Your eyes are lying to you. Nine purple dots prove it.

Adam Spencer
1 day ago
3 min read

A Harvard illusion with 9 purple dots exposes how your fovea, yellow eye-filters and brain context warp colour perception in real time.

( The Schulz-Hildebrandt illusion Physics Optics)

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

You’ll think you’re going dotty.

Look at the image above. Nine purple dots on a blue background. Simple enough.

Now stare at one dot. Really stare at it.

The dot you're focusing on looks purple. The other eight? They're shifting. Sliding. Going blue.

Move your gaze. Now that dot looks purple. And the others drift again.

Now move your phone or computer screen off to the side. Now all the dots seem purple.

Nothing in this image is changing.

The ‘change’ is entirely in your head.

Geeks on a plane.

Last August, Hinnerk Schulz-Hildebrandt was watching a flight path on his phone.

Who hasn’t?

But old HSH is a biomedical optics researcher at Harvard Medical School, so not surprisingly, he saw something most of us might miss.

The flight line kept changing colour depending on where he looked. Purple when he stared straight at it. Blue when it slipped into his peripheral vision.

Most of us would go "huh, weird" and get back to Avengers Age of Ultron.

Instead, Schulz-Hildebrandt formalised that ‘weird colour thing’ and ran an experiment.

He settled on nine purple dots on a blue background.

The resulting illusion was published in Physics Optics in October 2025 and it is, for something so stripped-back, genuinely mind-bending.

Friend or fovea?

Here's what's happening inside your eyeballs.

The centre of your retina, known to ophthalmologists as the good-old fovea centralis, has relatively few of the blue-detecting cone cells compared to the rest of your retina. So when you stare directly at one of those blue-purple dots, your eye is detecting less blue than it does when the same dot sits in your peripheral vision.

Result? The dot you're staring at reads as more purple. The other dots tend blue.

But wait, there's more.

Sitting right in front of your fovea is a layer of yellow pigments. Described by Nora Bradford in Scientific American as your eyeball's “internal sunglasses”, soaking up blue and near-ultraviolet light before it even reaches the cones.

Your brain has learned to "calibrate out" this difference over a lifetime of seeing the world, says Jenny Bosten, visual neuroscientist at the University of Sussex. You never notice it because your brain is quietly compensating every waking second.

Until someone shows you nine purple dots and your brain goes ‘information overload … I’m outta here’.

The background is doing heavy lifting too.

The blue background isn't accidental. It's crucial to the trick this grid is playing on your brain.

In a process called simultaneous contrast, your brain perceives colours relative to what surrounds them. The famous "what colour is this dress?" meme of a decade ago had the internet losing its collective mind for exactly the same underlying reason.

Btw the original dress was black and blue, whether you saw it as white and gold like I still do, or not.

A blue-purple dot on a blue background gets nudged toward "more purple" by your visual system as it tries to distinguish foreground from background.

Layer that on top of the fovea's blue-blindspot and the yellow pigment filter and you get the colour switching you can "see", even though it is not happening.

"It is exploiting several processes we already know about to create a nice effect."— Jenny Bosten, University of Sussex.

Purple haze.

Now, I've written before in NerdNews about how purple itself is a gorgeous hack.

It is your brain's workaround for a colour that doesn't even exist as a single wavelength of light [see that post HERE].

Your red and blue cones fire together, your brain invents a solution, and voila: purple.

Which is why this illusion works so beautifully on purple specifically. As Schulz-Hildebrandt puts it in his paper:

"Purple is a non-spectral colour... a fragile and unstable perception that is easily influenced by physiological and contextual factors."— Hinnerk Schulz-Hildebrandt, Physics Optics, October 2025.

Fragile and unstable. That's purple's whole deal. The nine dots aren't breaking anything. They're just catching purple purpleing.

And these nine dots are just the latest reminder that the world doesn't arrive pre-coloured. Your brain is building it, millisecond by millisecond, with creative accounting, shortcuts, and some very sophisticated guesswork.

And we haven't even begun on whether you and I see green as the same colour. That can wait.

Look at that dot again.

Still sure it's purple?