In late 2015, a 48-year-old woman presented herself to the psychiatric emergency service in Bern, Switzerland with several deep, self-inflicted stab-wounds to her chest. Some measured up to 7cm (2.75in) deep.

She told the physicians that she had followed direct orders from God.

It was a strange case, but not completely unusual, according to psychiatrist Sebastian Walther who first met Sarah in hospital and told BBC Future about her case, as she wished to remain anonymous. Walther recalls that she had a demeanour of “distinctly feeling blessed”, and that she heard voices every minute or so, sometimes lasting for hours. To her they were “divine” and “persistently enjoyable”, despite the fact that these voices gave her deadly instructions.

A brain scan revealed something even more surprising, Sarah had a tumour in a very critical place in her brain, disturbing a “sweet spot” of networks important for processing sound.

While most of us can clearly distinguish between outside sounds and our inner thoughts, many people hear voices – with estimates ranging from 5% to as high as 19% of the general population. Some are benign. But some, when coupled with other mental health issues, can be more distressing. These were the kind of voices Sarah heard.

Auditory hallucinations, like those she experienced, show just how fragile our brain’s auditory input can be. Her case provides a window into how our perceptual processes work and how easily they can be confused.