The human brain holds many mysteries, and this is illustrated most clearly by the existence of a range of phenomena, such as phantom limb pain. This particular phenomenon occurs when a person believes they can detect pain or other tactile sensations in a limb that they have lost through amputation.
Some people experience tactile hallucinations, in which they mistakenly believe they feel a sensation when, in fact, no factors could have induced it.
Tactile hallucinations usually occur in individuals living with a psychological condition, such as schizophrenia. However, people who are entirely mentally and physically healthy can also experience a similar phenomenon.
For instance, when a person receives a touch on their left hand, they may believe that they felt this touch in their left foot or vice versa. Scientists call this a phantom sensation, and researchers are still puzzled as to why this phenomenon occurs.
In a new study, whose findings appear in Current Biology, a team of researchers from New York University and the Universities of Hamburg and Bielefeld in Germany explain in more detail what characterizes phantom sensations. They argue that a better understanding of this phenomenon could help specialists decipher similar mysteries, including phantom limb pain.
“The limitations of the previous explanations for how and where our brain processes touch become apparent when it comes to individuals who have had parts of their bodies amputated or suffer from neurological diseases,” notes study coauthor Prof. Tobias Heed. He emphasizes that to this day, scientists know surprisingly little about how the human brain processes the sensation of touch.
“People who have had a hand or a leg amputated often report phantom sensations in these limbs,” Prof. Heed observes. “But where exactly does this false perception come from?”
A changing understanding of brain processes
“Previously, scientists thought that our conscious perception of where a touch occurred stems from a topographical map in the brain. Following this assumption, parts of the body, such as the hands, feet, or the face are represented on this map,” says Prof. Heed.
However, this new study, which focuses on fully healthy participants, indicates that the way the brain attributes sensations of touch is much more complicated.
“Our new findings […] demonstrate that other characteristics of touch are also used to attribute a touch to parts of the body,” Prof. Heed notes.
In the current study, the investigators conducted five different experiments, each of which involved the collaboration of between 12 and 20 healthy adults. In each experiment, the participants agreed to have tactile stimulators attached to their hands and feet.
The researchers used these stimulators to generate touch sensations in two different parts of the body in quick succession and then asked the participants to report where they had felt the touches. Prof. Heed and team repeated this test several hundred times for each participant.
“Remarkably, in 8% of all cases, subjects attributed the first touch to a part of the body that had not even been touched — this is a kind of phantom sensation,” says lead author Stephanie Badde.
3 factors contribute to phantom sensations
“The previous conception — that the attributed location of touch on the body depends on ‘maps’ of the body — cannot explain these new findings,” notes Prof. Heed.
“We show that phantom sensations depend on three characteristics. The most important is the identity of the limb — whether we’re dealing with a hand or a foot. This is why a touch on one hand is often perceived on the other hand,” he explains.
Two more factors contribute to the misattribution of touch:
- the side of the body — a person might think they sense touch in their right hand when, in fact, the touch occurred on their right foot
- the normal anatomical position of the limb (right or left)
For example, if a person crosses their arms or legs, positioning the right-side limb on the left of the body, they might mistakenly sense a touch on their right arm as a touch on their left foot.
“When parts of the body are positioned on the other side of the body than they usually are — for example, when crossing your legs — the two coordinate systems come into conflict,” says Prof. Heed.
Not only do the current findings contradict previous understandings of the brain’s mode of processing touch, but they might also, in the future, help guide research surrounding phantom limb sensations and other related phenomena.
“The findings could be used, for instance, to drive forward new research on the genesis of phantom pain.”
Prof. Tobias Heed