Finger illusions

A new class of illusions discovered at NeuRA

Researchers at Neuroscience Research Australia (NeuRA) have revealed a new class of illusions with the realisation that humans may be more attuned to their own bodies than previously thought.

The findings provide fundamental information into how the brain makes maps of the body so that it can then make accurate movements. The results expand the current understanding of proprioception and body representations. Researchers say the findings will provide new insight into clinical conditions where proprioception is disrupted.

Professor Simon Gandevia, Deputy Director of NeuRA and colleagues have tricked the brain into believing it owns a fake finger using only sensory inputs from the muscles for the first time.

“It may seem silly to ask yourself whether your index finger is part of your body. However, our current findings demonstrate that this question is far from silly and has led to important insights into key brain functions.”
The ground-breaking study highlights that sensing which parts of your body are yours (body ownership) and their positions in the world (proprioception) does not require vision, nor does it require touch. It can be achieved by muscle receptors alone.

PART 1. Main Finding
“The experiment relies on removing skin and joint messages from the relevant fingers both on the test side (the right) and the left, which we call the grasping side. This side grasps a fake finger” explains Professor Gandevia.
“When your grasping left fingers are made to move the fake right finger that is connected to your real right finger congruently, you will believe that you are holding your right index finger with your left hand. The fake finger becomes your own right index finger.

PART 2. A New Illusion
In addition, NeuRA researchers found a new grasp illusion in which the perceived distance between your hands shrink when you simply hold (and do not even move) the fake finger with your left hand.
“This second observation helps us to understand how our body maps are composed. It tells us that just touching something that is like a body part can dramatically reduce the distance between two relevant body parts. We suspect it is because the brain tests a strange scenario. It chooses to solve the scenario quickly by identifying the fake finger as close to your real finger”

What conditions may the findings seek to inform in the future?

“Ultimately these discoveries are relevant to any disorder where there is an abnormality in proprioceptive representation of the body and its position and the forces it’s generating. Stroke and schizophrenia and even phantom limb syndrome come to mind.

“In future it may be combined in to some therapies where you might manipulate the muscle signal and perhaps add a skin signal in order to try and change someone’s body image. Several possibilities will now open up given this work”

This paper ‘Is this my finger? Proprioceptive illusions of body ownership and representation’ has now been published in the Journal of Physiology.