What are optical illusions?

The following was written by Bill Evans and published in the Coolum and North Shore News, Ask the Professionals page on Friday, 30th October 2015.

Q: What are optical illusions?

A: An optical illusion occurs when a person perceives something different to the actual image presented to them. Our visual system can be fooled to give us different information than what is present in reality. Two common types of optical illusion are physiological and cognitive.

Physiological optical illusions are thought to arise from the compromises made by our visual system. One human eye contains over 100 million rod photoreceptors and over 3 million cone photoreceptors, yet the optic nerve from the eye only contains 1.7 million nerve fibres at most. Processing by different cell types within the retina mean that each nerve fibre can receive input from on average 100 rods and cones. If this wasn’t the case, our optic nerve would be thicker than our spinal cord, and our occipital cortex in the brain would be ten times the volume.

Each nerve fibre transmits information on colour, movement or changes in light intensity. This basic processing within the retina may be tricked by certain stimuli, giving for example the sensation of movement when no movement is present. You’re experiencing this in the image above; it’s not a video, it is a stationary image.

Cognitive optical illusions are thought to be caused by assumptions of the world based on experience interacting with what is observed. This may lead to the viewer having a different interpretation of the actual scene presented. Other cognitive optical illusions may have the viewer accept a physically impossible scene.

Here’s an incredible video of thirty of the best optical illusions: