Incapacitated subjects are, for example, elderly people with advanced dementia, mentally handicapped, coma patients or people with a severe psychological disorders. These are vulnerable people. They quite often have difficulty forming an idea of what the research entails, if at all. Others must do that for them.
To participate in a research, a representative must provide consent for these subjects. That person could be someone appointed by a judge, such as a curator or mentor. If there is no such person, an authorised person can give consent. And if such a person also does not exist, then the spouse, registered partner or other life partner can give consent on behalf of the participant. If these do also not exist, then adolescents who are deemed able to understand the context are allowed to give consent. And lastly, if there is/are no such person(s), then brothers or sisters who are deemed able to understand the context may provide consent on behalf of the participant.