Disadvantages of neuro-plasticity

[IMfez]

Why does our brain become less plastic when we become adults? Is there a disadvantage to it?

[thelurkinghorror]

Disadvantage to what? Becoming less plastic? Yeah, we become less resilient to injury, for one, and e.g. language acquisition is much harder.

It’s more efficient (or possible) to create a bunch of generalists who become specialized based on our environmental needs. Once we reach a certain age, we don’t need the voiced palatal fricative, or whatever, and then it is more efficient to stay specialized. As to why we recover better from brain injury at a younger age, WAG is that the benefits of keeping that aren’t very big.

[Der_Trihs]

@IMfez

Why does our brain become less plastic when we become adults? Is there a disadvantage to it?

A certain amount of mental instability, perhaps; teenagers and children aren’t known for their rationality and reliability. And in the ancient past when we evolved there was seldom much change during an adult lifetime; so there was less benefit to be gained by retaining that flexibility once you were an adult.

[deltasigma]

Our brains are still fairly plastic although there maybe a lot of variability between individuals. Just look at how many are able to recover after a stroke. But for truly extraordinary examples, there are the 50 or so reported cases of acquired savant syndrome.

[njtt]

@IMfez

Why does our brain become less plastic when we become adults? Is there a disadvantage to it?

Loss or distortion of memories (including learned skills), perhaps.

[bob_2]

The inability to ask unambiguous questions.

It is not clear whether the OP wants to know if there a disadvantage to **having **or to **losing **plasticity.

[Nametag]

@bob_2

The inability to ask unambiguous questions.

It is not clear whether the OP wants to know if there a disadvantage to **having **or to **losing **plasticity.

It is if you read the title…

[DSeid]

Little hard evidence to back this up but …

Becoming better at doing specific cognitive tasks means less ability to learn every cognitive task. We commit to that which is required for success in our environment, pruning away that which has not been used as often and making that which has worked consistently more hard wired and efficiently processed, faster and at less energy cost.

To that second part, the brain is an energy hog: 20% of a human’s energy budget while making up just about 2% of his/her mass (on average). Maintaining plasticity (lots more active synapses ready to be strengthened, constantly firing and testing out other alternate paths, lots of regrowth constantly going on, not just of dendrites and synapses but of all the supporting players like glial cells, to provide the raw material to be reshaped) would cost even more. There may not be enough energy available to an adult brain to do that. An 11 y.o.'s brain, more plastic than an adult’s, uses 33% of the body’s energy, a newborn’s, more plastic yet, 75%.

[deltasigma]

I don’t know if you’d consider this hard evidence, but it’s consistent with recent research into epigenetic methylation patterns of neurons and glial cells in the frontal cortex. From the NIH director’s blog.

Most researchers assume that methylation of cytosine happens almost exclusively in a specific DNA context—namely, when C (cytosine) is followed by G (guanine). That is certainly true in other tissues. But in the brain, the mappers found that non-CG methylation, an unconventional form of DNA methylation that’s almost non-existent in humans at birth, ramps up in neurons during the first two years of life. It then increases through adolescence until it finally plateaus in early adulthood, when the development of the frontal cortex is essentially complete. This period of escalating non-CG methylation overlaps with the time when connections between neurons are rapidly forming—suggesting that DNA methylation might be regulating which connections are formed or deleted.

[Learjeff]

Please correct me if I’m wrong, but I remember reading that within a couple years of birth, most connections between neurons have been formed, and during childhood to early adolescence, we tend to learn by switching off (inhibiting or losing) unnecessary connections.

At some point in adolescence the pendulum swings the other way and we tend to learn by adding new connections, retaining most of the old ones.

This might optimize our ability to rapidly adapt to the environment during childhood, but be relatively stable (reliable) thereafter, as long as there aren’t significant changes in the environment (changes requiring different behavior for survival/fitness). If true, this would be likely to also be true for our near relatives, the great apes.

