The darkness retreat restricts the sensory information to the brain, for recreational and 'spiritual' purposes and to encourage 'visions' and allow some sort of 'inner journey'. It uses blindfolds and restricts hearing by a 'sacred silence'.
The psychology of this process operates at a number of levels simultaneously - the sensory, biochemical and social. It is reminiscent of experiments by DO Hebb at McGill University in Canada in the early 1950s. Hebb restricted his subjects' visual sense by getting them to wear plastic visors; their auditory sense by using the hum of air conditioning to block out small sounds; and their sense of touch by the use of cotton gloves at all times. The effects often included various forms of hallucinations. Some subjects also got very anxious.
Why did subjects hallucinate? According to the psychologist James Gibson, it is because when the sensory input is inadequate 'the perceptual system hunts. It tries to find meaning, to make sense from what little information it can get.'
At the biochemical level, sensory deprivation also seems to have a profound effect. We know from animal research that in cases where the sensory input has been restricted levels of the neurotransmitter serotonin are significantly lower. (This sits oddly with the claim that the darkness retreat boosts the serotonin level). Then there is the social level. The Hebb research was funded by the Canadian Defence Research Board as an urgent attempt to understand sensory depri vation as a part of 'brainwashing', used by the Chinese on prisoners in Korea. Sensory deprivation was thought of primarily as an extremely negative experience; Hebb's subjects could press a 'panic button' to terminate the experiment.
The original thinking about sensory deprivation was thus in terms of coercion and torture, panic and stress. But the darkness retreat builds up a different set of expectations. Hallucinations are to be perceived as something to be enjoyed. And the darkness retreat restricts mainly vision, and this can allow individuals to appreciate other senses, like taste and touch, more fully.
Hebb would be surprised by people paying large sums of money to have their senses restricted; he had to pay his subjects to take part. But when people go to a situation with different expectations a lot changes. Their thoughts, feelings and emotions about their experiences all change. Plus they may feel in control: it is after all their inner journey.
My guess, however, is that some will still not enjoy the ride.
· Professor Geoffrey Beattie is head of Manchester University's psychology department.