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

Section 4241 provides that the district court, upon its own motion if necessary, must hold a competency hearing "if there is reasonable cause to believe that the defendant may presently be suffering from a mental disease or defect rendering him mentally incompetent to the extent that he is unable to understand the nature and consequences of the proceedings against him or to assist properly in his defense." Applying these rules, we have stated that a hearing must be held when there is reasonable ground for a district court to conclude that the defendant may not be competent to stand trial. See United States ex rel. Roth v. Zelker, 455 F.2d 1105, 1108 (2d Cir.), cert. denied, 408 U.S. 927, 92 S.Ct. 2512, 33 L.Ed.2d 340 (1972).