Tuesday July 26 2011
The mice's familiarity with an object was tested
“A continuous night's uninterrupted sleep may be the essential requirement for a good memory,” The Independent has reported. The newspaper added that it isn’t just the amount of sleep that is important for memory formation, but also enjoying sleep that is not interrupted.
The news is based on research carried out using mice that were genetically modified so that a particular group of nerve cells in their brains could be stimulated using light. Periodically stimulating the cells with pulses of light made the mice wake up briefly and then quickly return to sleep. This technique was unique in that it allowed the researchers to study specifically the “fragmentation” of sleep – in other words, the effect of interrupting a period of sleep but not altering the overall quality or length of the sleep.
The researchers then examined how fragmentation of sleep affected the mice’s ability to remember objects that they had previously seen. They found that, compared to ordinary mice, these mice showed less familiarity with the objects when subjected to fragmented sleep.
As with all animal studies, the results of this well-conducted research currently have limited implications in humans. It remains to be seen whether fragmented sleep can affect memory in humans and, if so, how often sleep would need to be disturbed over the course of the night for this to be the case.
Where did the story come from?
The study was carried out by researchers from Stanford University. No funding source was disclosed in its research paper. The study was published in the peer-reviewed scientific journal Proceedings of the National Academy of Science (PNAS).
The research was described well by the newspapers. All emphasised that the study was carried out in mice.
What kind of research was this?
This was an animal study that investigated the effect of disturbed sleep on memory in mice. The researchers say that sleep is characterised by several features, such as duration, intensity and continuity, but that previous techniques in sleep research were not able to manipulate single aspects of sleep without interrupting others. For example, most traditional methods of sleep deprivation used in experiments disrupted the composition of sleep by disturbing individuals so that the proportion of sleep spent in the rapid-eye-movement (REM) phase was reduced, which inadvertently disrupted sleep quality, continuity and in most cases sleep duration. In this experiment, the researchers created genetically modified mice in which they could activate certain brain cells (neurones) that play a role in the waking process. By doing this they could “fragment” sleep in mice without affecting its overall amount or intensity. They then carried out memory tests in the mice.
This exploratory research has the advantage of being able to manipulate selectively just one aspect of sleep, which would be difficult to do in humans. However, as with all animal studies the direct relevance to humans may be limited without follow-up work to see how similar the sleep and memory processes are in mice and humans.
What did the research involve?
The researchers used a technique called optogenetics, which allows certain genes to be switched on when they are stimulated with light. In this case, the researchers genetically engineered mice to possess a particular group of neurones that could be activated through light exposure. These genetically modified neurones are involved in regulating how much of a stimulus is needed for the mouse to be aroused from their sleep.
The first step was to develop a programme of light stimulation that would cause the genetically engineered the mice to have fragmented sleep without affecting other features of sleep such as sleep time. The researchers recorded the mice’s sleep patterns by monitoring their brainwaves with an EEG and observing muscle twitches. Their programme disrupted sleep continuity but did not appear to have an effect on the amounts of non-REM or REM sleep or the length of time the mice were asleep for when compared to control mice. By looking at the brainwaves the researchers could also verify that sleep quality was not affected.
The researchers said that one of the problems with researching the effect sleep has on memory is that the research procedures may lead to stress, which may itself have an effect on memory. This could mean that the research would be measuring the effect of stress rather than absence of sleep on memory. They therefore compared the amount of the stress hormone corticosterone in the genetically modified mice compared with control mice. They also looked at the behaviour of the mice for signs of anxiety: anxious mice tend to spend more times at the boundaries of a pen and are reluctant to spend time in the centre.
To test the mice's memory the researchers took advantage of the fact that mice tend to explore new objects more than familiar ones. The researchers allowed the mice to explore an object and 24 hours later placed a new object alongside the familiar one to measure how long the mouse spent with the new object relative to the familiar one. It is expected that if the mice remember the first object they will spend less time exploring it and more time with the novel object. In theory, if the mice do not remember the object from the training session they would spend a similar amount of time exploring each object.
What were the basic results?
The researchers found that the genetically modified mice were no more stressed than the control mice: they had similar corticosterone levels and showed similar levels of anxious behaviour.
Following “training” time spent with an object, the genetically modified and control mice were stimulated with a light shone into their brain every 60 seconds during their first four hours of sleep. When the mice were exposed to the familiar object and a novel object when they woke, the control mice spent a greater proportion of their time exploring the new object than the genetically modified mice did. This suggests the control mice had formed a greater memory of the object during training time.
The researchers found that decreasing the frequency of light stimulation to once every two minutes fragmented sleep to a lesser degree than the 60-second protocol they had used in earlier tests. Stimulation once every two minutes did not affect memory formation.
How did the researchers interpret the results?
The researchers said that their research methods were a “non-stressful, non-pharmacological means of manipulating a specific feature of animal sleep through the activation of its own arousal mechanisms”. They say that they have shown that sleep continuity, independent of the amount of sleep achieved, “is critical for learning and memory in mice”. They suggest that: “there is a minimum length of non-REM sleep necessary for memory consolidation”.
This animal research used an ingenious approach to disrupt one aspect of sleep to show that continuous sleep in mice improves the ability to consolidate memories. Although well-conducted research, this was an animal study and its current implications for humans is limited.
Several of the newspapers, although highlighting that the study was conducted in mice, related this research to human sleep conditions such as sleep apnoea, in which a temporary halt or disturbance in breathing pattern causes a person to wake up. However, although the current animal study has set a minimum length of uninterrupted sleep needed for memory formation in mice, this memory-formation threshold would need to be determined for humans. Although people with sleep apnoea are likely to have fragmented sleep, without direct study of the issue we cannot tell whether this might affect memory.
This mouse research warrants further follow-up, particularly using experimental designs that can quantify the degree of fragmented sleep in humans. These are needed to see if these findings relate to humans.