[dkfellows]

The brain definitely is able to use not just spiking frequency (which is approximately the same as EEG voltage, though not really) but also spiking patterns to encode information. We've observed some highly interesting phase locked loops showing various higher-order patterns in simulations (we simply don't have fine enough tools to look for the equivalent in the biology).

The variation in neurotransmitters allows for different sorts of activation, typically with different physical parameters (size of activation, time over which it decays) and multiple ways they interact with the other neurotransmitters.

Sparse networks are not understood to anything like the same extent as dense matrices. And another key property that most ML is missing is large numbers of feedback loops. Again, that makes predicting behaviours extraordinarily difficult.