Chapter 13. Memory, Learning, and Development
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By Jonathan Webb Science reporter, BBC News A cluster of cells in the brain of a fly can track the animal's orientation like a compass, a study has revealed. Fixed in place on top of a spherical treadmill, a fruit fly walked on the spot while neuroscientists peered into its brain using a microscope. Watching the neurons fire inside a donut-shaped brain region, they saw activity sweep around the ring to match the direction the animal was headed. Mammals have similar "head direction cells" but this is a first for flies. The findings are reported in the journal Nature. Crucially, the compass-like activity took place not only when the animal was negotiating a virtual-reality environment, in which screens gave the illusion of movement, but also when it was left in the dark. "The fly is using a sense of its own motion to pick up which direction it's pointed," said senior author Dr Vivek Jayaraman, from the Howard Hughes Medical Institute's Janelia Research Campus. In some other insects, such as monarch butterflies and locusts, brain cells have been observed firing in a way that reflects the animal's orientation to the pattern of polarised light in the sky - a "sun compass". But the newly discovered compass in the fly brain works more like the "head directions cells" seen in mammals, which rapidly set up a directional system for the animal based on landmarks in the surrounding scene. "A key thing was incorporating the fly's own movement," Dr Jayaraman told the BBC. "To see that its own motion was relevant to the functioning of this compass - that was something we could only see if we did it in a behaving animal." © 2015 BBC
Keyword: Learning & Memory
Link ID: 20933 - Posted: 05.14.2015
Thomas R. Clandinin & Lisa M. Giocomo An analysis reveals that fruit-fly neurons orient flies relative to cues in the insects' environment, providing evidence that the fly's brain contains a key component for drawing a cognitive map of the insect's surroundings. See Article p.186 Animals need accurate navigational skills as they go about their everyday lives. Many species, from ants to rodents, navigate on the basis of visual landmarks, and this is complemented by path integration, in which neuronal cues about the animal's own motion are used to track its location relative to a starting point. In mammals, these different types of navigation are integrated by neurons called head-direction cells1. In this issue, Seelig and Jayaraman2 (page 186) provide the first evidence that certain neurons in fruit flies have similar properties to head-direction cells, encoding information that orients the insects relative to local landmarks. Head-direction cells act as a neuronal compass that generates a cognitive map of an animal's environment. The activity of each head-direction cell increases as the animal faces a particular direction, with different cells preferentially responding to different directions1, 3. Rather than certain cells always responding to north, south and so on, the direction in which the cells fire is set up arbitrarily when the animal encounters new visual landmarks. The signals are then updated by self-motion cues as the animal navigates. Studying head-direction cells in mammals is challenging because of the complexity of the mammalian brain. By contrast, the small fly brain is a good model for studying neuronal activity. © 2015 Macmillan Publishers Limited.
Keyword: Learning & Memory
Link ID: 20932 - Posted: 05.14.2015
Alexandra Sifferlin Autism, already a mysterious disorder, is even more puzzling when it comes to gender differences. For every girl diagnosed with autism, four boys are diagnosed, a disparity researchers don’t yet fully understand. In a new study published in the journal Molecular Autism, researchers from the UC Davis MIND Institute tried to figure out a reason why. They looked at 112 boys and 27 girls with autism between ages 3 and 5 years old, as well as a control sample of 53 boys and 29 girls without autism. Using a process called diffusion-tensor imaging, the researchers looked at the corpus callosum — the largest neural fiber bundle in the brain — in the young kids. Prior research has shown differences in that area of the brain among people with autism. They found that the organization of these fibers was different in boys compared with girls, especially in the frontal lobes, which play a role in executive functions. “The sample size is still limited, but this work adds to growing body of work suggesting boys and girls with autism have different underlying neuroanatomical differences,” said study author Christine Wu Nordahl, an assistant professor in the UC Davis Department of Psychiatry and Behavioral Sciences, in an email. In other preliminary research presented at the International Meeting for Autism Research, or IMFAR, in Salt Lake City, the study authors showed that when girls and boys with autism are compared with typically developing boys and girls, the behavioral differences between girls with autism and the female controls are greater than the differences among the boys. Nordahl says this suggests that girls can be more severely affected than boys.
