It takes a careful hand because a goldfish’s brain, which looks a bit like a small cluster of lentils, is only half an inch long. “Under a microscope, we exposed the brain and put the electrodes inside,” said Lear Cohen, a neuroscientist and doctoral candidate at Ben-Gurion who performed the surgeries to attach the devices. Each of those electrodes was the diameter of a strand of human hair.

It was also tricky to find a way to perform the procedure on dry land without harming the test subject. “The fish needs water and you need him not to move,” he said. He and his colleagues solved both problems by pumping water and anesthetics into the fish’s mouth.

Once the electrodes were in the brain, they were connected to a small recording device, which could monitor neuronal activity and which was sealed in a waterproof case, mounted on the fish’s forehead. To keep the computer from weighing the fish down and impeding its ability to swim, the researchers attached buoyant plastic foam to the device.

After recovering from surgery, the fish debuted their headgear in an experiment. The goldfish navigated a two-foot-long, six-inch-wide tank. The closer the fish swam to the tank’s edges, the more the navigational cells in their brains lit up.

The fish’s brain-computer helped reveal that goldfish use a system of navigation that is subtly different from what scientists have found in mammals. For humans (and other members of our class), navigational cells specialize in pinging our precise location within our environment and building a map around that spot. Mammals have specialized neurons that create these “you are here” pins in their mental maps; the researchers didn’t find those cells in fish.