In May 2011, scientists announced that an autopsy of the brain of former Chicago Bears safety David Duerson, 50, who died from a self-inflicted gunshot wound, showed evidence of "moderately advanced" chronic traumatic encephalopathy, or CTE.

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If you finish Singh's code book and want to tax your brain even more, go back to David Kahn's 1, 200-page The Codebreakers, a classic that came out in 1967 and was recently updated.

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For the new research, published in the Archives of General Psychiatry, David Tolin of the Institute of Living in Hartford, Connecticut, recruited 107 people for brain scans using functional magnetic resonance imaging (fMRI).

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