After carrying out an autopsy on Albert Einstein at Princeton in 1955, pathologist Thomas Stoltz Harvey removed the scientist's brain, hoping to uncover the source of his immense intelligence, and stored the segments in two glass jars.

Harvey never discovered that genius element, but his obsession provided the inspiration for Incognito - the latest play from the pen of Nick Payne, who brought us the dazzlingly different Constellations.

The case of Einstein's 'stolen' brain (debate still rages about whether Harvey was authorised to take it for research purposes) is one of three interwoven storylines in Incognito about the grey matter within our skulls and how it defines us as humans.

Paul Hickey and Amelia Lowdell in Incognito (photo by Bill Knight)
 

In another strand, again based on true events, Henry M undergoes pioneering brain surgery in 1953 which cures his epilepsy but leaves him with severe amnesia.

The third tale follows the fortunes of neuropsychologist Martha, in modern day London, whose marriage breakdown sparks a bout of soul-searching.

Amelia Lowdell is part of the cast of four actors between them playing 21 characters. Among her roles are Harvey's wife and Martha.

"It's such an original piece. I'd never read a script like it. It really asks you to engage and follow the three storylines, which interweave so satisfyingly," she says.

"Martha's grappling with the conflicting concepts of whether it's our memory which makes us who we are or whether it's our personality.

"There's not so much science in her storyline, but I had to get my head around her understanding of what identity is on a more philosophical level.

"Through her work she's seen so many people who've been very damaged and she starts questioning what's left when the brain's that damaged. She's not sure people aren't just a collection of firing neurons."

Alison O'Donnell and Paul Hickey in Incognito (photo by Bill Knight)
 

There are obvious echoes of Constellations, which transferred successfully to the West End after opening at the Royal Court, particularly in the scientific eye Payne turns on his subjects.

The comic drama, in which a single love story is played out in myriad alternative universes, managed to be both compellingly complex and endearingly simple.

Despite the sometimes mind-bending subject matter, the life-affirming message that every relationship is unique won over critics and audiences alike.

Asked whether she feels any pressure to repeat the success of that metaphysical masterpiece, Lowdell replies 'no, that's Nick's torch to carry, and I don't know whether the felt that or not'.

Pressure or no pressure, her role involved plenty of research, but she insists that side of the job was a pleasure rather than a chore.

"Nick does a huge amount of research, and we had books piled high on the tables during rehearsals," she said.

"Paul Broks' book Into the Silent Land, about his experience as a neuropsychologist, was particularly helpful, but there's so much information available to you as an actress these days. It's so different to when I started out.

"I do enjoy the research but you have to know when to leave it behind. It gives you something to fall back on when you can't quite find the answer in the text."

Her preparations for the play, which previewed in Newcastle ahead of its run at the Bush Theatre, have given her plenty of time to ponder philosophically what makes us what we are. But what I really want to know is, given the chance, whose brain would she steal?

"The Dalai Lama's brain. He completely understands what it is to be human and happy and kind. I think one of the thing's Nick's play is examining is what it is to be happy and what makes us happy, and the Dalai Lama seems to have this pretty well figured out."

* Incognito, written by Nick Payne and directed by Joe Murphy, is at the Bush Theatre, in Uxbridge Road, Shepherd's Bush, from May 14 to June 21. Tickets, priced £10 (concessions) to £19.50, are available at www.bustheatre.co.uk or from the box office on 020 8743 5050.