Pruning and Plasticity - lydgate/mindmeld GitHub Wiki

• There's a lot of excellent work on the processes of neural pruning that will be important to dive into in relation to many of the ideas we've already explored.
  • In particular, pruning and plasticity and how our brains fluctuate across development between these phases seems (at least to me) related to the chaos-order-chaos ideas.
  • It's probably also important to explore this topic through the lens of ontogeny recapitulating phylogeny. For example, the move from a stable period of neural configurations in late childhood is followed by a massive proliferation of neurons and subsequent dramatic period of rapid pruning.
  • This may be likened to the concept of punctuated equilibria over evolutionary time that Gould in particular wrote about (periods of stability that are broken up nonlinearly by massive phase transitions that change basically everything, by first creating "chaos" and then creating pressure for restabilization).
• William Greenough's work in developmental neuroscience is very relevant here. I find it quite complex, so I think we should talk this through and think through the implications together.
  • Greenough's work was the basis for a relatively radical new model of brain 'plasticity' (before his work, it was a "given" that brain plasticity stops very early in development and we never have a chance to regain any of its flexbility to learn or adapt to radically different environments).
  • laid the foundation for a model in which lifelong learning processes were the fodder for long-term memories which are formed at the structural level in the brain.
  • In sum: Learning even in adulthood changes the structure of the brain (NOT just the function): "The most general conclusion that can be made confidently is that the brain is an extremely plastic organ, the structure of which is exquisitely sensitive to experience. A major function of the brain is thus to continuously re-organize itself, and it does so in a way that is specifically tailored to result in behavior that is adaptive in the context of the individual’s own unique environment."
  • He made the extremely important distinction between experience-expectant vs experience-dependent synapses (and learning more broadly).
    • experience-expectant synapses: produced early in development; incorporate the most relevant elements of the environment that have been encountered reliably as the species has evolved and help members of that species learn quickly what is commonly relevant to all in the species.
    • experience-dependent synapses: develop later in life; form in response to experiences that result in memories that are specific to that individual, and the unique environment encountered by that individual.

William T. Greenough

One study in Child Development (made a huge splash at the time and still relevant, despite being done in 1987): Experience and Brain Development (Child Development) ABSTRACT: experience can influence the developing and mature brain and proposes a new categorization scheme based upon the type of information stored and the brain mechanisms that appear to be involved in storing it. In this scheme, experience-expectant information storage refers to incorporation of environmental information that is ubiquitous in the environment and common to all species members, such as the basic elements of pattern perception. Experience-expectant processes appear to have evolved as a neural preparation for incorporating specific information: in many sensory systems, synaptic connections between nerve cells are overproduced, and a subsequent selection process occurs in which aspects of sensory experience determine the pattern of connections that remains. Experience-dependent information storage refers to incorporation of environmental information that is idiosyncratic, or unique to the individual, such as learning about one's specific physical environment or vocabulary. The neural basis of experience-dependent processes appears to involve active formation of new synaptic connections in response to the events providing the information to be stored. Although these processes probably do not occur entirely independently of one another in development, the categories offer a new view more in accord with neural mechanisms than were terms like "critical" or "sensitive period."

• Balancing Plasticity/Stability Across Brain Development - ScienceDirect feels important

  • so after puberty, after all that myelination, after stabilty, apropos your article
  • Also apparently critical periods are themselves plastic and they depend on the maturation of inhibitory circuits
  • You can make them happen sooner or later by fucking with GABA

• Myelin is important here too (excerpt from Wikipedia): The process of generating myelin is called myelination or myelinogenesis. In the CNS, cells called oligodendrocyte precursor cells (OPCs; the precursors of oligodendrocytes) differentiate into mature oligodendrocytes, which form myelin. In humans, myelination begins early in the 3rd trimester, although only little myelin is present in either the CNS or the PNS at the time of birth. During infancy, myelination progresses rapidly, with increasing numbers of axons acquiring myelin sheaths. This corresponds with the development of cognitive and motor skills, including language comprehension, speech acquisition, crawling and walking. Myelination continues through adolescence and early adulthood and although largely complete at this time, myelin sheaths can be added in grey matter regions such as the cerebral cortex, throughout life.