Sloppy brain, sloppy hands

I am so sloppy that I damage the things I love. I treasured the first house that I owned. So I wanted to paint it perfectly but failed blatantly. Why was I unable to miss the wall when painting the window frame? Unlike a robot, humans are generally a bit sloppy: we are unable to repeat the exact same movement twice. There are two sources of apparent sloppiness in your movements: brain sloppiness and ‘motor noise.’ Reading this blog you’ll find out that one may actually help you!

Brain sloppiness

When you intend to bring a paint brush towards a chosen spot, your brain needs to send a ‘movement plan’ to the muscles. Here I call imperfections in how the brain plans these movements ‘brain sloppiness.’ In the science of motor control we call it ‘planning noise.’

Brain sloppiness arises for two reasons: your brain cells are imperfect information transmitters and there may be uncertainty in the information that the movement plan is based on. For instance the sensory information about the movement target (the door post, not the wall) may be unreliable because it is getting dark and your tired eyes lose focus.

Uncertainty will cause errors in your movement planning. If you want to know how you correct for these errors check out this video. But noise can go either way. An accident may be more successful than the planned movement! You probably experienced the wonders of beginner luck in bowling or darts. The cool thing about brain sloppiness is that you can learn from these happy accidents. (yes, I did quote Bob Ross check out a much better painting video by Onno Lolkema).

One of my colleagues, Robert van Beers showed that the brain does not represent the aimpoint of the movement but the sloppy movement plan. Those interested can read his paper here. This means that the accidentally successful movement is represented in the brain. Therefore you can learn from it!


When humans repeatedly point to the same location, there is variability in their endpoints. This is caused by planning noise and execution noise. Illustrations taken from Journal of Neurophysiology

Learning from happy accidents

Learning from success is called ‘reinforcement learning.’ This type of learning relies on a combination of exploration and biasing future movement plans towards movement plans that turned out successful. Think of success as water carving out a path for future streams. Brain sloppiness can be considered a form of exploration and therefore it can help you discover better strategies. Maybe that is why my sloppiness in house painting goes hand in hand with producing paintings that sometimes look surprising beautiful.

Sloppy hands

Unfortunately, not all sloppiness is helpful. Sloppiness also arises in your muscles. Even if the brain instructed them perfectly, they are biologically limited in how they execute the movement. The scientific term for this type of sloppiness is ‘execution noise.’ This is the form of sloppiness that increases with fatigue and that decreases with expertise. This kind of noise doesn’t do much for you. It is not represented in the brain. Therefore you cannot reproduce it after a happy accident.

Do you have a sloppy brain or sloppy hands?

If you want to now whether you have sloppy hands or a sloppy brain, you could do a simple test. This test is based on the idea that when your brain repeats the sloppy movement plan, each bit of new sloppiness is stapled onto the previous sloppiness, causing errors to be dependent on each other.

First determine an aim point, then dip your finger in anything that will leave a mark (Nutella!). Now point a hundred times at the aim point you determined. Open your eyes and examine the pointing marks. If they form a circle, this indicates that each error was independent of the previous one. If the errors form a trace drifting away from the first mark, this indicates that each error was stapled onto the previous one. In other words: they are dependent. This causes errors to form a ‘random walk.’

When your traces form a random walk this indicates that brain sloppiness affected your movements. Lucky you! When your traces form a circle, this indicates that you have sloppy hands.


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