When what you perceive is different from what you are seeing in reality, you are having an eye illusion. Whenever you see something, the image of it gets captured on the retina. This is then transferred to the brain where it gets processed. Our outlook or perception sometimes determine what we see. As a result your brain will perceive the image differently from what it really is. This type of deception is mainly divided into two categories. They are physiological and cognitive.
When there is an after effect of seeing images like bright lights you are experiencing a Physiological eye illusion. In the primary stage of a visual processing, every stimulus has an individual dedicated neural pathway. In the case of repetitive stimuli however some channels may cause a physiological disturbance which can change how you perceive an image. The Mach Bands or Hermann’s grid illustrations are the two types of physiological deceptions which prove helpful when explaining this type of phenomenon. An increased sharpness and contrast of a visual stimulus is well explained by the Mach Bands and the Hermann grid. This is caused as a result of Lateral inhibition.
Another type of eye illusion that most people suffer from is the Cognitive type. This type is also widely known as “mind games” as it is a result of people’s individual perception differences about reality. Your brain as well as your eyes forms an unconscious interpretation about the reality. This is a result of having preconceived notions about things. For example, your beliefs about how things work or how they are supposed to be lead to you making unconscious assumptions. You can further divide this phenomenon into four categories.
Paradox and distorting.
Fiction – In this type you experience things not seen by others. People suffering from schizophrenia or hallucinations experience these perception problems.
Ambiguous – Alternating inferences can result in replacement of perception between certain things or images. The Necker Cube is a well known example of this type of illusion.
Paradox – When you see objects which appear paradoxical or impossible in reality you experience this form of hallucination. The Penrose triangle is a famed example for this hallucination which people form due to misunderstanding perceptions that all adjacent edges must enjoin.
Distorting – This results in seeing distorted images in either curvature, or size and length. The Müller-Lyer and the Café wall is a well known example which causes one to see distorted images.
You cannot always trust what you see. This has been proved time and again. Sometimes changed perception might be a result of illnesses. Most of the times however; changed perception is a result of the environment, which influences what we see.
Artists and film makers have always been incorporating these techniques since time immemorial. Professional photographers also use illusion techniques when developing photographs.