In early 2007, a 64-year-old Swiss woman was admitted to the emergency room of a local hospital after having suffered a moderate right hemispheric stroke. Several days following her hospitalization, the woman began to experience what she described to her physicians as a “pale,” “transparent” arm that began at her elbow, which she could move and utilize to complete actions. The phenomenon the Swiss woman experienced was a Supernumerary Phantom Limb (SPL), which is characterized by the sensation of possessing an extra limb that did not exist previously. Though uncommon, conditions such as SPL and phantom limb (the sensation that a missing limb is still attached to the body) typically arise due to some form of insult to the somatosensory region of the brain or from the removal or lack of body parts.

The third arm illusion at the Karolinska Institute in Stockholm, Sweden.

In the healthy brain, multisensory circuits organize visual, tactile, and proprioceptive inputs to the brain in order to compose a somatotopic map of which body parts are inherently our own. However, even the normal brain can be manipulated into believing in the existence of an extra limb. One method of manipulation is the rubber hand illusion, a technique that investigates bodily self-perception by placing a rubber hand next to a real hand and then stimulating both the real and fake limbs. Researchers Arvid Guterstam, Valeria Petkova, and Henrik Ehrsson of the Karolinska Institute in Stockholm, Sweden modified the techniques of the rubber hand illusion to study the representation of the real and fake hands in the brain and investigate whether, in order to accommodate for the new somatotopic representation of the fake limb, the brain would decrease its representation of the real hand.

By stimulating both limbs in a temporally and spatially congruent manner, The Karolinska team found that subjects effectively perceived the artificial hand as an additional limb without any significant disownership of the actual hand. However, in order to effectively elicit this illusion, both the false and real hands had to receive equivalent sensory, proprioceptive, and visual inputs. Results of this study indicate not only that identical, properly timed sensory input and integration is essential for initiating an SPL response in the healthy brain, but also that the brain is able to modify its somatotopic map to simultaneously accept two possibilities for self-identification.

Since current neuroprosthetic techniques only consider the functional limitations of paralysis, there is no compensation for the sensory limitations suffered by paralyzed individuals. However, the findings of the Karolinska team’s study suggest that it may be possible to create an illusion of a holistic multisensory experience for a paralyzed individual by incorporating supernumerary prosthesis into treatment techniques.  Hopefully, these discoveries will help researchers and bioengineers rethink their understanding of body-representation in the neuroprosthetic world.

A Mind Trick May Help Advance Prosthetics – LiveScience

The Illusion of Owning a Third Arm: Research Article – PlosOne

Seeing the phantom: A functional magnetic resonance imaging study of a supernumerary phantom limb – Annals of Neurology