# Not hardcoded

What if the brain is just a tangled, specialized mess?

• The brain contains around $$10^{11}$$ neurons and $$10^{14}$$ synapses.1
• If the exact connectivity of these elements is critical to the brain’s function, then there may not be underlying principles
• We’d be totally unable to identify units of functionality, or create effective abstractions
• “Hardcoding” is a programming term to describe handling cases individually, without abstraction or generality

Fortunately, this cannot be true.

• The human genome only contains 3.2 billion base pairs.2
• Each pair is 2 bits, so the genome is around 6.4 gigabits (0.8 gigabytes).
• Some Fermi estimation (in base 2):
• Suppose no more than $$\frac{1}{4}$$ of the genome talks about the brain. It’s probably much less.
• So we have $$1.6 \cdot 10^9 \approx 2^{30}$$ bits to work with.
• There are $$86 \cdot 10^9 \approx 2^{36}$$ neurons, so specifying a connection between two neurons requires 72 bits.
• Of course, most pairs of neurons are too distant to connect to each other. Let’s say that some sort of neighborhood-encoding scheme saves half of the data requirement (i.e. 36 bits per connection, where each neuron can choose from $$2^{18}$$ neighbors). This seems like a good ballpark since neurons tend to have something like $$2^{15}$$ synapses.1
• Then the brain-coding portion of the genome can only specify $$\frac{2^{30}}{36} \approx 2^{25}$$ synapses.
• This is only enough for 1024 neurons at $$2^{15}$$ synapses each.
• So the synapse-generation algorithm must be profoundly general and the brain is not hardcoded in any significant way.*
• This fits our experience: babies have to learn to walk, even though walking is a prime example of something that would make sense to hardcode.

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1. Chudler, E. Brain Facts and Figures. ↩︎

2. Chial, H. (2008) DNA sequencing technologies key to the Human Genome Project. Nature Education 1(1):219 ↩︎