Optical illusions, also known as a visual illusions, are images that are perceived by the viewer but differ from objective reality. There are three main types of optical illusion: literal optical illusions, physiological illusions, and cognitive illusions. This article will tell you more on how optical illusions work.
How they work
In very simple terms, an optical illusion works as the information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source.
The cognitive processes hypothesis
The hypothesis claims that visual illusions occur because the neural circuitry in our visual system evolves by neural learning, to a system that makes very efficient interpretations of usual 3D scenes based in the emergence of simplified models in our brain that speed up the interpretation process but give rise to optical illusions in unusual situations.
In this sense, the cognitive processes hypothesis can be considered as a framework for an understanding of optical illusions, as the signature of the empirical statistical way vision has evolved to solve the inverse problem.
Different types of Illusions
There are four types of optical illusions.
Ambiguous illusions are pictures or objects that elicit a perceptual 'switch' between the alternative interpretations. The Necker Cube is a well-known example of this.
Distorting illusions are characterised by distortions of size, length, position or curvature. A striking example is the cafe wall illusion.
Paradox illusions are generated by objects that are paradoxical or impossible, such as the famous impossible staircase. The final type of illusion are fictional illusions which are defined as the perception of objects that are genuinely not there to all. These are more properly called hallucinations, and can be very scary.
The Herman grid illusion
The Hermann grid illusion is one of the most famous and cool illusions, and is best explained using a biological approach. Basically, the receptive field of the retina - light and dark receptors in the eye - compete with one another to become active. This has been used to explain why we see bands of increased brightness at the edge of a colour difference when viewing Mach bands.
Once a receptor is active, it inhibits adjacent receptors. This inhibition creates contrast, highlighting edges. In the Hermann grid illusion, the grey spots appear at the intersection because of the inhibitory response which occurs as a result of the increased dark surrounded.