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Researchers study a small group of patients who underwent surgery that split their brains:

Through studies of this group, neuroscientists now know that the healthy brain can look like two markedly different machines, cabled together and exchanging a torrent of data. But when the primary cable is severed, information — a word, an object, a picture — presented to one hemisphere goes unnoticed in the other. Michael Gazzaniga, a cognitive neuroscientist at the University of California, Santa Barbara, and the godfather of modern split-brain science, says that even after working with these patients for five decades, he still finds it thrilling to observe the disconnection effects first-hand. ‘You see a split-brain patient just doing a standard thing — you show him an image and he can’t say what it is. But he can pull that same object out of a grab-bag,’ Gazzaniga says. ‘Your heart just races!’

“The Split Brain: A Tale of Two Halves.” — David Wolman, Nature