How We See What We Know Isn't There

Oliver Sacks, Hallucinations

In popular culture, we often associate having hallucinations with mental illness. Back in the middle Twentieth Century, simply hearing voices, with no other symptoms, was sufficient for the state to commit you involuntarily. But a growing body of evidence indicates that many ordinary, healthy people see and hear stuff that isn’t there. Rather than a sign of madness, hallucinations are a key into understanding how the human mind works.

Oliver Sacks compiles many anecdotes about individuals “suffering” a range of hallucinations, correlating their remarkable, and sometimes exciting, experiences with the most up-to-date science about how the brain generates images which have no substance. Far from the domain of mental illness, the process of generating hallucinations may reveal a mind at the peak of its performance. But it turns out, there’s no one phenomenon called “hallucinations.”

Instead, the brain generates hallucinations from a myriad of sources. For instance, a person slowly losing one physical sense may find the brain generating content to fill the growing gap. A person going blind may experience complex visual hallucinations, which, for some, may open the doors for all manner of art. People may hear intricate music when going deaf. Smell, the easiest sense to lose, also generates the most elaborate hallucinations.

No one knows exactly why this happens, though Sacks relates some interesting speculations. Perhaps the brain manufactures sensory input to keep idle neurons busy. Perhaps we need the input to maintain our sanity, and our brains would rather invent content than go mad. But in a later chapter, Sacks talks about how people suffering grief may hallucinate that which they have lost. I suspect the answer is very succinct: nature abhors a vacuum.

Neuroscience has looked into the process of how the brain creates hallucinations, and some of its conclusions are fairly surprising. For instance, the process of hallucinating is completely separate from the process of imagination. It is also distinct from the process of dreaming. The brain responds to hallucinations the same way it responds to something physically present. It just happens that the source of the response has no presence outside the brain.

This discovery led to the apparent hope that hallucinations, like dreams, could reveal something important about the inner workings of an individual psyche. Not so, as it turns out. Not only does the conscious mind have no control over hallucinations, neither, apparently, does the unconscious. We either completely invent our visions, or as in the case of mourners seeing the lamented dead, we see only what we have already seen before.

Thus, hallucinations have little application for clinical psychology, but provide a wealth of knowledge to neuroscience. Researchers have made great inroads mapping what parts of the brain govern what perceptions by charting electrical currents through a hallucinating neurosystem. In the case of some of the stranger hallucinations, like phantom limbs, they have even granted insight into how much of the mind dwells outside the cranium.

Hallucinations may even be far more common than we realize. The visual imagery we create on the cusp of sleep demonstrates identical traits to hallucinations, as does that weird effect most of us have from time to time, when we wake up and see something that isn’t there. Night terrors and narcolepsy have the same characteristics as hallucinations. The borderlands of consciousness are fertile ground for remarkably tangible phantoms.

Physical conditions may cause hallucinations, as well. For instance, many people who have migraine headaches, including Sacks himself, see lights and hear sounds prior to an attack. People suffering delirium may have short-term hallucinations as a symptom of their illnesses. Tracking blood flow and electrical currents in people whose hallucinations have outside causes teaches us a great deal about how bodily illnesses steer the mind.

Sacks even goes into how hallucinations arise from drug use. He is remarkably frank about his own experience: though hardly a “head” himself, he tried psychedelics and amphetamines back around the same time Abbie Hoffman made drugs look sexy. The difference is that, where Death came to clear Hoffman’s mess, Sacks learned what drugs had to teach, made a few mistakes, and put that part of his live behind him.

Oliver Sacks has been a prolific writer of popular neuroscience for many years now. The movie Awakenings was adapted from his writings. His years of experience come across in this book, which deals eruditely with complex science, but speaks in a clear voice, uncluttered by jargon. This smart, funny, incisive book turns a tough topic into an engaging read.