(a) In a proceeding for the enforcement of a claim for compensation under this chapter it is presumed, in the absence of substantial evidence to the contrary, that
(1) the claim comes within the provisions of this chapter;
(2) sufficient notice of the claim has been given;
(3) the injury was not proximately caused by the intoxication of the injured employee or proximately caused by the employee being under the influence of drugs unless the drugs were taken as prescribed by the employee's physician;
(4) the injury was not occasioned by the wilful intention of the injured employee to injure or kill self or another.
(b) If delay in giving notice is excused by the board under AS 23.30.100(d)(2), the burden of proof of the validity of the claim shifts to the employee notwithstanding the provisions of (a) of this section.
(c) The presumption of compensability established in (a) of this section does not apply to a mental injury resulting from work-related stress.