Beginning two decades ago, attention deficit hyperactivity disorder (ADHD) diagnoses jumped to 11 percent of American children aged 4 to 17 even though neuroscientists still did not know biologically what ADHD is. 

And one alarming fix for ADHD was the stimulant methylphenidate, commonly known as Ritalin, Between between 1991 and 1999, Ritalin sales in the United States increased 500 percent and the US still accounts for 85% of the entire world's consumption. All this despite the fact that Ritalin was considered so dangerous by 'speed freaks' in the 1960s that they shunned it. Like cocaine and amphetamines, Ritalin increases dopamine activity but where amphetamines stimulate dopamine release, cocaine and Ritalin block the transporters that re-uptake dopamine into the neuron that released it.  More dopamine remains in the brain.

One classic symptom is a relative rather than scientific measure: impulsivity, the tendency to act before thinking and choose a small but immediate reward over a larger one that requires some delay. Choosing between present and future rewards is a fundamental need in schooling, says Luis Populin, associate professor of neuroscience at University of Wisconsin-Madison. "If you say to an impulsive child, 'Do your homework so you will get a good grade at the end of the quarter,' that has less appeal than 'Let's play baseball this afternoon instead of studying chemistry.'"

To study impulsive behavior, Populin and graduate student Abigail Zdrale Rajala did a small study and selected two rhesus macaque monkeys with opposite behaviors. One was extremely calm, while the other was nervous, fidgety and impulsive. The monkeys were trained to stare at a dot on a screen and, when it went dark, to choose between two pictures placed to the side. Their choice of picture determined whether they got a small but immediate sip of water, or a larger sip, after a delay ranging up to 16 seconds. 

As expected, the calm monkey, but not the impulsive one, quickly figured out that waiting would bring the sweeter result.

This willingness to take a smaller reward right away rather than a larger, delayed reward, called "temporal discounting," is a common feature of "combined type" ADHD, which specifically lists impulsivity among its diagnostic criteria. 

When the monkeys were given a dose of methylphenidate, they chose the delayed reward more frequently. The impulsive monkey actually showed the same preference for delayed rewards as the un-medicated, calm monkey. However, identifiable differences in their performance mean that methylphenidate improved the condition, but did not eliminate it.

"There is no perfect animal model of ADHD," says Rajala, "but many studies are performed on rodents; this one was done in a non-human primate, which is much closer to humans." 

Some scientists have thought that temporal discounting in ADHD may result from cognitive processing, which relies on the highly evolved frontal cortex in the brain. The new results support an alternative: that temporal discounting is linked to the reward-processing mechanism, which is governed by more primitive parts of the brain.