Brain takes note when others err: some neurons light up only upon seeing a peer's mistake.

When one monkey sees another monkey messing up, the event ignites a small cluster of nerve cells in the brain that are sensitively tuned to others' failures. The results may help explain why members of another primate species are such exquisite connoisseurs of blame.

"We humans are very sensitive to others' mistakes," says Masaki Isoda of the Okinawa Institute of Science and Technology in Japan. He and his colleagues describe macaques' blunder detectors online August 5 in Nature Neuroscienee.

Catching other people's slipups isn't just schadenfreude. Noting another's lapse, be it a gymnast's step out of bounds or another animal's regurgitation of a poisonous berry, is a good way to learn about the world. "Everybody's life is a bit of a trial-and-error game," says neuroscientist Matthew Shane of the Mind Research Network in Albuquerque, who was not involved in the new study. An ability to sense others' errors helps people see what doesn't work without suffering the consequences firsthand.

Past studies have suggested that certain nerve cells in a brain region called the medial frontal cortex are general error catchers: The cells were thought to fire when a person makes a mistake and also when witnessing someone else err. But by listening in on single nerve cells in macaques, Isoda and his team found that some of these neurons don't seem to care about a personal mistake. Instead, these neurons are exclusively trained on other animals' errors.

Isoda and colleagues taught macaque monkeys to press either a yellow or a green button for a liquid reward. After every two rounds, the two monkeys switched between pushing the button and watching. If the button pusher got the right answer, both monkeys got a treat. But if the answer was wrong, both monkeys were denied.

Electrodes monitoring neuron behavior during the game found a small group of cells that fired away when a monkey watched its partner commit a treat-costing error, but not when the monkey itself messed up. (The researchers knew the observing monkey caught the error because it would not lick its lips in anticipation of a reward.)

People so readily pin the blame on a sister, neighbor or boss when things go wrong that it would make sense for people to also have nerve cells that can make these distinctions, Shane says.

Creating a full sense of another person's error involves other brain systems as well, says neuroscientist Ellen de Bruij n of Leiden University in the Netherlands. "You start to think about this other person and take the perspective of this other person," she says. That kind of sophisticated social reasoning probably involves brain areas outside the medial frontal cortex, she says.