We commonly say an art curator casts "an expert eye" over a painting, or an auditor over a set of accounts. But what is it that, for example, allows an experienced birdwatcher to identify an uncommon species rapidly, at a distance and against a chaotic background in the wild? And what separates the gaze of a qualified brain surgeon from that of a first-year medical student? How is that kind of visual expertise acquired? We commonly say an art curator casts "an expert eye" over a painting, or an auditor over a set of accounts. But what is it that, for example, allows an experienced birdwatcher to identify an uncommon species rapidly, at a distance and against a chaot... [4244 chars]