Visionary Neurologist Oliver Sacks on What Hallucinations Reveal about How the Mind Works

“We see with the eyes, but we see with the brain as well.”

While our delusions may keep us sane, hallucinations — defined as perceptions that arise independently of external reality, as when we see, hear, or sense things that aren’t really there — are an entirely different beast, a cognitive phenomenon that mimics mysticism and has no doubt inspired mystical tales over the millennia. In the 18th century, Swiss lawyer-turned-naturalist Charles Bonnet, the first scientist to use the term evolution in a biological context, turned to philosophy after deteriorating vision rendered him unable to perform the necessary observations of science. Blindness eventually gave him a special form of complex visual hallucinations, known today as Charles Bonnet syndrome, but he was otherwise fully lucid and marveled, as a cognitive scientist might, at “how the theater of the mind could be generated by the machinery of the brain.”

Some 250 years later, pioneering neurologist Oliver Sacks (b. July 9, 1933) — who has previously explored the necessary forgettings of creativity and how music impacts the mind — picked up Bonnet’s inquiry in his immeasurably fascinating book Hallucinations (public library). In this TED talk based on the book, Sacks draws on his extensive clinical experience of working with patients, illuminating that astounding “theater of the mind” to shed light on what hallucinations reveal about how the mind works.

We see with the eyes, but we see with the brain as well. And seeing with the brain is often called imagination. And we are familiar with the landscapes of our own imagination, our inscapes. We’ve lived with them all our lives. But there are also hallucinations as well, and hallucinations are completely different. They don’t seem to be of our creation. They don’t seem to be under our control. They seem to come from the outside, and to mimic perception.

In the book, Sacks offers a detailed definition of hallucinations, contrasting them with regular perception and peering into their promise for better understanding the brain and the human mind:

When the word “hallucination” first came into use, in the early sixteenth century, it denoted only “a wandering mind.” It was not until the 1830s that Jean-Étienne Esquirol, a French psychiatrist, gave the term its present meaning — prior to that, what we now call hallucinations were referred to simply as “apparitions.” Precise definitions of the word “hallucination” still vary considerably, considerably, chiefly because it is not always easy to discern where the boundary lies between hallucination, misperception, and illusion. But generally, hallucinations are defined as percepts arising in the absence of any external reality— seeing things or hearing things that are not there.

Perceptions are, to some extent, shareable — you and I can agree that there is a tree; but if I say, “I see a tree there,” and you see nothing of the sort, you will regard my “tree” as a hallucination, something concocted by my brain or mind, and imperceptible to you or anyone else. To the hallucinator, though, hallucinations seem very real; they can mimic perception in every respect, starting with the way they are projected into the external world.

When you conjure up ordinary images— of a rectangle, or a friend’s face, or the Eiffel Tower —the images stay in your head. They are not projected into external space like a hallucination, and they lack the detailed quality of a percept or a hallucination. You actively create such voluntary images and can revise them as you please. In contrast, you are passive and helpless in the face of hallucinations: they happen to you, autonomously — they appear and disappear when they please, not when you please.

Hallucinations are “positive” phenomena, as opposed to the negative symptoms, the deficits or losses caused by accident or disease, which neurology is classically based on. The phenomenology of hallucinations often points to the brain structures and mechanisms involved and can therefore, potentially, provide more direct insight into the workings of the brain.