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Mirror neurons are a recently discovery by Giacomo Rizzolatti and his fellow researchers at the University of Parma, Italy in the early 1990's. While studying macaque monkeys with implanted electrodes in their premotor cortex, they stumbled upon a neuron, and then a groups of neurons that fire in response to both action and observation of action. This region of the cortex, usually called F5, is usually implicated in the planning and carrying out of varying motor actions. The mirror neurons in this area are visuomotor neurons that send excitatory signals when observing a specific motor act or when the agent itself performs a similar task. Further research has found that different mirror neurons respond to observation and action stimuli at varying degrees, and show generalization abilities across tasks. For instance, many mirror neurons are goal selective and are excited by motor acts that are aimed at similar ends like bringing food to the mouth. Mirror neurons have also been located in other regions of the cortex since and have been individually isolated and studied in monkeys. They have also been largely implicated in the human cortex, especially at larger neuron network levels. It has only been within this past year that individual neuron recordings in humans have been proven to have mirror properties and much further research needs to be compiled in this area.
Physiology and Localization
The sensorimotor cortex:
The picture highlights the key areas involved in cortical action and sensory processing. It is here that most mirror neurons, their connections and their projections, can be found. They are mainly associated as apart of visuomotor neuron systems, although they connect to all sorts of sensory and motor processing neurons This shows the areas within the human cortex and while the macaque cortex differs significantly, however the f5 and other regions have been locationally and histologically correlated in both brains (see below lateral views of A) macaque and B) human motor cortices). The light blue premotory cortex seen above houses F5, which is located more ventrally.
The dorsolateral prefrontal associative cortex is usually implicated in motor planning and regulation involving an integration of sensory and intellectual information. It is also where much of the working memory activity takes place. The premotor cortex, mainly made up of Broadman's 6, is highly involved in motor planning. The supplementary motor area is also said to aid motor planning, however at a higher level, involving memory based movement at sequential levels. The primary motor cortex is the main initiator of all movements with its somatotopic mapping of the body. The somatosensory cortex also has a somatotopic representation of the body, however it processes sensory input and not motor actions. The posterior parietal cortex, areas 5 and 7 are largely associative, meaning they act as integrators of varying sensory information to conceptualize the localization of the body within and in relation to the external world. This information is key to making appropriate actions. These areas are all deeply interconnected and work together to piece together our experience to allow us to act in the world.
Mirror neurons are triggered by visually detected object-directed action. The generalization of detection varies significantly in these neurons but
"the actions most represented among those activating mirror neurons were grasping, manipulating and placing actions" (Gallese et al).
The categories of "strictly congruent" and "broadly congruent" neurons have been made to help specify a particular neuron's characteristics. "Strictly congruent" neurons make up about one third of mirror neurons found in F5. They detect more specific observed and executed action that correlate both in terms of goal and means of reaching a goal. "Broadly congruent" neurons make up the other two thirds of mirror neurons. They generalize more between actions towards a goal and are likely to be more characteristic of the human mirror neuron system.
The F5 area has been found to have strong hand and mouth mapping, usually dorsally and ventrally found respectively. Mouth mirror neurons are either ingestive or communicative in nature. Ingestive mirror neurons are related to the eating process, whether in biting or chewing etc... and are much more prevalent in both monkeys and humans. Communication mirror neurons on the other hand are obviously evolutionarily new and are much more developed in humans. Evidence for their newness in the evolutionary linkage is that most communicative neurons are also sensitive to ingestive information perceived.
Some neurons in the lateral hand areas of the F5 have been demonstrated to be activated for tool use in monkeys. Ferrari et al. named "tool-responding mirror neurons" that are specifically activated when manipulating particular tools suited for particular purposes and the observation of similar activity. In their study they used 2 monkeys to measure the activity of 209 neurons located in the implicated area, from this sample 208 neurons showed mirror properties. Of the mirror neurons found, 42 responded to tool usage, and strongly reacted to observation of an experimenter using a tool appropriately. Most of these were related to both mouth and hand usage of the tool. The majority also responded strongest with the use of a stick opposed to pliers.
