Scientists Unveil Purple-Blue Dot Illusion, Revealing How Eyes Misread Colour
A mind-boggling optical illusion has been developed by scientists to illustrate how our perception of colour can be easily skewed. At first glance, it may seem like a simple question: what colour are the dots in this image? However, if you spend several seconds carefully analysing the picture, you will notice the colours shift between a blue and purple-ish hue, depending on which dot you are focusing on.
The Science Behind the Illusion
Hinnerk Schulz-Hildebrandt, a biomedical optics engineer at Harvard Medical School, created this illusion to demonstrate how colour-detecting cells in our eyes function. He included it as part of a study, published in the journal Perception, that reveals our brains can misread colour. In the paper, he wrote: '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. 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, reflecting 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 cones are not distributed equally throughout the eye and are unevenly spread.
Why Blue Perception Is Unique
In the area of sharpest vision, S-cones which pick up blue tones are almost completely absent, Schulz-Hildebrandt explained. This means that our eyes are not as effective at seeing blue as they are other colours, especially when looking directly at it. Jenny Bosten, a visual neuroscientist at the University of Sussex, told Scientific American: 'We don’t notice this usually because our brains have learned to "calibrate" out the difference.'
In the nine-dot illusion, our brain changes how we perceive colour so it can stand out more. 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.
Implications and Comparisons
Schulz-Hildebrandt wrote: 'The combination of these mechanisms leads to a unique and impressive visual illustration. 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.'
This illusion adds to a long history of optical phenomena, such as the Café Wall illusion, first described by Richard Gregory in 1979. That illusion involves alternating columns of dark and light tiles creating the perception of sloping lines, helping neuropsychologists study visual processing. Both illusions highlight how our brains interpret visual information, with applications in fields like graphic design, art, and architecture.
Overall, this new purple-blue dot illusion provides a fascinating insight into the complexities of human vision and the ways our perceptual systems can be tricked by simple patterns.
