Mirror neurons are one of the most hyped concepts in psychology and neurocience. V.S. Ramachandran famously wrote that they will 'do for psychology what DNA did for biology'. Although recordings from single cells in the brains of monkeys have identified 'mirror' neurons that respond both to the execution of a movement and the observation of another agent performing that same movement, the existence of such cells in humans has, up until now, been inferred only from indirect evidence, particularly brain imaging. Now Roy Mukamel and colleagues have provided what appears to be the first ever direct evidence, using implanted electrode recordings of single cells, for the existence of mirror neurons in humans.
Mukamel's team seized the opportunity for single cell recording provided by the clinical investigations that were being carried out on patients with intractable epilepsy. These patients had electrodes implanted into their brains to identify the loci of their seizures. Mukamel and his colleagues recruited 21 of these patients and had them look at videos of hand gestures or facial expressions on a laptop in one condition, and perform those same gestures and expressions in another condition.
Most of the 1177 cells that were recorded showed a response either to the execution of an action or the sight of that action, not both. However, there was a significant subset of 'mirror' neurons in the front of the brain, including the supplementary motor area, and in the temporal lobe, including the hippocampus, that responded to the sight and execution of the very same actions.
Critics could argue that rather than having mirror properties, these cells were responding to a concept. For example, according to this argument, a cell that responded to the sight of a smile and the execution of a smile, was actually being activated by the smile concept. Mukamel's group reject that argument. They had a control condition in which the words for actions appeared on a screen, rather than those actions being seen or performed. The postulated mirror neurons responded to the sight and execution of an action, but not the word.
Another potential criticism is that the execution-related activity of a postulated mirror neuron is triggered by the sight of one's own action, rather than by motor-output per se. However, this can't explain the mirror neurons that responded both to the sight of a given facial expression and one's own execution of that facial expression (although proprioceptive feedback could still be a potential confound).
Mirror neurons make functional sense in relation to empathy and imitative learning, but a drawback could be unwanted imitation and confusion regarding ownership over actions. The researchers uncovered another subset of cells that could help reduce these risks - these cells were activated by the execution of a given movement but inhibited by the sight of someone else performing that same movement (or vice versa).
'Taken together,' the researchers concluded, 'these findings suggest the existence of multiple systems in the brain endowed with neural mirroring mechanisms for flexible integration and differentiation of the perceptual and motor aspects of actions performed by self and others.'
Roy Mukamel, Arne D Ekstrom, Jonas Kaplan, Maraco Iacoboni, & Itzhak Fried (2010). Single-Neuron Responses in Humans during Execution and Observation of Actions. Current Biology [In Press].