Series in which Dr David Eagleman takes viewers on an extraordinary journey that explores how the brain, locked in silence and darkness without direct access to the world, conjures up the rich and beautiful world we all take for granted.

This episode begins with the astonishing fact that this technicolour multi-sensory experience we are having is a convincing illusion conjured up for us by our brains.

In the outside world there is no colour, no sound, no smell. These are all constructions of the brain. Instead, there is electromagnetic radiation, air compression waves and aromatic molecules, all of which are interpreted by the brain as colour, sound and smell.

We meet a man who is blind despite the fact that he has eyes that can see. His story reveals that it's the brain that sees, not the eyes. A woman with schizophrenia, whose psychotic episodes were her reality, emphasises the fact that whatever our brains tell us is out there, we believe it.

Visual illusions are reminders that what's important to the brain is not being faithful to 'reality', but enabling us to perceive just enough so that we can navigate successfully through it. The brain leaves a lot out of its beautiful rendition of the physical world, a fact that Dr Eagleman reveals using experiments and street demonstrations.

Each one of our brains is different, and so is the reality it produces. What is reality? It's whatever your brain tells you it is. Show less