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Inarticulacy isn't normally a recommendation for the presenter of a television programme but it doesn't seem to have disqualified Oliver Sacks from the role. The very first sentence he uttered in The Mind Traveller (BBC2) stumbled about halfway through, fell sideways and converted itself into a plaintive cry of helplessness: "For a human being to be so intact" he said running his hand over the face of a man frozen by brain disease, "and yet I don't know what's going on." In what followed he stuttered, repeated himself, even abandoned speech now and then in favour of a bear- like growl. And yet The Mind Traveller also contained some wonderful writing, typical of Sacks's elegant published essays. He must have been panic-inducing to film with, but you don't really need to be articulate if you're as literate as this.

Particularly, it should be added, if you do some of your writing aloud, while the camera is running. There was plenty of this in evidence here - little passages of speech that seem to emerge from a kind of reverie about the subject, rather than any conventional communication with the other people present. Turning a human brain in his hands, like a giant pickled walnut, Sacks murmured with an almost private pleasure: "Very beautiful object... the most complex object in the world." Looking at a magnified image of a diseased brain cell he tried out metaphors: "This is the tombstone, this is the dead effigy of a cell," he said, which nicely captured the empty plaque you saw on screen, waiting for some kind of explanatory inscription. An alliteration that pleased him ("the lovely lethal cycad") was first heard on the voice-over but obviously had its birth as he stared over the foliage of Guam, making audible notes to himself for the article that is bound to follow. And when he wishes to cite an authority it will, as often as not, be a poet or a novelist rather than a scientist. Struggling to explain the timeless paralysis of the cases he was examining he turned to Joyce - "History is a nightmare from which I am struggling to awake," he quoted, going on to suggest that it was these frozen people who had somehow emerged into consciousness, a remark typical in its mix of erudition and charitable inversion. Personally I'd rather stay asleep if that's what wakefulness looks like but there was something undeniably tender in the effort.

You do need the tenderness because the programme itself is a punctilious freak show - a perfectly respectable genre both on television and in medical lecture rooms, provided it is handled with tact. Sacks achieves this, displaying dysfunctional marvels for our instruction, not our amusement (moral instruction too - there is a sense of "count your blessings" in many of the sequences). All this is a natural extension of his published work, which uses neurological wonders as a narrow window through which to glimpse the routine workings of the mind. The condition Sacks was examining in Guam, a tropical variation on Parkinson's Disease, promised to throw light on Alzheimer's and other degenerative brain diseases - the only problem being that the disease itself was dying, fading from the population before its mystery had been resolved. It might have been something in the water or it might have been that deadly cycad, a prehistoric plant from whose seeds the islanders make a kind of flour. The local researcher had decided that the flour was not to blame - once processed there was only a "pinch" of carcinogenic toxin left, he said, nibbling placidly on a tortilla. Dr Sacks turned a piece over in his hands with a wary look in his eyes: "There's no point in my eating it," he said, as if the director had mutely urged him to tuck in for the camera. "Why should I eat it?"

Quite right Doctor - for chocolate cake one might take the risk, but for a cold tortilla? This little stab of terror, breaking through the clinical fascination of the preceding 40 minutes, added another touch of humanity to a film which, like Sacks's speech, never quite achieved a formal structure but ended up making sense anyway.