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The earliest signs of dementia are rarely dramatic. They do not arrive as forgotten names or misplaced keys, but as changes so subtle they are almost impossible to notice: a slightly narrower vocabulary, less variation in description, a gentle flattening of language. The case of Sir Terry Pratchett
New research my colleagues and I conducted suggests that these changes may be detectable years before a formal diagnosis — and one of the clearest examples may lie hidden in the novels of Sir Terry Pratchett.
Pratchett is remembered as one of Britain’s most imaginative writers, the creator of the Discworld series and a master of satire whose work combined humour with sharp moral insight. Following his diagnosis of posterior cortical atrophy, a rare form of Alzheimer’s disease, he became a powerful advocate for dementia research and awareness. Less well known is that the early effects of the disease may already have been present in his writing long before he knew he was ill.
Dementia is often described as a condition of memory loss, but this is only part of the story. In its earliest stages, dementia can affect attention, perception and language before memory problems become obvious. These early changes are difficult to detect because they are gradual and easily mistaken for stress, ageing or normal variation in behaviour.
Language, however, offers a unique window into cognitive change. The words we choose, the variety of our vocabulary and the way we structure description are tightly linked to brain function. Even small shifts in language use may reflect underlying neurological change.
In our recent study, we analysed the language used across Terry Pratchett’s Discworld novels, examining how his writing evolved over time. We focused on “lexical diversity” — a measure of how varied an author’s word choices are — and paid particular attention to adjectives, the descriptive words that give prose its texture, colour and emotional depth.
Lexical diversity changed over time. Stephen Frost/Alamy
Across Pratchett’s later novels, there was a clear and statistically significant decline in the diversity of adjectives he used.The richness of descriptive language gradually narrowed. This was not something a reader would necessarily notice, nor did it reflect a sudden deterioration in quality. Instead, it was a subtle, progressive change detectable only through detailed linguistic analysis.
Crucially, the first significant drop appeared in The Last Continent, published almost ten years before Pratchett received his formal diagnosis. This suggests that the “preclinical phase” of dementia — the period during which disease-related changes are already occurring in the brain — may have begun many years earlier, without obvious outward symptoms.
This finding has implications that extend far beyond literary analysis. Dementia is known to have a long preclinical phase, during which opportunities for early intervention are greatest. Yet identifying people during this window remains one of the biggest challenges in dementia care.
Linguistic analysis is not a diagnostic tool in itself, and it would not work equally well for everyone. Factors such as education, profession, writing habits and linguistic background all influence how people use language. But as part of a broader approach — alongside cognitive tests, brain imaging and biological markers — language analysis could help detect early risk in a non-invasive and cost-effective way.
Importantly, language data already exists. People generate vast amounts of written material through emails, reports, messages and online communication. With appropriate safeguards for privacy and consent, subtle changes in writing style could one day help flag early cognitive decline long before daily functioning is affected.
Why early detection matters
Early detection matters more than ever. In recent years, new drugs for Alzheimer’s disease have emerged that aim to slow disease progression rather than simply manage symptoms. Drugs such as lecanemab and donanemab target amyloid proteins that accumulate in the brain and are thought to play an important role in the disease. Clinical trials suggest these treatments would be most effective when given early, before significant neuronal damage has occurred.
Identifying people during the preclinical phase would allow people and their families more time to plan, access support and consider interventions that may help slow progression. These may include lifestyle changes, cognitive stimulation and, increasingly, new drugs to slow the disease progression.
More than a decade after his death, Terry Pratchett continues to contribute to our understanding of dementia. His novels remain deeply loved, but hidden within them is another legacy: evidence that dementia may leave its mark long before it announces itself. Paying closer attention to language — even language we think we know well — could help transform how we detect, understand and ultimately treat this devastating condition.
Today marks the anniversary of the breaking of the siege of Leningrad (St Petersburg)
Vladimir Putins brother died of diphtheria during the German Siege, his grandmother killed by the Germans in 1941, his uncles died fighting Nazis on the Eastern Front pic.twitter.com/copYTB7Org
To the casual observer, a London taxi driver is just a guy who knows a shortcut to Heathrow and has strong opinions on local weather and politics (“This bloody Starmer and his leftie government”). But to a neuroscientist, that cabbie is a miracle of neuroplasticity.
