MOST novels, maybe all of them, are concerned with the question of what it means to be human. Few novelists would or could ask it as technically and specifically as Richard Powers. A central character puts it this way in The Echo Maker: "How does the brain erect a mind, and how does the mind erect everything else? What is the self, and where are the neurological correlates of consciousness?"

Powers has been working on it like a lab scientist for the past 20 years, with each successive fiction an advanced paper on some minute and peculiar aspect of the larger problem.

He was a physics student and computer programmer until he visited the Boston Museum of Fine Art and saw an old photograph by August Sandler titled Three Farmers On Their Way To A Dance, which inspired him to quit his job and write a debut novel of that name in 1985 - in itself an attempt to understand how one image could affect such a change in the mind of a beholder.

Since then, Powers has posited connections between Disney cartoons and nuclear war (Prisoner's Dilemma), genetics and musical structure (The Gold Bug Variations), a team of virtual reality researchers and a blindfolded teacher held hostage in Lebanon (Plowing The Dark). Powers himself calls his stories "dialogues between little and big". But little people have perhaps seemed too lost amid big thinking for this author to become a bestseller. After eight mostly excellent books, critics are still describing him as "an unknown star".

The Echo Maker, which won America's National Book Award last year, follows his popular semi-breakthrough The Time Of Our Singing (2002), an expertly composed study of racial politics, abstract mathematics and the human voice. If that novel was Powers's easiest on the brain, and hardest on the heart (long-term fans worry that he's becoming "accessible"), then this new one asserts that there is no simple difference between emotional and intellectual responses; what we think, and who we are, is decided on the dark landscape inside our skulls, changing with electrical weather systems that may never be accurately charted.

Mark Schulter, a young midwestern American male who could easily be a character from a song on Bruce Springsteen's Nebraska album, flips his truck off the road and into a river bed which serves as a rest stop for vast colonies of migrating sandhill cranes. Their wings are all he remembers of the accident, and he survives the resulting brain trauma with the rare disorder Capgras syndrome. His sister Karin, having escaped their ultra-Christian childhood and deadening but fast-developing home town, returns to find Mark convinced that she's a lookalike impostor at the centre of a plot against him. And the circumstances of the crash - two sets of unidentified tyre tracks on the road, an enigmatic note left in the aftermath - give his pathology some substance.

Famous neuroscientist Gerald Weber is called in, and thinks Schulter a possible case study for his next anecdotal bestseller about extraordinary cerebral glitches. But if Weber's work has already taught him that the self is absolutely not what we think it is - not altogether "whole, willful, embodied, continuous, or aware" - repeat visits to Nebraska trigger a fugue in his own brain which causes him to spin out from his career, marriage and sense of identity. Karin, meanwhile, falls back on old logic, habits, and lovers.

The more she learns about the hippocampus, the amygdala, and the brain's other unfathomable structures in trying to locate the root of her brother's denial, the more she wonders whether she was ever the better person he seems to remember. The novel itself is layered in waves of complexity, building outward from its core domestic drama to the idea that America itself is now experiencing a form of Capgras syndrome. The story begins in spring 2002, and ends with the invasion of Iraq a year later, when the point is finally made that Mark's problem may be universal: "Is this country anyplace you recognise?"

It's not surprising that a novelist so fixated on systems should eventually have produced a book that seems overly programmatic - plot, prose, and ideas come together here with a counter-intuitive, clinical smoothness. Humanity is always the convergence point for all Powers's parallel lines of theory - ultimately, if sometimes remotely, he does care about his characters, and the reader. But it's still his bigger picture that generates echoes.

The mystery surrounding the opening crash feels minor, generic and melodramatic when compared to the grander one suggested by his recurring descriptions of those cranes, passing through Nebraska on their way to the Arctic. Every year, for millions of years, they have followed a flight plan wired into their ancient memory. Something in their brains tells them to persevere. What does it mean to be human, if not to do the same?