Missing sleep may hurt your memory

Lack of sleep, already considered a public health epidemic, can also lead to errors in memory, finds a new study by researchers at Michigan State University and the University of California, Irvine.

The study, published online in the journal Psychological Science, found participants deprived of a night’s sleep were more likely to flub the details of a simulated burglary they were shown in a series of images.

Distorted memory can have serious consequences in areas such as criminal justice, where eyewitness misidentifications are thought to be the leading cause of wrongful convictions in the United States.

“We found memory distortion is greater after sleep deprivation,” said Kimberly Fenn, MSU associate professor of psychology and co-investigator on the study. “And people are getting less sleep each night than they ever have.”

The Centers for Disease Control and Prevention calls insufficient sleep an epidemic and said it’s linked to vehicle crashes, industrial disasters and chronic diseases such as hypertension and diabetes.

The researchers conducted experiments at MSU and UC-Irvine to gauge the effect of insufficient sleep on memory. The results: Participants who were kept awake for 24 hours – and even those who got five or fewer hours of sleep – were more likely to mix up event details than participants who were well rested.

“People who repeatedly get low amounts of sleep every night could be more prone in the long run to develop these forms of memory distortion,” Fenn said. “It’s not just a full night of sleep deprivation that puts them at risk.”

Making memories

Modifying the activity of neuronal networks that encode spatial memories leads to the formation of an incorrect fear memory in mice

The formation and retrieval of memories allows all kinds of organisms, including humans, to learn and thrive in their environment. Yet our memories are not always accurate, and mistaken remembrances can have important consequences, such as in the justice system and in our navigation of the world. Susumu Tonegawa, Steve Ramirez, Xu Liu and colleagues at the RIKEN-MIT Center for Neural Circuit Genetics, have gained insight into the creation of mistaken memories by using light activation of neurons to generate an incorrect fear memory in mice.

The researchers allowed mice to explore a novel location and used genetic techniques to label neurons in the hippocampus—a part of the brain linked to spatial memory—that were activated in the process with a special channel called channelrhodopsin-2. The cells that expressed this channel could then be artificially activated by light. In this way, the researchers were able to reactivate neurons that fired in that particular location, even if the mice were no longer there.

They then moved the mice to another location where they were exposed to foot shocks, causing the mice to exhibit immobility, a fear behavior. At the same time, the researchers used light to activate the channelrhodopsin-2-expressing neurons that had fired in the first location.

When Tonegawa and his colleagues moved the animals to a third location, they did not show fear behavior. Yet when the mice went back to the first location, where they had never experienced a foot shock, the mice now exhibited prominent freezing behavior. The researchers had generated a ‘false memory’ in the mice of foot shocks in a location in which they had never been exposed to them.

The researchers showed that light reactivation of neuronal networks in the central area of the hippocampus, called the dentate gyrus, could create false memories, while reactivation of the outer ‘CA1’ area of the hippocampus could not. Tonegawa and his colleagues suggest that this is because mouse exploration of different locations leads to activation of more overlapping neuronal networks in the CA1 than in the dentate gyrus. “This may reflect the fundamental differences of how memories are encoded in these two regions,” explains Liu.

The findings provide insight into how the brain encodes and processes memories and could one day lead to treatments for post-traumatic stress disorder. “Our work may also have implications for situations where patients mix reality with their own imaginations, such as in schizophrenia,” says Liu.

Reconstructing the Past: How Recalling Memories Alters Them

Recently the neurologist and author Oliver Sacks recalled a vivid childhood memory, recounted in his autobiography, Uncle Tungsten.

During WWII he lived in London during the Blitz, and on one occasion:

"…an incendiary bomb, a thermite bomb, fell behind our house and burned with a terrible, white-hot heat. My father had a stirrup pump, and my brothers carried pails of water to him, but water seemed useless against this infernal fire—indeed, made it burn even more furiously. There was a vicious hissing and sputtering when the water hit the white-hot metal, and meanwhile the bomb was melting its own casing and throwing blobs and jets of molten metal in all directions."

Except when his autobiography came out, one of his older brothers told him he’d misremembered the event. In fact both of them had been at school when the bomb struck so they could not have witnessed the explosion.

The ‘false’ memory, it turned out, was implanted by a letter. Their elder brother had written to them, describing the frightening event, and this had lodged in his mind. Over the years the letter had gone from a third-person report to a first-person ‘memory’.

Turning the memory over in his mind, Sacks writes that he still cannot see how the memory of the bomb exploding can be false. There is no difference between this memory and others he knows to be true; it felt like he was really there.

This sort of experience is probably much more common than we might like to imagine. Many memories which have the scent of authenticity may turn out to be misremembered, if not totally fictitious events, if only we could check. Without some other source with which to corroborate, it is hard verify the facts, especially for events that took place long ago.

That these sorts of distortions to memory happen is unquestioned, what fascinates is how it comes about. Does the long passage of time warp the memory, or is there some more active process that causes the change?

A study published recently sheds some light on this process and provides a model for how memories like Sack’s become distorted.