The newest region implicated in mirror neuron activity has been the rostal part of the inferior parietal lobe, also commonly called the PF or 7b area (see below image in pinkish). Of it's almost entirely sensory activated neuron population, about 50% also respond to action execution. Also implicated in this emerging "mirror neuron circuit" between differing areas is the STS or superior temporal sulcus area (also see below). Although this area shows no motor activity responses, it is heavily connected to the PF which in turn is connected to the F5 region of the premotor cortex. The STS is otherwise known for being activated when observing biological motion.
This same cortical circuit purposed in the mirror neuron circuit hypothesis is also known for coding intrinsic properties of objects and translating this information into the output necessary to perform actions concerning the object. "In monkeys, the transformation of an object's intrinsic properties into specific grips takes place in a circuit that is formed by the inferior parietal lobule and the inferior premotor area (area F5). Neurons in both these areas code size, shape and orientation of objects, and specific types of grip that are necessary to grasp them. Grasping movements are coded more globally in the inferior parietal lobule, whereas they are more segmented in area F5" (Jeannerod et al). It seems that mirror neurons aid this process and connect it more intimately to the agent and it's understanding of movement.
All this evidence is primarily used to suggest that mirror neurons are essential to action intention understanding in primates. The mechanism purposed in this case is that once a motor act is recognized visually, its understanding is based within a simulation of the actual action properties, in which the agent is already fully informed of its meaning. This therefore allows the monkey to apply intentions in terms of goals it understands to the agent outside of itself. It embeds a simple visual stimulus in a wealth of context known to the agent personally. A learning advantage has also been largely discussed as a key use of these neurons. This is especially noticeable when considered with the tool research discussed.
For about 15 years after the discovery of mirror neurons in monkeys, no individual neurons were isolated in order to prove that mirror neuron exist in the cortex. However in the search for the mirror neuron system in humans, much indirect evidence has been accumulated in attempt to both exemplify their purposed effects in our brains, and to understand such influences. In the search the F5, PF and other neural regions (far larger than the individual neuron level) have been shown to respond to both observation and action motor stimuli. Much of the evidence found has depended greatly on a the newest MEG, fMRI and TMS technology.
Trans-cranial magnetic stimulation (TMS) studies have been particularly helpful in revealing evidencial support for a mirror system in the human brain. The particular process used, first, stimulates a side of the motor cortex electrically with a specific intensity. This will then produce a motor-evoked potential (MEP) measurable in the contralateral muscle activated. The amplitude of the MEP is highly influenced by the context of the behavioral situation. What most studies have come to show is that observation of tasks often heightens the amplitude of the MEP created by motor cortex stimulation. Fadiga et al. (1995) found such results in human participants observing transitive and intransitive movement. Not only does this show possible mirror neuron activity, but it also would follow that human mirror neurons would be capable of far more generalizability because they would be reacting to intransitive movement (movement without a goal or specific task orientation) a capability not found in the monkey mirror neurons (Rizzolatti and Craighero 2004). This generalization of movements nature beyond the context of mere purpose driven action has been supported repeatedly by multiple studies and weighs heavily on theoretical conceptions concerning human and monkey differences. In other studies is has been shown that intransitive movements are processed by more frontal areas like the premotor and supplementary areas, sometimes implicated in preparation to act.
Results like these have lead to a highly favored cortical hypothesis for mirror neuron activity posing "Facilitation of the MEPs during movement observation may result from a facilitation of the primary motor cortex owing to mirror activity of the premotor areas, to a direct facilitating input to the spinal cord originating from the same areas, or from both" (Rizzolatti and Craighero 2004).
In other studies is has been shown that intransitive movements are processed by more frontal areas like the premotor and supplementary areas, sometimes implicated in preparation to act. Another distinct characteristic of a human mirror neuron system found in TMS experiments is a sensitivity to movements carried out before or in preparation of the performed an action. This has lead to hypotheses on imitation learning to be discussed later. There seems to be a greater timing and planning mechanisms involved in the human mirror neuron system, probably due to its interconnectedness with areas like the dorsolateral prefrontal cortex involved with higher level planning and timing and are also not present, or functionally similar in monkeys.
