(a) At the Trial. A member of the jury may not testify as a witness before that jury in the trial of the case in which he is sitting as a juror. If he is called so to testify, the opposing party shall be afforded an opportunity to object out of the presence of the jury.
(b) Inquiry into Validity of Verdict or Indictment. Upon an inquiry into the validity of a verdict or indictment, a juror may not testify as to any matter or statement occurring during the course of the jury's deliberations or to the effect of anything upon his or any other juror's mind or emotions as influencing him to asset [assent] to or dissent from the verdict or indictment or concerning his mental processes in connection therewith, nor may his affidavit or evidence of any statement by him concerning a matter about which he would be precluded from testifying be received, but a juror may testify on the questions whether extraneous prejudicial information was improperly brought to the jury's attention or whether any outside influence was improperly brought to bear upon any juror.