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Healthy Aging Brain:
The Neuroscience of Making the Most of Your Mature Mind
by Judith Horstman
(London Times)
Your brain is a work in process. From conception to death, it keeps on growing, changing and adapting as it creates and dissolves networks and memories. We used to think we were born with all the brain cells we'd ever have, and when they were gone, that was it. That's because your brain cells (neurons), unlike other cells in your body, can't reproduce themselves.
However, scientists now know that new neurons continue to arise in some parts of the brain, right up until the time of death. This is called neurogenesis. Scientists also thought that the brain was set in its ways and not able to change easily, but research has shown we are making new connections among and between brain cells all the time.
Functions thought to be hardwired are turning out to be adaptable. Even as you read this, your brain is changing in response to the very act of reading, the words you read, and the ideas the words provoke. This is neuroplasticity. And we recently learned that genes are not destiny. Yes, your genes are set in the DNA you inherit and can't be changed, but not all of them are active, or active to the same degree. Moreover, some of them can be affected dramatically by life experiences, including what we see, do, eat, feel, and think. All this is terrific news for your brain.
How memory works
Memory loss is the big bugaboo of ageing and for most of us, the canary in the coal mine: an early-warning system that something is about to go terribly wrong in the brain. Memory is essential for our very identity - we need it to create our sense of self, and some think immortality is the memory of you that is held by others.
It may seem that memory is all about the past, but in fact, memory is about the present and the future, helping us move through the now. It's the process of acquiring and storing information from our experiences that we will need to navigate similar situations in the future.
It's tempting to think of memories as bits of specific information stored in a specific place that you can simply retrieve at will. However, your memories are not held neatly inside individual neurons. Instead, they are created when messages are sent across synapses, the tiny gaps between neurons, from the outgoing axon on one neuron to the dendrite on another neuron that receives information. (An axon is an extension that sends information out from neurons, a dendrite receives it).
A memory is held in the connections this network makes and is firmly established when a network of synapses is strengthened — temporarily for a short-term memory and more or less permanently for a long-term one. Over time, this net of memories can be strengthened further, weakened or broken, depending on a combination of your brain chemistry, your genes, your actions and whether you acquire a brain-damaging disease or injury.
There are many different kinds of memory, stored in different ways and housed in different places in the brain.
Short-term or 'working' memory is fleeting - say, when you are surfing the net and need to recall what you've just seen for 30 seconds or less. Then the memory is discarded or transferred to long-term memory. The transition is called consolidation. The hippocampus, among other structures in the brain's medial temporal lobe, is key to converting short to long-term memories and it's an area that is damaged in Alzheimer's disease.
Long-term memories are complex and can be both conscious and unconscious. Explicit memory, or declarative memory, is what most of us think about as long-term memory. It requires conscious thought to recall, say, the name of your first lover or knowledge learned from a text book - such as the parts of the brain that process memories. It's primarily explicit memory that fails in Alzheimer's and most other kinds of dementia.
Implicit, or non-declarative, memory includes rote memory involving habits and motor skills. It doesn't require conscious thought for most of us to recall how to brush our teeth or how to ride a bicycle. Those with advanced Alzheimer's who no longer recognise their children may still play the piano beautifully.
What we retain - and what we don't.
Scientists have long puzzled over why and how we choose to retain some information and not others. Your brain is constantly filtering incoming messages, deciding what information to keep and how to go about it. Sometimes it takes repetition - for example, to learn that six times seven is 42, to memorise the periodic table of elements or to learn your bank number and the passwords to your social network and bank accounts.
And sometimes, says the memory researcher R. Douglas Fields, we don't need to be told twice. A single potent experience can burn an experience indelibly into your brain, never more so than when connected to survival. For example, a bad scare with a dog as a child.
Fields says, from a biological or evolutionary perspective, memory is about the future - it keeps only what you need. There's no survival value in having a recording system in your brain that accurately retains each and every event and experience. (Anyone struggling to manage burgeoning email files knows the solution is not a bigger inbox, it's to delete the files that aren't needed.) The trick for your brain is to weigh the minute-to- minute experiences and instantly pick out the ones to keep for reference and the ones to discard.
White matter matters
Why do we have trouble making new memories or retaining short-term ones when we are older? It has a lot to do, Fields says, with the gradual loss of myelin, or white matter. As your brain grows older, it shrinks somewhat, losing grey matter (neurons) and white matter (myelin). The loss of white matter seems to matter more because it plays the key role in connectivity and our ability to think depends on our brain circuitry.
The loss of white matter, which allows electrical signals to travel through the brain quickly and efficiently, means that it takes longer to connect a face with a name, a book with an author or any other facts. Its loss also makes the brain 'noisier', that is, less able to sort out important from unimportant input. But myelin loss, according to new research, only seems to occur in specific parts of the brain - the parts responsible for learning new things. The part responsible for long-term memory shows no such loss, as most of us can testify.
White matter is also crucial to how your different brain regions are able to communicate with each other, called cross-talk. As it dwindles, so does that function. Research by Randy Buckner, a cognitive neuroscientist at Harvard, has found that in older brains, communication between brain regions does appear to decline dramatically.
