Higher doses of ADHD drug might hinder learning ability

Kathleen Blanchard's picture
The ADHD drug Ritalin could impair memory and learning at higher doses.
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Higher doses of a commonly used drug for attention deficit hyperactivity disorder (ADHD) might harm learning ability and impair memory. Researchers from University of Wisconsin-Madison performed studies on monkeys, finding lower doses of the drug seem to boost learning ability. But when higher doses of the ADHD drug were used, deficits in learning and memory were found, even though symptoms of hyperactivity were reduced.

The study result is the same found in a 1977 investigation that Luis Populin, an associate professor of neuroscience at the University of Wisconsin-Madison School of Medicine and Public Health, said in a press release, "intrigued" many people, but "attempts to repeat the study did not yield clear-cut results.”

The new study, led by Populin, tested working memory in monkeys. Populin says though monkeys aren’t people, their behavior on higher doses of the ADHD drug reminded him of school children.

For the investigation, 3 monkeys were taught to look at a target ‘dot’ on a screen while another flashed by. The monkeys were rewarded with a sip of water if they waited until the central, or target dot turned off and then if they remained focused on where it had been.

During the tests, different doses of methylphenidate were used. Populin explained the system used tests working short-term memory and willingness to stay focused on a task. Doses of Ritalin used in the test were the same as those prescribed clinically for treatment of ADHD.

The Centers for Disease Control (CDC) estimates 5% of children in the U.S. population take drugs for attention deficit hyperactivity disorder, which are commonly believed to improve memory.

But Populin said in the media release “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.”

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In the study, the monkeys given higher doses of methylphenidate stuck with the task, but kept making the same errors over and over again. Populin said “…the subjects don’t seem to care” that it took them twice as long to get the task done.

Bradley Postle, a professor of psychology and expert on working memory at UW-Madison, who was not involved in the study explains Ritalin, generically known as methylphenidate, affects the brain’s executive function, “which can create an internal environment that, depending on the dose, is either more or less amenable to memory formation and/or retention.

If you can concentrate, and are able to process information without being interrupted by distracting thoughts or distractions in your environment, you will perform much better on a memory test. Apparently, the lower dose of methylphenidate helped create the conditions for success without actually improving memory itself.”

He likened the monkey’s behavior to that of children who raise their hand prematurely to answer a question in class. The monkeys wanted their reward before completing the task, even though they know they won’t get it.

Populin says the finding shows the importance of finding the right doses of drugs for treating ADHD, given the prevalence of the disorder among children and adults.” People think these drugs help improve memory, but our data say, ‘No, your memory is not getting better.’ At the higher dose, you get a behavioral improvement at a price, and that price is cognitive ability.”

The study, published in the Journal of Cognitive Neuroscience, shows higher doses of methylphenidate seem to control hyperactivity from ADHD, but, memory and learning may suffer. Ritalin is a stimulant that boosts activity of the central nervous system. Long-term effects of the drug on the developing brains of children are still under investigation.

Journal of Cognitive Neuroscience
"Dissociative Effects of Methylphenidate in Nonhuman Primates: Trade-offs between Cognitive and Behavioral Performance"
doi:10.1162/jocn_a_00225
Abigail Z. Rajala, Jeffrey B. Henriques, and Luis C. Populin
University of Wisconsin-Madison
March 8, 2012

Image credit: Wikimedia commons

Updated November 25, 2014

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