The last novel by the author Iris Murdoch reveals the first signs of Alzheimer's disease, experts say.
Experts analysed three of Dame Iris's books
A team from University College London say their examination of works from throughout Dame Iris's career could be used to help diagnose others.
They found the structure and grammar of her novels was relatively unchanged, but her language was noticeably simpler in her last novel, 'Jackson's Dilemma'.
The study is published online by the journal Brain.
It is part of the team's on-going research into the effects of Alzheimer's on language.
Dame Iris was diagnosed with Alzheimer's aged 76, shortly after the publication of 'Jackson's Dilemma' in 1995.
Tests carried out in 1997 showed that she was losing a range of cognitive abilities including arithmetic, spelling, and word production and showed other signs associated with Alzheimer's.
The diagnosis was confirmed in a post mortem after her death in 1999.
Experts from UCL and the Medical Research Council's Cognition and Brain Sciences Unit compared 'Jackson's Dilemma' with 'Under the Net', her first published work, and 'The Sea, The Sea' which was written at the height of her career.
They analysed the texts by converting them all to a digital format and using specialised software to look at how frequently certain types of words were used.
The researchers say this measures how varied a writer's vocabulary is.
It also shows how frequently new word types are introduced.
The experts found that the smallest number of word-types occurred in 'Jackson's Dilemma' and the largest in 'The Sea, The Sea'.
New word types were also found to be introduced much more frequently in the two earlier novels.
The vocabulary used in 'Jackson's Dilemma' was also found to be the most "commonplace" and that of 'The Sea, The Sea' the most "unusual".
It may be expected that the signs of Dame Iris's illness became evident in her later works.
Many people who have Alzheimer's find it difficult to find the words they want, particularly less common words such as equanimity or discretion - but can still produce grammatically-correct sentences.
But the team behind the study said their findings could help improve current diagnostic tests for Alzheimer's.
Around three quarters of a million people in the UK are estimated to have Alzheimer's.
Dr Peter Garrard of UCL's Institute of Cognitive Neuroscience, who led the research, said: "Iris Murdoch was known to write only in longhand, with few revisions of passages, sending the completed longhand manuscripts to her publishers with little allowance for editorial interference.
"Her manuscripts thus offer a unique opportunity to explore the effects of the early stages of Alzheimer's disease on spontaneous writing, and raises the possibility of enhancing cognitive tests used to diagnose the disease, for example by comparing correspondence or diary entries collected over someone's life."
He added: "Alzheimer's is known to disrupt the brain's semantic system, but this can happen subtly before anyone has the remotest suspicion of intellectual decline.
"Intriguingly, Murdoch experienced an intense and unfamiliar feeling of writer's block during this period.
"It would appear that the disease was already beginning to disrupt her cognitive abilities, which may go some way to explaining why critics were disappointed with the strangely altered quality of her final novel."
John Bayley, Iris Murdoch's husband, said: "I had felt all along that there was something different about Iris's last novel.
"She seemed to have become a different kind of writer, and therefore a different kind of person.
"That personality change is something you see in the early stages of Alzheimer's."
"I felt sure that Peter Garrard would find something unusual in her writing."
Professor Clive Ballard, Director of Research at the Alzheimer's Society, said: "Language does become impaired in numerous subtle ways in the early stages of Alzheimer's disease and other dementias.
"It remains to be established whether detailed analysis of written prose, when available across someone's lifespan, could be an aid to early diagnosis.
"Certainly on a more anecdotal level, changes in the characteristics of people's personal diary entries have been reported as a useful part of the clinical assessment."