These Feet Were Made for Walkin’

There’s an interesting new study that looks at how our hands and feet are wired in our brains. The results of the study suggest that hands for tool use evolved before feet for walking (the debate is over whether we evolved walking to free up our hands for tool use, or we evolved tool use after our hands were freed up because of upright walking).

But I was fascinated by another aspect of the study.

What the researchers did was compare the human somatosensory cortex with that of Japanese macaques, specifically in regards to the hands and the feet. You may recall we discussed the somatosensory cortex in regards to The F-Word. The paper is Hand before foot? Cortical somatotopy suggests manual dexterity is primitive and evolved independently of bipedalism.

We learned there that the different parts of the body all map to a specific part of the brain.

The Somatosensory Cortex

Then, when you look along the cortex there (the purple part), you can map what is called a “homunculus” that shows each location in the brain. Here’s the one from the new paper.

Human Homunculus

They also showed a similar homunculus (or wouldn’t that be a “simunculus”?) for the macaque.

Simian Homunculus

I have no idea why the latter is in more detail than the human one.

What they did in this study was stimulate individual fingers and toes to see how they mapped out in the brain.

For humans, it was non-invasive. They were put into an MRI machine for something called functional magnetic resonance imaging. With it you can see which part of the brain lights up when each individual digit is stimulated. For the monkeys, it was invasive as hell—you cannot make a monkey sit still for an MRI, so they surgically implanted electrodes.

Now, macaques have hands fairly similar to humans. They can manipulate their digits individually the way we do. They can also grasp things. And that shows up in their brains. Here’s the researchers representation of what they found.

Finger Processing in the Brain

Each digit is color coded. You can see that the positioning in the brains, for both species, is well separated.

When it comes to feet things are different. In the macaque, the feet are not used for upright walking. Yeah, they get around on them but they are also used for grasping tree limbs. When you look at how the brain processes that, it’s a smear.

Macaque Toe Processing in the Brain

There just isn’t the individual feedback from each digit. The brain concentrates on the grasping that uses all the digits at once. There’s a little bit of separation on the big toe, but that’s because it’s a grasping toe.

But what happens with humans? Here’s a picture of the actual locations in the brain (using the same color-coding). Because it’s fMRI, we can get an actual 3-D look at the locations.

fMRI Picture of Human Toe Processing

You can see now the big toe is well-separated, but the rest of the toes all pretty much overlap.

Here’s their picture showing the amount of “smear”.

Human Toe Processing in the Brain

Again, the lesser toes are quite a smear, while the big toe stands alone.

That’s because we have to use it to walk. We need it to provide good feedback to keep our balance and to navigate the various surfaces we are walking on. If you pay attention to what you do and what you feel when you walk barefoot, you’ll realize just how much you depend on that toe.

And I’m pretty sure that shoes turn a lot of that off. They already turn off a lot of feedback from the soles, because the soles in shoes only feel the bottom of the shoe and not the texture of the surface walked on. Well, shoes also turn off this extra degree of proprioception provided by that specialized portion of the brain just for the big toe.

That’s an important part of our evolution and an important part of our walking.

When you walk barefoot, you’re actually utilizing the way your body and brain are integrated, as a unit. Put shoes on that and the feedbacks get diminished and all screwed-up. This is an area of feedback that I was previously unaware of, but now that I know of it I can tell just how it does contribute to my extra awareness when walking barefoot.

And for the shod that all just smears away . . .