By Emily Underwood We’ve all heard how rats will abandon a sinking ship. But will the rodents attempt to save their companions in the process? A new study shows that rats will, indeed, rescue their distressed pals from the drink—even when they’re offered chocolate instead. They’re also more likely to help when they’ve had an unpleasant swimming experience of their own, adding to growing evidence that the rodents feel empathy. Previous studies have shown that rats will lend distressed companions a helping paw, says Peggy Mason, a neurobiologist at the University of Chicago in Illinois who was not involved in the work. In a 2011 study, for example, Mason and colleagues showed that if a rat is trapped in a narrow plastic tube, its unrestrained cagemate will work on the latch until it figures out how to spring the trap. Skeptics, however, have suggested that the rodents help because they crave companionship—not because their fellow rodents were suffering. The new study, by researchers at the Kwansei Gakuin University in Japan, puts those doubts to rest, Mason says. For their test of altruistic behavior, the team devised an experimental box with two compartments divided by a transparent partition. On one side of the box, a rat was forced to swim in a pool of water, which it strongly disliked. Although not at risk of drowning—the animal could cling to a ledge—it did have to tread water for up to 5 minutes. The only way the rodent could escape its watery predicament was if a second rat—sitting safe and dry on a platform—pushed open a small round door separating the two sides, letting it climb onto dry land. © 2015 American Association for the Advancement of Science
By Gareth Cook Much has been written on the wonders of human memory: the astounding feats of recall, the way memories shape our identity and are shaped by them, memory as a literary theme and a historical one. But what of forgetting? This is the topic of a new book by Douwe Draaisma, author of The Nostalgia Factory and a professor of the history of psychology at the University of Groningen. In Forgetting, Draaisma considers dreaming, amnesia, dementia and all of the ways that our minds — and lives — are shaped by memory’s opposite. He answered questions from Mind Matters editor Gareth Cook. What is your earliest memory and why, do you suppose, have you not forgotten it? Quite a few early memories in the Netherlands involve bicycles, and mine is no exception. I was two-and-a-half years old when my aunts walked my mother to the train station. They had taken a bike along to transport her bags. I was sitting on the back of the bike. Suddenly the whole procession came to a halt when my foot got caught between the spokes. I’m pretty sure this memory is accurate, since I had to see a doctor and there is a dated medical record. It’s a brief, snapshot-like memory, black-and-white. I don’t remember any pain, but I do remember the consternation among my mom and her sisters. Looking back on this memory from a professional perspective, I would say that it has the flash-like character typical for first memories from before age 3; ‘later’ first memories are usually a bit longer and more elaborate. It also fits the pattern of being about pain and danger. Roughly three in four first memories are associated with negative emotions. This may have an evolutionary origin: I never again had my foot between the spokes. And neither have any of my children. © 2015 Scientific American
Keyword: Learning & Memory
Link ID: 20918 - Posted: 05.13.2015
By Simon Makin After wandering around an unfamiliar part of town, can you sense which direction to travel to get back to the subway or your car? If so, you can thank your entorhinal cortex, a brain area recently identified as being responsible for our sense of direction. Variation in the signals in this area might even explain why some people are better navigators than others. The new work adds to a growing understanding of how our brain knows where we are. Groundbreaking discoveries in this field won last year's Nobel Prize in Physiology or Medicine for John O'Keefe, a neuroscientist at University College London, who discovered “place cells” in the hippocampus, a brain region most associated with memory. These cells activate when we move into a specific location, so that groups of them form a map of the environment. O'Keefe shared the prize with his former students Edvard Moser and May-Britt Moser, both now at the Kavli Institute for Systems Neuroscience in Norway, who discovered “grid cells” in the entorhinal cortex, a region adjacent to the hippocampus. Grid cells have been called the brain's GPS system. They are thought to tell us where we are relative to where we started. A third type—head-direction cells, also found in the entorhinal region—fires when we face a certain direction (such as “toward the mountain”). Together these specialized neurons appear to enable navigation, but precisely how is still unclear. For instance, in addition to knowing which direction we are facing, we need to know which direction to travel. Little was known about how or where such a goal-direction signal might be generated in the brain until the new study. © 2015 Scientific American
Keyword: Learning & Memory
Link ID: 20915 - Posted: 05.13.2015
