The Forest of Neurons

(This posting was not part of the recent talk  ‘What I Know and Why I Know It’ but it serves as an extended introduction to part 3 of 3 of that talk, not yet uploaded.)

Two major traditional concepts in the philosophy of mind are John Locke’s ‘tabula rasa’ and Immanuel Kant’s ‘synthetic a priori’ knowledge. The ‘Tabula Rasa’ concept falls within the British empiricist tradition and is that people are born with their mind as a ‘blank slate’ (the Latin term literally means a ‘scraped tablet’). Hence all knowledge is formed from experience. The ‘synthetic a priori’ concept falls within the Continental rationalist tradition and contradicts this. It maintains that there is some knowledge that exists before (‘a priori’) any experience – such as knowledge of time and space, which provides a framework into which other knowledge can fit.

These days, it is fairly common to hear news reports on developments in brain science, such as the large-scale Human Connectome Project that is ‘mapping’ the ‘wiring’ in our brains, and of the even larger Human Brain Project to simulate it. Even more common are reports that such-and-such a region of the brain has been associated with such-and-such a behaviour (invariably discovered from fMRI scans).

The impression that is given in these news reports is that there is a ‘wiring diagram’ of our brains and this wiring determines how different parts of the brain do different things and communicate with other parts (obviously there is some variation between you and me). This wiring diagram is presumably derived somehow from our genes. This seems to support the idea that our brains are somehow ‘pre-wired’ which fits into the Kantian idea.

But in the talk so far, I have presented a simple view of how the brain works in which I have emphasized the uniformity of the cortex – the basic structure of neurons within cortical columns is remarkably similar across the entire cortex and how each part of the brain is operating is also uniform. This seems to fit better with the Lockean idea that we are not ‘pre-wired’.

So which is it? Pre-wired or not?

Obviously, there’s a third option: neither! We normally associate ‘wiring diagrams’ with things like electronic circuits – hardware. But the brain is obviously a living organism. The brain grows. The question posed is akin to ‘which came first, the chicken or the egg?’.  This third way is neither ‘chicken’ nor ‘egg’, but it makes the idea that the answer could have been either ‘pre-wired’ or ‘not pre-wired’, indeed the very question, look slightly silly.

I want to make an analogy between neurons in the brain and other living entities – trees in a forest:

  • A forest is a collection of a large number of trees, spread out over a large area.
  • A cerebral hemisphere is a collection of a large number of neurons, spread across a ‘cortical sheet’ (it could be cut so that it could be flattened out to form a sheet).

(There are about 8 billion neurons in one hemisphere of the human cortex. It has been estimated that there are about 400 billion trees in the Amazonian rainforest.)

This analogy is mainly to help illustrate how different parts of the cortex can get built differently:

  1. with the same basic building blocks throughout the cortex,
  2. not by design (without a wiring diagram), and
  3. by factors outside of the cortex.

Obviously, there are limitations to the analogy:

  • The cortical sheet has multiple layers of neurons (e.g. 6 in the neocortical regions) whereas there is only one layer in the forest.
  • Neurons communicate directly with one another via synapses. Recall the common neuroscience maxim: ‘neurons that fire together wire together’, referring to the strengthening of synapses, i.e. the more both the post-synaptic neuron and pre-synaptic neurons are firing simultaneously, the easier it becomes for the post-synaptic neuron to fire when the pre-synaptic neuron fires. So how they change throughout their lifetime is strongly influenced by their neighbouring neurons. Trees on the other hand, obviously  just grow and are only influenced by their neighbours in competition for light and nutrients.

The forest will have a variation in tree species across its area. There is no ‘wiring diagram’ to specify which trees should go where.  As well as there being many internal factors, there are many factors external to those trees which determine which trees will tend to grow where, determined both by the underlying landscape on which the forest sits and the climate above, for example:

  • The ability to grow on steep slopes.
  • The effect of the Sun: North- versus South- facing slopes.
  • The effect of the Rain: Windward- versus Leeward slopes.
  • The soil and other properties of the soil.

Similarly, within the brain, there are many factors that affect the cortical sheet which are external to it:

  • the particular way the cortical sheet is folded up to fit within the skull.
  • the mix of chemicals to grow the neurons in the first place, and the cocktail of neurotransmitters produced in the lower parts of the brain.
  • the type of stimulus received from senses and the environment with which the brain is interacting.

To continue the analogy, imagine a forest after a fire has ravaged it. The surviving plants are free to grow and they will flourish, unimpeded by competition. In contrast, in an overgrown forest of gnarled old plants, it is difficult for any new growth because of the existing roots and foliage soaking up so much of the resources. This is analogous to:

  • the young brain being very plastic: neurons in the young brain being able to create new synapses at a prolific rate.
  • the difficulty of the elderly to learn new things or ‘unlearn’ i.e. form contrary habits  – it being difficult for neurons in an old brain to form new connections or ‘undo’ existing ones because of the mass of established synapses.

Note that a forest fire can rejuvenate a forest, whereas brains are periodically regenerated within new bodies that are culturally re-educated afresh. Perhaps the correct comparison for the brain is not a ‘scraped tablet’ but a ‘scorched earth’.

(Photo credit: Braden Piper Photography, Carlsbad Caverns NM)

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One Response to The Forest of Neurons

  1. Pingback: Changing Your Mind | Headbirths

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