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These past studies of exercise and neurogenesis understandably have focused on distance running. Lab rodents know how to run. But whether other forms of exercise likewise prompt increases in neurogenesis has been unknown and is an issue of increasing interest

new study, which was published this month in the Journal of Physiology, researchers at the University of Jyvaskyla in Finland and other institutions gathered a large group of adult male rats. The researchers injected the rats with a substance that marks new brain cells and then set groups of them to an array of different workouts, with one group remaining sedentary to serve as controls.

They found very different levels of neurogenesis, depending on how each animal had exercised. Those rats that had jogged on wheels showed robust levels of neurogenesis. Their hippocampal tissue teemed with new neurons, far more than in the brains of the sedentary animals. The greater the distance that a runner had covered during the experiment, the more new cells its brain now contained. There were far fewer new neurons in the brains of the animals that had completed high-intensity interval training. They showed somewhat higher amounts than in the sedentary animals but far less than in the distance runners. And the weight-training rats, although they were much stronger at the end of the experiment than they had been at the start, showed no discernible augmentation of neurogenesis. Their hippocampal tissue looked just like that of the animals that had not exercised at all.

“sustained aerobic exercise might be most beneficial for brain health also in humans.”

Just why distance running was so much more potent at promoting neurogenesis than the other workouts is not clear, although Dr. Nokia and her colleagues speculate that distance running stimulates the release of a particular substance in the brain known as brain-derived neurotrophic factor that is known to regulate neurogenesis. The more miles an animal runs, the more B.D.N.F. it produces. Weight training, on the other hand, while extremely beneficial for muscular health, has previously been shown to have little effect on the body’s levels of B.D.N.F.

As for high-intensity interval training, its potential brain benefits may be undercut by its very intensity, Dr. Nokia said. It is, by intent, much more physiologically draining and stressful than moderate running, and “stress tends to decrease adult hippocampal neurogenesis,” she said.

These results do not mean, however, that only running and similar moderate endurance workouts strengthen the brain, Dr. Nokia said. Those activities do seem to prompt the most neurogenesis in the hippocampus. But weight training and high-intensity intervals probably lead to different types of changes elsewhere in the brain. They might, for instance, encourage the creation of additional blood vessels or new connections between brain cells or between different parts of the brain.

Greg Sutherland from the University of Sydney agrees. In 2016, he came to similar conclusions as Alvarez-Buylla’s team, using similar methods. “Depending on your inherent biases, two scientists can look at sparse events in the adult brain and come to different conclusions,” he says. “But when faced with the stark difference between infant and adult human brains, we can only conclude that [neurogenesis] is a vestigial process in the latter.”

Alvarez-Buylla agrees that there’s still plenty of work to do. Even if neurogenesis is a fiction in adult humans, it’s real in infants, and in other animals. If we really don’t make any new neurons as adults, how do we learn new things? And is there any way of restoring that lost ability to create new neurons in cases of stroke, Alzheimer’s, or other degenerative diseases? “Neurogenesis is precisely what we want to induce in cases of brain damage,” Alvarez-Buylla says. “If it isn’t there to begin with, how might you induce it?”

forgetting seems to be an active mechanism that is constantly at work in the brain.

“To have proper memory function, you have to have forgetting.

Different types of memory are created and stored in varying ways, and in various areas of the brain.

Neurons communicate with each other through synapses — junctions between these cells that include a tiny gap across which chemical messengers can be sent

The more often a memory is recalled, the stronger its neural network becomes. Over time, and through consistent recall, the memory becomes encoded in both the hippocampus and the cortex

Because the hippocampus is not where long-term memories are stored in the brain, its dynamic nature is not a flaw but a feature

Neuroscientists often refer to this physical representation of a memory as an engram. They think that each engram has a number of synaptic connections, sometimes even in several areas of the brain, and that each neuron and synapse can be involved in multiple engrams

The brain is always trying to forget the information it’s already learnt,

Hardt’s lab showed that a dedicated mechanism continuously promotes the expression of AMPA receptors at synapses.

To forget certain things, it seemed that the rat brain had to proactively destroy connections at the synapse. Forgetting, Hardt says, “is not a failure of memory, but a function of it”.

Paul Frankland, a neuroscientist at the Hospital for Sick Children in Toronto, Canada, had also found evidence that the brain is wired to forget

Frankland was studying the production of new neurons, or neurogenesis, in adult mice. The process had long been known to occur in the brains of young animals, but had been discovered in the hippocampi of mature animals only about 20 years earlier. Because the hippocampus is involved in memory formation, Frankland and his team wondered whether increasing neurogenesis in adult mice could help the rodents to remember.

Eventually, it exists independently in the cortex, where it is put away for long-term storage.

Researchers think that the human brain might operate in a similar way

Studies of people with exceptional autobiographical memories or with impaired ones seem to bear this out

People with a condition known as highly superior autobiographical memory (HSAM) remember their lives in such incredible detail that they can describe the outfit that they were wearing on any particular day

Those with severely deficient autobiographical memory (SDAM), however, are unable to vividly recall specific events in their lives

As a result, they also have trouble imagining what might happen in the future

By better understanding how we forget, through the lenses of both biology and cognitive psychology, Anderson and other researchers might be edging nearer to improving treatments for anxiety, PTSD and even Alzheimer’s disease

Hardt thinks that Alzheimer’s disease might also be better understood as a malfunction of forgetting rather than remembering

But more memory researchers are shifting their focus to examine how the brain forgets, as well as how it remembers

In the past decade, researchers have begun to view forgetting as an important part of a whole

Why do we have memory at all? As humans, we entertain this fantasy that it’s important to have autobiographical details,

Forgetting enables us as individuals, and as a species, to move forwards.

And yet, immersion in water up to the level of the heart has been shown to increase blood flow to one of the brain’s major arteries by 14 percent over that which you’d expect on land. So perhaps there is something special about swimming that is distinct from exercise on land.