Brain imaging studies offer a much more complete image of what the mirror neuron system in humans looks like (see above for indicated areas). Various brain imaging techniques, especially the fMRI have implicated a few key regions as the human mirror neuron system. Rizzolatti and Craighero specifically point out, "the rostral part of the inferior parietal lobule and the lower part of the precentral gyrus plus the posterior part of the inferior frontal gyrus (IFG)." The inferior parietal region here corresponds with the PF reference before in monkeys.
The f5 region in humans is the highlighted box in the above image, and although it is known as a mirror neuron region the lines between and roles of ventral and the lateral sections are debated. More confusion arises from f5 because it is usually said to be a purely language devoted area in the human cortex. What brain imaging studies tell us is that this is not so, that much of this area is used for motor actions, especially of the hands and mouth. At this point in time is is usually agreed that the PMv (ventral premotor cortex) is congruent with the monkey f4 and the area 44 is congruent to the monkey f5.
What this greater image has given researchers is the ability to test more functional aspects of purposed mirror neuron systems. A great example of this is an fMRI study done by Iacoboni et al. (1999) at UCLA. He measured activity in the human PMv and 44 regions when observing actions between species. He had the participants watch other humans, dogs and monkeys performing ingestive tasks, and communicative tasks (for the dog condition this would be barking). What is showed was that activation of mirror neuron areas was strongest when observing humans, or when observing ingesting activities like biting and chewing. It is particularly interesting that the mirror neuron regions were only barely activated by the dog barking communicative condition. This study highlights the self-referential quality of mirror neurons.
In April 2010 it was announced that Dr. Itzhak Fried and Roy Mukamel of UCLA finally measured direct mirror neuron activity in humans. Their study tested already implanted electrodes in Dr. Fried's epilepsy patients in various areas of the cortex. Not only did they find mirror neuron activity in the previously implicated motor system, but also in areas that process memory and vision. They found 11 particular neurons in their search that behaved exactly like the broadly congruent mirror neurons in monkeys. Within this sample many had activity specialization, that fired selectively for facial of hand oriented movements (The types of movements they tested).
From their research they found that human mirror neuron activity was more complex than those recorded in monkeys. Often when patients observed actions, mirror neurons had much inhibition working on them. Researchers purpose that this is so we better differentiate between observed action and our own actions and that we don't activate movement every time we observe movement. Also purposed by Mukamel et al. is the possibility for "anti-mirror neurons" that may play a significant role in our ability to simulate motion without movement or "in our minds eye." These neurons were mainly found in the SMA and were highly activated during execution of movement, and seemed be been inhibited during observation tasks. Such new ideas could only be discovered through measurement of individual neuron activity because brain imaging like fMRI are blind to inhibitory and excitatory interactions.
Lastly, because Mukamel et al. could not choose where to look for mirror neuron activity, they were forced to test areas not usually directly associated as mirror neuron hubs, like the f5 or the PF. What they did find because of this was that mirror neurons, at least in humans, are more likely to be widespread within the brain, existing all over the cortex but only existing as a minority of neurons in each area.
This research is still in its infancy and still depends greatly on already started research based in patient treatment that involves electrode implants. While this will surely provide obstacles for ongoing research, the potential power of the information accumulated outweighs the difficulty. Already the conceptions of how our brain evolved and how it is able to do all that it does is being greatly challenged. The borders of separation between all the different cortex areas, whether premotor or somatosensory as previously delineated, are blurring. New ideas of consciousness and self understanding are being revealed to us through this research. It also abolishes the idea that the mirror neurons system is especially localized to areas like f5 and PF in humans.
