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

Hi,

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).

Sources:

• Wk 3-3 What is the role of cerebellum for consciousness? - Neural basis of Consciousness
• Are patients without cerebellum conscious? - Neural basis of Consciousness
• Cerebellar granule cells, in contrast to Purkinje cells, are among the smallest neurons in the brain. They are also the most numerous neurons in the brain: In humans, estimates of their total number average around 50 billion, which means that about 3/4 of the brain's neurons are cerebellar granule cells
• The adult human brain is estimated to contain 86±8 billion neurons, with a roughly equal number (85±10 billion) of non-neuronal cells.[40] Out of these neurons, 16 billion (19%) are located in the cerebral cortex, and 69 billion (80%) are in the cerebellum