The discovery that the human brain continues to produce new neurons in adulthood challenged a major dogma in the field of neuroscience, but the role of these neurons in behavior and cognition is still not clear. In a review article published by Cell Press February 21st in Trends in Cognitive Sciences, Maya Opendak and Elizabeth Gould of Princeton University synthesize the vast literature on this topic, reviewing environmental factors that influence the birth of new neurons in the adult hippocampus, a region of the brain that plays an important role in memory and learning.
The authors discuss how the birth of such neurons may help animals and humans adapt to their current environment and circumstances in a complex and changing world. They advocate for testing these ideas using naturalistic designs, such as allowing laboratory rodents to live in more natural social burrow settings and observing how circumstances such as social status influence the rate at which new neurons are born.
"New neurons may serve as a means to fine-tune the hippocampus to the predicted environment," Opendak says. "In particular, seeking out rewarding experiences or avoiding stressful experiences may help each individual optimize his or her own brain. However, more naturalistic experimental conditions may be a necessary step toward understanding the adaptive significance of neurons born in the adult brain."
In recent years, it has become increasingly clear that environmental influences have a profound effect on the adult brain in a wide range of mammalian species. Stressful experiences, such as restraint, social defeat, exposure to predator odors, inescapable foot shock, and sleep deprivation, have been shown to decrease the number of new neurons in the hippocampus. By contrast, more rewarding experiences, such as physical exercise and mating, tend to increase the production of new neurons in the hippocampus.
The birth of new neurons in adulthood may have important behavioral and cognitive consequences. Stress-induced suppression of adult neurogenesis has been associated with impaired performance on hippocampus-dependent cognitive tasks, such as spatial navigation learning and object memory. Stressful experiences have also been shown to increase anxiety-like behaviors that are associated with the hippocampus. In contrast, rewarding experiences are associated with reduced anxiety-like behavior and improved performance on cognitive tasks involving the hippocampus.
Although scientists generally agree that our day-to-day actions change our brains even in adulthood, there is some disagreement on the adaptive significance of new neurons. For instance, the literature presents mixed findings on whether new neurons generated under a specific experimental condition are geared toward the recognition of that particular experience or if they provide a more naive pool of new neurons that enable environmental adaptation in the future.
Gould and her collaborators recently proposed that stress-induced decreases in new neuron formation might improve the chances of survival by increasing anxiety and inhibiting exploration, thereby prioritizing safety and avoidant behavior at the expense of performing optimally on cognitive tasks. On the other hand, reward-induced increases in new neuron number may reduce anxiety and facilitate exploration and learning, leading to greater reproductive success.
"Because the past is often the best predictor of the future, a stress-modeled brain may facilitate adaptive responses to life in a stressful environment, whereas a reward-modeled brain may do the same but for life in a low-stress, high-reward environment," says Gould, a professor of psychology and neuroscience at Princeton University.
However, when aversive experiences far outnumber rewarding ones in both quantity and intensity, the system may reach a breaking point and produce a maladaptive outcome. For example, repeated stress produces continued reduction in the birth of new neurons, and ultimately the emergence of heightened anxiety and depressive-like symptoms.
"Such a scenario could represent processes that are engaged under pathological conditions and may be somewhat akin to what humans experience when exposed to repeated traumatic stress," Opendak says.
Because many studies that investigate adult neurogenesis use controlled laboratory conditions, the relevance of the findings to real-world circumstances remains unclear. The use of a visible burrow system--a structure consisting of tubes, chambers, and an open field--has allowed researchers to recreate the conditions that allow for the production of dominance hierarchies that rats naturally form in the wild, replicating the stressors, rewards, and cognitive processes that accompany this social lifestyle.
"This more realistic setting has revealed individual differences in adult neurogenesis, with more new neurons produced in dominant versus subordinate male rats," Gould says. "Taking findings from laboratory animals to the next level by exploring complex social interactions in settings that maximize individual variability, a hallmark of the human experience, is likely to be especially illuminating."
The above story is based on materials provided by Cell Press. Note: Materials may be edited for content and length.
This illustration shows two spiral galaxies - each with supermassive black holes at their center - as they are about to collide and form an elliptical galaxy. New research shows that galaxies' dark matter halos influence these mergers and the resulting growth of supermassive black holes.
Credit: NASA/CXC/M.Weiss
Every massive galaxy has a black hole at its center, and the heftier the galaxy, the bigger its black hole. But why are the two related? After all, the black hole is millions of times smaller and less massive than its home galaxy.
A new study of football-shaped collections of stars called elliptical galaxies provides new insights into the connection between a galaxy and its black hole. It finds that the invisible hand of dark matter somehow influences black hole growth.
