But then neuron imaging came along. We point a digital video camera at a bit of brain, and in that bit of brain each neuron contains a chemical we’ve injected that lights up when the neuron is active. Most often, that fluorescent chemical is responding to the amount of calcium in the neuron’s body, lighting up to an influx of calcium with every spike.2 By filming the bit of brain in sharp focus, we can see all the neurons with our eyes, see their outlines. And we can see which ones light up. It turns out for decades we’ve only been recording the tip of the iceberg. Most neurons we can see in
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