I’ve had another session with Claude (3.5 Sonnet) in connection with the article I’m writing. Tentative title: “Melancholy and Growth: Toward a Mindcraft for an Emerging World.” The post title carries the topics under discussion.
I’ve read widely in neuroscience over the years, and continue to dip into the technical literature now and then. Some years ago David Hays and I published on “Principles and Development of Natural Intelligence” in which we reviewed a wide range of material: neural processes and organization, behavioral correlates, the phylogeny of the nervous system, and growth in human development. That’s my basic framework. On that basis Claude’s remarks make sense to me. Still, I’d advise caution in making use of those remarks.
I’ve appended its remarks below. I’m using my standard convention, setting my words in bold Courier and Claude’s reply in plain Courier.
Version: Claude 3.5 Sonnet
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What can you tell me about the development of the nervous system in humans? I know something about the course of myelination, which I believe is mostly complete by the early 20s, when the sutures of the skull finally set. I also know about the proliferation and pruning of dendrites early. Give me some more information.
I see you already have some knowledge about neural development! Let me add to that while gently correcting a small misconception - the cranial sutures actually begin fusing in early childhood and are typically mostly fused by age 6, though some continue fusing into adulthood. This is separate from the process of myelination you mentioned.
Neural development is a fascinating process that begins very early in embryonic development. Here are some key aspects and timeline points:
During embryonic development, the nervous system starts forming from the neural tube around week 3 post-conception. This process, called neurulation, creates the foundation for the entire central nervous system.
The early stages involve rapid neurogenesis (creation of new neurons), with neurons being produced at a rate of about 250,000 per minute during peak periods of fetal development. Most of our neurons are actually produced before we're born!
You mentioned dendritic pruning - this is indeed a crucial process. After birth, there's massive synaptic proliferation (synaptogenesis) creating many more connections than we'll ultimately keep. This peaks in different regions at different times:
- Visual cortex peaks around 6 months
- Prefrontal cortex peaks around age 1-2
The subsequent pruning you referenced continues through adolescence, following a "use it or lose it" principle where frequently used connections are strengthened and unused ones are eliminated.
Regarding myelination, it proceeds in a predictable pattern:
- Begins in the spinal cord during fetal development
- Progresses from back to front in the brain
- Sensory pathways myelinate before motor pathways
- Higher-order association areas myelinate last
- The prefrontal cortex indeed continues myelinating into the early 20s
One fascinating aspect is how this development correlates with function. For instance, the areas controlling basic functions like breathing and temperature regulation are well-developed at birth, while areas involved in executive function continue developing into early adulthood.
Would you like me to elaborate on any particular aspect of neural development? I find the relationship between developmental timing and functional capabilities particularly interesting.
Tell me more about this, “though some continue fusing into adulthood,” and this “The prefrontal cortex indeed continues myelinating into the early 20s.”
Let me address both of those ongoing developmental processes.
Regarding cranial sutures, while most fusion occurs early, the sagittal suture (running along the top of the skull from front to back) can continue its fusion process into the 30s. The sphenofrontal and frontoethmoidal sutures typically complete fusion between ages 20-40. Some parts of the lambdoid sutures (at the back of the skull) may not fully fuse until around age 60, though this varies considerably between individuals.
As for prefrontal cortex myelination, this prolonged development has significant implications. The extended myelination period correlates with ongoing refinement of executive functions including:
- Complex decision-making
- Impulse control
- Planning ability
- Social cognition
- Risk assessment
Recent research has shown that myelination isn't simply a linear process of adding more myelin. Instead, it involves active remodeling where myelin sheaths can be modified based on neural activity and learning. This process is particularly active in the prefrontal cortex through adolescence and early adulthood.
This extended development period helps explain certain patterns of adolescent behavior and why capabilities like judgment and impulse control continue to mature into the early-to-mid 20s. It's also relevant to understanding vulnerability to certain psychiatric conditions that often emerge during this period.
Would you like me to expand on the functional implications of either of these developmental processes?
Could you compare the time course of suture fusing in humans with what happens in animals, particularly our primate relatives.
I'll address cranial suture fusion timing across primates, though I should note that comparative timelines aren't as well documented as human patterns.
In great apes (our closest relatives), suture closure generally happens earlier than in humans. Chimpanzees, for instance, show complete fusion of most cranial sutures by around age 12-15. This earlier fusion correlates with their earlier maturation overall - they reach adulthood faster than humans do.
Orangutans show an interesting pattern where males and females differ significantly in their closure timing. Female orangutans complete most suture fusion by around age 12-14, while males may continue some fusion into their late teens, correlating with their extended growth period and pronounced sexual dimorphism.
In Old World monkeys like macaques, the process is even more accelerated, with most sutures fusing by age 5-7. This matches their faster overall development - they reach sexual maturity around age 4.
The human pattern of extended suture fusion appears to be relatively unique among primates. This extended period might relate to our extended brain growth and development period, which is also unique among primates. Our brains continue growing for longer and reach a larger relative size compared to other primates.
