Why can't the human body multi-task?

11 October 2009

Part of the show Why does Water Expand when it Freezes?

Busy Highway

Question

Why can't the human body multi-task? If you turn your right foot and move it slowly in a clockwise circle. So you're going round in a circle with your foot. Then writing a number 6 with the same hand that you're moving your foot on a piece of paper. But the better way to show this actually is to take your hand on the same side of your body as you're moving your foot. Now, try and make that move in a circle in the opposite direction to your foot.The foot follows the hand. Why does that happen?

Answer

It's very, very difficult!

You can do it with practice, but it is incredibly hard to do. If you try and do that number 6, you'll find that number 6 flips around, and you start drawing it backwards. And the reason for this is to do with the way your brain codes for movement. Because you can easily do that if you use the opposite sides of the body.

If you've got a hand which you do a clockwise circle with and your right hand and then you use your left leg, you'll easily do a circle in the anticlockwise direction because you're using two different sides of your brain.

If you're trying to use the same side of your body, the motor cortex which is the bit of the brain which codes for movements, the way this is working is that it doesn't actually code for a brain cell, telling a muscle what to do.

The brain actually codes for movements by what's called a tuning curve. So you have a cluster of nerve cells which fire off when you want to make a movement with a part of the body into a certain direction in space.

And those nerve cells don't just switch on muscles that move say, just the arm. They switch on muscles which would move your leg in the same direction too, but they turn them on a bit less than the motor neurons that control the arm. So basically, you're facilitating or making it easier for your leg to move in the same direction as your arm. But it takes a little bit more switch on to make the leg move as well.

Therefore, if you try then to make a movement in the opposite direction with the leg, you're basically facilitating another group of nerve cells to move in the opposite direction. So the two things are trying to fight it out and it's, whichever one wins, actually ends up going in that direction, and the arm is such a dominant force that's somewhat brain devoted to it, that I think it probably overwhelms the signal for the leg which is why the leg finds it hard to be dominant in that way.

But it's an amazing demonstration, isn't it? It's great fun. You can have a lot of fun with that at parties!