From the strokes, he has complications with mobility, and a lower speed of cognitive processing, but no difficulty speaking.

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For example, one day we might be able to design apps that allow people to probe their own cognitive abilities (processing speed, memory, decision-making, etc.) at any time during the day and track these abilities over the years, transmitting the results from their smartphones to a health care provider if desired.

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And although imaging studies show that middle-aged people may use different brain regions to do the same old tasks, cognitive tests show that apart from brute speed, the brain's abilities are not diminished in middle age.

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This suggested to Dr Gabrieli and Dr Klingberg that the problem was not truly a cognitive one, but rather something to do with the speed at which signals are transmitted along nerve fibres.

ECONOMIST: Dyslexia