Disadvantages of neuro-plasticity

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

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.

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.

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.

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

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…

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%.

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.

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.

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.

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:

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.

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.

Thanks! Ignorance fought.