The Mirror Neuron system (MNS) is thought to underlie imitation in primates, and has been implicated in Autism Spectrum disorder in humans(1, 2). First observed in Macaques, mirror neurons are classified as units that selectively increase their firing rate both during the execution of a motor action by an individual and while that individual observes the same action performed by another . The interest in the MNS in relation to autism was sparked by the fact that two of its major symptoms are generalized social interaction & communication deficits which would seem to rely on something like the MNS. In order to explore how MNS properties might differ in normal vs. autistic patients, Hugo Théoret has been performing experiments in human subjects. His results suggest that a general deficit of something akin to the mirror neuron system is present in autistic individuals.
(EMG stuff)Dr. Théoret uses two techniques in his research on the mirror neuron system. These are electromyography (EMG) and transcranial magnetic stimulation (TCMS or TMS). EMG measures the voltage difference between ground and the skin nearby a muscle group. Muscle contraction is accompanied by currents which cause a change in voltage or potential. The is sensitive enough to detect voltage changes when an individual even considers a movement involving the measured muscle group. TMS is a coarse method of selectively activating cortical regions(3). The combined use of these tools has allowed Dr. Théoret to use simple experiments to draw interesting conclusions about individuals with Autism.
(TMS-er)Dr. Théoret’s main finding can be summarized by describing two experimental outcomes. First, in normal (non-autistic) individuals, there is a reliable deflection of the electromyogram produced by having the subjects watch a video of an action being performed which involves the measured muscle. For instance, if the right bicep is being measured, there will be an observable deflection of the potential in that muscle when the subject watches a video of an arm lifting an apple. There is also a measurable potential-deflection in that muscle when the proper area of motor cortex is stimulated via TMS. Beyond these individual effects, there is a summation effect such that the deflection is even larger when the subject both observes the video and receives the TMS.
Second, in autistic subjects, there is no deflection of the electromyogram upon a subject’s observation of the above described video. There is in these subjects a potential produced by TMS of the appropriate area, implying that there is no defect in the circuitry to produce such sub-threshold muscle activation. Needless to say there is no summation effect in these subjects.
Dr. Théoret feels that this work implies that understanding of others’ actions is achieved by an individual mapping actions onto their own motor cortex(4). This is an intriguing hypothesis, but there are really two possibilities which both fit with the data. One is as suggested by Dr. Théoret, the other would be that the mirror neuron system alone interprets the intention of the action, and (when possible) maps the action onto the motor cortex. The former possibility would require, for instance, that anybody receiving sufficiently strong TMS would necessarily experience the feeling that they were either observing somebody perform an action or that they were performing the action themselves. This is in keeping with the theory will laid out by Daniel M. Wegner in his book The Illusion of Conscious Will. Without getting too far afield, Dr. Wegner believes that we have a general ability to ascribe agency to observed acts, attributing them to either to ourselves or to others.
The implications of this work are that a defect in the mirror neuron system is responsible for social-interaction pathology in patients with autism. In fact, some researchers believe that defects in the mirror neuron system could lead to all the deficits associated with autism(5). Of course, others feel that such dysfunction cannot be responsible for all the symptoms of autism(6). It remains to be seen whether any definitive explanation of the role of the mirror neuron system in autism will arise, but it is clear that it plays some role in the interpretation of actions.
References
1. Rizzolatti, G., & Craighero, L., (2004) The Mirror Neuron System, Annu. Rev. Neurosci. 27, 169-192.
2. Oberman, L.M., & Ramachandran, V.S., (2007) The simulating social mind: the role of the mirror neuron system and simulation in the social and communicative deficits of autism spectrum disorders. Psychol. Bull., 133, 310-327.
3. Fitzgerald, P.B., Fountain, S. & Daskalakis, Z.J. (2006). A comprehensive review of the effects of rTMS on motor cortical excitability and inhibition. Clinical Neurophysiology 117, 2584-2596
4. Théoret, H., Halligan, E., Kobayashi, M., Fregni, F., Tager-Flusberg, H. & Pascual-Leone, A. (2005) Impaired motor facilitation during action observation in individuals with autism spectrum disorder. Curr Biol. 2005 15, R84-R85.
5. Iacoboni, M., Dapretto, M. (2006) The mirror neuron system and the consequences of its dysfunction. Nat Rev Neurosci. 7, 942-951.
6. Hadjikhani, N., Joseph, R.M., Snyder, J. & Tager-Flusberg, H. (2006) Anatomical differences in the mirror neuron system and social cognition network in autism. Cereb. Cortex. 16, 1276-1282.
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