The scientist flicked on a laser, filling the rat’s brain with blue light. The rodent, true to its past two weeks of training, scampered across its glass box to a tiny spout, where it was duly rewarded with a drink of water. From the outside, this would appear to be a pretty run-of-the-mill neuroscience experiment, except for the fact that the neurons directing the rat to its thirst-quenching reward didn’t contain any rat DNA. Instead, they came from a human “mini-brain” — a ball of human tissue called an organoid — that researchers at Stanford University School of Medicine had grown in a lab and implanted in the rodent’s cortex months before.
The experiment — part of a study published Wednesday in Nature — is the first describing human neurons influencing another species’ behavior. The study also showed that signals could go the other way; tendrils of human neurons mingled with the rodent brain cells and fired in response to air rustling the rats’ whiskers.