Why? Because you can’t become a London cabbie without mastering “The Knowledge.” As they cram the chaotic layout of one of the world’s most complex city grids into their heads, aspiring cabbies don’t just learn a map. They physically redesign and grow their brain.
A “brainbuilding” exercise with unexpected side effects
This “brainbuilding” exercise comes with a couple of unexpected side effects, one slightly negative, the other amazingly positive. But first, a bit of history.
Let’s rewind to 1851, when The Knowledge was born out of chaos. That year, the Great Exhibition — a Victorian extravaganza of crystal palaces and curious inventions — drew massive crowds to London, overwhelming the horse-drawn hacks of the day. Cabbies were getting lost, and fares were furious. To prevent further navigational anarchy, authorities created an exam to ensure cabbies actually knew where they were going.
To a neuroscientist, a London cabbie is a miracle of neuroplasticity. Each one has physically redesigned and grown their brain. (Credit: Will & Deni McIntyre via Getty Images)
The test that ensued was a beast of a challenge. “The Knowledge of London” is widely regarded as one of the toughest vocational tests in the world.It requires drivers to memorize the labyrinthine web of streets within a six-mile radius of Charing Cross, the traditional midpoint for measuring distances in London.
This is an area of about 113 square miles (293 km2) of urban spaghetti. For comparison, that’s about five times the size of Manhattan — but instead of straight streets and avenues, you have crooked lanes and ways.
The area encompasses about 25,000 streets, which candidates must learn by heart. But that’s just the skeleton. The meaty bit is the 100,000 landmarks they are expected to know and locate: hotels, hospitals, theaters, police stations, courts, clubs, parks, statues, and more.
The Knowledge takes two to four years to master.
The backbone of The Knowledge is the 320 so-called “runs”: basic routes canonized by Transport for London in the Blue Book, the Bible for would-be cabbies.
Studying The Knowledge is not something you do only in the evenings and weekends. It’s a full-time occupation that entirely takes over your life.
On average, it takes “Knowledge Boys” (the affectionate name for candidates; there is only a smattering of “Knowledge Girls”) two to four years to master The Knowledge. Five years is not an exception, but 18 months — the fastest so far — is. You can see them scootering around the city, clipboard strapped to the handlebars. They spend up to 15 hours a day on the road, literally shouting out the names of streets and landmarks as they drive — a popular memorizing method.
London taxi drivers must learn all 25,000 streets and some 100,000 landmarks within a six-mile radius around Charing Cross, as indicated by the black circle. (Credit: Learning The Knowledge: How London Taxi Drivers Build Their Cognitive Map of London – CC BY 4.0)
In total, candidates log anywhere between 30,000 and 50,000 miles (about 48,000 to 80,000 km) on the scooter. That’s roughly once or twice around the Earth. Without using GPS — that goes without saying.
Examiners grill candidates in a grueling one-on-one oral test. They are asked to recite several routes (“Manor House Station to Gibson Square”), naming every street, every turn, and every roundabout on their trip. At any point, they can be requested to name the landmarks to their left or right. Or the nearest police station or hospital. Or to reverse on the spot. A slight hesitation is enough to cost them points.
Only 20% to 30% of starters survive the process and earn their green badge. Famous failures include Roger Moore, the suavest James Bond, and Stephen Fry, Britain’s most famous polymath.
A new home for curious minds Magazines, memberships, and meaning.
Today, an army of 24,000 licensed cabbies (only 3% of whom are women) ply the streets of London in the traditional black cabs. Since 2021, they have been allowed to use satnav, but they must still be able to do without. The government protects the profession by allowing only black cabs to be hailed on the street (all other hired car services — minicabs, Uber, and the like — must be pre-booked).
As they learn and practice their profession, London cabbies don’t just fill their brain with knowledge — the knowledge literally changes their brain.
In 2000, the MRI scans that neuroscientist Eleanor Maguire took of London cabbies showed they all had a significantly larger posterior hippocampus — the seahorse-shaped nugget in our brain tied to spatial memory and navigation. The more years of experience a cabbie had, the larger their hippocampus.
Later research clarified that while those who had passed the exams had measurable hippocampal growth, this was not the case for those who failed the test. It was also shown that when cabbies retired, the hippocampus shrank back down — “use it or lose it,” indeed.
