We often confuse our own actions with those of other people.

That’s an important issue, to say the least. These same cells, Iacoboni proposes, help us distinguish between the self and others. In the epilepsy study, some neurons fired more during action and others fired more during observation.

The mirroring system includes a mechanism that helps the brain record the difference between seeing and acting. “It’s a lovely idea,” says Rizzolati, though he adds that it’s too early to say. Perhaps, he says, we form memory “traces” whenever we see or observe an action. The discovery suggests that memory is embedded in our mirror system, says Marco Iacoboni (University of California, Los Angeles), a leading authority in the field and a co-author of the epilepsy study. The cells showed up unexpectedly in an area known for memory, the medial temporal lobe, as well as in areas where they were expected. Last year, single mirror neurons were recorded in humans for the first time, using in-depth electrodes, in 21 epileptic patients. Because the lit-up areas contain millions of neurons, for humans most researchers speak of a “mirror system,” rather than mirror cells. By last year, a meta-analysis of 139 imaging studies confirmed mirroring activity in parts of the human brain where, in monkeys, mirror neurons are known to reside. As this has hardly been practical in humans, studies have had to rely on imaging, which shows which areas of the brain “light up” in different circumstances. Locating the tiny cells means attaching electrodes deep inside the brain. Like monkeys, humans have mirror neurons that fire when we both perceive and take an action. To tide us over until then, here’s a report on the state of mirror neuron science. Giacomo Rizzolati, the celebrated discoverer, will deliver the Keynote Address at the APS Convention in Washington DC, USA, on May 26, 2011, and report on his latest findings. In 1992, a team at the University of Parma, Italy discovered what have been termed “mirror neurons” in macaque monkeys: cells that fire both when the monkey took an action (like holding a banana) and saw it performed (when a man held a banana).

How many neurons in a monkey brain