Mirror
neurons are one of the most important discoveries in the last decade of
neuroscience. These are a variety of visuospatial neurons which
indicate fundamentally about human social interaction. Essentially,
mirror neurons respond to actions that we observe in others. The
interesting part is that mirror neurons fire in the same way when we
actually recreate that action ourselves. Apart from imitation, they are
responsible for myriad of other sophisticated human behavior and thought
processes. Defects in the mirror neuron system are being linked to
disorders like autism. This review is a brief introduction to the
neurons that shaped our civilization.
Keywords: Autism, neurons, visuospatial
INTRODUCTION
Mirror
neurons represent a distinctive class of neurons that discharge both
when an individual executes a motor act and when he observes another
individual performing the same or a similar motor act. These neurons
were first discovered in monkey's brain. In humans, brain activity
consistent with that of mirror neurons has been found in the premotor
cortex, the supplementary motor area, the primary somatosensory cortex,
and the inferior parietal cortex [Figure 1].
The
mirror neuron system in the human brain. (1) SMA: Supplementary motor
area, (2) PSSC: Primary somato sensory cortex, (3) IPC: Inferior
parietal cortex, (4) VPMA: Ventral premortal area, neurons having mirror
properties, BA: Broca's area, WA: Wernicke's ...
Originally
discovered in a subdivision of the monkey's premotor cortex, area F5,
mirror neurons have later been also found in the inferior parietal
lobule (IPL).[1]
IPL receives a strong input from the cortex of the superior temporal
sulcus (STS), a region known to code biological motion, and sends output
to ventral premotor cortex including area F5.[2]
Neurophysiological
(EEG, MEG, and TMS), and brain-imaging (PET and fMRI) experiments
provided strong evidence that a fronto-parietal circuit with properties
similar to the monkey's mirror neuron system is also present in humans.[3]
As in the monkey, the mirror neuron system is constituted of IPL and a
frontal lobe sector formed by the ventral premotor cortex plus the
posterior part of the inferior frontal gyrus (IFG).
DEVELOPMENT
Human
infant data using eye-tracking measures suggest that the mirror neuron
system develops before 12 months of age, and that this system may help
human infants understand other people's actions. Two closely related
models postulate that mirror neurons are trained through Hebbian or
associative learning.[4,5]
THE HEBBIAN THEORY
Donald
Hebb in 1949 postulated that a basic mechanism for synaptic plasticity
wherein an increase in synaptic efficacy arises from the presynaptic
cell's repeated and persistent stimulation of the postsynaptic cell.
When an axon of cell A is near enough to excite a cell B and repeatedly
or persistently takes part in firing it, some growth process or
metabolic change takes place in one or both cells such that A's
efficiency, as one of the cells firing B, is increased. The theory is
often summarized as “Cells that fire together, wire together.” This
Hebbian theory attempts to explain “associative learning”, in which
simultaneous activation of cells leads to pronounced increases in
synaptic strength between those cells. Such learning is known as Hebbian
learning.
DISCOVERY
In
1990s, a group of neurophysiologists placed electrodes in the ventral
premotor cortex of the macaque monkey to study neurons specialized for
the control of hand and mouth actions.[6]
They recorded electrical signals from a group of neurons in the
monkey's brain while the monkey was allowed to reach for pieces of food,
so the researchers could measure their response to certain movements.
They found that some of the neurons they recorded from would respond
when the monkey saw a person pick up a piece of food as well as when the
monkey picked up the food.
In another experiment, they
showed the role of the mirror neuron system in action recognition, and
proposed that the human Broca's region was the homologue region of the
monkey ventral premotor cortex. Subsequently, a study by Ferrari Pier
Francesco and colleagues described the presence of mirror neurons
responding to mouth actions and facial gestures.[7]
A
recent experiment by Christian Keysers and colleagues have shown that,
in both humans and monkeys, the mirror system also responds to the sound
of actions.[8]
Functional magnetic resonance imaging (fMRI) can examine the entire
brain at once and suggests that a much wider network of brain areas
shows mirror properties in humans than previously thought. These
additional areas include the somatosensory cortex and are thought to
make the observer feel what it feels like to move in the observed way.[9]
Neuropsychological studies looking at lesion areas that cause action
knowledge, pantomime interpretation, and biological motion perception
deficits have pointed to a causal link between the integrity of the IFG
and these behaviors.[10,11] Transcranial magnetic stimulation studies have confirmed this as well.[12]
Mukamel et al.
