epistemic status: still a student but quite sure of myself on this topic and pretty sure that this misconception plausibly has a non negligeable impact on some debates
Medical student here, I just wanted to shed some light on what I think is a common misconception.
Yes the human brain contains about 86 billion neurons. But about 60 billions are in the cerebellum and have little to do with consciousness. Those neurons can plausibly be approximated as just filtering the noisy signal going from the rest of the brain (cortex etc) to the limbs.
A person without cerebellum can be perfectly conscious.
But reducing the number of neurons by two thirds like that does not change the order of magnitude of the number of synapses, which arguably are a more important number to consider when looking for an upper bound needed for consciousness.
But you can't just assume that all those neurons are fully connected, the length of axons is not the same for all neurons and the number of dendrites vary etc.
According to this study, the average total number of synapses in the neocortex of young male brains was 164x10^12 (so 164 'trillion', using the american short scale).
This is the standard understanding in neuroscience (and for what its worth is my working belief), but there is some evidence that throws a wrench into this idea, and needs to be explained, for instance this review "Consciousness without a cerebral cortex: a challenge for neuroscience and medicine" where evidence towards the idea that consciousness without a cortex can occur. in particular this is a famous case of a human with hardly any cortex that seemed to act normally, in most regards.
The hydranencephalic people in that paper do indeed exhibit certain behaviors that we intuitively associate with consciousness. But it’s equally true that deep-RL agents & LLMs exhibit certain behaviors that we intuitively associate with consciousness. So is this paper “evidence towards the idea that consciousness without a cortex can occur”? Yeah sure, but I don’t think it’s strong evidence. (More of my opinions here.)
What is evidence of consciousness, then?