The Mind Benders (1963 - UK) [Dirk Bogarde] thriller

Major John Hall with the British military's intelligence service is conducting direct surveillance on elderly Oxford Physiology professor, Dr. Sharpey, who he suspects of treason in selling secrets to the other side, when he witnesses Sharpey do an act which can only be construed as an attempted suicide, a successful one at that. Hall speaks to a number of Sharpey's Oxford colleagues, including Dr. Danny Tate, who do not believe Sharpey could have embarked on treasonous activities, especially in his long known patriotism and pacifism, despite the evidence such as Sharpey having an extraordinarily large amount of cash on his person when he killed himself. Out of those conversations, Hall, who was aware of Sharpey's work on space physiology for the space program in the US, learns of Sharpey's latest associated research, what started out as the effects of extreme cold on physiology, but which morphed due to circumstances, that research which they believe could have contributed to Sharpey's emotional state leading to his suicide: the effects of isolation, such as what astronauts may experience, on physiology, to the extreme of extended periods of total sensory deprivation. Also not convinced that Sharpey could have been involved in treasonous activities is Dr. Harry Longman, who had worked closely with Sharpey on much of this research but who had only recently stepped away from it in part in seeing what it was doing to Sharpey. To discover what happened to Sharpey in large measure to prove he innocent of treason, Longman returns to the lab, this time as the subject as Sharpey had done, to go into the sensory deprivation tank for an extended period. What Hall witnesses of Longman's physiology in the process leads to him embarking, with Tate's willing assistance, on a dangerous subsequent experiment on Longman to prove Sharpey's acts of treason, but which has the potential consequence of putting Longman and his loving wife, Oonagh Longman, in jeopardy.