recorded activity from 1177 brain neurons of 21 patients suffering from
intractable epilepsy. The patients had been implanted with intracranial
depth electrodes to identify seizure foci for potential surgical
treatment. Electrode location was based solely on clinical criteria; the
researchers, used the same electrodes to “piggyback” their research.
The experiment included three phases; making the patients observe facial
expressions (observation phase), grasping (activity phase), and a
control experiment (control phase). In the observation phase, the
patients observed various actions presented on a laptop computer. In the
activity phase, the subjects were asked to perform an action based on a
visually presented word. In the control task, the same words were
presented, and the patients were instructed not to execute the action.
The researchers found a small number of neurons that fired or showed
their greatest activity both when the individual performed a task and
when they observed a task. Other neurons had anti-mirror properties,
that is, they responded when the participant saw an action but were
inhibited when the participant performed that action. The mirror neurons
found were located in the supplementary motor area and medial temporal
cortex.[13]
POSTULATED FUNCTIONS OF MIRROR NEURONS IN HUMANS
Intention understanding
Mirror
neurons are associated with one of the most intriguing aspect of our
complex thought process, that is “Intention understanding”. There are
two distinct processes of information that one can get observing an
action done by another individual. The first component is WHAT action is
being done? And the second more complex component is WHAT FOR or, WHY
(Intention) the action is being done. Figure 2
is a representation of the consequences described. The complex beauty
of the discussed subject is the second component where our mirror
neurons premonate the future action which is yet to occur. Two
neuroscientists[14]
first hypothesized that mirror neurons are involved in intention
understanding, which was later supported by fMRI study. In this
experiment, volunteers were presented with hand actions without a
context and hand actions executed in contexts that allowed them to
understand the intention of the action agent. The study demonstrated
that actions embedded in contexts yielded selective activation of the
mirror neuron system. This indicates that mirror areas, in addition to
action understanding, also mediate the understanding of others’
intention.[15]
These data indicate that the mirror neuron system is involved in
intention understanding, though, it fails to explain the specific
mechanisms underlying it.
In order to explain this hypothesis, a study[16] was carried out on two rhesus macaque monkeys [Figure 3]. The monkeys were trained to perform two actions with different goals. The schematic representation is shown in Figure 2.
In
the first, the monkey had to grasp an object in order to place it in a
container. In the second, it had to grasp a piece of food to eat it. The
initial motor acts, reaching and grasping, were identical in the two
situations, but the final goal oriented action was different. The
activity of neurons was recorded from the IPL, which has long been
recognized as an association cortex that integrates sensory information.
The results showed that 41 mirror neurons fired selectively when the
monkey executed a given motor act (e.g. grasping). However
interestingly, only specific sets (15 neurons) within the IPL fired
during the second goal constrained acts.
Some
of these “action-constrained” motor neurons had mirror properties and
selectively discharged during the observation of motor acts when these
were embedded in a given action (e.g., grasping-for-eating, but not
grasping-for-placing). Thus, the activation of IPL action-constrained
mirror neurons give information not only about, but also on why grasping
is done (grasping-for-eating or grasping-for placing). This specificity
allowed the observer not only to recognize the observed motor act, but
also to code what will be the next motor act of the not-yet-observed
action, in other words to understand the intentions of the action's
agent.
Autism and intention understanding
It
has been postulated and proved by neuroscientists that the inability of
autistic children to relate to people and life situations in the
ordinary way depends on a lack of a normally functioning mirror neuron
system.[17–19]
EEG recordings mu waves from motor areas are suppressed when someone
watches another person's move, a signal that may relate to the mirror
neuron system. This suppression was less in children with autism.
