This blog entry is about one of the most interesting discoveries of the 90's in Neuroscience -- Mirror Neurons -- and a recent research paper that adds to their intrigue.

Mirror neurons are found in the premotor cortex, and what has made them so interesting is that they fire both when the individual performs a goal-directed action and when they watch someone else perform the same action. It is as if the mirror neurons encode an understanding about the intentions of someone else. For example, when my husband reaches for his coffee cup I understand that he intends to take a drink before he even raises the cup to his lips. Neuroscientists think it is the mirror neurons that encode the "understanding" when we watch what others are doing.

A recent study suggests that mirror neurons may do more than just reflect understanding others. A group of Neuroscientists in Tubingen Germany and Parma Italy has shown that mirror neurons may contribute to thinking about how to interact.
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They ran a clever study to find out if the mirror neurons in the ventral premotor cortex encodes how far away from you someone else is performing an action. Is it within reach (peripersonal space) or just out of reach (extrapersonal space)? About half of the neurons responded differently depending on whether the action was performed within reach or just beyond. Some of the mirror neurons fired more if the observed action was within reach, while others fired more when the action was out of reach. This result shows that some mirror neurons are space-selective and are providing information about understanding and whether the action is occurring close or further away.

The clever twist came when the researchers tested what happens to the response of the space-selective mirror neurons when the action is performed within reach but there is a glass barrier that blocks reaching. Would the space-selective mirror neurons continue to encode the distance to the action or would they change because the barrier makes it out of reach?

Surprisingly, the response of some neurons changed. Some mirror neurons that responded when actions were within reached stopped responding when there was a barrier. While other neurons that responded to actions out of reach changed and responded to the near actions performed behind the barrier. As if they "knew" that they could not reach the action because of the barrier.

This study adds an intriguing new dimension to the mirror neuron story. Some mirror neurons are space-selective and encode if an observed action occurs within or outside of reach, like a ruler to measure if the action is close or further away. And some space-selective mirror neurons change their response when a barrier blocks access. As the authors suggest, mirror neurons may contribute to understanding "what others are doing" and these space-selective properties might help to decide "how I might interact with them".

It is not hard to see that these space-selective properties, and the plasticity of these mirror neurons, may be part of the neural system that controls our social interactions.

But why is it that only some space-selective mirror neurons change their responses to the barrier while others continue responding to the distance? Perhaps the neurons that don't change simply code whether the action is within reach and the neurons that do change their response identify that something needs to be done to be able to interact -- move the barrier.

The answer to why there are 2 types of space-selective mirror neurons will have to come from future experiments. But it is intriguing to consider if comparing their responses is the key to deciding on a plan for interacting.

Maybe these are  the neurons that make us think before we act!

For more information about Dr. Casile's research you can look at his website.

Article: Vittorio Caggiano,1 Leonardo Fogassi,2,3 Giacomo Rizzolatti,3 Peter Thier,1 Antonino Casile1*,   Mirror Neurons Differentially Encode the Peripersonal and Extrapersonal Space of Monkeys, Science 17 April 2009: Vol. 324. no. 5925, pp. 403 - 406

1 Department of Cognitive Neurology, Hertie Institute for Clinical Brain Research, University of Tübingen, 72076 Tübingen, Germany.
2 Dipartimento di Psicologia, Università di Parma, 43100 Parma, Italy.
3 Dipartimento di Neuroscienze e Istituto Italiano di Tecnologia, Università di Parma, 43100 Parma, Italy.