Critics of "eyewitness ghost sightings" suggest that limitations of human perception and ordinary physical explanations can account for such sightings; for example, air pressure changes in a home causing doors to slam, or lights from a passing car reflected through a window at night. Reports of ghosts "seen out of the corner of the eye" may be accounted for by the sensitivity of human peripheral vision, which can easily mislead, especially late at night, when the brain is tired and more likely to misinterpret sights and sounds. Similarly, pareidolia, an innate tendency to recognize human forms in random patterns, often causes people to believe they have seen ghosts. In addition, some feel human psychology plays a part in such sightings; a person's belief that a location is haunted may cause them to interpret mundane events as confirmation of haunting.
External influences, such as sound waves are thought to be another cause of ghost sightings. Frequencies lower than 20 hertz are called infrasound and are normally inaudible, but British scientists Richard Lord and Richard Wiseman have concluded that infrasound can cause humans to feel a "presence" in the room, or unexplained feelings of anxiety or dread. And Carbon monoxide poisoning, which can cause powerful auditory and visual hallucinations, depression, and a generalized sensation of illness and dread, was recognized as a possible cause for "haunted houses" as early as 1921.
Some sceptics dispute popular explanations for the existence of ghosts, such as the perception that ghosts are produced from the victims of unsolved violent murders. They suggest that if the former is true, then the number of such murders (approximately 1 million over the course of the twentieth century in America alone) should produce overwhelming numbers of ghost sightings. They also argue that the traditional perception of ghosts wearing clothing is illogical given the supposed spiritual nature of ghosts, suggesting that the basis of what a ghost is said to look like and consist of is quite dependent on preconceptions made by society. Sceptics also cite a total lack of scientifically testable and verifiable evidence in favour of the existence of ghosts, despite centuries of interest in the subject.
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