Now that doesn't contradict findings that the brain remains plastic throughout middle and old age - if your brain remains active. Older brains can continue to learn. Intensive training causes neurons to fire - to generate an electrical charge and release neurotransmitters - and that activity has been shown to stimulate myelination, which appears to be connected to better cognition.
Forgetting is a useful skill
The next time a word or name stays frustratingly out of reach, consider this: your brain may be just doing its job. Forgetting not only helps the brain conserve energy, it also improves short-term memory and recall of important details, according to two Stanford University studies.
Scientists had students study 240 word pairs and then instructed them to memorise only a small subset of the list. Then the researchers performed MRI scans while testing them to see how well they had learned all the pairs. Those who were best at summing up the retained pairs were also the worst at remembering the ones to discard.
'When we want to remember things that are relevant, we put in much less neural effort if we have forgotten the things that are irrelevant,' says the psychologist Anthony Wagner, a co-author of the study.
A second study on mice, by William Flew, a neuroscientist at Columbia University, revealed short-term benefits from an inhibition of long-term memory. So if you accidentally called your neighbour by the wrong name, don't worry - your brain probably just chose to dump his name in favour of a more crucial fact, such as where you left those damn keys.
Use those words or lose them
We all know the maddening experience of not being able to think of a certain word that is undoubtedly in our repertoire - and our memory - if only we could retrieve it. Researchers have discovered an association between a specific region in the neural language system and these tip-of-the-tongue experiences that are a normal part of ageing.
Deborah Burke, of Pomona College in California, found that TOT moments became more frequent as grey matter density in the left insula declined. This is an area of the brain implicated in sound processing and production. The findings support a proposal by Burke predicting that when we do not use a word often, the connections among all its various representations in the brain become weak.
'Words aren't stored as a unit,' says Burke. 'Instead you have the sound information connected to semantic information, connected to grammatical information, and so on. However, the sounds are much more vulnerable to decay over time than other kinds of information. And that leads to the TOT experience.' So if you spend time alone but find yourself talking to yourself - it has a purpose.
Slower is sometimes better
So your ageing brain is slowing down - but that might not be so bad. In fact, in some cases, it could be a downright advantage. Several studies suggest that an ageing brain might be more accurate, more thoughtful, more social, and better able to use more of its parts.
As your brain ages, perception of sights, sounds and smells take a bit longer, and laying down new information into memory becomes more difficult. The ability to retrieve memories quickly also slides and it is sometimes harder to concentrate and maintain attention.
However, studies show that, though perception and reaction time do indeed take longer, slowing down doesn't necessarily undermine mental acuity. Indeed, evidence shows that older brains can be as mentally fit as younger ones - they just work in different and creative ways to compensate for some kinds of declines in ways that can keep older people nearly as sharp as youngsters, especially when tackling challenging tasks.
Perhaps the biggest trick older people have is to use both hemispheres simultaneously to handle tasks for which younger brains rely predominantly on only one side. Researchers at the University of Michigan used functional magnetic resonance imaging (fMRI) to observe the brains of young adults (aged 18 to 30) and older people (aged 65 to 92) as they tackled simple and difficult mental exercises.
For the easy tasks, brain activity was very similar, but tougher challenges prompted differences. The older people activated several frontal brain regions that the younger brains did not. In addition, the younger people 'turned off' parts of the brain during the tasks, whereas the elders kept those regions active.
Using both sides of the brain gives elders a tactical edge, even if the pure speed of each hemisphere's processing is slower. Old brains may also think more. A study at the University of Dortmund in Germany found that elders presented with new computer exercises paused longer before reacting and took longer to complete the tasks. Yet they made 50 per cent fewer errors.
Why multitasking becomes harder with age
You leave the living room and head off for the kitchen and forget what you wanted when you get there. This may not be due completely to a memory issue but rather to an interaction between memory and attention. The older brain may slow down because it's more easily distracted and is less able to block out irrelevant information.
In one study, Adam Gazzaley of the University of California and his colleagues asked two groups - one made up of 19 to 33-year-olds and the other of 60 to 72-year-olds — to perform a memory task. The researchers recorded the electrical signals from the participating brains during the tasks. During the first 200 milliseconds after exposure, the older group could not screen out distracting stimuli as well as the younger adults could.
The ability to ignore wasn't abolished, it was just delayed. By then, however, the irrelevant information had interfered with the memory task, making the older group less accurate. But not all older adults were distracted compared to younger adults. When Gazzaley divided the older group in half according to performance, only the low-scorers had the problem. Gazzaley poses the theory that older brains also can't switch back and forth as easily between working memory and long-term memory. Working memory is when you hold information in your mind for a brief period, such as that walk to from the living room to the kitchen.
In a recent study using fMRI, researchers had two age groups view a scene for about 14 seconds, then interrupted them with a face and some questions, then asked them to recall the original scene. The older group had more trouble doing so. When researchers examined the fMRIs they saw why. When interrupted, the brains of both groups disengaged from the task and reallocated brain resources toward processing the interruption. However, after the interruption the younger adults were better able to easily reconnect with the task.
They say the ability to ignore distractions is key to memory formation, and becomes more difficult with age - and with the increasingly complex and distracting world we live in.
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