Mirror Neurons in Primary Motor Cortex?

The mirror neurons, it would seem, dissolve the barrier between self and others. I call them "empathy neurons" or "Dalai Llama neurons".

-- MIRROR NEURONS AND THE BRAIN IN THE VAT
by V.S. Ramachandran

Everyone knows what mirror neurons are, those darlings of the pop neuroscience world. First observed in the ventral premotor area F5 of macaque monkeys, mirror neurons increase their rate of firing when the animal performs an action, and when the animal watches someone else perform the action (Rizzolati et al, 1996). These "monkey see, monkey do" neurons have taken on a life far beyond their originally postulated role in imitation. However, not everyone believes that mirror neurons can account for all aspects of human language, culture, and social cognition -- from empathy to altruism to autism to aesthetics to

certain listeners' misattribution of anger in the music of avant garde jazz saxophonists (Gridley & Hoff, 2006)

-- as explained by Alison Gopnik [see also The Neurocritic, Mixing Memory, Neurofuture, et al.]:

The idea that these particular cells might underlie a fundamental human impulse [altruism] reflects the emergence of a new scientific myth. Like a traditional myth, it captures intuitions about the human condition through vivid metaphors.

An intriguing new study in the Journal of Neuroscience (Tkach et al., 2007) describes neuronal activity in the primary motor cortex (MI, also called M1) and dorsal premotor cortex of monkeys that looks an awful lot like that of mirror neurons, heretofore confined to ventral premotor cortex and the inferior parietal lobe. The authors conclude that

congruence between observation and action is a general feature of the motor system, even outside of canonical "mirror" areas.

Why is this so interesting? Primarily because of what's going on in M1. If [some] cells there show similar activity during both the execution and observation of actions, what's preventing the arm from moving in the latter case?

If our interpretation of this phenomenon is correct and the monkeys are generating covert motor commands during observation that are congruent with the commands generated during the behavior itself, a natural question is how the dissociation between motor cortical modulation and action occurs. One possibility is that the motor cortical activity we observe is being actively gated by other cortical areas. Results from human EEG studies along with our LFP results suggest that an increase in power in the beta range is associated with inhibition of the excitatory state of the motor cortex (Gilbertson et al., 2005). There is some clinical evidence regarding the origin of this inhibition in patients with frontal lobe damage that exhibit "unwilled" automatic movements (Archibald et al., 2001). These clinical studies suggest that the prefrontal, anterior cingulate, and supplementary motor cortices may contribute the necessary inhibition to prevent triggering of movement commands realized in activated motor and premotor cortical areas. Another possibility is that the motor cortex is part of a more distributed network responsible for movement. Therefore, motor cortical activity alone may not be sufficient to elicit action. Without knowing more about the functional roles of the cells from which we are recording, it is difficult to say anything further about the mechanisms intervening between stimulus and response during the observation phase of the experiment.

In the end, we return to the question, "What's so special about mirror neurons?" We know virtually nothing about the morphological properties of these magical cells (unlike their media cousins, the spindle neurons), so the answer awaits another day (and results from some highly impractical experiments).

References

Gridley MC, Hoff R. (2006). Do mirror neurons explain misattribution of emotions in music? Percept Mot Skills 102:600-2.

Rizzolatti G, Fadiga L, Gallese V, Fogassi L. (1996). Premotor cortex and the recognition of motor actions. Cog Brain Res. 3:131-41.

Tkach D, Reimer J, Hatsopoulos NG. (2007). Congruent activity during action and action observation in motor cortex. J Neurosci. 27:13241-50.

A variety of studies have shown that motor cortical areas can be activated by observation of familiar actions. Here, we describe single-neuron responses in monkey primary motor (MI) and dorsal premotor (PMd) cortices during passive observation and execution of a familiar task. We show that the spiking modulation, preferred directions, and encoded information of cells in MI and PMd remain consistent during both observation and movement. Furthermore, we find that the presence of a visual target is necessary to elicit this congruent neural activity during observation. These findings along with results from our analysis of the oscillatory power in the beta frequency of the local field potential are consistent with previous imaging and EEG studies that have suggested that congruence between observation and action is a general feature of the motor system, even outside of canonical "mirror" areas. Such congruent activity has proposed relevance to motor learning, mimicry, and communication and has practical applications for the development of motor-cortical neuroprostheses in paralyzed patients.