Have you ever seen something that wasn't really there? Heard someone call your name in an empty house? Sensed someone following you and turned around to find nothing?Hallucinations don't belong wholly to the insane. Much more commonly, they are linked to sensory deprivation, intoxication, illness, or injury. People with migraines may see shimmering arcs of light or tiny, Lilliputian figures of animals and people. People with failing eyesight, paradoxically, may become immersed in a hallucinatory visual world. Hallucinations can be brought on by a simple fever or even the act of waking or falling asleep, when people have visions ranging from luminous blobs of color to beautifully detailed faces or terrifying ogres. Those who are bereaved may receive comforting "visits" from the departed. In some conditions, hallucinations can lead to religious epiphanies or even the feeling of leaving one's own body.Humans have always sought such life-changing visions, and for thousands of years have used hallucinogenic compounds to achieve them. As a young doctor in California in the 1960s, Oliver Sacks had both a personal and a professional interest in psychedelics. These, along with his early migraine experiences, launched a lifelong investigation into the varieties of hallucinatory experience.Here, with his usual elegance, curiosity, and compassion, Dr. Sacks weaves together stories of his patients and of his own mind-altering experiences to illuminate what hallucinations tell us about the organization and structure of our brains, how they have influenced every culture's folklore and art, and why the potential for hallucination is present in us all, a vital part of the human condition.
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, 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.1
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.
Hallucinations tend to be startling. This is sometimes because of their content — a gigantic spider in the middle of the room or tiny people six inches tall — but, more fundamentally, it is because there is no “consensual validation”; no one else sees what you see, and you realize with a shock that the giant spider or the tiny people must be “in your head.”
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.
There is another mode of hallucination, sometimes called pseudo-hallucination, in which hallucinations are not projected into external space but are seen, so to speak, on the inside of one’s eyelids — such hallucinations typically occur in near-sleep states, with closed eyes. But these inner hallucinations have all the other hallmarks of hallucinations: they are involuntary, uncontrollable, and may have preternatural color and detail or bizarre forms and transformations, quite unlike normal visual imagery.
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.
Though the phenomena of hallucination are probably as old as the human brain, our understanding of them has greatly increased over the last few decades.2 This new knowledge comes especially from our ability to image the brain and to monitor its electrical and metabolic activities while people are hallucinating. Such techniques, coupled with implanted-electrode studies (in patients with intractable epilepsy who need surgery), have allowed us to define which parts of the brain are responsible for different sorts of hallucinations. For instance, an area in the right inferotemporal cortex normally involved in the perception of faces, if abnormally activated, may cause people to hallucinate faces. There is a corresponding area on the other side of the brain normally employed in reading — the visual word form area in the fusiform gyrus; if this is abnormally stimulated, it may give rise to hallucinations of letters or pseudowords.
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.
