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Just published in PLOS ONE: Caffeine Improves Left Hemisphere Processing of Positive Words. Authors Lars Kuchinke and Vanessa Lux from Ruhr University Bochum in Germany tested how quickly people processed positive emotional words when they had been given a 200 mg caffeine tablet (about 2-3 cups of coffee) compared to people given a placebo. They found that caffeine didn’t affect word processing in general, but it did affect processing of positive words, specifically those processed in the left hemisphere of the brain, which is responsible for language processing in right-handed people (and everyone in the study was). Positive words were processed more quickly and accurately by people that had caffeine.
How can they know if the words were processed in this hemisphere? The words were processed visually (the participants saw them on a screen), and some were presented in the right visual field and some in the left visual field. Because the nerves from each of our eyes split and then criss-cross towards the back of our brain, it means that everything in our right visual field (from both eyes) is processed by the left hemisphere of the brain. Here’s a site that explains it.
What’s really interesting about this study is that the caffeine didn’t have an effect on word processing in general, and didn’t have an effect on processing negative emotion words, either. It was only positive emotion words that were better processed. The authors think that the dopaminergic pathways play a role in this effect, as caffeine stimulates the brain to produce dopamine.