This prolonged development period appears to be a derived trait in humans, possibly related to our extended childhood and adolescence, which are also uniquely human features among primates. Would you like me to elaborate on any particular aspect of these comparative patterns?
How does this relate to cerebral plasticity?
The relationship between cranial suture fusion, brain development, and plasticity is quite interesting. There are several key connections:
First, the extended period of human suture fusion allows for continued brain growth and remodeling. While the sutures themselves don't directly control plasticity, they accommodate the physical changes that come with neural reorganization and growth. This is particularly important during childhood and adolescence when significant learning and skill acquisition occur.
However, it's important to note that plasticity isn't directly limited by suture fusion. The brain remains plastic well into adulthood, long after the sutures have fused. This plasticity is more directly related to:
- The ongoing myelination we discussed earlier, particularly in the prefrontal cortex
- Synaptic modification and reorganization
- Adult neurogenesis in specific regions like the hippocampus
- Changes in dendritic spines and axonal sprouting
What's particularly fascinating is how plasticity varies across brain regions and across the lifespan. Primary sensory areas tend to have critical periods of heightened plasticity early in life - think of how children more easily learn languages or recover from sensory deficits. Higher-order association areas, including the prefrontal cortex, retain more plasticity for longer, which aligns with their extended myelination timeline.
Would you like me to elaborate on any of these aspects of plasticity and their developmental timing?
Let me offer a speculation I’ve had. I used to teach in a university. During the school year – Fall, Winter, and Spring – I taught three courses in each of the two semesters. So much of my time was taken up by teaching, preparing for classes, giving the classes, interacting with students, reading student papers, etc. But I didn’t teach during the summer. Once the Spring semester was over I noticed, however, that it took 2 or 3 weeks for me to get in a really good frame of mind for conducting research, reading, extensive note taking, and writing.
I’m wondering if this pattern can be attributed to neural plasticity? During the school year my brain was “set up” to deal with the demands of teaching. When that was over the brain had to reorganize in order to meet the demands of research more effectively.
That's a fascinating self-observation and a plausible hypothesis! What you're describing aligns with our understanding of task-dependent neural plasticity. The brain does indeed reorganize based on changing cognitive demands, though typically not as rapidly as semester-to-semester changes.
What you might be experiencing is likely a combination of:
1. Functional plasticity - your neural networks reconfiguring to optimize for different types of cognitive tasks. Teaching requires rapid response to diverse situations, constant social awareness, and quick decision-making. Research, by contrast, often demands sustained focus, deep analytical thinking, and complex problem-solving over longer periods.
2. Changes in attention networks - your brain's priority system essentially "retunes" to filter information differently. During teaching, you're likely primed to notice student engagement cues, potential disruptions, and opportunities for immediate intervention. During research, you might find yourself more attuned to patterns in data or theoretical connections.
That said, the transition period you're noticing might also reflect the brain's need to shift between different "modes" of cognitive operation rather than structural plasticity per se. The fatigue or disorientation during these transitions could be your brain adjusting its default networks and executive function priorities.
Would you be interested in exploring specific aspects of how these cognitive demands differ between teaching and research? It could help illuminate what types of neural adaptation might be occurring.
I wonder if the same thing could be going on when people take vacations. I believe that vacationing is a relatively new phenomenon, dating mostly back to the early 20th century. When you get away from work for two weeks and spend you time doing something else, that may be just enough time for neural plasticity to do a bit of re-jiggering its connections. Perhaps a bit like simulated annealing in optimization problems in computing.
That's a really thought-provoking analogy with simulated annealing! The parallel works quite well - just as simulated annealing allows a system to escape local optima by temporarily increasing randomness/energy before settling into potentially better configurations, vacation might serve a similar function for neural organization.
During regular work, our neural networks become highly optimized for specific tasks and routines - essentially stuck in a "local optimum." The complete change of environment and activities during vacation could act like that temperature increase in annealing, temporarily disrupting established patterns and allowing for new configurations.
The two-week timeframe you mention is interesting because it aligns with some timescales of neural plasticity. While major structural changes take longer, changes in synaptic strength and local circuit organization can occur over days to weeks. This might explain why very short breaks (like weekends) don't provide the same psychological reset that a proper vacation does.
Your historical observation about vacation being relatively new is also intriguing. Traditional agricultural societies had seasonal variations in work intensity that might have served a similar function, but the modern pattern of sustained work punctuated by brief, intense periods of different activity is quite novel from an evolutionary perspective.
This could suggest that regular vacations aren't just a social construct but might serve an important biological function in allowing our brains to reconfigure and optimize - preventing us from getting too deeply stuck in potentially suboptimal patterns of neural organization.
Would you be interested in exploring how different types of vacations might affect this process differently? For instance, whether active adventure vacations versus relaxing beach holidays might facilitate different types of neural reorganization?
You last paragraph is very suggestive, but I'm afraid I've got to eat breakfast. 'Till later.
Thanks for this fascinating discussion about neural plasticity, optimization, and vacations! The intersection of cognitive demands and neural reorganization is a rich area to explore. Enjoy your breakfast!
