A curious feature of hallucinations is you can’t conjure them up at will. Yet they demonstrate the mind is stocked with images, and can produce involuntary but realistic perceptions that are immediate and vivid, to the point the perceiver mistakes the apparition for a real physical entity or the voice in the head as emanating from a real speaker. Nevertheless, they are often bizarre, and don’t correspond to actual events from the past. In contrast, you can consciously retrieve memories and even construct imaginary scenes, but these are diffuse and vague, and are not perceived as though happening in the present.
Hallucinations, then, just happen, but they are provoked by a wide range of circumstances. Sometimes it’s a matter of sensory loss: the deaf may hear voices, the blind may see vivid visual displays and people who have lost the sense of smell may experience bizarre odours.
Depriving people of sensory stimulation, as in an isolation cell, can induce hallucinations, as can conditions such as migraine, epilepsy, Parkinsonism, delirium, narcolepsy and psychosis. Oliver Sacks covers these and more, using his copious case notes, along with historical accounts, to provide vibrant descriptions of the wide range of forms hallucinations can take. Amazingly, he also seems to have experienced most of them himself.
He participated enthusiastically in the drug culture of the 1960s, and seems to have tried almost everything on offer, including cannabis, LSD, mescaline, morphine and even a mixed cocktail of amphetamine, LSD and a pinch of cannabis. This concoction produced a rare case in which he was able to consciously conjure a visual hallucination by demanding that indigo, the most elusive of the spectrum of colours, appear on a blank wall. It did, and he remained obsessed by indigo for some time afterwards.
Some hallucinations, such as heightened colours, distortions of size or movement, and the appearance of zigzag lines, can be plausibly explained in terms of what we know of visual neurophysiology. Less explicable are those giving the appearance of a real event, as when Sacks, high on a drug called Artane, greeted friends who had arrived at his door and proceeded to cook ham and eggs for them, only to discover they were not there at all.
Other hallucinations are simply bizarre – distorted faces, battle scenes, grotesque animals, visions of heaven and hell. Again, though, these are not like memories and don’t replay past events. They seem to be made up of stored fragments that normally go into our perceptions of the world, but are somehow disengaged, distorted and randomly combined.
It is a matter of wonder that there are so many otherwise unpalatable plants that induce hallucinations and altered states of consciousness. Sacks suggests they evolved psychoactive properties either to deter animal predators or to attract animals to eat the fruit and disseminate the seeds through defecation. But the symbiotic relation with humans seems especially pronounced, to the point we encourage the survival of plants with psychoactive properties by cultivating them, not always legally. All cultures have found chemical ways of inducing hallucinatory or stimulant effects, whether as part of religious ecstasy, simply for enjoyment or to enhance creativity – although the sense of creativity may itself be hallucinatory.
Sacks’s book is a fascinating compendium of hallucinatory effects and what causes them, written with his usual flair for exploring the minds of others, as well as of himself. Hallucinations unlock weird, wonderful but sometimes alarming aspects of the mind, and my only complaint is the book doesn’t really dig far enough into what it might all mean.
HALLUCINATIONS, by Oliver Sacks (Picador, $40).
Michael Corballis is emeritus professor of psychology at the University of Auckland and author of Pieces of Mind: 21 Short Walks Around the Human Brain.