What causes psychosis? Horizon: Why Did I Go Mad? tries to help us understand

Trying to imagine such a thing is less than halfway to understanding it, but it’s a pretty good place to start.

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In 1999, Dr David Strange, who was then studying for a PhD in epidemiology at Oxford, was with his supervisor when something horrible happened. Looking down, he saw that there were “rat-like things” all over his feet. Naturally, he was terrified: these creatures, he understood, were going to eat him alive from the inside. And yet his supervisor seemed not to have noticed them. Making his excuses, Strange ran all the way home and hid under his bed.

In the years since, his life has not been easy. There have been several suicide attempts and stays in psychiatric units. Pulling a plastic crate from a cupboard, Strange showed the camera the many drugs that supposedly keep him stable, listing their side effects (“These stop me from shitting myself,” he said, with a black laugh). How effective are the medicines? Not very. At a coffee shop where he was meeting a psychiatrist, his eyes suddenly widened. A crab-like beast covered with poisonous spikes was, he announced, lurking by the legs of their chairs. In that moment, his stare hardening, I understood why our forebears used the word “possession” in such cases.

Strange was the touchingly game star of Horizon: Why Did I Go Mad? (2 May, 9pm), a deft and enlightening film about psycho­sis, a condition that one in 100 people will experience at some point. An eloquent and drily witty man with painfully expressive features, he was our tour guide in a netherworld he fears he will never be able to leave.

What causes psychosis? It occurs when the brain produces too much dopamine, the chemical that helps us to recognise threat. Such overproduction may have a genetic component, but the circumstances of one’s childhood play the bigger role: David was violently bullied by his stepfather as a boy, an experience that put his brain on high alert. Those who grow up in urban environments are at greater risk, as are migrants. Dislocation is not good for mental health, which is something local councils might consider as they send tenants to far off cities where they know absolutely no one.

Experts were lined up to tell us of the latest potential treatments. Another patient, Rachel Waddingham, who constantly hears voices, tried out “avatar therapy” courtesy of Professor Tom Craig of King’s College London. He created a computer-generated image to match the cruellest voice she hears, a man who speaks of her worthlessness and threatens to torture her (sketching him, she drew a devil figure, black and spiny). Craig then asked her to stand up to it as it abused her, the idea being that this would help her learn to control the voice itself. I looked at Waddingham, pulpit-solemn and deathly white, telling the avatar to leave her alone, and thought of Gustave Doré’s illustrations for Paradise Lost. What must it be like to be accompanied by an Infernal Council every­where you go? Trying to imagine such a thing is less than halfway to understanding it, but it’s a pretty good place to start.

And so – another kind of madness – to Jed Mercurio’s Line of Duty (ended 30 April). I still love it, of course. On the sofa, mesmerised by the sheer speed of the action, you forgive the convenience of its final, too neat (and yet not remotely neat) plot twists. Even so, I’m still amazed that Huntley (Thandie Newton) chose to bury the forensic evidence relating to the murder of Tim Ifield in a spot where she’d found evidence in another case. How did the crooked Hilton enlist the young detective, Jamie, so quickly? And what power did Hilton have over Maneet, who leaked AC12 secrets to him?

Above all, why did Mercurio have Huntley insist, during her confession, that she isn’t a bad person? She only killed Ifield and chopped three of his bloody fingers off! It is this, rather than any Knowing Looks on the part of Kate and Steve, that is making me worry for the magnificent Hastings (Adrian Dunbar), Brexit Britain’s soothing new ­father figure. If Mercurio can have Huntley spew this kind of line, what’s to stop him from turning our oak tree into a serpent? 

Horizon: Why Did I Go Mad? and Line of Duty are available now on BBC iPlayer.

Rachel Cooke trained as a reporter on The Sunday Times. She is now a writer at The Observer. In the 2006 British Press Awards, she was named Interviewer of the Year.

This article first appeared in the 04 May 2017 issue of the New Statesman, The Russian Revolution