A tourist visits Hirbet Madras archaeological site. Reuters/Baz Ratner What happens if you spend six months alone in a dark cave?

On Valentine’s Day in 1972, geologist-turned-researcher Michel Siffre was about to find out.

He kissed his wife goodbye and descended a hundred feet to the aptly named Midnight Cave in Texas. He did not emerge for six months.

Those months of isolation and sensory deprivation had a profound effect on his mental and physical health.

Ten years earlier, Siffre had been the sole participant in a pioneering study in which he spent nine weeks alone in a cave with no way of telling whether it was day or night. He later wrote that he had become ‘a half-crazed disjointed marionette’ and suffered severe physical and emotional distress.

If that experience had been so unpleasant, then why did he choose to repeat it, and for much longer? Part of the answer is that Siffre’s missions had a scientific purpose.

He wanted to discover what would happen to his biorhythms when all external clues to time were removed. In this he was successful.

His solitary confinement produced new findings in the field of chronobiology and helped to advance the understanding of long-term isolation. But this scientific knowledge was won at considerable personal cost.

Compared to the freezing cave that Siffre had occupied ten years previously, Midnight Cave was a balmy 21 degrees Celcius. He lived in a nylon tent atop a wooden stage of 17 square metres.

The living quarters were dominated by scientific equipment, but he had some creature comforts including a freezer, furniture, books, and a music player. NASA supplied his food and monitored the experiment as part of its research for long-duration space missions. Siffre was consuming the same diet as the Apollo astronauts.

There were no clocks in the cave and Siffre had no other way of telling the time. His assistants camped at the mouth of the cave and acted as ‘ground control’, turning on the lights in the cave when he woke and turning them off when he felt drowsy. When the lights were off he was in absolute darkness.

Even towards the end of the experiment, when Siffre was depressed and despairing, he continued diligently to collect data. Every day he completed a battery of physical and cognitive assessments: memory tests, mental acuity tests (similar to IQ tests), tests of manual dexterity, and so on. Throughout his stay a rectal probe recorded his body temperature and electrodes on his chest recorded his cardiac rhythm. Every night he attached additional electrodes to his body so that his sleep patterns could be recorded. The data reached the surface by means of a cable. As well as acting as a tether that effectively confined Siffre to his platform, the cable occasionally delivered painful electric shocks when lightning hit the surface above him.

Siffre had prepared for the loneliness and tedium by taking books and records. But the damp caused his record player to malfunction, and within a few weeks all his books and papers were covered with mildew.

The lack of sensory and social stimulation led to a gradual deterioration in Siffre’s psychological state. At one point he even contemplated suicide, only to reject the idea because he did not want to saddle his parents with the debts he had run up financing his experiment. By the seventy-seventh day his memory had deteriorated to such an extent that he forgot things unless he wrote them down immediately. He wrote in his journal: ‘What am I doing here in this silly experiment while my professional life ebbs away?’ ground control: he’d had enough. Ground control told him everything was fine. He stayed.

Michel Siffre emerges from Midnight Cave on July 9, 1972. YouTube/AP Archive When he eventually emerged, Siffre was a changed man. Though many of the negative effects were temporary, some were long lasting. Three years after leaving Midnight Cave, Siffre still had memory lapses, his eyesight remained poor, and he had what he described as inexplicable ‘psychological wounds.'

He divorced and, according to one report, retreated to a South American jungle to recover from the ordeal. At face value, Siffre’s psychological deterioration was not surprising.

Deleterious effects of isolation have been widely reported from observational studies in prisons and anecdotal accounts of voluntary and involuntary solitary confinement.

Fictional representations of individuals in conditions of solitary confinement or sensory deprivation often end in their madness. However, the evidence is by no means clear that sensory deprivation is necessarily always a bad thing. So where does the popular conception come from?

The story starts with some extraordinary laboratory studies conducted between the 1950s and the 1980s to find out what happens when humans are subjected to sensory deprivation, social isolation, and extended periods of confinement. Some of these experiments were funded by the US military, in response to alarming accounts of ‘brain washing’ techniques used in the Soviet show trials of the 1940s and by the Chinese government against Western prisoners in the Korean War. Further impetus came from NASA, whose interest in isolation studies as an analogue for long space missions continues to the present day. However, the Canadian government funded much of the most important research in the 1950s and 1960s, and a great deal of it was carried out at Montreal’s McGill University.

Deleterious effects of isolation have been widely reported from observational studies in prisons and anecdotal accounts of voluntary and involuntary solitary confinement. Reuters/Daniel Munoz

One of the first of the Canadian studies, published in 1954, was by Bexton, Heron, and Scott. It remains one of the best-known academic papers on sensory deprivation, both for the remarkable results it revealed and the entertaining way in which those results were communicated. The paper described the disturbing experiences of a group of male student volunteers who spent up to a week in solitary confinement. They wore translucent goggles, which meant they could tell the difference between light and dark but had no other visual input. The cubicle in which each individual was confined was soundproofed, and all they could hear was the background hum of the air conditioner. Cardboard cuffs, to restrict tactile stimulation, extended from the elbow and covered the hands, precluding any self-stimulation.

The volunteers tended to sleep a lot at the start of the study, but as time passed they lay awake, agitated and bored. In the absence of external sensory input, they generated their own noise by whistling, singing, or talking to themselves. They found it difficult to concentrate and unable to focus on simple cognitive tasks such as basic mental arithmetic. ‘Blank periods’ occurred, during which they had no thoughts. They became restless and found the whole experience exceedingly unpleasant. Most of the volunteers could not tolerate the conditions beyond three days and dropped out, despite generous financial inducements.

Several of the volunteers who did stay the course reported experiencing a variety of visual, aural, and tactile hallucinations or delusions. Some found these amusing, others irritating. Visual hallucinations ranged from simple dots and lines to complex images such as ‘a row of little yellow men with black caps and their mouths open’ and ‘a procession of squirrels with sacks over their shoulders, marching purposefully across a snow field’. Some heard voices or music, or felt objects hitting their skin.

The McGill isolation studies captured the imaginations of psychologists and public alike. By the end of the 1970s, more than a thousand scholarly papers had been published on the topic. Yet many of the most startling results resisted replication in subsequent research. One review in the late 1960s suggested that fewer than one in five participants had actually experienced hallucinations. The climate of opinion was also changing, as sensory deprivation experiments increasingly became associated in the public mind with stories of solitary confinement, torture, and CIA ‘mind-control’ experiments. Personal (and in some cases physical) attacks on researchers by political activists prompted many to leave the field.

Whatever the limitations of the early isolation experiments, it is clear that people do sometimes have hallucinatory experiences under conditions of sensory deprivation. Moreover, those conditions do not have to be as artificial as in the laboratory experiments. Extreme environments that expose people to sensory (and particularly visual) monotony can have strange effects on perception, especially when the monotony is combined with stressors such as sleep deprivation and hunger. Perhaps surprisingly, hallucinations are not uncommon in everyday life either.

Reprinted from "EXTREME: Why Some People Thrive at the Limits" by Emma Barrett and Paul Martin with permission from Oxford University Press. Copyright © 2014 by Emma Barrett and Paul Martin.