Forget about our brains, even the most simple nerve networks defy understanding.

If the artificial intelligence researchers who want to ‘reverse engineer the brain’ as a model for artificial general intelligence want yet another sobering read, they should have a look at Kerri Smith’s recent Nature review of work in many labs on different simple animal models that are vastly less complicated than the human brain (nematodes, fruit fly larvae, zebrafish embryos, etc.). They are bloody complicated:

...neural-network diagrams are yielding surprises — showing, for example, that a brain can use one network in multiple ways to create the same behaviours...Circuits vary in layout and function from animal to animal. The systems have redundancy that makes it difficult to pin one function to one circuit. Plus, wiring alone doesn't fully explain how circuits generate behaviours; other factors, such as neurochemicals, have to be considered.

...Eve Marder of Brandeis University in Waltham, Massachusetts, has been working on a simple circuit of 30 neurons in the crab gastric system...although the circuits of individual animals may look the same and produce the same output, they vary widely in the strength of their signals and the conductance at their synapses