The Valley of Astonishment (Young Vic)

Can you see colours in music? Can you smell fire in light? Can you put words into feelings, or tangible objects? The answer to all three questions is, well, yes.

It’s the most extraordinary thing about this new, deceptively slight and subtle 75-minute theatre piece by Peter Brook and Marie-Hélène Estienne, that a study of the phenomenon of synaesthesia – a neurological terms for muddling up the senses – should be so instantly recognisable.

One case history, that of a journalist, Sammy, played by Kathryn Hunter, runs like a filigree thread through the laboratory of theatrical demonstration and example; she has everything inscribed in her mind, not her notebook, and is despatched to a department of cognitive science and diagnosed a synaesthete.

But already the diagnosis bends. Surely she is merely gifted with a phenomenal memory, and so she goes on the halls, like an old variety act. There, Marcello Magni functions as a sleight-of-hand merchant and performs card tricks with the participation of audience members.

But while her brain skitters off in these uncharted territories, her body is beset by another condition, that of impaired proprioception, a sensory failure leading to paralysis, causing every day in her life to become a mental marathon.

Like stroke victims re-learning mental and physical functions, so Sammy is forced to start again, just as a child will learn its first language through mnemonics, word association and imitation. The miracle of the brain was the subject of Harold Pinter’s A Kind of Alaska, and Brook’s own The Man Who, both "theatricalising" the work of neurologist Oliver Sacks.

Sacks is acknowledged here, too, and the piece – it’s not really a play – references another famous Brook production, The Conference of the Birds, in its poetic allusion to the Persian epic story of thirty birds crossing seven valleys in search of a king.

There is indeed something bird-like about Hunter, frail and wounded but imbued with an indomitable spirit and determination, and a growly, gravelly voice. Magni and Jared McNeill are at first the doctors on her case, later diverting into character cameos and artists, one of them painting the floor with his music.

That music glows. A tantalising picture in the programme shows the peeling, pock-marked, vermilion wall of the Bouffes du Nord, Brook’s Paris venue which is co-producing this show with theatres in New York and Europe, including the Warwick Arts Centre and the Young Vic.

The Young Vic doesn’t have the magic of the Bouffes, but it is otherwise a perfect venue for something so simple and resonant, played on a plain white rectangle, with a few neutral chairs and the exquisite musical accompaniment of Raphaël Chambouvet (piano) and Toshi Tsuchitori (Japanese strings and percussion).

Some, perhaps, will find the piece po-faced and pretentious. I found it obvious in a good way, a life-confirming exercise in the mystery of our existence through a detailed and penetrating observation of how we all tick, and how the body is subject to the unpredictable workings of the brain.