1.  A defendant is not criminally responsible by reason of insanity if, at the time of the criminal conduct, as a result of mental disease or defect, the defendant lacked substantial capacity to appreciate the wrongfulness of the criminal conduct.  

2.  As used in this section, "mental disease or defect" means only those severely abnormal mental conditions that grossly and demonstrably impair a person's perception or understanding of reality. An abnormality manifested only by repeated criminal conduct or excessive use of alcohol, drugs or similar substances, in and of itself, does not constitute a mental disease or defect.  

3.  Lack of criminal responsibility by reason of insanity is an affirmative defense.