at Princeton University’s Cutaneous Communications Laboratory. Geldard and Sherrick discovered the phenomenon by accident, while designing experiments to investigate the perception mechanical stimuli applied to the forearm. The underlying mechanisms, however, remained unknown for years afterwards. Some researchers attributed the illusion to activity in the primary somatosensory cortex, the first stop in the brain for tactile sensations entering from the body, while others argued that cognitive processes such as selective attention are involved. This was settled in 2006, with the publication of a functional neuroimaging study by researchers from UCL, which showed that the phantom tactile sensations perceived during the illusion are associated with activity in the corresponding regions of the somatosensory cortex.
In the new study, Makoto Miyazaki of the Kochi University of Technology and his colleagues show that the cutaneous rabbit is not confined to the body, but can “hop out”, so that the illusory sensations are perceived to be emanating from an object. They recruited 8 participants and sat them at a table in front of a device consisting of a number of piezoelectric contactors attached to a flat 10cm-long aluminium rod. The participants wore earphones which emitted beeps; each time they heard a beep, they were required to place the tips of their index fingers onto the underside of the aluminium rod and close their eyes. The device then applied trains of mechanical pulses to one index finger and then the other, in quick succession. A second experiment, in which the pulses were delivered by small metal plates instead of a stick, was also performed.
At the end of each trial, the participants used a set of pointers to indicate where the tactile sensations they “felt” had come from. In the first experiment, but not the second, all the participants experienced the cutaneous rabbit, and reported that they felt sensations from two points on the aluminium stick as well as from their fingertips. The cutaneous rabbit had hopped out of the body and onto the aluminium stick.
How can this illusion occur, given that the stick lacks a corresponding receptive field in the somatosensory cortex? It can be explained in terms of the body schema (or body image), a representation of the physical body encoded by the brain. It has long been known that the body schema can be temporarily extended – the brain temporarily incorporates external objects such as clothes and tools into the schema, treating them as if they were a part of the body. Thus, the cutaneous rabbit illusion occurs because the brain regards the aluminium stick as an extension of the fingers, and incorporates it into the body schema. As a result, the stimuli applied to the fingertips are mislocalized, and perceived to originate from the stick and not from the fingers.
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Miyazaki, M., et al. (2010). The “Cutaneous Rabbit” Hopping out of the Body. J. Neurosci. 30: 1856-1860. DOI: 10.1523/jneurosci.3887-09.2010.[Full text]
Blankenburg, F., et al (2006). The Cutaneous Rabbit Illusion Affects Human Primary Sensory Cortex Somatotopically. PLoS Biol 4 (3): e69. DOI: 10.1371/journal.pbio.0040069. [Full text]