Jane Brody With people worldwide living longer, marketers are seizing on every opportunity to sell remedies and devices that they claim can enhance memory and other cognitive functions and perhaps stave off dementia as people age. Among them are “all-natural” herbal supplements like Luminene, with ingredients that include the antioxidant alpha lipoic acid, the purported brain stimulant ginkgo biloba, and huperzine A, said to increase levels of the neurotransmitter acetylcholine; brain-training games on computers and smartphones; and all manner of puzzles, including crosswords, sudoku and jigsaw, that give the brain a workout, albeit a sedentary one. Unfortunately, few such potions and gizmos have been proven to have a meaningful, sustainable benefit beyond lining the pockets of their sellers. Before you invest in them, you’d be wise to look for well-designed, placebo-controlled studies that attest to their ability to promote a youthful memory and other cognitive functions. Even the widely acclaimed value of doing crossword puzzles has been called into question, beyond its unmistakable benefit to one’s font of miscellaneous knowledge. Although there is some evidence that doing crosswords may help to delay memory decline, Molly Wagster, a neuroscientist at the National Institute on Aging, said they are best done for personal pleasure, not brain health. “People who have done puzzles all their lives have no particular cognitive advantage over anyone else,” she said. The institute is one of several scientific organizations sponsoring rigorous trials of ways to cash in on the brain’s lifelong ability to generate new cells and connections. One such trial, Advanced Cognitive Training for Independent and Vital Elderly, or Active, was a 10-year follow-up study of 2,832 cognitively healthy community-dwelling adults 65 and older. © 2015 The New York Times Company
Andrew Griffin Scientists have created an electronic memory cell that mimics the way that human brains work, potentially unlocking the possibility of the making bionic brains. The cell can process and store multiple bits of information, like the human brain. Scientists hope that developing it could make for artificial cells that simulate the brain’s processes, leading to treatments for neurological conditions and for replica brains that scientists can experiment on. The new cells have been likened to the difference between having an on-off light switch and a dimmer, or the difference between black and white pictures or those with full colour, including shade light and texture. While traditional memory cells for computers can only process one binary thing at a time, the new discovery allows for much more complex memory processes like those found in the brain. They are also able to retain previous information, allowing for artificial systems that have the extraordinary memory powers found in human beings. While the new discovery is a long way from leading to a bionic brain, the discovery is an important step towards the dense and fast memory cells that will be needed to imitate the human brain's processes. “This is the closest we have come to creating a brain-like system with memory that learns and stores analog information and is quick at retrieving this stored information,” Sharath Sriram, who led the project, said.
Margaret Wente Child psychiatrist Susan Bradley was a pioneer in treating kids with gender-identity disorders. In the 1970s, she founded the child and adolescent gender identity clinic at the Clarke Institute in Toronto, which eventually became part of the Centre for Addiction and Mental Health (CAMH). Back then, the field was virtually unknown. Today, it is Ground Zero in a fierce battle between oldfangled psychiatry and transgender activists who insist that practitioners like Dr. Bradley are guilty of child abuse. Caught in the middle are confused parents, well-meaning schools, and – most important of all – troubled and bewildered kids. The new rush to turn little Jason into Janey, or Sally into Sam, is generally regarded (in the media, at least) as progress – proof of what a tolerant and progressive society we’ve become. But what if it’s just another fad? What if the radical step of changing genders isn’t always the right answer for a child’s emotional distress – especially when that child is only 10 or 6, or 3? “Some of these kids are quite significantly ill,” says Dr. Bradley. “They often have serious family problems and anxiety disorders. Or they’ve had serious trauma. A girl I saw had been raped, and after that she decided she was going to be a male. If you didn’t pay attention to the trauma you’re not doing that kid a service.” These days, that eminently reasonable view is being challenged by people who believe that children’s sexual confusion should automatically be taken at face value. The clinic that Dr. Bradley helped to found – which does, in fact, support gender transition for a sizable minority of its patients – is being pilloried as transphobic. “Is CAMH trying to turn trans kids straight?” screamed a headline in NOW magazine. Under pressure from activists, CAMH has put its gender clinic under six-month review. And a new bill before the Ontario legislature, which is supported by Premier Kathleen Wynne, would explicitly bar the therapeutic approach taken by the clinic, wrongly equating it to the notorious “conversion” therapy that seeks to turn gay people straight. © Copyright 2015 The Globe and Mail Inc.
