In what circumstances will a defendant be found competent to testify at trial?

specifically raised the issue of capacity to testify, the district court, in order to insulate this fundamental constitutional right from an involuntary waiver, must determine whether the defendant is in fact physically able to testify. Cf. United States v. Purnett, 910 F.2d 51, 56 (2d Cir. 1990) (holding that, "where a trial court has sufficient cause to doubt the competency of a defendant to make a knowing and intelligent waiver of the right to counsel," it must take affirmative measures to ensure that the competency issues are properly resolved in order for any waiver of the right to counsel to be valid, despite the fact that there is no general requirement that a trial court find a defendant competent whenever a defendant has invoked the right to self-representation.).