"There seems to be a mysterious link between the amount of dark matter a galaxy holds and the size of its central black hole, even though the two operate on vastly different scales," says lead author Akos Bogdan of the Harvard-Smithsonian Center for Astrophysics (CfA).
This new research was designed to address a controversy in the field. Previous observations had found a relationship between the mass of the central black hole and the total mass of stars in elliptical galaxies. However, more recent studies have suggested a tight correlation between the masses of the black hole and the galaxy's dark matter halo. It wasn't clear which relationship dominated.
In our universe, dark matter outweighs normal matter -- the everyday stuff we see all around us -- by a factor of 6 to 1. We know dark matter exists only from its gravitational effects. It holds together galaxies and galaxy clusters. Every galaxy is surrounded by a halo of dark matter that weighs as much as a trillion suns and extends for hundreds of thousands of light-years.
To investigate the link between dark matter halos and supermassive black holes, Bogdan and his colleague Andy Goulding (Princeton University) studied more than 3,000 elliptical galaxies. They used star motions as a tracer to weigh the galaxies' central black holes. X-ray measurements of hot gas surrounding the galaxies helped weigh the dark matter halo, because the more dark matter a galaxy has, the more hot gas it can hold onto.
They found a distinct relationship between the mass of the dark matter halo and the black hole mass -- a relationship stronger than that between a black hole and the galaxy's stars alone.
This connection is likely to be related to how elliptical galaxies grow. An elliptical galaxy is formed when smaller galaxies merge, their stars and dark matter mingling and mixing together. Because the dark matter outweighs everything else, it molds the newly formed elliptical galaxy and guides the growth of the central black hole.
"In effect, the act of merging creates a gravitational blueprint that the galaxy, the stars and the black hole will follow in order to build themselves," explains Bogdan.
Credits: ScienceDaily
A senior researcher at the Large Hadron Collider says a new particle could be detected this year that is even more exciting than the Higgs boson.
The accelerator is due to come back online in March after an upgrade that has given it a big boost in energy.
This could force the first so-called supersymmetric particle to appear in the machine, with the most likely candidate being the gluino.
Its detection would give scientists direct pointers to "dark matter".
And that would be a big opening into some of the remaining mysteries of the universe.
"It could be as early as this year. Summer may be a bit hard but late summer maybe, if we're really lucky," said Prof Beate Heinemann, who is a spokeswoman for the Atlas experiment, one of the big particle detectors at the LHC.
"We hope that we're just now at this threshold that we're finding another world, like antimatter for instance. We found antimatter in the beginning of the last century. Maybe we'll find now supersymmetric matter."
The University of California at Berkeley researcher made her comments at the annual meeting of the American Association for the Advancement of Science.
In the debris
Supersymmetry is an addition to the Standard Model that describes nature’s fundamental particles and their interactions.
Susy, as it is sometimes known, fills some gaps in the model and provides a basis to unify nature's forces.
It predicts each of the particles to have more massive partners. So the particle that is light – the photon – would have a partner called the photino. The quark, the building block of an atom’s protons and neutrons, would have a partner called the squark.
But when the LHC was colliding matter at its pre-upgrade energies, no sign of these superparticles was seen in the debris, which led to some consternation among theorists.
Now, with the accelerator about to reopen in the coming weeks, there is high hope the first evidence of Susy can be found.
The machine is going to double the collision energy, taking it into a domain where those theorists say the gluino really ought to emerge in sufficient numbers to be noticed. The gluino is the superpartner of the gluon, which "glues" the quarks together inside protons and neutrons.
The LHC’s detectors would not see it directly. What they would track is its decay, which scientists would then have to reconstruct.
But importantly, those decay products should include the lightest and most stable superparticle, known as the neutralino – the particle that researchers have proposed is what makes up dark matter, the missing mass in the cosmos that binds galaxies together on the sky but which cannot be seen directly with telescopes.
"This would rock the world,” said Prof Heinemann. "For me, it’s more exciting than the Higgs."
'The other side'
So, not only would supersymmetry proponents be elated because they would have their first superparticle, but science in general would have a firm foot on the road to understanding dark matter.
Dr Michael Williams, from the Massachusetts Institute of Technology, said: "We sometimes talk about the dark matter particle, but it’s perfectly plausible that dark matter is just as interesting as [normal] matter, [which] has a lot of particles that we know about.
"There might be just as many dark matter particles, or even more.
"Finding any particle that could be a dark matter candidate is nice because we could start to understand how it affects the galaxy and the evolution of the universe, but it also opens the door to whatever is on the other side, which we have no idea what is there."
Particle physicists have three major conferences in August and September, one of which is the main gathering of the supersymmetry community. All these meetings are bound to draw huge interest.
But Prof Jay Hauser, who works on the CMS detector at the LHC, added a little caution on timings. "Even if we did see something, remember it might be complicated enough that it takes us a while to explain it," he told reporters.