By Lisa Sanders, M.D On Thursday we challenged Well readers to solve the difficult case of twin sisters who, in the prime of youth, developed a weakness that forced them to use their arms to rise from a chair. Nearly 300 of you wrote in with thoughts on this difficult case. Many of you recognized that this was likely to be a genetic disorder, though I greatly admired the “House”-ian thinking that led to a host of possible reasons why two sisters, living in different states, might develop the same symptoms independent of their shared DNA. It took this patient, Katie Buryk, four years to get her answer, which was: Late onset Tay-Sachs disease Although several of you made this difficult diagnosis, the first to do so was George Bonadurer, a second year medical student at Mayo Medical School in Rochester, Minn. He says he recently read about this disease in a book of unusual cases that had come to the Mayo clinic for help. This is actually Mr. Bonadurer’s second win of this contest. Strong work! Tay-Sachs disease was first identified by two physicians, independently, in the 1880s. Dr. Warren Tay was an ophthalmologist in London. Dr. Bernard Sachs was a neurologist in New York City. Each described a disease in infants that caused profound weakness, blindness and, usually by age 4, death. Careful consideration of cases over the following decades showed that the disease was inherited and often seen in children of Ashkenazi descent. Studying the patterns of inheritance, it became clear that both parents had to have the abnormal gene and that each of their children would have a one in four chance of being born with the disease. The terrible manifestations of the disease derive from an inherited inability to make an essential protein in the brain. This protein acts to break down discarded components of the cells. Without this protein, these discarded cell parts accumulate, interrupting normal nerve and brain cell functioning. This mechanism and the missing protein was identified in 1969, allowing for the development of a test for carriers. Since the development of this test, the incidence of Tay-Sachs in the United States has dropped by 90 percent. © 2015 The New York Times Company
Douwe Draaisma When we sleep, wrote English psychiatrist Havelock Ellis over a hundred years ago, we enter a ‘dim and ancient house of shadow’. We wander through its rooms, climb staircases, linger on a landing. Towards morning we leave the house again. In the doorway we look over our shoulders briefly and with the morning light flooding in we can still catch a glimpse of the rooms where we spent the night. Then the door closes behind us and a few hours later even those fragmentary memories we had when we woke have been wiped away. That is how it feels. You wake up and still have access to bits of the dream. But as you try to bring the dream more clearly to mind, you notice that even those few fragments are already starting to fade. Sometimes there is even less. On waking you are unable to shake off the impression that you have been dreaming; the mood of the dream is still there, but you no longer know what it was about. Sometimes you are unable to remember anything at all in the morning, not a dream, not a feeling, but later in the day you experience something that causes a fragment of the apparently forgotten dream to pop into your mind. No matter what we may see as we look back through the doorway, most of our dreams slip away and the obvious question is: why? Why is it so hard to hold on to dreams? Why do we have such a poor memory for them? In 1893, American psychologist Mary Calkins published her ‘Statistics of Dreams’, a numerical analysis of what she and her husband dreamed about over a period of roughly six weeks. They both kept candles, matches, pencil and paper in readiness on the bedside table. But dreams are so fleeting, Calkins wrote, that even reaching out for matches was enough to make them disappear. Still with an arm outstretched, she was forced to conclude that the dream had gone. © 2015 Salon Media Group, Inc
by Laura Sanders On a test of visual perception, children with autism perceive moving dots with more clarity than children without the disorder. The results, published in the May 6 Journal of Neuroscience, reveal a way in which children with autism see the world differently. When asked to determine the overall direction of a mess of dots moving in slightly different directions, children with autism outperformed children without the disorder. Other tests of motion detection didn’t turn up any differences. The results suggest that children with autism may be taking in and combining more motion information than children without autism, says study coauthor Catherine Manning of the University of Oxford. This heightened ability may contribute to feelings of sensory overload, the researchers suggest. © Society for Science & the Public 2000 - 2015
Ian Sample, science editor Brain scans of children who were born prematurely have revealed differences in the connectivity of key regions that may play a role in developmental disorders. Previous studies have already highlighted that children who are born preterm are more at risk of autism and other behavioural conditions, such as the poor attention that is associated with ADHD, or attention deficit hyperactivity disorder. The new findings could help doctors understand why preterm children are so often affected, and work out whether medications or different styles of care could help the children reach their full potential. Researchers at King’s College London scanned the brains of 66 infants on average 42 weeks after their mothers’ last period before the birth. Forty seven of the babies were born prematurely, at less than 33 weeks. The other 19 babies were born on average after 40 weeks gestation. In their final weeks in the womb, babies’ brains are building connections at an incredible rate, which makes them particularly sensitive to changes in the last trimester. If a baby is born prematurely, the crucial period of brain growth happens in the radically different environment of the neonatal unit. From the MRI scans, the scientists found that infants born prematurely had increased connectivity in only one part of the brain they tested. A region called the thalamus, a kind of neural relay station, was better connected to a part called the lateral sensory cortex, which handles signals from the mouth, lips and jaw. The result might be explained by pre-term babies breast or bottle feeding much earlier, or being given dummies while on supportive breathing machines. © 2015 Guardian News and Media Limited
