Susannah wonders about her many hallucinations—which, incidentally, are the only things she truly remembers from the hospital. She admits she still struggles to distinguish fact from fiction. She describes some of the research that's been conducted on hallucinations in which researchers inject subjects with the drug ketamine, which somehow breaks down the subjects' sense of reality. However, all that's truly known about hallucinations is that they happen when the brain perceives something that isn't there; the brain fails to recognize that there's no external source. The fact that the brain generated the hallucination is also why they're easier to remember.
By thinking about her hallucinations this way, Cahalan basically asks whether they are "real" or not. In light of the way that she handles ideas of memory throughout the memoir, they are real: they're just a different kind of real. They were not factual or real to others, but they were extremely real to the version of Susannah who was ill. Because they're all Susannah remembers of that time, they're instrumental in remembering what it was like inhabiting that alternate identity.
Susannah's hallucinations are also easier to remember because many of them are highly emotional, which means her brain flagged them as important anyway. She'll never forget the aging psychiatrist, or waking up restrained with the orange "FLIGHT RISK" band on her wrist. Neither of these events actually happened, and the "FLIGHT RISK" bands don't even exist. Susannah explains that memory is fallible anyway, and she wonders if she truly remembers even the hallucination of seeing the band on her wrist.
Cahalan again throws the entirety of her narration into question, though these unreliable, untrue events did effectively happen. This finally makes it clear that memory itself is very individual, and is subject to each person's personal experience of an event. Susannah is the only one who will truly remember these hallucinations, as she's the only one who experienced them.