What are the effects of prolonged boredom, for example as experienced by 17 months of interplanetary travel? This is the question investigated by a new European Space Agency project in which 12 volunteers will be locked in an isolation tank for 500 days. (In the comments, A.R. points out that this is probably bad reporting by the BBC, as the project actually seems to involve an isolated living space rather than a true isolation tank).
An "isolation tank" is a dark, soundproof container of salt water heated to skin temperature which is intended to induce sensory deprivation - an absence of all sensory stimulation. Such tanks were first used by John Lilly to research the dissociative and psychomimetic effects of the drug ketamine (see "Altered States.")
Even earlier work from the 1930's into the effects of isolation by Nathaniel Kleitman involved locking human subjects in an underground cave, without exposure to natural light, in which light was provided according to scheduled artificial day-lengths.
The European Space Agency is looking for volunteers - you can find the application form for the Mars500 project here.
(Thanks to J.N. for the tip)
I think this is different to an "isolation tank", that's bad reporting on behalf of the BBC. ESA are planning to put all 12 volunteers in a bunker, sealed off from the outside world, except communication via delayed video link to a control centre to simulate the long space flight.
I think if you were locked in a sealed box of saltwater for 500 days you'd either be dead or the skin would probably be floating off your body.
Upon closer inspection you must be right - I shudder to think what 500 days in an isolation tank would do to your mind, even if you had a drysuit (etc)!
I remember an old talk by Niels Birbaumer on patients with advanced ALS, to the point that they no longer had sphincter or eyelid control (and thus no voluntary motor control). His hypothesis was that they would remain conscious for some time, but without the ability to effect their environment and experience stimulation, they would eventually lapse into brain death. He thinks that by using biofeedback while they are still conscious he can teach them to interact with their environment by intentionally activating certain brain areas.
So advanced ALS may be exactly this: dying of boredom.