Connections: A Story of Human Feeling by Karl Deisseroth (Penguin), £20.

Mental illness is one of the greatest mysteries of the human condition. Some of the earliest accounts of psychopathology appeared in ancient Mesopotamian texts in West Asia more than 4,000 years ago, when “melancholia” – or depression – was seen as a symptom of demonic possession and treated with often highly intrusive therapies by local priests.

Since then, civilisations have used everything from bloodletting and lobotomy to modern drugs and talking therapy to grapple with the uneasy partnership between the body and the mind and ultimately answer the question: why do we feel what we feel?

In 2021, there is still much we don’t know about mental illness. But exciting scientific breakthroughs studying the brain intact have now made it possible to home in on the individual cells – and even the invisible electrical currents – behind some of the most complex human emotions.

One of the researchers behind these findings is a world-renowned psychiatrist and neuroscientist Karl Deisseroth, author of the new book Connections: A story of Human Feeling. Deisseroth is most famous for his breakthrough work on optogenetics – a technology that renders individual, highly specific brain cells photosensitive and then activates those cells using flashes of light delivered through a fibreoptic wire. He has won numerous prizes for his discoveries. 

In Connections, Deisseroth uses each of the seven chapters to explore a broad area of mental illness – eating disorders, sociopathy, dementia, depression, psychosis, schizophrenia and borderline personality disorder – weaving in findings from his own research to explain the origin and symptoms of these illnesses on a cellular level.  

As a humanities student, I had never heard of most of the scientific ideas Deisseroth tackles in the book but had no issues following his explanations. As well as being a world-renowned, award-winning psychiatrist and neuroscientist, Deisseroth is a masterful writer and breaks down even the most complex scientific ideas into striking images that drive home the sometimes profoundly upsetting emotional toll of our fallible human bodies.  

He explains, for example, how the life-saving infantile reflexes that are suppressed during most of adult life reappear in dementia patients as inhibition and cognitive function are stripped away. Patients are often seen to sweep up their arms in a reflex to sudden motion – echoing the life-saving instincts of our tree-climbing ancestors. “As the fabric frays and texture is lost, the original self finds voice again in a heart-wrenching grasp for safety,” Deisseroth explains.

But some of the most thought-provoking scientific passages are not those that explain recent discoveries but those that lay out the progress that still needs to be made in our understanding of the impact of human evolution on mental illness. 

In one passage, Deisseroth explains the evolutionary link between malaria resistance and the life-threatening blood genetic disorder sickle-cell disease. To be born with sickle cell disease, a child must inherit a copy of the sickle cell gene from both their parents. However, when someone carries just one copy, they enjoy the “harsh evolutionary bargain” of resistance to a deadly form of malaria. “These mutations are ragged measures, quick hacks, still jousting in the torturously slow arena of natural selection,” he writes.

But even though we are at a “tipping point” with our understanding of mental illness, understanding its evolutionary role is still far behind that of physical ailments. “With the sickle-cell trait, those who receive the benefit are not the same as those who suffer. Is this also the case for mental illness, that there is some benefit only for close relatives? Or alternatively, could it be that the mentally ill do directly benefit – at some time, in some way?” he asks.

In a particularly upsetting passage, Deisseroth tackles the evolutionary conundrum of eating disorders and the bodily suffering that is facilitated by the sheer force of the mind. “At what moment in evolution did the balance of power finally tip toward cognition becoming stronger than hunger?” he asks. 

In fact, one of the main strengths of the book is that Deisseroth acknowledges how medicine and science alone are inadequate to explain, least of all describe, much of the internal human experience. For example, in the prologue, he writes: “Ideas from literature have long seemed to me just as important for understanding patients – at times providing a window into the brain more informative than any microscope objective”.

Deisseroth displays a profound love of literature throughout Connections. He explains how he was inspired to follow psychiatry by a chance encounter with a patient who abused him with novel words, including one resembling “telmetale”, a word invented by James Joyce – and peppers the book with epigrams from the writer, along with Milton and Millay, to illustrate his points.

But it is Deisseroth’s own intimate, and at times lyrical, rendering of the case studies he came across during his work as an emergency psychiatrist that gives the book its sometimes heart-wrenching emotional depth. 

First and third-person passages – sometimes with altered language to reflect altered states – introduce the reader to a young woman with an elusive eating disorder, an older man smothered into silence by dementia, a woman coming to terms with a potential schizophrenia diagnosis, a young man with borderline personality disorder and a newlywed inexplicably unable to shed tears for his recently deceased bride.

Even though one in four Britons will experience a mental health problem of some kind each year – a number that is likely to rise as we come to grips with the impact of continuous lockdowns – a diagnosis of a mental illness still carries a stigma that a physical ailment does not, mainly because mental illness has been so poorly understood for so long. 

This is why Connections is such a profoundly important piece of work. Both through his generous explanation of the scientific phenomena behind mental illnesses and his compassionate portraits of the people who suffer from them, Deisseroth invites you to reconsider everything you think you know about the brain, mental illness and the power of human emotions.