The debate over whether or not mirror neurons really exist, and whether or not they have been satisfyingly proved to exist in humans rumbles on. One bug-bear is the lack of direct observations of neurons firing in humans due to the inability of investigators to push probes into human brains except in very rare cases where brains are already being investigated for things like Parkinsons Disease. However, in this breezy piece in Nature Reviews Neuroscience, Leonie Walberg reports on some work where recordings of single-cell activity have been made, from several areas of human brain, that behave with mirror neuron properties.
The authors, she says, report activity in patients with implanted electrodes when they observed or executed grasping actions and facial expressions. Recordings were made from medial frontal cortex (which includes the anterior cingulate cortex, an important component of the 'social brain'), medial temporal cortex (which includes the amygdala - another important 'social brain'structure, and the hippocampus).
"Of the 68 'matching cells' detected, 33 showed increased firing during both the observation and execution of a particular action and 21 cells showed decreased firing in both conditions. Interestingly, in the remaining 14 neurons, the firing rate increased in one condition but decreased in the other."
The authors speculate that the decreased firing in some cells, which is not the classical mirror neuron condition, may have a role in suppressing execution of observed actions to ensure that we do not imitate every action we see. However this observation seems destined to further fuel the rather rancorous debate over mirror neurons being all things to all people. After all, mirror neurons were first reported in monkeys - a species that is very poor at imitation, whereas humans, if anything, over-imitate.
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