GEAUGA COUNTY -- The teenage gunman in the Chardon shooting massacre has changed his plea to 'not guilty by reason of insanity'.

Lane's attorney, Mark DeVan, entered the change on Monday.

Not guilty by reason of insanity is a rare plea for teenage defendants.

Just one percent of defendants enter the plea; of that one percent, only 15 percent prove successful at trial.

Former Cuyahoga County Public Defender and longtime defense attorney Gerald Gold says not guilty by reason of insanity is challenging, because the burden goes beyond proving mental illness alone.

Through testimony and evidence when Lane was in juvenile court, it was learned he suffered from migraine headaches, and experienced auditory and visual hallucinations.

"Now the question is, at the time this happened, was a mental illness the cause of what happened. Did the defendant not understand the difference between right and wrong at the time," Gold said.

For the last three weeks, Lane has undergone psychiatric evaluations.

"It is tough to fake out a good psychologist or psychiatrist," Gold said.

Lane is charged in the murders of three Chardon high school students.

"I think it adds to the idea that a person has to have something really wrong with them. To know that he goes into a place where everybody really knows him and he's not going to get away with something," Gold remarked.

Forensic psychiatrist Dr. Sara West says insanity has become much more difficult to prove in modern cases than it was in the past.

In Ohio, the standard of proof is: severe mental illness and a causal link. Meaning, the mental illness was the direct reason for committing the crime.

Expert psychiatrists will likely be brought in on both sides of the courtroom.

TJ Lane's trial is scheduled for November. There is still the issue of whether his trial will be held in Chardon.

If found guilty, he could serve up to life in prison.

If found not guilty by reason of insanity, he will likely be sentenced to a mental institution.