First, they obtained mice lacking the serotonin-1A receptor gene, as well as some of the mice’s littermates that had the gene. They then operated on each of the mice to implant electrodes in the two hippocampi of its brain. The hippocampus was formerly thought to be involved only in learning and memory, but now it seems to be implicated in schizophrenia, bipolar disorder, depression, and anxiety as well. What’s more, the hippocampus is normally rich in serotonin-1A receptors. The scientists were also careful to implant the electrodes in such a way that they could record electrical signals from individual neurons in each mouse’s hippocampi.