Over a year ago Derek sent dad an email, describing a meeting he'd attended in England. Several people at the meeting remembered dad in his professional capacity and asked after him. Derek thought dad would like to know.

Dad printed out Derek's email and read it many times over several days. He kept saying that he would write a reply, but never did. He delayed so long that I stepped in, encouraging him to sit at the keyboard and make a start. After many more days of procrastination he eventually did as I suggested, but it took him several minutes just to get the first three or four words on the screen. I swapped seats with him and took dictation. It was the last letter dad came close to writing.

Something funny has happened to his ability with letters. When he writes by hand, about every fifth letter is missing (when he types, he never gets anywhere because when he spots an error he frequently backspaces too far and erases good text along with the bad.)

Watching odd fragments of words (always in capitals nowadays) struggling to appear on dad's pieces of card is very strange - it is an intimation of something not quite right, a program with a bug, or a hand working without the guidance of a brain.

After seeing my dad fight to put together a shopping list, I once wrote out a reusable one for him. It has about eight items on it and if, when he got to the shops, he bought everything on the list it would do no harm. Instead, he looks at it, puzzles over where it came from, loses it for days on end, often tries to copy some of it to another list, gets lost halfway, then finds an old list from a previous day and cannot remember which was the one he was working on, starts again and ends up with something like:

'CA FOOD'

That's cat food, by the way - yes, all by itself. He will then take this to the shops and buy bread and milk.

Dad's signature is still fairly good, just a little shaky. Mum's, on the other hand, is an elusive thing. She needs to be prompted letter by letter, often requiring examples. Even with a copy in front of her, she is sometimes unable to sign her name. Like dad, she also seems to be resorting to capitals most of the time. We obviously signed the enduring power of attorney forms just in time; I just wish I knew where we put them.

Every now and again, usually when we are at the doctor's, mum manages a good signature. Sometimes she just looks helplessly at us and tries to explain that she can't do it. Other times, something strange appears on the paper: homophones, anagrams, etc. I wish I'd kept copies, they look so odd. They are so close to being right, but so terribly wrong.

In my studies, I came across some deeply odd conditions. One of them is 'alexia without agraphia'. This is the inability to read, whilst retaining the ability to write. In other words, sufferers can write something down correctly, but are then unable to read it. How can this be, you're tempted to ask, but the answer is that a malfunctioning brain is not just a good brain with a small problem, it can give rise to a radically different consciousness in which all bets are off, nothing is obvious any more, and distinctions between real and unreal have little meaning. Being able to see letters, and knowing the structure of words is, so it turns out, not enough to be able to read.

We've tried to help mum and dad by putting signs in the house. For example, they could never find the cat food, so I put a sign 'CAT FOOD IS IN HERE' on the appropriate cupboard. It didn't work. After a while mum asked dad to take it down. She didn't like it.

I left a card on the table for dad, explaining how many fibre tablets to take each day. It said 'DON: TAKE TWO TABLETS WITH EACH MEAL'. He called several times a day to ask if WITH meant before, after, or during. I toyed with the idea of making the note more specific, using AFTER instead of WITH, but then I thought perhaps this would simply result in questions about how long 'AFTER' actually implied, or debates about why it was once OK to take tablets before meals but now they had to be taken after.

It is these situations that make you marvel at how well we normally communicate, drawing on vast reserves of common knowledge and shared experience. When these resources start to fail, language doesn't really compensate.