Terry Pratchett speaks of Alzheimer's impact on writing

Sir Terry Pratchett has spoken about how the onset of Alzheimer's disease is affecting his ability to write.

The Discworld author, whose new novel Unseen Academicals will be published in October, told the Daily Telegraph he can still 'turn a phrase', despite suffering from posterior cortical atrophy, a form of the degenerative disease.

However, he continued: 'My eyesight, as eyesight, is perfectly good. But how the brain deals with what my eyes can see can be pretty ropy.'

The 61-year-old illustrated this by explaining that he could glance at the floor and fail to see a cup there because his 'brain is filling up the space with something else'.

In the interview, he stressed this has not yet affected his ability to craft words in his trademark style, although he now makes use of a speech-to-text computer programme rather than typing.

Pratchett has brought Alzheimer's into the public eye since he disclosed he had the condition in 2007, recently addressing the Liberal Democrat party conference and calling for additional funding for research into the condition.

The writer claimed that the world is facing a 'tsunami of Alzheimer's' unless a cure is developed which can lessen its impact.