Your eyes believe the most important thing in the world is edge detection. This is the process of noticing contrast between two colours, in order to identify borders and the edges of shapes. This means you know where things end and where they begin, so there’s less chance of you bumping into, or falling off of them. However, sometimes this edge detection has unintended consequences: that’s where optical illusions come in.
Mach bands are a type of optical illusion where gradient bands of uniform colour seemingly darken towards the lighter colour next to them. Take a look at this one below:
They can be any shape or size and come in a variety of complexities. This however is a simple one. If you take a glance at it, it seems as though the bands start lighter and get darker, when in fact each band is the same tone the whole width.
Strangely enough, we have the horseshoe crab to thank for our understanding of why this happens. Horseshoe crabs have very simple but very large visual systems, so we can understand it relatively easily as well as actually being able to work out what we’re seeing. In particular, their visual system is similar to humans in a process called lateral inhibition. Lateral inhibition, in short, refers to how the cells in your eyes, when activated by light, inhibit (quiet down) all the cells around them. This is the process responsible for edge detection. As edge detection is the most important thing in the world, it informs how your vision works all the time.
The light sensitive cells in your eyes are activated by looking at something bright, like a light colour. When they are active, they try to make all the other light-sensitive cells around them reduce their activation. They do this via a network of connections that send inhibitory signals to the cells next to them. When lots of cells all around each other are doing this, you perceive the colour at a uniform brightness. However, this changes when there is an edge nearby. If the light you are focusing on is next to a section of darker colour, there will be cells hit by light reflected from this darker area. These will not be as active. This means they won’t be trying to stop the cells next to them from activating.
Furthermore, those cells next to them that are active, will be making them be even quieter than they already are. This means that not only is the non-active cell not perceiving much brightness at all, but its neighbour is only half as inhibited as all the other active cells. This means it sees the colour as much brighter than its neighbours. Therefore, an exaggerated contrast is created between the dark and light. This is fueled by the edge detection abilities of your brain, as the edge is being purposefully highlighted. The arrangement and inhibitory connectivity of your eye cells is what causes this. This explains why the left side of the mach bands appears lighter than the right side, and the right side appears darker in comparison: the dark band beside the band you are focusing on is allowing the cells hit by light from the side nearer the dark to activate more, and seem lighter than the cells on the right. The super-activation of these cells also inhibits the cells on both sides more, increasing the gradient seen within the band.
A similar phenomenon is found in the scintillating grid illusion. When you’re not focusing on them, the junctions between the white grid lines seem to hold little grey dots. When you focus, these disappear. Have a look:
If we think about the idea of lateral inhibition again, the most inhibited areas of the grid are going to be the junctions, because there’s the most white light converging on these areas. When four lines of light, and therefore four times the inhibitory signals are hitting the cells focusing on the junction, they will be strongly inhibited. Therefore, it looks like there is a dark dot in the middle because everything around those areas is telling the cells looking there to stop firing. However, as soon as you look at them, they disappear. This is because of a part of your eye called the fovea. This is located in the centre of your retina (the layer of cells at the back of your eye where the light hits), and is full of light sensitive, colour-vision cells at a much higher population than the rest of your eye. Because of this number and quality of cells, the effect disappears.
The idea of lateral inhibition (cells telling cells around them to reduce firing) is what informs ‘colour theory’, and how colours vary in shade depending on what colours they are adjacent to.
While it can be fun to see how you can trick your brain with optical illusions, it’s interesting to be able to highlight such a complex process through just a few colours on a screen!
