THINKING hard for several hours can leave us feeling mentally tired, and now we may know why. Prolonged concentration leads to the build-up of a compound called glutamate in regions at the front of the brain. This may explain why we avoid difficult tasks when mentally fatigued: a glutamate overload makes more mental work difficult. Excess glutamate can be harmful, says Antonius Wiehler at the Paris Brain Institute in France, who led the research. "The brain wants to avoid this, so it is trying to reduce activity." Very high glutamate can be toxic to brain cells.
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