Rare neurons found in monkeys’ brains

A mysterious kind of nerve cell that has been linked to empathy, self-awareness, and even consciousness resides in Old World monkeys. The finding, published May 10 in Neuron, extends the domain of the neurons beyond humans, great apes and other large-brained creatures and will now allow scientists to study the habits of a neuron that may be key to human self-awareness.

“People have been reluctant to say, but want to believe, that these neurons might be the neural correlate of consciousness,” says neuroscientist and psychiatrist Hugo Critchley of the University of Sussex in England. Finding the neurons in macaques, which can be studied in laboratories, “opens up the possibility to study directly the role of these cells,” he says.

An earlier study saw no signs of the cells, called von Economo neurons, in macaques. But while carefully scrutinizing a small piece of a macaque brain for a different experiment, anatomist Henry Evrard  of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany, stumbled across the rare, distinctive cells. About three times bigger than other nerve cells, von Economo neurons have long, fat bodies and tufts of message-receiving dendrites at each end.

Evrard compares the first sighting to seeing the tip of an iceberg. After many additional tests, he and his colleagues concluded that the cells, though smaller and sparser than their human counterparts, were indeed the elusive von Economo neurons.

No one knows what these hulking, strangely-shaped neurons do, but scientists have hints that the job may be very important. One reason for this assumption was that initially, von Economo neurons were found exclusively in big-brained animals with complex social lives: people, great apes, elephants, whales and dolphins, for instance. (A recent sighting in zebras’ brains presented a puzzle.)

In people, von Economo neurons are located in the anterior insula and anterior cingulate cortex, hubs for empathy and self-awareness in the brain. And when the neurons die off, as they do in a rare form of dementia, people lose their capacity for relating to others.

Finding these cells in similar brain regions in monkeys could present a blow to the idea that the cells bestow self-awareness. Although it’s difficult to assess a monkey’s state of mind, macaques don’t reliably recognize themselves in a mirror, one simple test of self-awareness. In the macaque, their location suggests that the von Economo neurons may handle a more basic kind of bodily awareness. The anterior insula helps the brain sense internal body states such as hunger, stress or pain. “The von Economo neurons may play a more sophisticated role in humans,” says Evrard.

The results have clinical implications, says William Seeley of the University of California, San Francisco, who recently discovered that von Economo neurons are selectively lost in people with a certain type of frontotemporal dementia that affects people’s sense of empathy and self-awareness. Learning more about the neurons’ behavior in macaques may help scientists understand what goes wrong in particular human diseases.