In Lucy, Scarlett Johansson gains access to her other 90 per cent and becomes a telepathic, telekinetic, murderous genius.

Bradley Cooper takes a pill in Limitless and becomes a celebrated author and later, through an unlikely turn of events not worth describing here, a (also murderous) senate candidate.

Is it true? Are large parts of our brain locked away from us?

Well ... no.

"We are always using all the time 100 per cent of our brain. Even when we're asleep," says Dr Steve Kassem, a neuroscientist based at NeuRA in Sydney.

But don't feel too disappointed.

Because the science of what really is going on up there is far more interesting than any Hollywood blockbuster.

And, kind of like Lucy, you really can turbocharge your brain, making you temporarily smarter. But not for too long. Because you would die.

Yep. Every second of every day.

The thing is, says Dr Kassem, you have a very narrow experience of what your brain is doing. You experience it reading this sentence right now. But in the background it is doing so many other things that you are not aware of.

Let's start at the bottom of your skull. This region is called the brainstem. It is responsible for, among other things, keeping you alive.

It sends out signals to make your heart beat and your lungs breathe. It can minutely vary those signals depending on what the body needs; if you start running, it knows to raise your heartbeat to pump oxygen to your hardworking muscles.

On top of the brainstem sits the cortexes of the brain. First is the motor cortex, which sends signals to our muscles to make us walk.

As we stroll along, our somatosensory cortex - responsible for monitoring signals from nerves in our skin - is checking how much pressure our feet are putting on the ground, so we don't crush them. It is also monitoring our sense of balance and keeping us upright, while our visual cortex looks ahead to make sure we don't trip over.

Walking upright without falling over is very complicated and needs constant micro adjustments by our brain - adjustments we are blissfully unaware of.

The temporal cortex is monitoring what our ears are hearing. If we are crossing a road, it knows to listen for incoming cars, while our visual cortex (which makes up a huge 30-40 per cent of the brain) knows to watch for them through our peripheral vision - even if we aren't looking.

"Everyone's had that experience of walking and being on their phone, and you go to walk across a road and your brain just goes 'stop!'," says Dr Kassem. "It's always doing that - but you don't notice it until you bring attention to it."

The part of the brain that you can experience, the bit that you think of as you, is the prefrontal cortex, which sits behind your forehead. Our uniquely large cortexes are what give us the ability to logic and reason and have abstract thoughts. It is here that our consciousness sits, scientists suspect.

But you're not even fully aware of all the work it is doing. For example, this part of the brain is the one that studies the bricks in a wall and compares them to what we know about bricks and tells you 'please don't try to run through that wall'.

You can. But it's not good for you.

All the brain is working all the time. But it can increase the amount of power it has by ramping up its energy use. This is what a surge of adrenalin does.

"You feel it when you're in a really stressful situation: you're giving a public speech or about to get punched in the face," says Dr Kassem.

This boosts your processing power. You can feel a level of clarity and focus about your decisions, plus reaction speed. Some people report a sense of time slowing down in life-or-death situations; the big boost in brain power might be the reason.

But running on adrenalin all the time is bad. Long term, it can cause your neurons to die.

So save it for special occasions. Like, Dr Kassem says, when you are about to get punched in the face.

- SMH/The Age