New research with monkeys sheds light on how the drug methylphenidate may affect learning and memory in children with attention deficit hyperactivity disorder.
The results parallel a 1977 finding that a low dose of the drug boosted cognitive performance of children with ADHD, but a higher dose that reduced their hyperactivity also impaired their performance on a memory test.
“Many people were intrigued by that result, but their attempts to repeat the study did not yield clear-cut results,” says Luis Populin, an associate professor of neuroscience at the University of Wisconsin-Madison School of Medicine and Public Health.
Populin was senior author of the new study exploring the same topic, now available in the early access section of the Journal of Cognitive Neuroscience, published early this month.
In the study, three monkeys were taught to focus on a central dot on a screen, while a “target” dot flashed nearby. The monkeys were taught that they could earn a sip of water by waiting until the central dot switched off, and then looking at the location of the now-vanished target dot.
The system tests working short-term memory, impulsiveness and willingness to stick with the task, as the monkeys could quit “working” at any time, says Populin. The study used different doses of methylphenidate — the generic name for Ritalin — that were comparable to the range of clinical prescriptions for ADHD.
According to the Centers for Disease Control, almost five percent of American children are taking medications for ADHD.
Strikingly, dosage had a major and unexpected impact. “At a low dose, the performance scores improved because the monkeys could control their impulses and wait long enough to focus their eyes on the target. All three were calmer and could complete a significantly larger number of trials,” says Populin, who collaborated with Jeffrey Henriques and graduate student Abigail Rajala on the study.
At the higher dose, “performance on the task is impaired,” Populin says, “but the subjects don’t seem to care, all three monkeys continued making the same errors over and over.”
The monkeys stayed on task more than twice as long at the higher dose, even though they had much more trouble performing the task.
Although ADHD drugs are commonly thought to improve memory, “If we take the accuracy of their eye movements as a gauge of working memory, memory was not helped by either dose,” says Populin.
“It did not get better at the lower dose, and there actually was a small negative effect at the higher dose.”