Q. Is it possible that someone could be so mentally ill, in some form or another, that the reality is so off base, that the person may not know he or she is lying?
It is very rare.
A person with alzheimer's, for example, might perseverate on something from 30 years ago, but speak as if it was a present tense issue, without being deceptive.
"I am going to see my wife" said an alzheimer's patient, even though she had been dead for 20 years. "We are going to the movies later today."
His sentence structure does not show deception.
This is similar to the repetition of a lie.
I inform the subject that I drive a blue car, but I do not own a blue car. The subject later says "Pete drives a blue car", and there is nothing within the language to indicate that the subject is deceptive.
People with brain damage, adult mental retardation, or adult autism may function in a way in which the brain does not process language like the rest of the population.
For example, a 50 year old woman with brain damage from a car accident when she was 7 years old may have the functioning level of a 7 year old, but a social worker trained in interviewing children may not find that an interview with the woman works the same as a normally developed 7 year old child, replete with a strong grasp on verb tenses. The reference point for adults with brain injuries, retardation, autism, or other issues may not apply.
Reader, have you had an elderly loved one slip into a state of forgetfulness where conversations of the past were spoken of as if present?
With deception in analysis, there is an intent to deceive that our process picks up. The subject has a need to deceive and will deliberately skip over time, or avoid answering questions.
When someone suffers from alzheimer's or has had a serious brain injury, the language is the picture taken by their mind's camera, the brain.
If the brain is damaged, the 'camera' being injured or diseased, the picture (the language) will reflect this very thing.