There's a whole lot of medical science going on in the theatre at the moment. Lucy Prebble's The Effect, which opened at the National Theatre last night, takes a hard satirical look at drugs experimentation in neuroscience and psychiatry, while Nick Payne's Constellations, opening in the West End on Friday, takes a similar line in placing romance and personal health issues in the centre of a flirtation with multiverse theories and quantum mechanics.

And as if our heads weren't spinning enough, having lately recovered from a series of brainy, cerebral plays by Tom Stoppard and Michael Frayn, along comes Jonathan Miller once more, proving science's place at the centre of the arts, in Kate Bassett's long-awaited, exhaustive and authorised biography of the great man, In Two Minds.

Bassett's title hints at Miller's long-rehearsed schizophrenia about "wasting" his time in theatre instead of pursuing a career in neurology, his first love. But it's precisely that tension in his make-up (and I'm not talking Leichner No 9, dearie) that has made him such a remarkable director of plays and opera. He infuses the stage with behavioural observation, the sort of relish of nervous tics, spiritual hesitations and intellectual quirks (and quarks?) that only a medical man knows about.

His great friend the neurologist Oliver Sacks is one of Bassett's key witnesses, and Bassett wisely points out that Miller stands in a great historic line of trainee and qualified doctors who have migrated into comedy and drama: Oliver Goldsmith, Schiller, Chekhov, Ibsen, Strindberg, Somerset Maugham, Christopher Isherwood and Mikhael Bulgakov. Their artistry was rooted in their medicine.

And Oliver Sacks himself has become a familiar name in the theatre, most notably in Harold Pinter's play, Awakenings, based on his memoir about the effects of the L-Dopa drug on a catatonic patient coming out of a coma after 30 years, providing Judi Dench with one of her most memorable performances.

The intriguing thing in Prebble's play is: what exactly are the effects of these drugs, administered experimentally in clinical conditions? And does a brain scan necessarily supply all the answers? And, of course, does the heart hold ultimate sway over everything?

I wonder, though, if Miller and Sacks would go along with Steve Poole's aggressive and cynical programme note at the National in which he says that "the idea that a neurological explanation could exhaust the meaning of experience was already being mocked as medical materialism by the psychologist William James a century ago."

All the uncertainty arises because we really don't know, in the end, how and why the brain actually works, and it's a chilling moment at the National when Anastasia Hille, playing a depressed doctor (who sardonically tells her psychology student guinea pig that "it's never too late to become a real doctor") holds up a rubber brain and gleefully invites our wonder at its sponge-like anonymity.

Of course one of the things a thinking actor always does is mould a character to his or her own personality; there is no other way of giving a true performance, scientifically or artistically. And this is the real joy of Billie Piper and Jonjo O'Neill in The Effect: they understand the emotional charge of their characters because of the scientific mystery of what they go through as drug triallists. Their physical presence is defined by their spiritual and indeed medical confusion.

This use of personality in acting is beautifully defined for Kate Bassett by Peter Eyre, one of Miller's favourite actors, who once played Hamlet, Oswald in Ghosts and Konstantin in The Seagull - the three great Freudian filial roles - for him in the one season, at Greenwich Theatre.

In finding correspondences between the actors' personalities and the dramatis personae, the performers really could be themselves on stage. This process was particularly helpful on Ghosts. "It was a very subtle and painless transformation," says Eyre. "When we finally performed the play it was as natural for us as breathing... a slightly creaking melodrama became a modern and powerful tragedy."