Linguistic Changes in Terry Pratchett's Writing as an Indication of Posterior Cortical Atrophy

Posterior cortical atrophy (PCA) is a neurodegenerative condition characterized by impaired visuospatial and visuoperceptual skills. PCA sufferers typically have some degree of language impairment, particularly in word retrieval and the processing of words that encode spatial relationships. Terry Pratchett was a renowned British author who was clinically diagnosed with PCA. He continued to publish books after his diagnosis until his death in 2015 from PCA-related complications. This paper investigates longitudinal trends in vocabulary richness, propositional idea density, and proportions of words from five different lexico-semantic word categories (spatial prepositions, colors, numbers, adjectives of form, and function words) in 35 of Pratchett’s novels as potential markers of PCA. Mean type-token ratio and mean rate of unique ideas (propositions) were found to decrease significantly with age in Pratchett’s novels. Spatial preposition word token rate was found to be significantly negatively correlated with age. The results presented in this paper support the hypothesis that damage to brain regions involved in the processing of spatial prepositions caused by PCA may manifest as decreasing rates of spatial prepositions in written text over time.