With the advent of functional brain imaging in the 1990s it became possible to visualize, at least in gross terms, how the brain might respond to sensory deprivation—and, if one was lucky (hallucinations are notoriously fickle, and the inside of an fMRI machine is not an ideal place for delicate sensory experiences), one might even catch the neural correlates of a fugitive hallucination. One such study, by Babak Boroojerdi and his colleagues, showed an increase in the excitability of the visual cortex when subjects were visually deprived, a change that occurred within minutes. Another group of researchers, in the neuroscience lab led by Wolf Singer, studied a single subject, a visual artist with excellent powers of visual imagery (an article on this by Sireteanu et al. was published in 2008). The subject was blindfolded for twenty-two days and spent several sessions in an fMRI machine, where she was able to indicate the exact times her hallucinations appeared and disappeared. The fMRI showed activations in her visual system, both in the occipital cortex and in the inferotemporal cortex, in precise coincidence with her hallucinations. (When, by contrast, she was asked to recall or imagine the hallucinations using her powers of visual imagery, there was, additionally, a good deal of activation in the executive areas of the brain, in the prefrontal cortex—areas that had been relatively inactive when she was merely hallucinating.) This made it clear that, at a physiological level, visual imagery differs radically from visual hallucination. Unlike the top-down process of voluntary visual imagery, hallucination is the result of a direct, bottom-up activation of regions in the ventral visual pathway, regions rendered hyperexcitable by a lack of normal sensory input.
The deafferentation tanks used in the 1960s produced not only visual deprivation but every other sort of deprivation: of hearing, touch, proprioception, movement, and vestibular sensation, as well as, to varying degrees, deprivation of sleep and social contact—any of which may in themselves lead to hallucinations.
Hallucinations engendered by immobility, whether from motor system disease or external constraints, were frequently seen when polio was rampant. The worst afflicted, unable even to breathe by themselves, lay motionless in coffinlike “iron lungs” and would often hallucinate, as Herbert Leiderman and his colleagues described in a 1958 article. The immobility produced by other paralyzing diseases—or even splints and casts for broken bones—may likewise provoke hallucinations. Most commonly these are corporeal hallucinations, in which limbs may seem to be absent, distorted, misaligned, or multiplied; but voices, visual hallucinations, and even full-blown psychoses have been reported, too. I saw this especially with my post-encephalitic patients, many of whom were, in effect, enclosed in immoveable parkinsonism and catatonia.
Sleep deprivation beyond a few days leads to hallucination, and so may dream deprivation, even with otherwise normal sleep. When this is combined with exhaustion or extreme physical stress, it can be an even more potent source of hallucinations. Ray P., a triathlete, described one example:
Once, I was competing in the Ironman Triathlon in Hawaii. I was not having a good race, I was overheated and dehydrated—miserable. Three miles into the marathon portion of the race, I saw my wife and my mom standing on the side of the road. I ran over to them to say I would be late to the finish line, but when I reached them and began telling my tale of woe, two complete strangers who did not even remotely resemble my wife and mother looked back at me.
The Hawaiian Ironman Triathlon, with its extreme temperatures and long hours of monotony under grueling conditions, can provide an athlete with a fertile venue for hallucination, much the same as the vision quest rites of passage of Native Americans. I have seen Madame Pele, the Hawaiian Volcano and Fire Goddess, at least once out there in the lava fields.
Michael Shermer has spent much of his life debunking the paranormal; he is a historian of science and the director of the Skeptics Society. In his book
Mushers go for 9–14 days on minimum sleep, are alone except for their dogs, rarely see other competitors, and hallucinate horses, trains, UFOs, invisible airplanes, orchestras, strange animals, voices without people, and occasionally phantom people on the side of the trail or imaginary friends. . . . A musher named Joe Garnie became convinced that a man was riding in his sled bag, so he politely asked the man to leave, but when he didn’t move Garnie tapped him on the shoulder and insisted he depart his sled, and when the stranger refused Garnie swatted him.
