A person is not criminally responsible for an act or omission if at the time of doing the act or making the omission he or she is through any disease affecting his or her mind incapable of understanding what he or she is doing or of knowing that he or she ought not to do the act or make the omission; but a person may be criminally responsible for an act or omission, although his or her mind is affected by disease, if that disease does not in fact produce upon his or her mind one or other of the effects mentioned in this section in reference to that act or omission.