The Illusion of a Sixth Finger

Proprioception, or the sense of how our body and limbs are positioned, is central to our self-concept. Damage to the proprioceptive system can have devastating effects on the ability to coordinate movements, grasp objects, or even walk. However, even the healthy proprioceptive system is susceptible to influence by the other senses.

The most famous case of the proprioceptive system being influenced by other senses is in the treatment of phantom limb pain. Patients who have endured an amputation of a hand or foot, for example, often feel intractable pain associated with the missing limb. The inability to see a missing hand prevents the patient from relaxing the muscles that produce a grip response, leading to a vicious cycle of gripping and pain.

Ramachandran and Rogers-Ramachandran developed an innovative technique in 1996, which they called the "mirror box," that could break this cycle and provide relief to suffering amputees. The technique involved placing a mirror to occlude the missing hand such that it reflects the preserved hand in the position where it would normally be. By gripping and ungripping the preserved hand, the patient "sees" their amputated hand doing the same movements, and over time this can lead to a substantial reduction in the phantom pain.

Later, Botvinick & Cohen (1998) showed that a similar effect could be produced in individuals without amputated limbs. In their "rubber hand" experiments, the researchers occluded a participant's hand and placed a rubber hand in its place. By simultaneously stroking the occluded hand and the rubber hand in the same anatomical position, participants reported the feeling that the rubber hand was their own. Subsequent researchers have manipulated the appearance of the rubber hand and shown that as long as the stroking between the rubber hand and hidden hand is synchronized, ownership can still happen, even for unusual-looking hands.

Source: Nicolas Davidenko

A sixth finger?

Using a similar technique, Cadete & Longo (2020) recently examined whether participants can be induced to feel like they have an extra finger in one of their hands. In their experiments, participants placed their left hand behind an occluding mirror while they looked at a reflection of their right hand being stroked by an experimenter. When the experimenter stroked the right thumb, they would also stroke the left thumb; then the right index and the left index, and so on. However, when they stroked the right pinky finger, they only stroked the inside part of the left pinky finger. And then, the experimenter stroked empty space next to the right pinky finger while stroking the outside of the left pinky finger.

In experiment 1, the synchronous stroking was done only once per finger, and the results were mixed: Only about half of participants reported any sensation of a sixth finger, and if it happened, it only lasted for a fleeting moment. In experiment 2, the experimenters extended the procedure to include 20 simultaneous strokes per finger, and this time, the results were undeniable. Most of the 19 participants reported feeling like they had six fingers in the occluded (left) hand, and in particular, reported feeling like they had two pinky fingers in that hand. Furthermore, this feeling was often felt for much longer than a fleeting moment, compared to experiment 1.

Neural plasticity

Polydactyly, a condition where a person is born with extra fingers or toes, is quite rare, affecting only about 1 in 1000 individuals. For the rest of the population, having five fingers per hand is as stable and internalized as any other physical trait. As such, the ability to quickly induce the feeling of a sixth finger in a simple mirror-box experiment is truly remarkable. By providing synchronized visual and tactile feedback, the brain can abandon years of knowledge about its own body and come to accept an extra finger as its own.

These findings point to the astounding plasticity and flexibility of the proprioceptive system, suggesting intriguing possibilities for other perceptual body modifications that might be achieved. With the advent of virtual reality, body ownership can be manipulated much more broadly than what a simple mirror box allows for. For example, researchers have used VR to induce the sensation of having a third arm or even a tail. The malleability of our own body representation may thus provide avenues for therapy to those suffering from body dysmorphic disorders.