Your eyes ‘can deceive you'

People rely on their eyes for most things they do. But, researchers say that even your eyes can deceive you.

An international team, led by the Vision Centre in Australia, says that the information provided by people's visual sensing systems is often distorted, unreliable and subject to illusion.

“We tend to regard what we see as ‘the real world' In fact a lot of it is distortion, and it is occurring in the early processing of the brain, before consciousness takes over,” said team leader Dr Isabelle Mareschal.

Role of visual cortex

He added: “Our latest work shows that the cells of the primary visual cortex create small distortions, which they then pass on to the higher levels of the brain, to interpret as best it can.”

A common example of this that is often exploited by artists and designers is known as the tilt illusion where perfectly vertical lines appear tilted because they are placed on an oriented background, say the researchers.

For their study, the researchers analysed a series of groundbreaking experiments in which they trace the origins of the tilt illusion to the cells of the primary visual cortex — the first stage of vision processing, before the conscious mind takes over.

“We wanted to test at what level the illusion occurs in the brain, unconscious or conscious — and also to see if the higher brain is aware of the illusions it is receiving and how it tries to correct for them.

“The answer is that the brain seeks more contextual information from the background to try to work out the alignment of the object it is seeing,” Dr Mareschal said.

The team subjected volunteers to a complex test in which they had to indicate the orientation of a vertical line, which was perceived as constantly tilting from side to side, against a fuzzy background that was also changing.

“These illusions happen very fast, perhaps in milliseconds. And we found that even the higher brain cannot always correct for them, as it doesn't in fact know they are illusions,” she added.

The findings have been published in the Proceedings of the National Academy of Science journal.