Compelling New Hypotheses
The mirror neuron hypothesis of autism, now referred to as autism spectrum disorder (ASD), states that autism is due to a mirror neuron dysfunction of some kind. The reasoning behind this is that autism symptoms are most prevalent within social situations and many believe this to be because they lack a "theory of mind" (TOM). A theory of mind is based in a person's ability to understand that other people have minds and intentions beyond their own. Without this understanding of the world most of our daily social interactions could not be possible, as is seen in those with autism. This connects to mirror neurons because mirror neurons are purposed to be an essential attribution of a properly functioning TOM. Mirror neurons allow us to connect others actions to our own and then infer or directly understand the others intention. From this humans developed the ability to recognize that others intention cannot be our own because they usually act differently from us. Through this process we have a theory of mind. If mirror neurons are no-existent or deficient in some way, then essentially no proper TOM can be developed.
There is heavy debate of this theory. Much of this debate is because not all people believe that a missing TOM can explain all of the manifestations of autism found. Also there seems to be conflicting evidence on both sides of the issue. There are many studies looking into this hypothesis and supporting it, and some big neuroscience names, like VS Ramachandran behind its popularity.On the other hand there are multiple studies coming out saying that either its simply not true, or that it is not telling the whole story of autism.
Role in Evolution
Evolution of Language
Research on communicative mirror neurons are of especially significant interest to language researchers. By distinguishing between human language mirror neurons and those found in monkeys we could gain great insight into how it developed evolutionarily.
Hebbian learning is based on the basic neurological principle of "fire together wire together"
Glossary of terms
Autism or Autism Spectrum Disorders (ASD)
Broadly Congruent Mirror Neurons
Motor-Evoked Potencials (MEP's)
Strictly Congruent Mirror Neurons
Superior Temporal Sulcus (STS)
Transcranial Magnetic Stimulation(TMS)
"Cells that Read Minds" NY Times.
The news release article describing the discovery of mirror neurons in humans.
Have an hour? Listen to Marc Iacoboni, mirror neuron researcher at UCLA.
Or watch a NOVA special episode how mirror neurons make us specially human.
Really interested in the mirror neuron hypothesis for autism? Heres an in depth look.
True and False
Gallese, V., L. Fadiga, L. Fogassi, and G Rizzolatti (1996). Action Recognition in the Premotor Cortex. Brain 119: 593-609
Iacoboni M, I. Molnar-Szakacs, V. Gallese, G. Buccino, JC. Mazziotta, et al. (2005) Grasping the Intentions of Others with One's Own Mirror Neuron System. PLoS Biol 3(3): e79. doi:10.1371/journal.pbio.0030079
Iacoboni M, RP. Woods, M. Brass, H. Bekkering, JC. Mazziotta, and G. Rizzolatti (1999). Cortical mechanisms of human imitation. Science 286:2526-8.
Jeannerod M., MA. Arbib, G. Rizzolatti, and H. Sakata (1995). Grasping objects: the cortical mechanisms of visuomotor transformation. Trends in Neuroscience 8(17): 324-320.
Kandell, ER., JH. Schwartz, and TM. Jessell (2000) eds.
Principles of Neural Science
. 4th ed. New York City: McGraw-Hill Companies, Inc.
Keysers, C and V. Gazzola (2010). Social Neuroscience: Mirror Neurons Recorded in Humans. Current Biology 20(8)
Lingnau A., B. Gesierich, and A. Caramazza (2009). Asymmetric fMRI adaptation reveals no evidence for mirror neurons in humans. PNAS 106(24): 9925-9930.
Ramachandran V.S., and LM. Oberman (2006). Broken mirrors: a theory of autism. Scientific American, 5: 62-9
Rizzolatti G., and L. Craighero (2004). The Mirror-Neuron System. Annual Rev. Neurosci. 27 169-192
Schulte-Ru¨ ther M., HJ. Markowitsch, GR. Fink, and M Piefke (2007). Mirror Neuron and Theory of Mind Mechanisms Involved in Face-to-Face Interactions: A Functional Magnetic Resonance Imaging Approach to Empathy.Journal of Cognitive Neuroscience 19(8): 1354–1372.
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