What is the test for determining whether a defendant is mentally competent to stand trial?

rational as well as factual understanding of the proceedings against him." ' [Citation.] [] When a trial court is presented with evidence that raises a reasonable doubt about a defendant's mental competence to stand trial, federal due process principles require that trial proceedings be suspended and a hearing be held to determine the defendant's competence. [Citations.] Only upon a determination that the defendant is mentally competent may the matter proceed to trial. [Citation.] [] California law reflects those constitutional requirements. Section 1368, in subdivision (a), requires a trial court to suspend criminal proceedings at any time 'prior to judgment' if the court reasonably doubts 'the mental competence of the defendant.' A defendant can create reasonable doubt through substantial evidence of mental incompetence, or the trial court can raise the issue on its own. . . ." (People v. Ary (2011) 51 Cal.4th 510, 517.)

"A defendant is presumed to be mentally competent to stand trial. ( 1369, subd. (f).) [] . . . [S]ection 1368 provides that if the trial court has any doubt as to the defendant's competence to stand trial, it must state that doubt in the record and inquire of counsel whether, in his or her opinion, the defendant is mentally competent. ( 1368, subd. (a).) The trial court is authorized to conduct a competency hearing on its own motion and at the request of counsel. ( 1368, subd. (b).) [] . . . '[O]nce the accused has come forward with substantial evidence of incompetence to stand trial, due process requires that a full competence hearing be held as a matter of right. [Citation.] In that event, the trial judge has no discretion to exercise. [Citation.] As we also have noted, substantial evidence of incompetence is sufficient to require a full competence hearing even if the evidence is in conflict. [Citation.] We have concluded that where the substantial evidence test is satisfied and a full competence hearing is required but the trial court fails to hold one, the judgment must be reversed. [Citation.]' " (People v. Young (2005) 34 Cal.4th 1149, 1216-1217, fn. omitted.)