James Gorman If modern science is right, the great mystery of embryonic development is less about how life unfolds, and more about how it folds. Embryos of many organisms grow from two cells to four, then eight, and so on until there are thousands in a kind of ball. Then sheets of cells start to make folds or furrows as the basic shape of the creature — fly or fish or human — begins to emerge. One of the most striking examples is a moment in the development of Volvox, a kind of algae that forms one of the simplest multicellular organisms. When it is a sphere of a few thousand cells, it reaches adult size, but not adult shape. So it turns itself inside out. Scientists at the University of Cambridge in England have made a time-lapse recording of the process that shows it in three dimensions for the first time and has enough detail that researchers can check their mathematical descriptions of the transformation. © 2015 The New York Times Company
Keyword: Development of the Brain
Link ID: 20888 - Posted: 05.05.2015
Roger Dobson Tapping your fingers on the table is usually a sign of boredom or irritation. But not all tappers are equal, it seems. Men drum their digits slightly faster than women and people in their twenties tap substantially faster than people twice their age. The results of the first study into finger-tapping speeds also found that smokers tap a little faster than non-smokers and fit people tap faster than those who avoid exercise. The research, carried out by scientists at two universities in Istanbul – Bogazici University and Fatih University – examined the tapping rates and “finger load capacities” of 148 people aged between 18 and 85. Each participant was asked to perform a one-minute tapping exercise on a keyboard at “maximum volitional tempo”. Researchers found that the index finger on the right hand of both men and women was the fastest digit, achieving a tapping rate of up to five beats a second among those in their twenties. The middle finger was almost as nifty as the index finger, but the little finger – the slowest digit in the bunch – was capable only of a sluggish 3.8 taps a second among people in the same age group. At first glance, the study might appear to be rather frivolous. But a deeper understanding of finger tapping could aid the design of computer keyboards and musical instruments. It may also aid researchers who use finger-tapping tests for medical assessment of neurological conditions such as Parkinson’s disease, schizophrenia and Alzheimer’s.
|By Michele Solis An individual with obsessive-compulsive disorder (OCD) is overcome with an urge to engage in unproductive habits, such as excessive hand washing or lock checking. Though recognizing these behaviors as irrational, the person remains trapped in a cycle of life-disrupting compulsions. Previous studies found that OCD patients have abnormalities in two different brain systems—one that creates habits and one that plays a supervisory role. Yet whether the anomalies drive habit formation or are instead a consequence of doing an action over and over remained unclear. To resolve this question, a team at the University of Cambridge monitored brain activity while people were actually forming new habits. Lapses in supervision are to blame, the researchers reported in a study published online in December 2014 in the American Journal of Psychiatry. They scanned 37 people with OCD and 33 healthy control subjects while they learned to avoid a mild shock by pressing on a foot pedal. Pressing the pedal became a habit for everyone, but people with OCD continued to press even when the threat of shock was over. Those with OCD showed abnormal activity in the supervisory regions important for goal-directed behavior but not in those responsible for habit formation. The finding suggests that shoring up the goal-directed systems through cognitive training might help people with OCD. The growing understanding of OCD's roots in the brain may also help convince individuals to engage in standard habit-breaking treatments, which expose a person to a trigger but prevent his or her typical response. “It's hard for people to not perform an action that their whole body is telling them to do,” says first author Claire Gillan, now at New York University. “So if you have an awareness that the habit is just a biological slip, then it makes OCD a lot less scary and something you can eventually control.” © 2015 Scientific American
