Like a kind of neural Voodoo doll, there’s a representation of our body in our brain, so that when we’re touched on, say, our left arm, the brain’s representation of our left arm is activated. So what do you think would happen in a tactile illusion when being touched on one part of the body leads to the sensation of having been touched somewhere else? Would it be the brain’s representation of the body part that was touched that was activated, or would it be the brain’s representation of where the touch was ‘felt’ to have occurred, that was activated.
That’s what Felix Blankenberg and colleagues have investigated using a brain scanner and the Rabbit illusion. In this illusion a person’s wrist is tapped several times followed by tapping of their elbow. Under optimal conditions it can lead to the sensation that the tapping continued up the arm from the wrist to the elbow, like a ‘rabbit’ hopping up the arm.
Blankenberg placed electrodes along the arms of thirteen participants and compared the brain activity that occurred when pulses were delivered all the way up their arm; when six pulses on the wrist were followed by three at the elbow (inducing the Rabbit illusion, as confirmed by participants’ reports); and in a control condition, in which three pulses were given at the wrist, three at the elbow, followed by three at the wrist again – a pattern that does not induce the illusion.
They found the brain’s representation of the middle part of the forearm (in the primary somatosensory cortex) was activated when pulses were actually delivered there, and crucially, also when, during the illusion, sensations were felt there even though no pulses were actually delivered there. By contrast, the region was not activated during the control condition.
“The intervening hops of the rabbit that get mislocalised and filled-in for conscious phenomenology evidently also get filled in and appropriately re-localised within human primary somatosensory cortex”, the researchers concluded.
Blankenburg, F., Ruff, C.C., Deichmann, Rees, G. & Driver, J. (2006). The cutaneous rabbit illusion affects human primary sensory cortex somatotopically. PLoS Biology, 4, e69.