We all love an illusion. They are fun, intriguing and can mess with our head.
There are many kinds of optical illusions, and science cannot always explain the reasons they play tricks on human perception. What we see in our minds is the result of our brains receiving information from our eyes and filling in the gaps. And they don’t always get it right.
We have gathered together some of our favourite visual tricks to give your brain a quick workout.
The checker shadow illusion
Square A and square B in the illustration above are the same colour. It seems impossible but it is true. To prove this, we can demonstrate with a modified image:
No, we can't get our heads around it either! This is an example of a contrast illusion, which causes two areas of identical colour to appear differently depending on the context.
In the case of the checker shadow illusion, the shadow cast by the green shape seems to darken a brighter region. This amazing illusion was created by the vision scientist Edward Howard Adelson in 1995 and was designed to show how well our visual system can break down image information.
Therefore, this illusion isn't a fault but a demonstration of the strength of our sight.
Café wall illusion
The café wall illusion is a geometrical-optical illusion where straight dividing lines between staggered dark and light blocks appear to be curved.
The theory is that our brains determine a white area to be larger than a black area, when it is actually the same size (known as the irradiation illusion). This can be proved by swapping black and white colours for any other colours with lower contrast.
This illusion has many names but was termed the 'café wall illusion' by scientist Richard Gregory, who worked near a café (located in Bristol, UK), which was decorated with such a design on its frontage.
The Penrose triangle
The Penrose triangle is an example of a paradox geometrical–optical illusion. This is an impossible object that cannot exist in 3D form within a physical space. It was popularised by the scientist Lionel Penrose in the 1950s, although versions of it (such as the Reutersvärd triangle illusion) had existed before.
Perhaps the best-known examples of impossible objects feature strongly in the works of famous Dutch artist M.C. Escher, such as 'Relativity' and 'Belvedere'.
Motion-induced blindness
Bonneh's illusion, or 'motion-induced blindness' is a recently-discovered visual illusion. A continuously moving pattern rotates around a flashing green dot in the centre of the screen. Three yellow dots arranged in a triangular shape sit within the rotating pattern. If we watch the green dot for approximately 10 seconds without deviation, some of the yellow dots start to disappear and reappear at random intervals.
The reasons this illusion occurs are complicated, but in essence, it is due to a lack of concentration when viewing the moving image.
Necker cube
The Necker cube is not an illusion as such, but more of a shape with multiple viewpoints, known as 'multistability'. Because there are no visual clues as to its orientation, it can be interpreted in multiple ways.
Most people see the lower-left face of the cube as being the front. This is probably because we tend to see things from above and front on, therefore our brains automatically perceive this as the preferred interpretation.
Other examples of multistability include Rubin's vase illusion, where either two faces or a vase can be seen in the same image.
Scintillating grid illusion
The scintillating grid illusion is one of those visual tricks that can make your brain hurt. When looking at the white circles that appear on the interconnecting lines of the grid, black dots seem to form and quickly disappear in other areas of the grid. The speed of this formation is what gives this illusion its name.
It was developed fairly recently but is based on a similar illusion from 1870 discovered by Ludimar Hermann – known as the Hermann grid illusion.
Peripheral drift illusion
The peripheral drift illusion has many variations, including sawtooth patterns, but is most easily seen within circular designs such as the image above. This illusion was described by Jocelyn Faubert and Andrew Herbert back in 1999, showing that the effect is stronger when the eye is moving or when blinking.
Faubert and Herbert have speculated that this illusion is caused by a combination of factors including eye movement, differences in light intensity and how our eyes perceive depth.
The rabbit-duck illusion
The rabbit-duck illusion is an example of a 'figure-ground organisation', a type of ambiguous drawing where two objects can be seen. It originally appeared in a humourous German magazine called 'Fliegende Blatter', and captioned 'Which animals are most like each other? Rabbit and duck'.
What you perceive depends on what your brain decides to see, which can depend on several factors including how creative you are. Other examples of this illusion include the classic drawing of two opposing faces, which can also be viewed as a vase.
Colour constancy
Back in 2015, an image of a dress on sale in a shop in Cheshire went viral when disagreements arose as to its colour. Some saw it as black and blue, and others thought the dress was white and gold.
The reason that some people saw the dress in different colours cannot be completely explained. One theory, put forward by neuroscientists Bevil Conway and Jay Neitz, puts the differences down to the way that our brains perceive different hues in daylight. So, in the case of the dress image, your perception of the colour would have been dependent on whether you believed the image of the dress was taken in natural or artificial light.
Roman Originals, the makers of the dress, later confirmed that the dress was black and blue. However, the number of people who saw a white and gold dress makes it one of the most well-known examples of differences in human colour perception.
Ponzo illusion
The Ponzo illusion is a geometrical-optical illusion that takes its name from the Italian psychologist Mario Ponzo, although he never claimed to have discovered the illusion himself.
There are a few different explanations as to why we judge the top line to be longer than the bottom one. The two lines converge towards the vanishing point in the illusion, therefore our brains are hardwired to assume that the top line is further away, and therefore longer.
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