How your eyes deceive you...

27 April, 2012

People rely on their eyes for most things they do – yet the information provided by our visual sensing system is often distorted, unreliable and subject to illusion.

Researchers at The Vision Centre, Australia, are throwing new light on the tricks the brain plays on its owner as it struggles to make sense of the visual and other sensory signals it constantly receives.

"We tend to regard what we see as ‘the real world’," says Dr Isabelle Mareschal, a researcher with the Vision centre and University of Sydney. "In fact a lot of it is distortion, and it is occurring in the early processing of the brain, before consciousness takes over."

"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.

In a new article in the scientific journal Proceedings of the National Academy of Science, Dr Mareschal and Professor Colin Clifford report a series of groundbreaking experiments 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," she explains.

"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."

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," Dr Mareschal says. "And we found that even the higher brain cannot always correct for them, as it doesn’t in fact know they are illusions."

This is one of the reasons that people’s eyes sometimes mislead them when looking at objects in their visual landscape.

Normally she says, it doesn’t matter all that much – but in the case of a person driving a car fast in traffic, an athlete performing complex acrobatic feats, a pilot landing an aircraft or other high-speed uses of sight, the illusion may be of vital importance by causing them to misinterpret the objects they ‘see’.

The brain uses context, or background, to interpret a host of other visual signals besides the orientation of objects – for example it uses it to tell colour, motion, texture and contrast – so the Vision Centre research has wide scope for understanding how the brain interprets the world visually – and indeed how the brain itself works.

"We propose that our technique is a powerful tool that could be applied to contextual effects in other realms, such as colour or depth," the researchers conclude. "Distinguishing visual processes by their dependence on a complete, conscious representation of the stimulus is a critical step toward formulating a comprehensive model of cortical processing."

Their paper "Dynamics of unconscious contextual effects in orientation processing" by Isabelle Mareschal and Colin W. G. Clifford, appears in the journal the Proceedings of the Academy of Science (PNAS).

The Vision Centre is funded by the Australian Research Council as the ARC Centre of Excellence in Vision Science.

Source: ARC Centre of Excellence in Vision Science