What is the test for striking a witness's entire testimony because he invokes the Fifth Amendment?

Striking a witness's entire testimony because he invokes the Fifth Amendment is "an extreme sanction." United States v. Lord, 711 F.2d 887, 892 (9th Cir.1983). However, "[w]here a defense witness refuses to answer questions that go to the heart of the direct testimony on a central issue ... the truth seeking function of the court is impaired." Denham v. Deed, 954 F.2d 1501, 1504 (9th Cir.1992). Therefore, a district court may bar "a defense witness's testimony when the witness has refused on cross-examination to respond to questions on non-collateral matters." Id.

We have previously noted that "[t]he distinction between matters which are 'collateral' and those which are 'direct' is not precise or easy." United States v. Seifert, 648 F.2d 557, 561 (9th Cir.1980). Since the distinction between matters which are collateral and those which are direct can only be made by referring to the particulars of the case at hand, " '[a] trial court has wide discretion to determine whether a witness's testimony must be stricken because cross-examination was restricted.' " Seifert, 648 F.2d at 561-62, (quoting United States v. Star, 470 F.2d 1214, 1217-18 (9th Cir.1972)).