What is the current state of the law on the grounds that a witness can be deemed incompetent to testify in a mental health case?

Finally, as noted above, the grounds upon which a trial court may disqualify a witness as incompetent, or exclude the witness's testimony for lack of personal knowledge, are exceptionally narrow. The witness must be allowed to testify unless he or she cannot communicate intelligibly or understand the duty to tell the truth, or unless no rational jury could believe the witness actually saw the events he or she claims to have seen. In many cases, psychiatric testimony, itself "inherently [subject to] expert debate" (People v. Gonzalez (1990) 51 Cal.3d 1179, 1247, 275 Cal.Rptr. 729, 800 P.2d 1159), would be less useful on these issues than the court's own evaluation of the witness's demeanor and responses in light of all the evidence. (See, e.g., Alcala, supra, 4 Cal.4th 742, 781, 15 Cal.Rptr.2d 432, 842 P.2d 1192.)