You think you know your right from your left, and your foot from your hand; but a study published by Bielefeld University at the University of Hamburg with New York University challenges the way scientists explain how we know these things—or don’t know, as they reveal.
The brain perceives touch due to neighboring neurons which respond to corresponding areas of skin. Scientists believed that our perception of a touch to our body was based on an anatomical system of reference: a map that placed a stimulus, such as a touch to a hand, in a context of right/left, above/below, etc.
Scientists believed that there was a map of the body that existed in the brain and represented the parts of the body, and that the external map of where they are in space was also being used by the brain.
This belief did not explain what happens when the body is crossed, such as sitting cross-legged, and a touch on the right hand is interpreted by the brain as a touch to the right, or even left, foot!
The external mapping system should, theoretically, locate the left leg on the right side but still represent the information about what side of the body the limb belongs to, but it did not.
The experiment conducted at Bielefeld University attached little stimulators on participants’ hands and feet and generated impulses and asked the participants to tell the scientists where they had felt the “touch”. Participants went through several hundreds of these impulses, and some participants crossed their feet or hands. In 8% of the cases, participants said the first touch was to a part of the body that had not been touched yet.
When the researchers crossed the body parts and the left hand was on the right side of the body, the brain sometimes believed that the touch was to the right foot, which is a part of the body that does not belong to the same side of the body or the same position in space.
It seems that the brain believed that since the left hand was positioned in the place near the right foot, the right foot was perceived as the part that was touched.
This study is the first to conclude that even healthy adults can misattribute touch and shows promise for a new understanding of phantom pains and sensations.
- Stephanie Badde, Brigitte Röder, Tobias Heed: Feeling a Touch to the Hand on the Foot. Current Biology, https://doi.org/10.1016/j.cub.2019.02.060, published online on 4 April 2019, in print on 6 May 2019.