Bendigo scientists believe this "Size-Weight Illusion" is much more than a bizarre quirk.

They are using it to plumb the depth of the human mind and say it could help reveal how we process reality, as well as the deeper mysteries of consciousness itself.

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The 134-year-old illusion still mystifies psychologists and philosophers, La Trobe University's Philippe Chouinard said.

"It is an example of how what we perceive and feel is very different from the information we take in," Dr Chouinard said.

It shows that people don't need an accurate reading of the world around us to make exact movements, he said.

It makes us very different from machines, that need exact readings of the world around them.

No-one is entirely certain how us humans do it. That is where Dr Chouinard and his team come in.

They are based in La Trobe Bendigo's Vision & Action Laboratory, which sits at the end of an unassuming hallway on the university's Bendigo campus.

Behind its doors are tools ranging from virtual reality equipment to eye tracking software. There are even machines that measure pinch strength and can give scientists an insight into how tightly a person is holding the items they pick up.

The lab is one of a number worldwide delving into visual illusions.

It is not just humans that world experts are studying, either.

Dr Chouinard himself has contributed to recently published research showing how dogs react to a range of optical illusions.

Researchers have discovered dogs can see some but not all of the illusions that humans perceive.

They can, for example, see the Ebbinghous Illusion (pictured) but the circle they think is bigger is the opposite to the ones humans perceive.

Dogs are not tricked by other illusions that humans are fooled by the scientists have found, raising the opportunity to compare how different species process the world.

Much of that research was led by experts who have cut their teeth at the Bendigo campus.

Dr Chouinard and his laboratory's focus centres on humans and how they perceive optical illusions.

His PhD students have recently delved into how people perceive the Rubber Hand Illusion, where people hide their hand while watching, say, a huntsman spider crawling across a fake limb.

Scientists have used the illusion to discover how and when people start feeling a tingling on the back of their own hands, as if the venomous arachnid is creeping across their own skin.

Humans and spiders have not been harmed when Bendigo scientists have replicated that illusion.

Dr Chouinard has a particular interested in the Size-Weight Illusion because it could reveal how the conscious and unconscious minds interact with each other.

The illusion, Dr Chouinard says, exposes this "dual mind" and allows scientists to study the mechanics of how the body senses something and then notifies conscious and unconscious parts of the mind.

There are three broad theories that could explain the Size-Weight Illusion, though none seem to do so on their own.

The first was proposed by the man who first wrote about the illusion in 1886 by French physician Augustin Charpentier.

Charpentier noticed that people always thought the smaller of any two objects used appeared were the ones people thought were heavier.

He wondered whether that was because the smaller object's mass was more concentrated in the hand.

Other experts have wondered whether it is less about the object itself and more about people's brains mistakes an object's density for it's weight.

Maybe, they have reasoned, a small object has to be denser than a large one if it weighs the same amount.

Still more experts have argued that it is actually about people's expectations about what they are lifting.

They say that sensations of lifting something unexpectedly light or heavy will change our perceptions of its weight.

Dr Chouinard suspects there is some combination of all three theories at work.

"That's certainly what we find in our lab," he said.

"Some a probably more involved (than others), depending on the situation."

Obviously, our minds have found a way around their apparent shortcomings.

"This illusion might us that our perception of these objects is different from reality, but we have to successfully grasp and lift them and it has to be done very accurately," Dr Chouinard said.

"That's driven by a completely different body system - the motor system."

Dr Chouinard said a theory that satisfactorily explains what is happening is still some way off.

When we do, the theory will likely tell us a lot about the surprisingly ill-defined concept of consciousness.

"What's weird about consciousness is that you and I experience it, so we can feel really confident that it exists," Dr Chouinard said.

"But at the same time it is so hard to define. There's no consensus among psychologists or philosophers about it.

"It's also one of the hardest things for scientists to test because we have to rely on all these subjective reports and tricks, like the ones we use in my lab."

Dr Chouinard sometimes feels like thinkers 400 years ago who made a major breakthrough when they came up with a theory on how disease spreads.

"Before then people didn't know where diseases came from. Then 'germ theory' came out and all of a sudden everything was explained," he said.

"I think it's going to be the same thing with consciousness. We have this vague concept that we are studying but we don't really understand it.

"Hopefully, one day, somebody will unlock that secret."

To learn more about Philippe Chouinard and his team's work at La Trobe University Bendigo's Vision & Action Laboratory visit the website: www.pachouinard.com