What Is a Visual Illusion?

A visual illusion is a distortion of movement, form, size or color

Visual Illusions can be the result of a stroke in the occipital lobe, seen in blue at the back of the brain. Photo © A.D.A.M.

A visual illusion is a distortion of movement, form, size or color in the visual field. Visual illusions can be the result of a stroke in the occipital lobe, which is located at the back of the cerebral cortex and is the main center for visual processing.

Stroke can be caused either by a clot obstructing the flow of blood to the brain (called an ischemic stroke) or by a blood vessel rupturing and preventing blood flow to the brain (called a hemorrhagic stroke).

 A transient ischemic attack, or "mini stroke", is caused by a temporary clot.

Effects of Stroke

If a stroke occurs and blood flow can't reach the area that controls a particular body function, that part of the body stops working as it should. If the stroke occurs toward the back of the brain, for example, it's likely that some level of vision will be impaired. 

The effects of a stroke depend on several factors, including the location of the blockage and how much brain tissue is affected. However, because one side of the brain controls the opposite side of the body, a stroke affecting one side will result in neurological complications on the side of the body it affects. For example, if the stroke occurs in the brain's right side, the left side of the body (and the left side of the face) will be affected, which could produce any or all of the following:

  • Paralysis on the left side of the body   
  • Vision problems including visual illusions   
  • Quick, inquisitive behavioral style   
  • Memory loss

If the stroke occurs in the left side of the brain, the right side of the body will be affected, producing some or all of the following:

  • Paralysis on the right side of the body   
  • Speech/language problems   
  • Slow, cautious behavioral style   
  • Memory loss

When stroke occurs in the brain stem, depending on the severity of the injury, it can affect both sides of the body and may leave someone in a ‘locked-in’ state. When a locked-in state occurs, the patient is generally unable to speak or achieve any movement below the neck.

Types of Visual Illusions

Examples of visual illusions include when a person sees two or more objects in front of them when there is only one; when everything appears to be the same color all of the time; or when people or objects appear to be much larger or smaller than they actually are.


References:

Allan Ropper and Robert Brown, Adam's and Victor's Principles of Neurology, 8th Edition McGraw-Hill Companies Inc, United States of America, 2005, pp 417-430.

American Stroke Association, www.strokeassociation.org

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