The world is not as it seems — and not just because of COVID.
Your brain is continually playing around with your sense of reality.
But you may only notice this when you get caught out by a magic trick, a blue-and-black striped (or is that a white-and-gold striped) dress, or an enormous moon rising above the horizon.
Illusions have been used throughout history to amaze, entertain, trick and even scare us.
But they are also a mind-boggling window into how our brain works.
"Illusions help us understand the rules our brain uses to create reality, based on the input it receives from our senses," says Mark Williams, an honorary professor of cognitive science at Macquarie University.
"Because we don't see the world as it actually is, illusions explain to us how we are creating the world we actually perceive."
We tend to separate what we think of as "the real world" and "illusions", says Branka Spehar, a psychologist at UNSW, who studies visual perception and attention.
"On some level, everything is an illusion and nothing is an illusion."
Our brain does not sense the world in absolutes; instead, it compares the sum of all parts of what it perceives and filters out information it doesn't think is important.
"Our sensory systems respond to the sum of all the contextual information in which the relative information is more important than the absolute," Dr Spehar says.
"So for example, you perceive colour relative to the background, or you see orientation relative to the frame of reference."
And with so much going on, sometimes we can overlook the simplest of details.
So let's put your view of reality to the test with a few illusions, starting with a magician's favourite. Watch carefully …
The jellybean and the cup
Congratulations if you kept up with the jellybean, but the chances are you might have missed the fifth hand, the fox and the change in colour of the cups. So what's going on?
This illusion shows we don't pay attention to everything that's going on, even in simple scenes, Dr Spehar says.
"As you are tracking the movement of the cups, your attentional capacity is just absorbed by those things and you just don't have capacity to encode other things that are happening in that location."
This illusion has real-world implications. The ability to filter information is important for performing tasks such as driving.
The whole illusion is showing we have a capacity to select information that's relevant for our task, which in this case is tracking the jellybean under one of the cups when everything is moving.
Rotating snakes
This is a static image. Or is it?
The image appears to move, but if you stare at the central point, it stops moving. So what's going on?
Known as rotating snakes, this illusion triggers receptors in your retina that are responsible for detecting movement in your peripheral vision.
Dr Spehar says we don't exactly know how this illusion works, but it appears to be tied to differences in contrast between the black and white and coloured areas, as well as eye movements.
"The illusion has lots of light and dark elements, of both high and low contrast scattered everywhere," Dr Spehar explains.
"[These] stimulate motion-sensitive neurones in the periphery of our visual field."
When you blink or move your eyes, the elements of the illusion are projected onto different parts of your retina so you experience the sensation of clockwise or counterclockwise motion.
Ames room
This is just an ordinary room. Or is it?
Instead of being shaped like a box, the back wall of an Ames room is on an angle.
In this example from Catalyst, the right-hand corner is closer to us than the left-hand corner, so it distorts our view of Lily.
When she stands on the right she looks like a giant, but when she moves to the left she appears to shrink.
So what's going on?
The Ames room plays with our assumptions about how spaces work.
So when the walls in a room aren't at right angles, we still think they are.
"Therefore, if you're a certain size compared to the ceiling on one side then you must be relatively the same size on the other side, so it makes you look smaller than you really are," Professor Williams explains.
3D Schröder staircase
The 3D Schröder staircase, created by Japanese mathematician Kokichi Sugihara, was the winner of last year's Best Illusion of the Year contest.
"I made this optical illusion as an experimental material to examine the brain behaviour that, when seeing a 2D picture mixed with a real 3D object, perceives the picture part as 3D too," Dr Sugihara says on his blog.
Professor Williams says the illusion is like an Escher painting, where the perspective is all off.
"We've got these rules about what stairs should look like and if they don't fit with that, our brains will create what they should look like," he says.
He says it's also reminiscent of another illusion known as the Necker cube — a 3D line drawing of a cube that shifts between two perspectives.
If you stare intently at this diagram, you'll see how the orientation of the cube changes.
Young woman or old woman?
Like the Necker Cube, this classic illusion is an ambiguous figure. You can only see one woman at a time. Which do you see?
Ambiguous figures test what's known as figure-ground perception. You can either see the figure or the ground, but never both at the same time.
"If you see a younger woman, you can't see the old woman, and if you see the old woman, you can't see the young woman, because there is a simple principle that in every part of our visual field, we see a certain region either as a figure, or as a ground," Dr Spehar says.
But this particular illusion also tests our social biases, according to research led by Mike Nicholls, director of the Brain and Cognition Laboratory at Flinders University.
"What we found is that younger people tended to see the younger figure, and older people tended to see the older person," Professor Nicholls says.
He says it fits with the idea that younger people are more likely to socialise with other younger people, and the same applies to older people.
"In other words, almost social psychology is governing what's happening with a very fundamental bit of visual perception."
But, he adds, that's not to say people can't see the opposite figure.
"It's not an all or nothing thing. You only see the effect when you test in big groups."
What do you see in the tree?
The ability to see faces, or other objects, in random patterns is known as pareidolia.
Look carefully and you'll see faces all around us; in trees, rocks, clouds, the Moon and dodgy photos of paranormal phenomena on the internet.
Pareidolia is an unusual illusion in the sense that the brain is creating something from a lot of noise, says psychologist Simon Cropper from the University of Melbourne.
"It's indicative of a system that's really, really good at making extremely good guesses at stuff," Dr Cropper says.
"It does a lot of filling in the gaps and there are certain things that if you give it just enough evidence, it will leap to a conclusion."
But, he says, because we expect faces to be animate, we are usually able to correct the illusion and recognise the face as simply a pattern in tree or rock, for example.
"We make mistakes all the time, we hallucinate all the time and we generally correct it pretty well," Dr Cropper says.
"But if your visual system is consistently making those errors and you never actually correct them very well, then that could be problematic and distressing."
It can be much harder, however, to change how you see something in other illusions, such as those based on colour and depth perception, even when you are given more information.
This brings us to what Dr Cropper believes is the best illusion in recent years …
#TheDress
In 2015, a badly lit photo of a sparkly striped dress took the internet by storm.
"It caused such disagreement about what a very simple stimulus actually showed," Dr Cropper says.
Were the stripes blue and black or white and gold?
"If you see that dress in real life there is no question about what the colour it is," Dr Cropper says.
"It's only because it's in this photograph in this particular presentation that it is hard to work out what the stripes are."
Dr Cropper says the illusion illustrates how individual differences in perception and simple aspects of quite a rudimentary stimulus can actually have a really strong effect on how you see something.
Cone receptors in the central part of our retina detect different wavelengths of light to produce the effect of seeing colour.
Everyone sees colour slightly differently, depending upon, among other things, the proportion of the three types of cones in their retina.
The colour you see is complicated by the context in which the object is place. In the case of the dress, it's the effects of different lighting between the foreground and background.
"In order to make the judgement about the stripes on the dress, you have to have an anchor, a reference," Dr Cropper says.
"The side to which you happen to anchor your reference on will dictate how you then interpret these stripes."
On top of this, we rely on very few colour names to describe very complex stimuli.
But we still don't know exactly what's going on in #TheDress.
"There's no good explanation I've seen of exactly why the colours in that photograph are so dramatically different," Dr Cropper says.
To explore the science behind more illusions, watch Catalyst at 8:30pm on ABC TV, or catch up later on iview.
