I was asked this question on Quora:
Here’s how I responded:
Here’s a question: in a system composed of, is there a “code”?
I ask this because I find that the “code” metaphor is often misleading when thinking about biology. Codes are composed of symbols. But it is not clear that neurons communicate using symbols.
The way a neuron affects other neurons is more like how a gear affects other gears. There is no code — there is causality. An active neuron releases some neurotransmitter, and this in turn makes other neurons more active. It’s like a complex network of dominoes.
Does the idea of a “code” help us understand how one domino affects the next one in the chain?
I admit that by the time a human is thinking in terms of words and symbols, “code” is probably a useful metaphor for what is going on. But the origin of coding schemes remains a great mystery in neuroscience, cognitive science, and artificial intelligence. So I recommend starting with a much less loaded metaphor, such as clockwork or dominoes. Thinking in mechanical terms helps us realize what exactly neuroscience and AI research are trying to achieve.
For now, there is a fascinating gap in our understanding of what exactly codes are in the first place.
Anyway, if you are interested in the causal “domino effect” that starts at the retina, have a look at this answer: