Perceptual distortions are incorrect understandings or abnormal interpretations of a perceptual experience. A perceptual distortion occurs when a person's response to stimuli varies from how it is commonly perceived. Perceptual distortions can relate to either sensory or psychological perception and can occur as a result of cognitive bias, psychological disorders, medication or drugs, or physical damage to the brain or sensory organs.
An example of perceptual distortion is found in people suffering from an eating disorder. People afflicted with anorexia nervosa holds a distorted self-image. They see their bodies as overweight and unsightly, whereas other people see the sufferer as malnourished and underweight.
Another example of perceptual distortion is commonly found in people with the mental illness schizophrenia. When a person is unable to recognize that he has an illness, then he is said to have anosognosia, or lack of insight.
Perceptual distortions can result from cognitive biases, or patterns of thought and deviations in judgment in particular situations. Common examples are the observer expectancy effect, known also as the selective perception bias, which skews interpretation of results to be in line with a theory one already believes, and attentional bias, which is the tendency for one’s emotions to determine or affect one's focus.