Your brain contains a highly organised map of your body. Not a normal kind of map – this one will vary ever so slightly because of the particular way you use your body. What you do for a living might affect this, for example – your brain’s hand map might show subtle clues that you are a pianist or a surgeon. Or reflect that you rock climb or write a lot.
We’ve known subtle details of the brain’s body map can change as a result of our daily life experiences for a while. But new research by myself and colleagues has now demonstrated how powerfully experience can affect the brain.
We used ultra high-res brain scanning to reveal clear maps of individual toes in two foot painters, born without either arm. While these organised toe maps are not found in typically developed humans, they are found in monkeys – who, like the foot painters, use their toes in skilled ways.
This suggests that all humans could have the potential for toe maps, but modern life in shoes prevents them by limiting individual movement of our toes.
The body in the brain
These maps were found in the somatosensory cortex of the brain, which contains a map of our whole body. All body parts are represented by an individual section of brain, and these sections are laid out in the brain as they are on the body. In the brain’s hand map, for instance, there are small sections representing each of the five fingers – with the thumb next to the index finger, which is next to the middle finger, and so on.
It is because of this beautiful and clear organisation that this area is of big interest to scientists studying how the brain changes in response to experience – known as brain plasticity. If we know how the body map normally looks, we can easily document any changes caused by how we use our body.
As an example, it has been shown that learning a musical instrument leads to increases in the size of finger maps for those fingers highly used to play. In a more extreme case, when two fingers are fused together with surgery, the brain maps of the two fused fingers also combine into one.
The foot map
Until very recently, it was generally assumed that the typical foot map should have clear sections to represent individual toes, like the hand map has fingers. Only recently did we find out that this, surprisingly, is not the case. In fact, most people don’t have a sections for each of the five toes. And, those they do have are scattered all over the foot area, in no clear order.
This lead my colleagues and I to wonder whether this is how the human foot map is naturally. Or, could it result from the fact that modern humans don’t really use their toes separately?
To help solve this mystery, we approached two incredible individuals for help. These two people were born without either arm, and subsequently had to learn to use their toes to perform all tasks of daily life. This includes almost any typical-hand task most of us can do: including typing on a keyboard, answering the phone, putting on clothes (in one case, including doing buttons) and feeding themselves with a fork or spoon.
It also includes some tasks that most two-handers would struggle with, like administering injections to animals with a syringe (one was a farmer), and – my favourite – one would apply nail varnish to his wife’s nails for her.
This skill with a brush made total sense because both individuals were actually sufficiently skilled with their toes so as to support their profession as foot artists: they make art with their feet better than most people do with their hands.
Looking in the brain
We put these two artists in an ultra high-field fMRI scanner and stimulated each of their toes, one at a time. When we looked in the foot area of the artists’ brains, we found that they had individual, organised toe maps – just like the hand maps of you and I. We compared this to a group of two-handed people, who showed no such organised toe maps – replicating previous findings.
Using new analysis methods, we showed the pattern of brain activity in the foot area resulting from touching the artists’ toes was highly similar to a typical hand pattern. That is, the pattern generated by touching the fingers of two-handed people, in their brain’s hand area.
We next moved from looking in the foot area of the brain, to see what was happening in their (missing) hand area when we touched the artists’ toes. This could provide more extreme examples of brain plasticity. We found that the pattern of activity in the hand area was also “hand-like” in the artists. This might indicate the artists are recruiting some of the “unused” hand area to support their skilled toe movement.
All in all, our results suggest that using your toes in a hand-like manner causes hand-like activity in the brain.
Losing toe maps
Our results make sense from a brain plasticity perspective – if you don’t use your toes separately in action, your brain does not need to represent each toe separately. The results also make sense given our primate cousins have organised toe maps, in a similar brain position and orientation to the artists.
This could indicate one of two things. One, either all primates (human and non-human) have the genetic potential for toe maps, but typical humans don’t develop them because we don’t use our toes individually. Or, it could mean that we are born with toe maps as babies, but lose them over time if we don’t use our toes the right way.
Whether toe maps fail to develop or fail to persist remains to be determined. But looking at the toe maps of babies – or even populations who live without shoes – could be the key to unlocking this mystery of brain plasticity.