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More dopamine can make you worse
The pattern is called the inverse U-shaped curve for dopamine. Studies that sort people by baseline dopamine, low, medium, or high, find that a drug raising dopamine boosts working memory, the ability to keep thoughts online for short periods, in the low and medium groups. Someone already ceilinged out on dopamine gets no benefit and may do worse.
The same reading holds across the psychopharmacology literature on cognitive enhancement. People starting from a low baseline of performance get a positive boost that is quite small. High performers risk getting worse. Real individual differences in neurochemistry decide where a person sits on the curve.
For university students, who are unlikely to sit in a low-baseline state, the evidence suggests that taking a stimulant to earn a better grade could backfire.
Why does dopamine have an optimal level?
Amy Arnsten's lab at Yale studied how dopamine D1 receptors affect the prefrontal cortex. In a 2007 study led by Susheel Vijayraghavan, stimulating those receptors in monkeys improved working memory at moderate levels and impaired it at higher ones. Too little leaves neural signals weak, and too much floods the circuit with noise that blurs which signals matter.
Why did a stimulant help some people and hurt others?
In a 2003 study in the Journal of Neuroscience, Venkata Mattay and colleagues gave amphetamine to people grouped by a variant of the COMT gene, which breaks down dopamine in the prefrontal cortex. People with the variant that clears dopamine faster improved on a working memory task. People with the slower-clearing variant, who start with more dopamine, got worse.
Does the inverse U apply outside dopamine?
Robert Yerkes and John Dodson reported in 1908 that mice learned best at a moderate level of electric shock, with weaker and stronger shocks giving worse results. Psychologists later extended the pattern to arousal and performance in general. The Yerkes-Dodson law is still taught as a rule of thumb, even though the original experiment involved mice.
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