Capacity to make a choice requires understanding, weighing of information, and choosing:
"the case law on capacity has for some time recognised that, to be able to make a decision, the person concerned must not only be able to understand the information relevant to making it but also be able to "weigh [that information] in the balance to arrive at [a] choice": see Re C (Adult: Refusal of Treatment)  1 WLR 290, 295, approved in Re MB (Medical Treatment)  2 FLR 426."
per Baroness Hale a para 24 of R v C  UKHL 42 (30 July 2009). The choice must be autonomous (free) and must not be driven, for example by a compulsion, delusion, or phobia (25).
Inability to communicate a decision to refuse consent amounts to an inability to make a decision (29).
This was said in the context of the Sexual Offences Act 2003[UK], s 30(1) and (2), but its core is no doubt of wider relevance. It could apply wherever a "decision" is required (avoiding the words "consent" or "refuse"), as in Crimes Act 1961[NZ], s 138.