No expert witness testifying with respect to the mental state or condition of a defendant in a criminal case may state an opinion or inference as to whether the defendant did or did not have the mental state or condition constituting an element of the crime charged or of a defense thereto, except that such expert witness may state his diagnosis of the mental state or condition of the defendant. The ultimate issue as to whether the defendant was criminally responsible for the crime charged is a matter for the trier of fact alone.