Expert opinion evidence may be rebutted by showing the incorrectness or inadequacy of the factual assumptions on which the opinion is based, the reasoning by which he progresses from his material to his conclusion, the interest or bias of the expert, inconsistencies or contradictions in his testimony as to material matters … Also, in cases involving the opinions of medical experts, the probative force of that character of testimony is lessened where it is predicated on subjective symptoms, or where it is based on narrative statements to the expert as to past events not in evidence at the trial.