It turned out the spikes were identical in all cases; the only difference was how many spikes were fired. The heavier the weight, the higher the frequency of spikes (figure 1.9). This was Adrian’s second discovery, what is now known as rate coding. The idea is that neurons encode information in the rate that they fire spikes, not in the shape or magnitude of the spike itself. Since Adrian’s initial work, such rate coding has been found in neurons across the animal kingdom, from jellyfish to humans. Touch-sensitive neurons encode pressure in their firing rate; photosensitive neurons encode
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