In 2012, Stevens’s team published evidence that microglia 'eat' synapses, especially those that are weak and unused.[2] These findings pinned down a new role for microglia in wiring the brain, indicating that adult neural circuitry is determined not only by nerve cells but also by the brain’s immune cells. This helped to explain how the brain, which starts out with a surplus of neurons, trims some of the excess neurons away.