In any prosecution for an offense, it shall be an affirmative defense that the defendant, at the time he committed the proscribed act or acts, lacked substantial capacity, as a result of mental disease or defect, either to appreciate the wrongfulness of his conduct or to control his conduct within the requirements of the law. General Statutes 53a-13 (a). "Sanity . . . is an independent fact and not an element of any existing criminal offense. As to such an independent fact, as with regard to other affirmative defenses, the legislature has the constitutional authority to allocate the burden of proof to the defendant rather than to the state." State v. Joyner, 225 Conn. 450, 464-65, 625 A.2d 791 (1993). "The credibility of expert witnesses and the weight to be given to their testimony and to that of lay witnesses on the issue of sanity is determined by the trier of fact. . . . In its consideration of the testimony of an expert witness, the trial court might weigh, as it sees fit, the expert's expertise, his opportunity to observe the defendant and to form an opinion, and his thoroughness. It might consider also the reasonableness of his judgments about the underlying facts and of the conclusions which he drew from them." (Citation omitted; internal quotation marks omitted.) State v. DeJesus, 236 Conn. 189, 201, 672 A.2d 488 (1996).