By Aleksandra Sagan, CBC News In a Dutch town about 20 kilometres outside of Amsterdam, a small community lives in what at first glance seems like a real-life version of The Truman Show. Hogewey has a grocery store, a theatre and a barber shop. The only twist is that many of its 152 residents live unaware that their orderly community is actually a nursing home for people with severe dementia. "We protect our residents from the unsafe world. They do not understand the world outside this because the outside world doesn't understand them," says Yvonne van Amerongen, an employee at Hogewey who also helped develop the concept. Hogewey was officially opened in 2007, but the idea has now caught the attention of health-care professionals in Ontario and Alberta. Rhonda Desroches, who helped create a smaller-scale Hogewey in Penetanguishene, Ont., says relatives of the residents are pleased with how happy their family members seem to be in the new facility. Dementia is a growing problem. According to the Alzheimer Society Canada, one out of 20 Canadians over 65 has Alzheimer's Disease, and that figure jumps to one in four for Canadians over 85. In 2012, the World Health Organization declared dementia a public health priority. Many dementia patients move into nursing homes, where they are monitored in a safe setting. But some medical professionals want to shift patients away from unfamiliar, clinical settings and into spaces that resemble more typical surroundings. Hogewey creates a familiar, "normal" environment that dementia patients understand, says van Amerongen. The citizens of Hogewey share a house with about six others, and are classified according to one of seven lifestyles. ©2015 CBC/Radio-Canada
Link ID: 20875 - Posted: 05.04.2015
Neuroscientists have discovered brain circuitry for encoding positive and negative learned associations in mice. After finding that two circuits showed opposite activity following fear and reward learning, the researchers proved that this divergent activity causes either avoidance or reward-driven behaviors. Funded by the National Institutes of Health, they used cutting-edge optical-genetic tools to pinpoint these mechanisms critical to survival, which are also implicated in mental illness. “This study exemplifies the power of new molecular tools that can push and pull on the same circuit to see what drives behavior,” explained Thomas R. Insel, M.D., director of NIH’s National Institute of Mental Health (NIMH). “Improved understanding of how such emotional memory works holds promise for solving mysteries of brain circuit disorders in which these mechanisms are disrupted.” NIMH grantee Kay Tye, Ph.D. External Web Site Policy, Praneeth Namburi and Anna Beyeler, Ph.D., of the Massachusetts Institute of Technology (MIT), Cambridge, and colleagues, report their findings April 29, 2015 in the journal Nature. Prior to the new study, scientists suspected involvement of the circuits ultimately implicated, but were stumped by a seeming paradox. A crossroads of convergent circuits in an emotion hub deep in the brain, thebasolateral amygdala, seem to be involved in both fear and reward learning, but how one brain region could orchestrate such opposing behaviors – approach and avoidance – remained an enigma. How might signals find the appropriate path to follow at this fork in the road?
Children who were often bullied by their peers may experience more anxiety and depression than children who were abused by adults, a finding that U.S. and British researchers say highlights an "imbalance" in school services to tackle bullying. Researchers followed the mental health of more than 4,000 children in Avon, south west England from birth to age 18 and 1,400 others in North Carolina from age nine up to age 26 through parent questionnaires and clinical interviews. In the Avon study, maltreatment was defined as physical, emotional, or sexual abuse or "maladaptive parenting" such as hitting, shouting and hostility. Children were interviewed about the frequency of bullying, which included overt threats, physical violence and nasty names as well as social exclusion or spreading lies or rumours. The results consistently showed an increased risk of anxiety, depression, self-harm and suicidal tendencies in children who were bullied, whether or not they had a history of abuse by adults, Prof. William Copeland, a clinical psychologist at Duke University School of Medicine in Durham, N.C. and his co-authors concluded in Tuesday's issue of Lancet Psychiatry. "What was a surprise was to see [the results] were as significant and pervasive as what we see for children that are physically abused, sexually abused or neglected," Copeland said. Government policies have focused almost exclusively on providing services for child abuse but much less attention and resources are devoted to bullying, the researchers said. Copeland's previous research showed long-term repercussions from bullying persist — and that includes impacts on physical health, dropping out of school and trouble with authorities. ©2015 CBC/Radio-Canada
By Tina Hesman Saey People with depression have more mitochondrial DNA and shorter telomeres than nondepressed people do, an international team of researchers reports online April 23 in Current Biology. Mitochondria are organelles that produce energy for cells. Mitochondria seem to become inefficient under stress, the team found, so more mitochondria may be needed to produce enough energy. Telomeres are the DNA endcaps on chromosomes that prevent the genetic material from unraveling. Short telomeres are associated with shorter life spans. The altered DNA may reflect metabolic changes associated with depression, the researchers say. Experiments with mice showed that these DNA changes are brought on by stress or by stress hormones. Four weeks after scientists stopped stressing the mice, their mitochondrial and telomere DNA had returned to normal. Those results indicate that the molecular changes are reversible. Researchers also studied DNA from more than 11,000 people to learn whether past stress was responsible for the molecular changes seen in people with depression. Depression was associated with the DNA changes, but having a stressful life was not. For instance, people who had experienced childhood sexual abuse but were not depressed did not have statistically meaningful changes to their DNA compared with people who had no history of abuse. The findings suggest that stress can change DNA but many people can bounce back. Depressed people may have a harder time recovering from the molecular damage. © Society for Science & the Public 2000 - 2015.