The thinking, conscious part of you – your executive function – can only pay attention to a little information at a time. Try to do two things at once, and you’ll do neither well.
But… but that doesn’t sound right.
Do you have but face? That’s the expression you have when you’re about to say “But…”
You can walk and chew gum at the same time. You can drive and talk to a passenger. How can that be? The answer is that you learn some things so well that they become automatic. Your brain (and body) does the tasks without bothering the “you” part of you. These are called automatic or over learned processes.
This is one of the differences between experts and novices. Experts do some things automatically, without paying attention to them. They have attention left over for other things.
Brains interrupt automatic processes when they need to. You’re driving, chatting with a friend in the passenger seat. An idiot in a black SUV pulls out in front of you. Driving needs attention. You stop talking to your friend, slam on the brakes, and give a hand signal to that jerk. What the signal is depends on what country you’re in. In Australia, you might use two fingers. In the US, just one.
Attention and neurons
Why do brains have limited attention? Some researchers think it has to do with neurotransmitters. One neuron activates another by sending chemicals (neurotransmitters) across synapses. Synapses are small gaps between neurons. This temporarily depletes the neurotransmitter stores in the sending neuron. The depletion is the memory trace. It fades as the neuron absorbs the neurotransmitters, building up its store.