Correcting a misconception: consciousness does not need 90 billion neurons, at all

post by bvbvbvbvbvbvbvbvbvbvbv · 2023-03-31T16:02:30.282Z · LW · GW · 19 comments

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).



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comment by Adam Shai (adam-shai) · 2023-03-31T16:47:42.008Z · LW(p) · GW(p)

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.

Replies from: steve2152
comment by Steven Byrnes (steve2152) · 2023-03-31T17:38:22.989Z · LW(p) · GW(p)

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 [EA · GW].)

Replies from: MSRayne
comment by MSRayne · 2023-04-01T02:56:58.871Z · LW(p) · GW(p)

What is evidence of consciousness, then?

comment by Razied · 2023-03-31T16:18:46.098Z · LW(p) · GW(p)

A person without cerebellum can be perfectly conscious.


You have absolutely no idea that this is the case, neither does anyone else. That's the whole point of the hard problem of consciousness. We don't even have any idea whether a single neuron (or a single atom, or region of space containing a field, or any other subset of the universe smaller than a human brain) has even a little bit of consciousness. I think consciousness is probably the result of information processing and not dependent on specific physical processes, but claiming that we 1. know exactly what the cerebellum is doing and that 2. it's definitely not involved or required in consciousness seems extraordinarily premature to me.

Replies from: interstice, bvbvbvbvbvbvbvbvbvbvbv, korin43, gilch, TAG
comment by interstice · 2023-03-31T17:02:16.614Z · LW(p) · GW(p)

You have absolutely no idea that this is the case, neither does anyone else

Yes we do, because there are people with cerebellar agenesis who are able to live normal lives, so we have about as much evidence of them being conscious as we do for anyone.

comment by bvbvbvbvbvbvbvbvbvbvbv · 2023-03-31T18:10:43.371Z · LW(p) · GW(p)

Yes we do, it's in the sources.

Replies from: Razied
comment by Razied · 2023-03-31T20:51:58.119Z · LW(p) · GW(p)

If what you mean by "consciousness" is something like "ability to utter the words 'I am conscious' ", then sure, but then why do we care about the number of neurons required to make a system utter those words? The main thing of interest here is trying to use baselines from neuroscience to infer things about which AI systems are truly conscious (what other debates were you referring to?), in the it's-something-to-be-like-it sense. Being able to say "I am conscious" does not confer a system moral worth, it is its subjective experience that does that, and the observation that people without a cerebellum can live normal lives doesn't tell us anything about whether it has affected the intensity of their subjective experience.

Replies from: bvbvbvbvbvbvbvbvbvbvbv
comment by bvbvbvbvbvbvbvbvbvbvbv · 2023-03-31T22:51:31.987Z · LW(p) · GW(p)

Ley's put it lile this : if you had hours of interaction with this individual you'd have no reason to doubt it's conscious. I indeed don't know if it has the exact same sense of consciousnes as someone with a cerebellum but this is also true for everyone else : I don't know if you and I have the same conscious experience either.

Replies from: TAG
comment by TAG · 2023-04-01T18:50:17.463Z · LW(p) · GW(p)

So there is still a possibility that the cerebellum is responsible for some of the inaccessible aspects of consciousness?

Replies from: bvbvbvbvbvbvbvbvbvbvbv
comment by bvbvbvbvbvbvbvbvbvbvbv · 2023-04-02T13:18:03.681Z · LW(p) · GW(p)

I don't understand what you mean by "inaccessible"

comment by Brendan Long (korin43) · 2023-03-31T16:38:16.842Z · LW(p) · GW(p)

It seems like the important thing to ask isn't whether we can know for certain if a person without a cerebellum is concious (since we don't really know that for non-self people in general). More interesting to me is the question of if we've asked someone without a cerebellum if they feel concious. If the answer is "Yes, and they said they're concious", then I think that's about as much evidence as we're likely to get.

comment by gilch · 2023-03-31T21:22:44.561Z · LW(p) · GW(p)

What are you talking about? There has been at least one case of a woman literally born without a cerebellum and nobody noticed until she had a brain scan. The neocortex was plastic enough to be able to take over the functions. She was much less coordinated than average as a child (she was able to speak intelligibly and walk unassisted by age 7), but otherwise behaved like a normal human. Are you seriously asking me to believe she's a zombie? How do I know you're conscious?

Replies from: Razied
comment by Razied · 2023-03-31T21:25:45.147Z · LW(p) · GW(p)

How do I know you're conscious?

Exactly! You don't! And all this talk of who is born without which brain region and how they went through life gets us no closer at all to actually understanding which physical systems are not zombies.

Replies from: bvbvbvbvbvbvbvbvbvbvbv
comment by bvbvbvbvbvbvbvbvbvbvbv · 2023-03-31T22:54:55.439Z · LW(p) · GW(p)

Don't you agree that certain brain lesion definitely make you not conscious? I think identifying which region is indispensable is important.

If I had to guess human can be conscious without a cerebellum but not without basal ganglia fwiw

Replies from: Razied
comment by Razied · 2023-03-31T23:07:59.087Z · LW(p) · GW(p)

No, I wouldn't agree that there are brain lesions that definitely makes you not conscious, I would certainly agree that something like complete brain death makes you less conscious, but we don't have anything more precise than our vague intuitions born from N=1 subjective data to go by here. We have no idea which kinds of information processing actually lead to qualia. Is consciousness like mass and gravity? in that every tiny bit of the universe has it, and we only notice it when enough coalesces together? or is it more like a phase transition? with simple systems having exactly 0 consciousness, and it suddenly turning on at some level of complexity of computation? Or is the "complexity" even irrelevant, and consciousness refers to just very specific kinds of information processing? None of these questions can be resolved by looking at the behaviour of humans with brain damage, they require a full theory of how subjective feelings map onto physical systems.

Replies from: gilch, bvbvbvbvbvbvbvbvbvbvbv
comment by gilch · 2023-04-01T00:19:06.798Z · LW(p) · GW(p)

Fine. Hard Problem. Maybe panpsychism. I'm not claiming anybody understands how consciousness works. I only object to the "absolutely no idea that this is the case, neither does anyone else." Given the case I linked to above, we have more than sufficient Bayesian evidence (on balance) to believe that a person can be conscious [LW · GW] without a cerebellum, and if that's what you're objecting to, I don't believe you're still arguing in good faith. If we were talking about the cortex instead, then I'd be with you, but we're not. You are not allowed to claim everyone has literally zero knowledge [? · GW] given the state of the evidence. That's just Bayes. You were surprised [? · GW] by the evidence. You should update [? · GW], not double down [LW · GW].

comment by bvbvbvbvbvbvbvbvbvbvbv · 2023-04-01T08:24:35.386Z · LW(p) · GW(p)

I don't like how it sounds but : i think you are missing a lot of biological facts about consciousness and that we're not as clueless as you seem to think. I definitely recommend reading the book "consciousness and the brain" by stanislas dehaene which is basically a collection of facts onbthe topic.

comment by TAG · 2023-03-31T17:10:21.146Z · LW(p) · GW(p)

A person without cerebellum can be perfectly conscious.

You have absolutely no idea that this is the case, neither does anyone else

Putting it a bit more mildly, that depends on what you mean by consciousness.

Replies from: bvbvbvbvbvbvbvbvbvbvbv