Both “normal” perception and “abnormal” hallucination involve internally generated predictions about the causes of sensory inputs, and both share a core set of mechanisms in the brain. The difference is that in “normal” perception, what we perceive is tied to—controlled by—causes in the world, whereas in the case of hallucination, our perceptions have, to some extent, lost their grip on these causes. When we hallucinate, our perceptual predictions are not properly updated in light of prediction errors.
