The most extensive took place at McGill University Medical Center in Montreal, led by the psychologist Donald Hebb. The McGill researchers invited paid volunteers – mainly college students – to spend days or weeks by themselves in sound-proof cubicles, deprived of meaningful human contact. Their aim was to reduce perceptual stimulation to a minimum, to see how their subjects would behave when almost nothing was happening. They minimised what they could feel, see, hear and touch, fitting them with translucent visors, cotton gloves and cardboard cuffs extending beyond the fingertips. As Scientific American magazine reported at the time, they had them lie on U-shaped foam pillows to restrict noise, and set up a continuous hum of air-conditioning units to mask small sounds.

After only a few hours, the students became acutely restless. They started to crave stimulation, talking, singing or reciting poetry to themselves to break the monotony. Later, many of them became anxious or highly emotional. Their mental performance suffered too, struggling with arithmetic and word association tests.

But the most alarming effects were the hallucinations. They would start with points of light, lines or shapes, eventually evolving into bizarre scenes, such as squirrels marching with sacks over their shoulders or processions of eyeglasses filing down a street. They had no control over what they saw: one man saw only dogs; another, babies.