Analogue brains

Digital computers and mammalian brains share not only a talent for computation but also a method that favors absolutes. The ones and zeros of a computer are analogous to the all-or-none character of action potentials in the brain. Neurons sum all incoming depolarizing signals until, at a threshold level of depolarization, an action potential fires along the axon. Now, however, Henrik Alle and Jörg Geiger (Max Planck, Frankfurt, Germany) show that the rat brain also uses graded signals for communication.

This analogue or subthreshold processing has been seen in invertebrates, but “in the central mammalian brain it was never shown,” says Geiger. The presumption was that the all-or-none action potentials were the whole story. “To be honest,” says Geiger, “I don't know how this kind of thinking developed.”

Alle and Geiger looked in the...