Optical illusion reveals how your perception of colour can be skewed

Do these dots look blue or purple? Mind-bending optical illusion reveals how your perception of colour can be easily skewed

  • The optical illusion was developed by researchers at Harvard University
  • It involves a series of 1,000 dots featuring different shades of blue and  purple
  • Volunteers were asked to sort the dots into different colour categories
  • When fewer blue dots were shown, volunteers were still convinced they could see roughly the same number of blue and purple dots

A new optical illusion has revealed just how easily our perception of colour can be skewed.

The illusion, by Harvard University, involves a grid of 1,000 individual dots, which range from very blue to darker hues of purple.

Researchers asked a group of volunteers to say if they thought each individual dot was either blue or purple.  

During the trials, dots that the participants previously identified as purple looked increasingly blue to them.

Researchers say this could be because the human brain does not make decisions based on hard rules, but rather based on context of what it has seen before.

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Interpretation of dot colour changes based on how often we are exposed to certain things. The more purple dots we see when asked to identify blue dots, the more blue dots we think we can see 

Participants in the study were students from Harvard University, who took part in more than 1,000 trials with the grid.

Each dot is the same size, with colours ranging from ‘very blue’ to ‘very purple’, according to the Harvard University researchers.

For the first 200 trials, participants were shown an equal number of blue and purple dots.

However, over the course of the remaining 800 trials, the researchers steadily decreased the number of blue dots until almost only shades of purple remained.

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But while you might expect the number of blue dots identified by participants to decrease as they disappeared from the grid, they didn’t.

In fact, the number remained fairly consistent – despite warnings from the researchers that the number of dots could vary during the trials. 

In the final 200 tests, participants saw dots they had previously identified as purple as blue, suggesting their concept of the colour had shifted.

‘When the prevalence of blue dots decreased, participants’ concept of blue expanded to include dots that it had previously excluded,’ said David E. Levari, of Harvard University. 

Harvard University researchers showed the grid to participants and asked them to judge how many of the 1,000 points were blue

Even when researchers told participants blue dots would become rarer and warned them about their perception of blue expanding in the subsequent tests, the number of dots counted remained consistent.

In later tests, Harvard researchers even offered to pay participants who managed to keep their judgement of the colours consistent a $10 (£7.50) reward – but despite this, the shift still occurred.

When the Harvard University researchers increased the number of blue dots in the grid, it triggered the complete reverse effect with the participants.

As the number of very blue dots increased, participants were less likely to identify a purple dot as blue – suggesting their definition of the colour had narrowed.

To demonstrate a potential real-world consequences of this design flaw in the brain, researchers showed participants a range of computer-generated faces.

The expressions on these faces ranged from neutral to threatening.

Like the blue and purple dots, as the number of obviously threatening faces decreased, participants expanded their definition to encompass expressions they had previously identified as neutral. 

In the new study, which was published in Science magazine, the researchers noted: ‘This phenomenon has broad implications that may help explain why people whose job is to find and eliminate problems in the world often cannot tell when their work is done.’

The optical illusion involves a series of 1,000 dots ranging from very blue to very purple (pictured). When the blue dots became less common, the human brain expands the definition of ‘blue’ to include dot it had previously excluded 


The human brain has a pre-determined idea of something, it has its own understanding of almost everything. 

It can recognise the colour blue, threatening body language and other things based on their integral qualities.  

However, the way we perceive something to be can be altered in certain situations.

This happens without conscious thought or effort and there is little we can do to prevent the shift. 

When something is seen less often, we expand our idea of what constitutes it.

For example, when seeing a large amount of blue dots among purple dots, it is easy to detect the blue ones. 

When the blue dots become rarer, participants’ concept of blue expanded to include dots that it had previously excluded, blurring the boundary between blue and purple.  

This also happens in social environments and under more complex situations, 

For example, it can be applied to a person’s view on ethical matters, and how this changes based on recent exposure. 

This theory is called ‘prevalence-induced concept change’.  

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