What colour are the dots in this optical illusion?

What colour are the dots in this image?

A mind–boggling optical illusion has been developed by scientists to show how our perception of colour can be easily skewed.

At first glance it may seem like a simple question.

But if you spend several seconds carefully analysing the picture you’ll notice the colours shift between a blue and purple–ish hue, depending on which dot you are focusing on.

Hinnerk Schulz–Hildebrandt, a biomedical optics engineer at Harvard Medical School, created the illusion to show how colour–detecting cells in our eyes work.

He included it as part of a study, published in the journal Perception, that reveals our brains can misread colour.

‘In this paper a novel optical illusion is described in which purple structures (dots) are perceived as purple at the point of fixation, while the surrounding structures (dots) of the same purple colour are perceived toward a blue hue,’ he wrote.

‘As the viewing distance increases, a greater number of purple structures (dots) revert to a purple appearance.’

He explained that there are three types of cones, or colour–detecting cells, in our eyes: L–cones, S–cones and M–cones.

These letters stand for long, short and medium and reflect the wavelength each is responsive to.

For example, L–cones pick up red tones the best, while S–cones can detect blues and M–cones are most sensitive to greens and yellows.

These types of cone are not distributed equally throughout the eye and are unevenly spread.

In the area of sharpest vision, S–cones which pick up blue tones are almost completely absent, he explained.

This means that our eyes aren’t quite as good at seeing blue as they are other colours, experts explained – especially when you’re looking directly at it.

‘We don’t notice this usually,’ Jenny Bosten, a visual neuroscientist at the University of Sussex, told Scientific American.

‘[That’s] because our brains have learned to “calibrate” out the difference.’

In the nine–dot illusion, our brain is changing how we perceive colour so it can stand out more, Mr Schulz–Hildebrandt explained.

It means that at first look, the dots and background colours appear quite similar.

Our brain sees that combination and interprets the dots as ‘more purple’ to help distinguish them from the background.

The effect of individual dots becoming ‘more purple’ as surrounding dots become ‘more blue’ can even be seen in real–time as you scan the image.

However, the effect becomes less pronounced if you look at the illusion from further away.

‘The combination of these mechanisms…leads to a unique and impressive visual illustration,’ he wrote.

‘A pattern of purple objects on a blueish background appears only purple where the viewer looks directly at it.

‘In the periphery, the perception shifts towards blue. As the viewing distance increases, the number of objects perceived as purple also changes.’

WHAT IS THE CAFÉ WALL OPTICAL ILLUSION?

The café wall optical illusion was first described by Richard Gregory, professor of neuropsychology at the University of Bristol, in 1979.

When alternating columns of dark and light tiles are placed out of line vertically, they can create the illusion that the rows of horizontal lines taper at one end.

The effect depends on the presence of a visible line of gray mortar between the tiles. 

The illusion was first observed when a member of Professor Gregory’s lab noticed an unusual visual effect created by the tiling pattern on the wall of a café at the bottom of St Michael’s Hill in Bristol.

The café, close to the university, was tiled with alternate rows of offset black and white tiles, with visible mortar lines in between. 

Diagonal lines are perceived because of the way neurons in the brain interact. 

Different types of neurons react to the perception of dark and light colours, and because of the placement of the dark and light tiles, different parts of the grout lines are dimmed or brightened in the retina. 

Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges. 

These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.

Professor Gregory’s findings surrounding the café wall illusion were first published in a 1979 edition of the journal Perception.

The café wall illusion has helped neuropsychologists study the way in which visual information is processed by the brain. 

The illusion has also been used in graphic design and art applications, as well as architectural applications.

The effect is also known as the Munsterberg illusion, as it was previously reported in 1897 by Hugo Munsterberg who referred to it as the ‘shifted chequerboard figure.’

It has also been called the ‘illusion of kindergarten patterns’, because it was often seen in the weaving of kindergarten students.