Delusions are beliefs that reflect an abnormality of thought processes. They are usually bizarre and completely unrelated to conventional cultural or religious belief systems, or a person’s intelligence.

Sometimes they are categorized according to their theme. For example, schizophrenics will often believe that an external force is controlling their thoughts or actions (delusions of control). They may also have delusions of grandeur (belief that they are a famous person or historical figure) or they could believe that they are being followed, attacked or conspired against when they’re not (delusions of persecution).

Delusions are often associated with psychiatric disorders, but can also be a symptom of neurodegenerative disorders such as Alzheimer’s or brain damage. There is no main theory today about how the brain generates delusions, but it has been proposed that it is generated by a combination of right hemisphere damage and left hemisphere hyperactivity. When the right hemisphere is damaged, the left hemisphere tries to compensate by creating creative stories.

One interesting delusional syndrome is Capgras Syndrome. People with Capgras Syndrome act as though they are in a parallel universe in which the people that they know are impostors or doubles. When they see themselves or a relative in the mirror they believe that they are seeing an exact double. People with Capgras will often remove all mirrors from the home because of this and some of them even believe that inanimate objects or pets have been replaced by exact replicas.

Sometimes people with Capgras Syndrome have epilepsy or strange-looking temporal lobes in the brain.  

The kind of delusion that a person has and how the delusion can be addressed varies from person to person. When a person is suffering from a delusion, they may not recognize you on a conscious level, but there is often an alternative way that one can communicate with them. One may not know what this way is, but there is likely to be one because of how the brain is wired. For example, the part of the brain that processes visual information is different from the part of the brain that processes auditory information, just as the part of the brain that processes emotional responses  is different from other areas of the brain as well. Therefore, the person who is suffering from a delusion has not really forgotten everything; they simply do not know how to access their knowledge. In some instances, a person may still be able to get the delusional person to communicate with them by talking to them over the phone where they are receiving only auditory input from the other person and are not receiving visual input that would make them think that the delusional person’s friend is a doppelganger.