Proper brain function depends on the balance between the activity of excitatory and inhibitory neurons. In the synthetic brain circuit seen here, engineered light-activated neurons (blue and white) respond to stimulation patterns that mimic excitatory signals from the developing brain. The electrodes in the foreground record the transmission of signals between cells, revealing important information about the development of neural networks. The Tsai Lab studies how rhythms generated by synchronicity between excitation and inhibition are impaired in Alzheimer’s disease.