Ask Oliver Sacks

Renowned neurologist Oliver Sacks answers your questions about the secret world of hallucinations.  These queries came to us via Twitter, Facebook and e-mail.

Q. Are the visual or auditory hallucinations in blind or deaf people analogous to sensations in a phantom limb?

A. Not really. Phantom limbs occur because there is a stable, lifelong image of the limbs in the brain—the body-image. If a limb is amputated, part of this body-image is now exposed, so to speak (normally, it is seamlessly incorporated), and intrudes into consciousness as a phantom. There is no such schema underlying visual or auditory perceptions, and if visual or auditory hallucinations occur, they will be concocted from fragments (often disassembled and reassembled) of memory images.

Q. What is the relationship between Lewy Body Dementia and hallucinations?

A. Lewy bodies are abnormal aggregates of protein inside nerve cells, and Lewy Body disease is a malignant form of Parkinson’s disease involving progressive dementia, as well. Visual misperceptions and hallucinations are much commoner in Lewy Body dementia than in Alzheimer’s disease; this is because Lewy bodies are especially common in the visual association cortex, as well as in the brainstem.

Q. Why do hallucinations such as auras and phosphenes take on geometric patterns?

A. The simpler, stabler geometric patterns mirror the functional architecture of the primary visual cortex (V1), especially its columns of neurons with their specific orientation sensitivities. The more complex geometric patterns, which form and reform and transform, probably reflect the ever-changing, often chaotic, self-organizing patterns of activity in a large pool of visual neurons. In either case, one is, so to speak, seeing the basic organizational features of one’s own visual cortex, visible as hallucinations because of their abnormally excited states.

Q. Is déjà vu a type of hallucination?

A. One can regard déjà vu as a hallucination, but it is more properly regarded as an illusion—an illusion of familiarity. Déjà vu does not invent anything, like a hallucination—it transforms what is actually present and perceived.