Scientists have identified a link between disrupted brain activity during deep sleep, declining memory and the accumulation of a protein associated with Alzheimer’s disease. The findings could help explain why some people experience greater memory problems as they age.
The study, published in Nature Neuroscience on 11 September, found that the buildup of a protein called tau in the brain’s frontal cortex was associated with changes in slow brain waves that help preserve memories overnight.
Researchers at the University of California, Berkeley, discovered that these waves became shorter and less coordinated as tau accumulated. They called the disconnected patterns ‘lonely waves’.
“This is the first time we’ve shown this relationship between tau and how it messes with memory consolidation by attenuating traveling slow waves that originate in the frontal cortex,” lead researcher Omer Sharon told ScienceDaily.
How deep sleep helps preserve memories
During deep non-rapid eye movement (non-REM) sleep, large groups of neurons briefly switch off and on together, producing slow electrical waves that travel across the brain.
These coordinated waves are believed to play an important role in consolidating memories, allowing the brain to retain information acquired during the day.
The researchers compared brain activity in healthy young adults in their early twenties with older participants aged between their mid-sixties and mid-seventies.
Using electroencephalograms (EEGs), which record electrical activity in the brain, they found that slow waves in younger adults travelled across larger regions. In older adults, the waves covered shorter distances and appeared more independently.
The researchers also used positron emission tomography (PET) scans to examine tau accumulation in older participants. They found that greater concentrations of tau in the frontal cortex were associated with more fragmented slow-wave activity.
‘Lonely waves’ and memory loss
To understand how these changes affected memory, participants were asked to memorise pairs of words before sleeping and recall them the following day.
Those whose brain waves travelled shorter distances generally performed worse in the memory tests.
The researchers also tracked a smaller group of participants over several years. They found that increasing tau levels were associated with less coordinated slow waves and declining overnight memory retention.
Interestingly, these changes were observed in people who had not developed Alzheimer’s disease.
“These people did not have Alzheimer’s,” Sharon said. “They had tau in their brains, but with subclinical impacts. Their memory decline was within the normal range for their age.”
The researchers further examined spinal fluid samples from an independent group of older adults. They found that participants with more isolated slow waves also had higher ratios of tau to amyloid, another protein associated with Alzheimer’s disease.
Could sleep patterns help detect Alzheimer’s?
The findings suggest that changes in slow-wave activity could eventually help researchers identify early memory problems associated with Alzheimer’s disease.
However, the study does not determine whether tau accumulation disrupted the waves or whether changes in sleep activity contributed to the buildup of the protein.
“We can see tau and lonely waves rising together over time,” Sharon said. “What we cannot yet say is which leads.”
Further research will be needed to establish whether restoring coordinated brain-wave activity during sleep could help protect memory or slow cognitive decline.
(Edited By Raunaq Lekhi)