This supports the theory that the brain is not so much a bucket to be filled, but a muscle to be trained. The transformation in the hippocampi of London cabbies is so dramatic that they’re regularly cited alongside musicians, bilinguals, and jugglers in textbooks about adult neuroplasticity (i.e., the ability of the adult brain to reshape itself by training).
Trainee cabbies must learn 320 “runs” in within the six-mile radius around Charing Cross. These maps indicate the chronological progression of learning the origin and destination pairs of each route. The network gets denser as time goes on.(Credit: Learning The Knowledge: How London Taxi Drivers Build Their Cognitive Map of London – CC BY 4.0)
But cabbies are in a league of their own: Their brains don’t just learn maps — they become maps.
A 2021 study shows how trainee cabbies learn by layering streets like a living GIS system, turning abstract geography into visceral knowledge. This gives them lightning-fast planning abilities, which is why experienced cabbies can navigate more efficiently than “Butter Boys,” the slang term for newly licensed cabbies.
Crossing Vauxhall Bridge on a windy day
Training for The Knowledge is both spatial and embodied. Trainees do not merely read maps — they trace the city with their bodies, building a three-dimensional mental atlas through repeated, multi-sensory immersion. This teaches them what crossing Vauxhall Bridge feels like on a windy day and the angle of the left turn onto Shaftesbury Avenue.
Another study made a useful distinction between taxi drivers and bus drivers. Although both spend long hours behind the wheel, cabbies creatively navigate London’s spaghetti bowl of streets, while bus drivers follow fixed routes. That explains why bus drivers don’t have enlarged hippocampi.
All that work building a mental map of London pays off in more ways than one. The specific area that cabbies bulk up is often the first to atrophy as Alzheimer’s strikes. Recent research suggests that taxi drivers (and ambulance drivers) have some of the lowest mortality rates due to Alzheimer’s. It seems that a lifelong practice of spatial processing builds up a cognitive reserve against the onset of the disease, which is often experienced as spatial disorientation.
The anterior and posterior hippocampus, the latter being the locus of our spatial memory and navigation skills. (Credit: Maguire et al, via turor2u – public domain)
There is, however, also one downside to having a pumped posterior (back) hippocampus. Mental real estate being finite, there is bound to be a trade-off, and studies show that it is a slimmer anterior (front) hippocampus. This has negative implications for cabbies’ non-spatial visual memory.
Specifically, cabbies performed below average on the Rey-Osterrieth Figure Test, which requires subjects to draw complex, abstract shapes from memory. In other words, by hyper-specializing their brains to store a massive, detailed map of the city, it seems cabbies sacrificed some ability to process other types of visual information. Not knowingly or willingly. But the brain enforces its own urban planning.
Most cabbies would be willing to pay that small price for their mastery of The Knowledge. In fact, their cognitive maps are so unique that some computer scientists have studied cabbie navigation to improve GPS routing algorithms by mimicking their more intuitive routing practices. Yes, humans may yet teach machines something about how to navigate a city.
The Knowledge is living proof that the brain is a gloriously malleable instrument that can change with our evolving needs even into adulthood. We can train it to contain an entire city, even one as notoriously untamable as London.
The next time you take a black cab in London, marvel at the living atlas behind the wheel. Know that the driver has a posterior hippocampus larger than yours. And that he or she’s earned it, one London street at a time.
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January 27, 2026
Closer to catastrophe than ever before. It is now 85 seconds to midnight.
A year ago, we warned that the world was perilously close to global disaster and that any delay in reversing course increased the probability of catastrophe. Rather than heed this warning, Russia, China, the United States, and other major countries have instead become increasingly aggressive, adversarial, and nationalistic.
Hard-won global understandings are collapsing, accelerating a winner-takes-all great power competition and undermining the international cooperation critical to reducing the risks of nuclear war, climate change, the misuse of biotechnology, the potential threat of artificial intelligence, and other apocalyptic dangers. Far too many leaders have grown complacent and indifferent, in many cases adopting rhetoric and policies that accelerate rather than mitigate these existential risks.
Because of this failure of leadership, the Bulletin of the Atomic Scientists’ Science and Security Board today sets the Doomsday Clock at 85 seconds to midnight, the closest it has ever been to catastrophe.
canisgallicus 