Hallucinations may overlap with misperceptions or illusions. If, looking at someone's face, I see only half a face, this is a misperception. The distinction becomes less clear with more complex situations. If I look at someone standing in front of me and see not a single figure but five identical figures in a row, is this “polyopia” a misperception or a hallucination? If I see someone cross the room from left to right, then see them crossing the room in precisely the same way again and again, is this sort of repetition (a “palinopsia”) a perceptual aberration, a hallucination, or both? We tend to speak of such things as misperceptions or illusions if there is something there to begin with – a human figure, for example – whereas hallucinations are conjured out of thin air. But many of my patients experience outright hallucinations, illusions, and complex misperceptions, and sometimes the line between these is difficult to draw.
— Hallucinations (Oliver Sacks)
