One example of an error in perception is hallucinations. People experience hallucinations for several reasons, including mental illness, brain damage, sleep deprivation, and drug use. In a case study I found, a man sees faces in trees due to abnormal neural activity in his cortical regions. This disorder is called Hallucinogen Persisting Perceptual Disorder. The man in the study started hallucinating while taking LSD, but at the time of the study had not used drugs in over ten years and was still experiencing hallucinations. While the patient was looking at trees and hallucinating, his neural activity was significantly different than when he did not see faces in the trees. The only other item this man claimed caused him to hallucinate were brick walls, which also caused him to see faces occasionally. The biological cause for this disorder is unknown; however it may have something to do with the face processing regions of the brain.