AN ILLUSION device that makes one object look like another could one day be used to camouflage military planes or create "holes" in solid walls.

The idea builds on the optical properties of so-called metamaterials, which can bend light in almost any direction. In 2006, researchers used this idea to create an "invisibility cloak" that bent microwaves around a central cavity, like water flowing around a stone. Any object in this cavity is effectively invisible.

Now a group of researchers has gone a step further. "Invisibility is just an illusion of free space, of air," says Che Ting Chan, a physicist at the Hong Kong University of Science and Technology and a co-author of the study. "We are extending that concept. We can make it look like not just air but anything we want."

Instead of bending light around a central cavity, the team has worked out the mathematical rules for bending light in other ways. For example, a material could be designed to bend light in the same way as a spoon would. So the light hitting the material would be distorted to make it look as if a spoon were there.

It is also possible to design a complementary material that has the opposite effect - to exactly cancel out the effect that an object has on light. So light distorted by a spoon could be passed through a complementary material to eliminate these distortions.

The new illusion device uses these two ideas together. To make a cup look like a spoon, for example, light first strikes the cup and is distorted. It then passes through a complementary metamaterial which cancels out the distortions to make the cup seem invisible. The light then moves into a region of the metamaterial that creates a distortion as if a spoon were present. The result is that an observer looking at the cup through the metamaterial would see a spoon (Physical Review Letters, vol 102, p 253902).

The idea has some surprising applications. Chan says the technique could be used to change the optical properties of an opaque material, allowing light to tunnel from one side to the other. That could lead to a device that when attached to a wall, creates a "hole" through which viewers could see the other side.

But although illusion cloaks are theoretically sound, there are numerous engineering challenges to overcome before they might become practical.

Metamaterials are difficult to build. Their components need to be much smaller than the wavelength of the light they distort, which for visible light is less than a micrometre. It would also be challenging to design both parts of the device so that they don't interfere with each other.

John Pendry, the physicist at Imperial College London who devised the theory behind invisibility cloaking in the 1990s, thinks these difficulties can be overcome. "I don't see any obstacles to this device being built," he says.

This schematic shows the brain regions (green) from which the outcome of a participant's decision can be predicted before it is made. Courtesy John-Dylan Haynes.

You may think you decided to read this story -- but in fact, your brain made the decision long before you knew about it.

In a study published Sunday in Nature Neuroscience, researchers using brain scanners could predict people's decisions seven seconds before the test subjects were even aware of making them.

The decision studied -- whether to hit a button with one's left or right hand -- may not be representative of complicated choices that are more integrally tied to our sense of self-direction. Regardless, the findings raise profound questions about the nature of self and autonomy: How free is our will? Is conscious choice just an illusion?

"Your decisions are strongly prepared by brain activity. By the time consciousness kicks in, most of the work has already been done," said study co-author John-Dylan Haynes, a Max Planck Institute neuroscientist.

Haynes updated a classic experiment by the late Benjamin Libet, who showed that a brain region involved in coordinating motor activity fired a fraction of a second before test subjects chose to push a button. Later studies supported Libet's theory that subconscious activity preceded and determined conscious choice -- but none found such a vast gap between a decision and the experience of making it as Haynes' study has.

In the seven seconds before Haynes' test subjects chose to push a button, activity shifted in their frontopolar cortex, a brain region associated with high-level planning. Soon afterwards, activity moved to the parietal cortex, a region of sensory integration. Haynes' team monitored these shifting neural patterns using a functional MRI machine.

Taken together, the patterns consistently predicted whether test subjects eventually pushed a button with their left or right hand -- a choice that, to them, felt like the outcome of conscious deliberation. For those accustomed to thinking of themselves as having free will, the implications are far more unsettling than learning about the physiological basis of other brain functions.

Caveats remain, holding open the door for free will. For instance, the experiment may not reflect the mental dynamics of other, more complicated decisions.

"Real-life decisions -- am I going to buy this house or that one, take this job or that -- aren't decisions that we can implement very well in our brain scanners," said Haynes.

Also, the predictions were not completely accurate. Maybe free will enters at the last moment, allowing a person to override an unpalatable subconscious decision.

"We can't rule out that there's a free will that kicks in at this late point," said Haynes, who intends to study this phenomenon next. "But I don't think it's plausible."

That implausibility doesn't disturb Haynes.

"It's not like you're a machine. Your brain activity is the physiological substance in which your personality and wishes and desires operate," he said.

The unease people feel at the potential unreality of free will, said National Institutes of Health neuroscientist Mark Hallett, originates in a misconception of self as separate from the brain.

"That's the same notion as the mind being separate from the body -- and I don't think anyone really believes that," said Hallett. "A different way of thinking about it is that your consciousness is only aware of some of the things your brain is doing."

Hallett doubts that free will exists as a separate, independent force.

"If it is, we haven't put our finger on it," he said. "But we're happy to keep looking."

How Believing Can Be Seeing: Context Dictates What We Believe We See

ScienceDaily (Feb. 19, 2008) - Scientists at UCL (University College London) have found the link between what we expect to see, and what our brain tells us we actually saw. The study reveals that the context surrounding what we see is all important -- sometimes overriding the evidence gathered by our eyes and even causing us to imagine things which aren't really there.

The paper reveals that a vague background context is more influential and helps us to fill in more blanks than a bright, well-defined context. This may explain why we are prone to 'see' imaginary shapes in the shadows when the light is poor.

Eighteen observers were asked to concentrate on the centre of a black computer screen. Every time a buzzer sounded they pressed one of two buttons to record whether or not they had just seen a small, dim, grey 'target' rectangle in the middle of the screen. It did not appear every time, but when it did appear it was displayed for just 80 milliseconds (80 one thousandths of a second).

"People saw the target much more often if it appeared in the middle of a vertical line of similar looking, grey rectangles, compared to when it appeared in the middle of a pattern of bright, white rectangles. They even registered 'seeing' the target when it wasn't actually there," said Professor Zhaoping, lead author of the paper. "This is because people are mentally better prepared to see something vague when the surrounding context is also vague. It made sense for them to see it -- so that's what happened. When the target didn't match the expectations set by the surrounding context, they saw it much less often.

"Illusionists have been alive to this phenomenon for years," continued Professor Zhaoping. "When you see them throw a ball into the air, followed by a second ball, and then a third ball which 'magically' disappears, you wonder how they did it. In truth, there's often no third ball - it's just our brain being deceived by the context, telling us that we really did see three balls launched into the air, one after the other.

"Contrary to what one might expect, it is a vague rather than a bright and clearly visible context that most strongly permits our beliefs to override the evidence and fill in the blanks. In fact, a bright and clearly visible context actually overrides the evidence in the opposite direction - suppressing our 'seeing' of the vague target even when it is present.

"Mathematical modelling suggests that visual inference through context is processed in the brain beyond the primary visual cortex. By starting with a relatively simple experiment such as this, where visual input can be more easily and systematically manipulated, we are gaining a better understanding of how context influences what we see. Further studies along these lines can hopefully enable us to dissect the workings behind more complex and wondrous illusions."

Journal reference: Filling-in and suppression of visual perception from context -- a Bayesian account of perceptual biases by contextual influences, by Professor Li Zhaoping and Dr Li Jingling appears in the 15 February issue of the journal PLoS Computational Biology.

The research was funded by the Gatsby Charitable Foundation and a Cognitive Science Foresight Grant.