What do mirror neurons look like?

You have probably heard about mirror neurons, but I bet you don't know what they look like. While we know exactly what Von Economo neurons look like, but know nothing about their activity patterns, the only thing we know about Mirror Neurons is their activity pattern.

Rene Magritte's Mirror (neuron)

Mirror neurons are the neurons in our brains that fire when we move a certain way and also fire when we see other people move in that same way. Exciting studies have shown that some mirror neurons are modulated by the specific intent of the action. That is a particular mirror neuron will fire strongly when a monkey picks up a piece of food to eat it, but fired less strongly when the monkey picks up the food to move it. The same neuron fired with the same intensity difference when the monkey watched someone pick up the food to eat it or pick up the food to move it. (reviewed in Casile et al., 2011)

Pretty exciting stuff, really. But what does it mean? There is some speculation that these neurons are essential for empathy, and for theory of mind. But the real question is even deeper than that.  What does it mean when a neuron fires in response to something (an animal, a motion). Does it mean that that particular neuron encodes that thing? Or does it just mean that that particular neuron is a part of a huge network in which gets activated in response to that thing?

If that single neuron were to die, would it affect your thoughts?

Forest for Trees

Or are there so many neurons activated in the network in response to something that one neuron dying would be like one tree falling in a forest?

Let's leave that question there for a moment.

Another, slightly more answerable question is: What do mirror neurons look like? They are often found in the motor cortex (area F5), but not all the neurons there have mirror properties. So which ones are mirror neurons? where do mirror neurons go? what is their chemical signature? Kraskov et al. (2009) have started to look at these qualities. They anti-dromically stimulated the neurons in F5 to determine if they went through the pyramidal tract or not (which would suggest that they lead to the spinal cord, though this is not certain).  They found that about a quarter of the neurons which follow this tract have mirror properties, and a quarter have anti-mirror properties (meaning they are active during the motion, but are drastically quieted during observation of the motion).  This in an interesting finding, and Kraskov et al. suggest that these anti-mirror neurons might serve to suppress actual motion while one is watching a motion.

Kraskov et al., figure 2 (mirror neuron activated on left, anti-mirror neuron suppressed on right)

In conclusion, some mirror neurons might send information to the spinal cord, but we still don't know how they are morphologically or chemically different from the (non-mirror) motor neurons right next to them.

I hope to soon see studies investigating the cellular, molecular, and physiological characteristics of mirror neurons. 


© TheCellularScale

Kraskov A, Dancause N, Quallo MM, Shepherd S, & Lemon RN (2009). Corticospinal neurons in macaque ventral premotor cortex with mirror properties: a potential mechanism for action suppression? Neuron, 64 (6), 922-30 PMID: 20064397

Casile A, Caggiano V, & Ferrari PF (2011). The mirror neuron system: a fresh view. The Neuroscientist : a review journal bringing neurobiology, neurology and psychiatry, 17 (5), 524-38 PMID: 21467305