With the advent of language (and then written language) and the use of tools, we dramatically increased our ability to respond quickly to a change in the environment, which led to humankind’s domination of the planet. This might change the fitness calculus regarding the optimum age of loss of plasticity, but I doubt it’s been long enough for any kind of new optimum to work itself out genetically.

[DSeid]

First part correct. Second not. Not so much new connections so much as stengthening some and weakening others. (Which is not to say that there are no new synapses … heck there are even a few new neurons being made … but not many and possibly no more than are lost along there way, if that.)

The best work was done by Peter Huttenlocher’s group at University of Chicago and is summarized some here.

By measuring the number of synapses – connections between nerve cells – Huttenlocher and Dabholkar were able to establish when the production of synapses reached its peak and when it began to decline in three portions of the brain: the auditory cortex, the primary visual cortex and the middle frontal gyrus (which controls higher-order thinking skills) …

… Huttenlocher found that the auditory cortex reaches its highest synaptic density at age three months and then begins to decline until age 12, when it levels out. For the visual cortex, the decline appears to begin even earlier.

In the case of the middle frontal gyrus, the peak in synaptic density is not reached until a child is three and a half years old. The slow decline of the number of synapses does not appear to stop until mid-adolescence, the researchers found.

By the time a person is an adult, the number of synapses in the various parts of the brain are relatively equal.

The subject is covered in much more detail in a book that Huttenlocher wrote, some of which is available on-line.

In the context of this loss of synapses with aging, and the consequent decrease in plasticity (as it becomes more limited to unmasking pathways that are already extant and weakening others) is the finding that sometimes more synpatic denisty means poor learning and less means better.

This article is also on point:

Young brains are capable of forming many new synapses, and they are consequently better at learning new things. That is why we acquire vital skills – walking, talking, hearing and seeing – early on in life. The adult brain stabilises the synapses so that we can use what we have learned in childhood for the rest of our lives.
Disappearing inhibitors
Earlier research found that approximately one fifth of the synapses in the brain inhibit rather than excite other nerve-cell activity. Neuroscientists have now shown that many of these inhibitory synapses disappear if the adult brain is forced to learn new skills …

In addition to regulating the strength of synapses, and modifying (typically lessening) inhibition of different pathways, the adult brain also makes certain pathways more efficient by varying the myelination.

So the course does seem to be throwing down the mass of raw material in some rough overapproxiamation of the shape that works, and then pulling off off bits that don’t seem to be needed and making the rest that is there more exact right shapes within a narrower range.

[deltasigma]

Not sure how much this adds to the discussion, but it might be interesting. It seems that autism might be the result of an excess of neural connectionswhich in turn seems to result from a surplus of neurons in certain regions of the brain.

Gradually he saw a pattern. At birth, children with autism had normal-size brains. But by the time they were a year old, the brains of most autistic children had grown far beyond average. The average adult human brain weighs 1,375 grams, but Courchesne encountered one 3-year-old autistic boy whose brain weight was estimated at 1,876 grams.
The MRI scans further revealed that only certain parts of the brain became larger. The growth was striking in the prefrontal cortex, the region just behind the eyes that is responsible for language, decisions, and other sophisticated thinking. Courchesne also saw an increase in both the gray matter (consisting of dense clusters of neurons) and the white matter linking different regions of the brain. This explosive neural expansion continued in many autistic children until the age of 5, and then it stopped. Past that age, Courchesne found, the rate of brain growth slowed in autistic children, falling behind that of ordinary children. By the teen years, some brain regions actually started to shrink.
.
.
.
When autistic children are born, Courchesne’s research suggests, they have an abundance of neurons jammed into an average-size brain. Over the first few years, the neurons get bigger and sprout thousands of branches to join other neurons. The extra neurons in the autistic brain probably send out a vast number of extra connections to other neurons. This overwiring may interfere with normal development of language and social behavior in young children. It would also explain the excess brain size seen in the MRI scans.

[Learjeff]

@DSeid

First part correct. Second not.

Thanks! Ignorance fought.