Spots Before Your Eyes Explained

A simple grid of thick grey lines on a black background, punctuated with white dots at the intersections, leads to an intriguing result.

The white spots elsewhere in the grid become discoloured by your brain! Much of what we see in our peripheral vision is interpolated by our brain, rather than being 100% pure information from the eyes. This is particularly true with large areas of constant colour - and this grid is essentially all black.

As a result, your brain tries to fill in the white circles with the predominant shade of black. In other words, there appears to be slightly smaller black or grey circles dancing around inside the ones you're not looking directly at.