|Photo: Rashbehari Das
Staying up all night studying does more harm than good — it leads to fuzzy memories the next day. In other words, our mental scrapbook’s ability to register fresh memories is seriously compromised if a good night’s sleep is denied, scientists say.
Neuroscientists have known for a while that sleep deprivation does hamper the consolidation of long-term memory — the exact mechanism of which was unraveled recently by an India-born scientist and his colleagues in the US and Germany. But now a team of researchers from Harvard Medical School, Boston, has shown that lack of sleep not only fetters memory retention but also its very formation.
Matthew Walker and co-workers at the Harvard centre reported on February 11 in the online version of Nature Neuroscience that sleep before learning is crucial to preparing the brain for the next day’s memory formation. The findings, they say, are “worrying… considering society’s increasing erosion of sleep time”.
The Harvard scientists scanned the brain’s hippocampus region — where everyday events are minted into fresh memories — using sophisticated functional Magnetic Resonance Imaging (fMRI) technique. The study involved 28 volunteers in the age group of 18-30 years. The individuals were divided into two groups, with one made to stay awake for nearly 35 hours (two days and one night), and the other permitted to have a normal night’s sleep. The group that kept up was allowed to read books, take short walks, surf or chat on the Internet or play board games.
Towards the end of the second day, all participants were shown a slideshow of 150 pictures of landscapes, objects and non-celebrities. As they watched, their brains were mapped using fMRI. The scientists found that the mean recognition levels of the sleep-deprived group were about 20 per cent less than that of the other. The participants were then recalled after a full day’s break and asked to identify the slides they had earlier seen as a set of 75 fresh slides were added to the lot. “The volunteers who lacked sleep on the first day performed poorly, despite having had two nights to recover the lost sleep,” the scientists said.
“Your ability to learn is 20-40 per cent worse, that is, the difference between acing the exam and failing it miserably,” Walker told KnowHow.
While the work done by Walker and his associates conclusively proved that sleep before learning is vital, scientists had little clue about the brain mechanisms that help sleep to move and consolidate newly learnt things into long-term memory. All they knew was that for long-term storage, memories move from the hippocampus, one of the oldest regions of the brain, to the neocortex, the grey matter covering the hippocampus. This, they knew, occurred during deep, dreamless sleep.
All along, nearly for a generation, scientists thought that the hippocampus pushes the memory meant for long-term storage, or consolidation, to the neocortex. But Brown University scientist Mayank Mehta (who completed his doctoral studies from the Indian Institute of Science, Bangalore, and worked for a few years in quantum physics before moving to the US and neuroscience) and his colleagues recently proved this wrong. Their work, published in the November 2006 issue of Nature Neuroscience, showed that it is not the hippocampus that uploads information to the neocortex in a burst of brain cell communication but the neocortex that drives the dialogue.
To strike up a conversation between the hippocampus and the neocortex, the neurons from both the brain regions should be in sync. In other words, if the neocortical neurons display any activity, there should be corresponding firing among the hippocampal neurons. The previous studies failed to exhibit any such synchronous firing — which neuroscientists call phase locking — in the two regions. While neocortical neurons showed rhythmic activity during deep sleep, excitatory neurons in the hippocampus showed erratic activity.
What set Mehta thinking was that if these two parts of the brain talk during deep sleep, why didn’t they appear to be speaking the same language'
“There were many reasons why scientists were unable to establish this link. One reason is that they were looking at the excitatory neurons in the hippocampus. Second, they were looking at the activity using extracellular electrodes where they can only measure the spiking activity (the rush of neurons),” Mehta told KnowHow.
Mehta and his colleagues demonstrated that neurons from the neocortex work in tandem not with excitatory hippocampal neurons but what they call interneurons — inhibitory brain cells in the hippocampus. The study conducted in rats hence showed that the timing of activity or talk was the same in both the brain regions, with a small delay in the hippocampus — as if the inhibitory neurons in the hippocampus were echoing the speech in the neocortex.
What really helped Mehta to crack the mystery was his association with Nobel prize-winning German scientist Bert Sakmann. Mehta’s team used a ground breaking single-cell recording technique developed at Sakmann’s laboratory at the Max Plank Institute for Medical Research in Heidelberg for recording electrical activity in rats’ brains. “This technique has helped us in simultaneous measurement of electrical potential in single neurons from the hippocampus and the neocortex, Sakmann, who was in Delhi earlier this month for an Indo-EU science conference, told KnowHow. “This is by far the best technique available to accurately record electric activity of individual neurons,” Sakmann claims.
“The technique of looking inside a neuron and identifying the neural type was very important for the study. If you don’t differentiate which neuron you are recording from, it all seems like a mess,” says Mehta.
Sumantra Chattarji at the Bangalore-based National Centre for Biological Sciences admits that scientists knew the hippocampus records episodic memory whereas the neocortex plays a critical role in long-term memory storage. But they haven’t been able to get the correlation correct. “The new technique made all the difference,” says Chattarji.
This method of experimentally seeing how the two brain regions “talk” to each other may help them study other aspects of brain function such as perception and emotion, hopes Mehta.