Three years ago, Alan Yentob made a documentary with neurologist Oliver Sacks. The two spent quite a bit of time working together, yet when Yentob returned to make a new programme with him he knew in advance that Sacks would almost certainly not recognise him.

The truth is that the 77-year-old Sacks has never recognised Yentob, or indeed anyone else.

It turns out that the man who has written intelligently and poignantly about some startling neurological conditions has spent a lifetime dealing with one of his own – face blindness.

Sacks has an inability to recognise anyone's features, even on occasion his own when he looks unexpectedly in the mirror.

But this was not a programme purely about Sacks (although his insights and his humanity made for compelling viewing), but rather about his latest research into sight. We met a novelist who, after suffering a stroke, lost the ability to read but who could still write. We encountered a woman who saw only in two dimensions until she was nearly 50 when her world unexpectedly burst into 3D, and a group of people who can communicate only by touch.

This was all illuminating but frustratingly brief. Any one of the four conditions investigated would have made an hour's documentary in itself and there were dozens of questions left unanswered.