Don't believe everything you see

Optometry Tomorrow keynote speaker, Sir Colin Blakemore, can make an object disappear in front of a person’s eyes, not that they are likely to notice.

The seeming-magic trick – where one shape of three changes form or colour in front of a participant’s eyes – is part of one experiment Sir Blakemore has done to demonstrate the role the brain plays in that picture of the world we all see.

The University of London professor set out to show the practitioners gathered at the Optometry Tomorrow (Hilton Birmingham Metropole, 13–14 March) Charter Lecture that vision is about more than just healthy eyes.

The constant movement of the eyes – from the flicks three times a second to eye drift and tremor – is evidence of the brain’s input, as our view of the world does not include these rapid shifts, he told OT.

Another clue is our lack of visual memory. A person may say with confidence they can see the room clearly, but if spontaneously asked to close their eyes, they can often struggle to name the colour of their companion’s shirt or the number of objects on the table in front of them.

Sir Blakemore explained: “We have no visual image past. We retain a tiny part … and the rest just goes.”

This loss is deliberate, he said, adding: “The brain is always making judgements about what is the most salient part of the image. It has very limited resources.”

Another limitation is in the eye itself: “There are more than 120 million photoreceptor cells but only 1.5 million optic nerves. There’s a large compression of information.”

Therefore, the majority of our view of the world is the brain’s assumption of what is there, he stressed.

“Anyone who goes to a magic show knows they can’t trust their vision. [But] from reading to driving, it’s remarkable we do so well.”

Evolution has selected for sufficient but not perfect vision, Sir Blakemore explained. However, the danger comes when we put our full faith in what we saw – or worse, what we think we remember we saw.

“The legal profession has very substantially changed its position on eyewitnesses … When implanting a suggestion in someone’s mind, it can become as secure as the real visual experience,” he said.

One of the most fascinating discoveries – which raised almost philosophical questions – was that the part of the brain that produces our constant ‘picture’ of the world in front of us is different to the area that uses visual information to pick up a cup or tap the correct letter on a keyboard, the neuroscientist said.

“Why do we need a picture of the world, a personal visual experience? It’s a puzzle,” he mused.