1 {the bright shade of dull reddish grey fades ever so slightly to a darker, duller reddish grey}
2 {fading...}
3 {fading...}
4 {fading...}

2021-05-16 Rerun commentary: This image shows an interesting optical illusion, known as Mach bands. Each rectangle of colour is completely uniform in colour. However, each rectangle appears to our visual system to have a slight gradient of brightness across it, slightly darker on the left and brighter on the right.

This is an effect produced in the retinas of our eyes, by the neurological processing that takes place there before the visual signals get sent to our brain. The light detecting cells in our retinas experience a phenomenon known as lateral inhibition. This is a physiological effect in which the triggering of a receptor neuron dampens the response in neighbouring neurons.

Our eyes evolved to do this because this processing is a simple form of edge enhancement, that enables us to better discern boundaries between light and dark regions. This is an important ability for our visual system, as good detection of edges leads to a precision in our ability to see and to visually distinguish different objects. It essentially sharpens our vision a bit - without it, our vision would look a little bit blurrier.

In normal circumstances, this retinal processing is advantageous. However, for various contrived images it leads to visual effects that we term optical illusions. These are images for which the neural processing of our visual systems results in an impression in our brains that is a demonstrably inaccurate representation of the image content. Mach bands, the effect you can see in the above image, is one of those.

Another is the Hermann grid illusion, in which you can see fuzzy grey blobs in the white intersections of this grid that you are not looking directly at. As you move your eyes around the grid, you can see that when looking directly at an intersection, it is white, but the intersections away from your centre of vision appear to be darker.

Optical illusions are not a sign that "something has gone wrong" in your visual system - indeed they are a sign that everything is working normally. They also provide a fascinating insight into the way our eyes and brain work to provide us the sense of vision.