‘It was an extraordinary sight.’ Dahl was recalling Pat's slow recovery from the stroke. ‘Extraordinary to walk into the intensive care, with all its intravenous drips and tubes and oxygen tents, to see Pat propped up in her bed with the nurse beside her singing songs, and yet she could barely utter a word.’

‘It's an example of localisation of function.’ I drew a sketch on a scrap of paper. ‘The speech centres are here on the left side of the brain …’

‘That's Broca's area.’

‘Yes, exactly, and music and singing are on the right.’

Early on as a medical student you learn about Wernicke's area in the left temporal lobe, and the fibres that pass forward, connecting it to Broca's area in the frontal lobe. Damage to either site can cause aphasia, difficulty with speech and language. If Wernicke's area is damaged, then patients have a receptive aphasia – they can't understand what is being said. In contrast, when Broca's is affected people can understand clearly – they know exactly what they want to say – but they have expressive aphasia, and just cannot get the words out.

‘That's what Pat had, of course,’ Dahl said, ‘expressive aphasia. But she also struggled to understand what was being said, especially if many people were talking at once.’

‘She probably had elements of both. But clearly the right side of her brain was fine. This is where music and melody are based, and also foreign language.’

Years later I saw a patient, a young man, whose case would have fascinated Dahl.

Near the North Pole

Darren is in his twenties and doesn't remember much about his illness. He remembers being a bit unwell, and the doctor diagnosing flu. He remembers being at his girlfriend's house, and the food smelling disgusting and making him sick. He goes to bed feeling tired and with a headache. When he wakes, his speech isn't making sense. He thinks he is talking normally, but he isn't.

He cannot understand why everyone is looking at him strangely. He doesn't remember much after that.

His dad, Raymond, realises that this is more than flu because Darren is mumbling incomprehensibly. They get him to hospital. A young doctor rushes into the accident and emergency department, and Raymond fears the worst.

‘It's either meningitis, a growth on the brain, or encephalitis,’ says the doctor.