I heard some time ago that the works of Iris Murdoch, a literary figure famous not just for her works but also her descent in the hell of Alzheimer's disease, trace the decline of her mind at an early stage, namely through analysis of the vocabulary she used. Her writing betrayed the subtle decline of her mind before it was officially diagnosed. A similar piece of analysis has been performed on the novels of Agatha Christie. Textual-analysis software was used to analyse a number of her books, exploring:
"vocabulary size and richness," an increase in repeated phrases (like "all sorts of") and an up tick in indefinite words ("anything," "something") — linguistic indicators of the cognitive deficits typical of Alzheimer's disease. The results were statistically significant; Christie's lexicon decreased with age, while both the number of vague words she employed and phrases she repeated increased. Her penultimate novel, "Elephants Can Remember," exhibits a "staggering drop in vocabulary" — of 31 percent — when compared with "Destination Unknown," a novel she wrote 18 years earlier"
Similar work is currently being conducted in research labs such as TRIL on speech – in terms not just of content but also prosody, and other formal characteristics – opening up the possibility that biomarkers of such diseases can be identified in those of us who don't write books, but simply use the phone.