Unless required for the disposition of ex parte matters authorized by law, members of the governing board or officers or employees of an agency assigned to render a decision or to make findings of fact and conclusions of law in a contested case may not communicate, directly or indirectly, in connection with any issue of fact, with any person or party, nor, in connection with any issue of law, with any party or the party's representative, except upon notice and opportunity for all parties to participate. If one or more members of a board or commission or a member or employee of an agency, who is assigned to render a decision in a contested case, took part in an investigation upon which the contested case is based, the member or employee may not participate in the conduct of the hearing nor take part in rendering the decision on the contested case. However, the member or employee may appear as a witness and give advice as to procedure. If, because of the disqualification, there is no person assigned to conduct the hearing or render the decision, the agency shall appoint a person to fulfill those duties. A person assigned to render a decision:
(1) May communicate with other members of the agency; and
(2) May have the aid and advice of one or more personal assistants.