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Although it remains deeply mysterious, researchers have in recent decades begun to understand the importance of sleep, and how it relates to brain function in both health and disease.
We now know, for example, that sleep is essential for memory consolidation—while offline, our brains are in fact very busy processing new information by modifying synaptic connections. More recently, it has emerged that sleep disturbances are linked to neurodegenerative brain changes, and may therefore serve as an early warning sign of diseases such as Alzheimer’s and Parkinson’s.
In a symposium held at the 10th FENS Forum for Neuroscience in Copenhagen last month, sleep researchers described new work that shows how certain physiological characteristics of sleep seem to be related to cognitive processes and mental illnesses.
Measuring IQ via sleep waves?
Sleep consists of five different stages; As we sleep, we cycle through them, with each cycle lasting approximately 90 minutes. Each of the stages is characterized by distinct patterns of brain waves that can be detected by electrodes attached to the scalp (electroencephalography, or EEG).
During sleep stage II, oscillating bursts of neuronal activity occur, lasting about half a second, with a frequency of between 12 and 24 per second. These so-called sleep spindles are generated by the thalamus, and are believed to play a role in memory consolidation and other sleep-related functions. Some studies link sleep spindles to intellectual ability—their size, number, and duration is apparently closely correlated to IQ measurements.
Martin Dresler of the Max Planck Institute of Psychiatry in Munich, described his group’s recent investigation into this link, the largest such study to be performed to date. Dresler and his colleagues recruited 160 participants with a broad range of IQ scores, and used EEG to record their sleep spindles overnight.
The researchers found a modest association between the size and duration of sleep spindles and intelligence: Participants with higher IQ scores tended to have larger and longer sleep spindles than those with lower scores. Importantly, though, this association was only seen in women. A follow-up study performed on 86 men revealed a similar but weaker relationship, between IQ scores and the duration of sleep spindles that occurred during a 100-minute afternoon nap.
“The association between sleep spindles and intelligence is more complex than we assumed,” said Dresler, adding that the results “give us a more accurate framework for the next phase of research into individual differences in sleep patterns.”
Dresler thinks that the parameters of sleep spindles might be an indicator for the integrity of the brain’s white matter tracts, which contain bundles of insulated nerve fibers that transmit information between distant brain regions.
“IQ scores correlate much higher with measures of white matter integrity in females compared to males,” he says. “In females there is a more straightforward relationship between these neural structures and their function in intelligence compared to males.”
The reason for these observed sex differences is still unclear; they might point to important differences between daytime and night-time sleep, or to daily fluctuations in hormone levels, which could subtly influence brain function.
Weaker signalling a sign of disease?
Since the pioneering research of neuropsychologist Brenda Milner, who started to work with the famous amnesic patient Henry Molaison in the early 1950s, we have known that a brain structure called the hippocampus is critical for memory formation. [See: One Man’s Continuing Contribution to the Science of Memory]
And while the role of sleep in memory consolidation is now well established, we still know very little about the underlying mechanisms. In the past few years, however, it has emerged that interactions between the hippocampus and cerebral cortex during sleep are critical for strengthening newly formed memories.
Lisa Genzel of the University of Edinburgh described her recent work investigating these interactions, and how they are disrupted in various mental illnesses.
A study published in 2012 showed that mice with schizophrenia-like symptoms exhibit abnormal sleep spindles, as well as de-coupling of the hippocampal-cortical interactions that occur during sleep. Genzel and her colleagues have not only extended these findings to humans, but also linked them to the consolidation of memories.
To do so, they, recruited 16 patients diagnosed with schizophrenia, 16 with depression, and 16 volunteers without a diagnosis (“healthy controls”), and trained them to perform a simple motor task that involved learning to tap their fingers in a specific sequence. The researchers used functional magnetic resonance imaging (fMRI) to examine the participants’ brain activity during learning, and EEG to record their brain waves while they slept, then tested their performance on the same task the following day.
They found that participants with schizophrenia and depression exhibited weaker hippocampal-cortical interactions during sleep than the controls, and that this was associated with significantly worse performance on the motor memory task the next day.
“Patients [with schizophrenia and depression] show less connectivity between the hippocampus and prefrontal cortex, and that is associated with decreased consolidation,” says Genzel, adding that this de-coupling may contribute to the progression of these diseases.
Another brain imaging study, published in 2011, showed that functional connectivity between the hippocampus and frontal cortex is strongest during sleep stage II, especially during the brief periods during which spindles are being produced. This suggests that spindles contribute to coordinating the activity of these brain regions, or to transferring information between them.
This was confirmed in an animal study published earlier this year, which showed that electrical stimulation that increases the coordination between sleep spindles and other types of brain waves—short-wave ripples and delta waves—enhances memory consolidation in rats, apparently by reorganizing neuronal networks in the frontal cortex. Conversely, consolidation was interrupted by stimulation that reduced coordination of the different brain wave patterns.
“Sleep spindles are necessary but not sufficient for memory consolidation, and it’s the slow-wave ripples that seem to be most important,” says Genzel. “The function of sleep seems to be transferring hippocampal-dependent memories to long-term storage in the cortex, and the sequence of ripples, slow waves, and ripples seems to be key to this process.”