Except after six months of examinations, doctors could find nothing wrong with Yvonne’s eyes. She was eventually admitted to the neurology unit where O’Sullivan was working. During the observations, Yvonne’s eyes would flicker between her husband and the doctors; as the consultant moved an ophthalmoscope close to her eyes, she blinked. It certainly seemed like her eyes were responding to her surroundings, yet she continued to claim that she was enveloped in an impenetrable darkness.

O’Sullivan’s colleagues assumed she was faking it, perhaps for some kind of lawsuit. “There’ll be no Oscar for that performance,” one muttered after they had left the ward. O’Sullivan herself was unconvinced. “I liked Yvonne. I felt sorry for her. But I did not believe she was blind,” she writes in her new book It’s All in Your Head, recently shortlisted for the Wellcome Book Prize.

Now she knows better. At the Royal London Hospital she has become an expert in “psychosomatic” illnesses. She has treated people who are paralysed from the waist down, or who have such severe cramp in their fingers that their hand has become little more than a claw; one woman could not even empty her bladder without a medical catheter. Yet when doctors look, they can find no physiological cause – suggesting the problem originates in the mind, not the body.

In this light, it’s perfectly possible that Yvonne really wasn’t conscious of what she was seeing – somehow, her unconscious mind was discarding the information before she became aware of it.

Keen to know more, I spoke to O’Sullivan about her career and her recent book detailing these remarkable case studies.