Basically
autism is characterized by two neuropsychiatric abnormalities. First is
the defect in the social-cognitive domain which presents as mental
aloneness, a lack of contact with the external world and lack of
empathy. The second is sensorimotor defects like temper tantrums, head
banging, and some form of repetitive rituals. All these are now
suggested to be because of some anomaly of the mirror neuron
development. One interesting phenomena in autism is the inability to
comprehend abstract reasoning and metaphors, which in normal humans is
subserved by left supramarginal gyrus rich in mirror neurons. Mirror
neuron abnormalities have also been blamed for a number of other
autistic problems like language difficulties, self-identification, lack
of imitation, and finally intention understanding.
However,
the autistic enigma continues as whether the primary deficit in
intention understanding found in autistic children is due to damage of
the mirror neuron system as it is responsible for understanding the
actions of others, or rather there exists more basic defects in the
organization of the motor chains. In other words, the fundamental
deficit in autistic children resides in the incapacity to organize their
own intentional motor behavior.
Emotions and empathy
Many studies have independently argued that the mirror neuron system is involved in emotions and empathy.[20–23]
Studies have shown that people who are more empathic according to
self-report questionnaires have stronger activations both in the mirror
system for hand actions and the mirror system for emotions, providing
more direct support for the idea that the mirror system is linked to
empathy. Functions mediated by mirror neurons depend on the anatomy and
physiological properties of the circuit in which these neurons are
located. Emotional and empathetic activations were found in
parieto-premotor circuits responsible for motor action control. In a
fMRI experiment[24] represented schematically below, [Figure 4]
one group of participants were exposed to disgusting odorants and, the
other group, to short movie clips showing individuals displaying a
facial expression of disgust. It was found that the exposure to
disgusting odorants specifically activates the anterior insula and the
anterior cingulate. Most interestingly, the observation of the facial
expression of disgust activated the same sector of the anterior insula.[25]
In agreement with these findings, the data are obtained in another fMRI
experiment that showed activation of the anterior insula during the
observation and imitation of facial expressions of basic emotions.
The mirror neurons of the anterior insula fires at a basic emotional theme, irrespective of different modality of portrayal
Similar results[26,27]
have been obtained for felt pain and during the observation of a
painful situation, which was involved another person loved by the
observer. Taken together, these experiments suggest that feeling
emotions is due to the activation of circuits that mediate the
corresponding emotional responses.
Evolution of language and mirror neurons
The
discovery of mirror neurons provided strong support for the gestural
theory of speech etymology. Mirror neurons create a direct link between
the sender of a message and its receiver. Thanks to the mirror
mechanism, actions done by one individual become messages that are
understood by an observer without any cognitive mediation. The
observation of an individual grasping an apple is immediately understood
because it evokes the same motor representation in the parieto-frontal
mirror system of the observer. On the basis of this fundamental property
of mirror neurons and the fact that the observation of actions like
hand grasping activates the caudal part of IFG (Broca's area),
neuroscientists proposed that the mirror mechanism is the basic
mechanism from which language evolved.[28]
Humans
mostly communicate by sounds. Sound-based languages, however, do not
represent the only natural way for communication. Languages based on
gestures (signed languages) represent another form of complex,
fully-structured communication system. This hypothesis argues that
speech is the only natural human communication system, the evolutionary
precursor of which is from animal calls. The argument goes as follows:
Humans emit sound to communicate, animals emit sounds to communicate,
therefore human speech evolved from animal calls.
The contradictions of the above syllogism are:
- The anatomical structures underlying primate calls and human speech are different. Primate calls are mostly mediated by the cingulate cortex and by deep, diencephalic, and brain stem structures. In contrast, the circuits underlying human speech are formed by areas located around the Sylvian fissure, including the posterior part of IFG.
- Animal calls are always linked to emotional behavior contrary to human speech.
- Speech is mostly a dyadic, person-to-person communication system. In contrast, animal calls are typically emitted without a well-identified receiver.
- Human speech is endowed with combinatorial properties that are absent in animal communication.
- Humans do possess a “call” communication system like that of non-human primates and its anatomical location is similar. This system mediates the utterances that humans emit when in particular emotional states (cries, yelling, etc.). These utterances are preserved in patients with global aphasia.
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