Chris Birch woke up after having a stroke and realised, despite being "100% straight before", that he was now attracted to men.
Telling his story, which features in a BBC 3 documentary, he told the Today programme's James Naughtie that it was "a very lonely time" - he had to figure out who he was all over again.
Colin Blakemore, professor of Neuroscience at the University of Oxford, explained that there have been cases of "bizarre changes of attitude, emotion and perception" as a result of brain damage.
"A lack of insight as to what's happened is very common, after brain damage," he said. "In many cases people just feel that they are a new person."
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