Although many are familiar with the tremors and slow movements associated with Parkinson’s, working memory — the ability to temporarily store and manipulate information, rather than simply repeat it — is frequently compromised.
Deficits in working memory interfere with daily activities by impeding planning, problem solving and independent living. The new research has shown that people with Parkinson’s disease perform markedly better on a test of working memory after a good night’s sleep.
The findings underline the importance of addressing sleep disorders in the care of patients with Parkinson’s. Researchers are learning that training can improve working memory capacity, a finding that has implications for the biology of sleep and memory.
Research scientists from the Emory University School of Medicine have published their findings in the journal Brain.
“It was known already that sleep is beneficial for memory, but here, we’ve been able to analyze what aspects of sleep are required for the improvements in working memory performance,” said postdoctoral fellow Michael Scullin, first author of the paper.
The performance boost from sleep was linked with the amount of slow wave sleep, or the deepest stage of sleep. Several research groups have reported that slow wave sleep is important for synaptic plasticity, the ability of brain cells to reorganize and make new connections.
Researchers also discovered that sleep apnea, the disruption of sleep caused by obstruction of the airway, can effect memory. Study participants who showed signs of sleep apnea, if it was severe enough to lower their blood oxygen levels for more than five minutes, did not see a working memory test boost.
During the study, participants took a “digit span test,” in which they had to repeat a list of numbers forward and backward. The test was conducted in an escalating fashion: the list grows incrementally until someone makes a mistake.
Participants took the digit span test eight times during a 48-hour period, four during the first day and four during the second. In between, they slept.
Repeating numbers in the original order is a test of short-term memory, while repeating the numbers in reverse order is a test of working memory.
“Repeating the list in reverse order requires some effort to manipulate the numbers, not just spit them back out again,” Scullin said. “It’s also a purely verbal test, which is important when working with a population that may have motor impairments.”
The study was performed on 54 participants with a diagnosis of Parkinson’s disease with 10 of the participants also exhibiting dementia with Lewy bodies, a more advanced condition in which patients may have hallucinations or fluctuating cognition as well as motor symptoms.
Those who had dementia with Lewy bodies saw no working memory boost from the night’s rest. As expected, their baseline level of performance was lower than the Parkinson’s group.
Participants with Parkinson’s who were taking dopamine-enhancing medications saw their performance on the digit span test jump up between the fourth and fifth test. On average, they could remember one more number backwards. The ability to repeat numbers backward improved, even though the ability to repeat numbers forward did not.
Researchers found individuals receiving dopamine-enhancing medication for Parkinson’s displayed the greatest memory improvement from better sleep.
Patients not taking dopamine medications, even though they had generally had Parkinson’s for less time, did not experience as much of a performance benefit. This may reflect a role for dopamine, an important neurotransmitter, in memory.
Researchers are planning an expanded study of sleep and working memory, in healthy elderly people as well as patients with neurodegenerative diseases.
“Many elderly people go through a decline in how much slow wave sleep they experience, and this may be a significant contributor to working memory difficulties,” Scullin said.
Source: Emory University