Download mirror neurons: our current understanding

STUDENT PSYCHOLOGY JOURNAL VOLUME II
MIRROR NEURONS: OUR CURRENT UNDERSTANDING
Georgina Mullen
Senior Freshman, Psychology, UCD
[email protected]
ABSTRACT
Mirror neurons are a relatively new phenomena, first observed in the
premotor cortex of macaque monkeys when a number of neurons were
observed to respond both when a monkey performed a goal orientated
task, and when the monkey watched another (human or monkey) perform
that task. A number of researchers have suggested that mirror neurons
also exist in humans. It is proposed that a human mirror neuron system
may contribute to a number of cognitive functions such as action
understanding; ‘theory of mind’, humans’ abilities to infer another’s
mental state through experiences or others’ behaviour; emotion
understanding; imitation; and speech perception. Faulty human mirror
neurons have even been suggested to underpin social impairments such as
those characteristic of Autistic Spectrum Disorder (ASD). However, there
has been much debate regarding the existence and functional roles of
mirror neurons in humans. While there is much literature regarding
human mirror neurons, the majority consist of reviews while few concern
empirical experiments. Additionally concern has been expressed for some
of the experimental methods used in empirical studies. A recent
experiment from Mukamel et al. (2010) is the first of its kind to directly
gather evidence for the existence of mirror neurons in humans and for
their function subserving action understanding. The present review
critically outlines the growth in this controversial field of research, taking
into account the recent direct recording of human mirror neurons, and
what implications this may have on our understanding of social cognition.
INTRODUCTION
Social cognition involves any process among conspecifics, allowing for
individuals to understand the actions, intentions and emotions of others.
Such social abilities are a crucial aspect of human survival and success
(Blakemore et al., 2004). For this reason much research has been devoted
to exploring what mechanisms and processes underlie social cognition.
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This expansive research has resulted in what has been suggested by some
to be the greatest recent discovery in neuroscience; mirror neurons
(Ramachandran, 2000). Ramachandran (2000) suggests that the discovery
of mirror neurons will impact the field of psychology as DNA influenced
biology; that mirror neurons can provide a unifying framework which can
explain a number of humans’ mental abilities. However, such grand
claims have not gone undisputed. While it is largely accepted that mirror
neurons exist in macaque monkeys where they were originally recorded
(Casile et al., 2011), researchers have been skeptical regarding the
existence and proposed functions of a human mirror neuron system
(Dinstein et al., 2008).
MIRROR NEURONS IN MONKEYS
Direct recordings in macaque monkeys have found that both observation
of an action and performance of that action can lead to the discharge of a
subset of neurons, called mirror neurons (Gallese et al., 1996). These
mirror neurons were first observed in the F5 area of the premotor cortex
(di Pelegrino et al., 1992; Gallese et al., 1996) but were later observed in
the inferior parietal lobule (Fogassi et al., 2005; Rozzi et al., 2008) (see
Figure 1).
Further research investigated mirror neuron activity in two
conditions; when a monkey watched a human hand reach and grasp an
object (the visible condition), and when the monkey watched a human
hand reach and then disappear behind a screen (the hidden condition)
(Umilta et al., 2001). Some mirror neuron activation was recorded during
the hidden condition, but only when the monkeys had first seen an object
at the location behind the screen, suggesting that mirror neurons in
monkeys are responsible for action understanding (Umilta et al., 2001).
Furthermore Fogassi et al. (2005) suggested that mirror neurons
contribute to monkeys’ understanding of intention, based on their
experiment in which different populations of parietal neurons fired when a
monkey grasped an object which was subsequently eaten, and when the
monkey grasped an object which was subsequently placed in a box.
A HUMAN MIRROR NEURON MECHANISM
Based on the discovery of mirror neurons in monkeys, researchers began
to question whether a similar mirror mechanism may exist in humans
(Gallese et al., 1996). Experiments that support the existence of a human
STUDENT PSYCHOLOGY JOURNAL VOLUME II
mirror neuron mechanism have mostly used indirect methods which
indicate neural activation. Some of the first evidence was derived from
Hari et al.’s (1998) study which recorded neuromagnetic oscillatory
activity in participants’ precentral cortex. Hari et al. (1998) measured
brain activity using magnetic resonance imaging (MRI) and
magnetoencephalography (MEG) while participants manipulated a small
object with their hand. They observed a significant modification in
neuromagnetic activity when participants’ observed others manipulating
objects. This indicated the existence of an action observation/execution
matching system in the human brain, similar to the one previously
observed in monkeys (Hari et al., 1998). Subsequently a number of
empirical studies investigated the presence and possible functions of a
human mirror system. These appeared to support a mirroring mechanism
within humans located in the frontal and parietal areas of the brain
(Iacoboni et al., 1999; Iacoboni et al., 2005), and in other motor regions
(Hari et al., 1998; Koski et al., 2003; Gazzola & Keysers, 2009) (see Figure
2). Additionally multisensory mirroring mechanisms have been observed
in nonmotor regions such as the amygdala and insula (Hutchison et al.,
1999; Calder et al., 2000; Wicker et al., 2003; Keysers et al., 2004).
These studies used a number of techniques to measure brain
activation, including functional (f)MRI, MEG, electroencephalography
(EEG), positron emission tomography (PET), transcranial magnetic
stimulation (TMS), and lesion studies (observation of neurological
patients). These methods do not allow for a direct and exclusive measure
of mirror neuron activity, which has led a number of researchers to
question the validity of the assumption that a mirror neuron system exists
in humans (Dinstein et al., 2008; Hikock, 2009). While it is clear that these
indirect methods cannot provide definite evidence, they are thought to
correlate well with direct measures (Iacoboni, 2009). Dinstein et al. (2008)
caution the inferences drawn from indirect methods such as fMRI as
responses may not be generated by mirror neurons but by other neural
populations. These interpretations therefore fail to take into account the
fact that mirror neurons in monkeys only make up a small minority of
neurons in these areas and thus the activation may not be from mirror
neurons but rather from neighbouring visual, motor, and visuomotor
neurons (Dinstein et al., 2008).
Recent research from Mukamel et al. (2010) recorded single neuron
activity in humans, rather than using indirect methods. This recording
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during action observation and execution appears to support previous
indirect suggestion that a mirroring mechanism exists in humans. While
past research has focused on recording activity in areas of the brain
homologous to regions containing mirror neurons in monkeys, Mukamel et
al. (2010) detected a mirroring mechanism in areas of the medial frontal
and temporal cortices which were not previously suggested to contain
mirror neurons. Mukamel et al. (2010) did not record in areas where
human mirror neurons had been suggested as placement of electrodes was
determined only by clinical considerations (participants consisted of
patients with pharmacologically intractable epilepsy). Despite Mukamel
et al.’s (2010) influential evidence, some criticisms of a human mirroring
mechanism remain valid. For example, the argument that evidence for
mirror activation in humans does not have key features in common with
the mirror neuron system seen in monkeys (Heyes, 2009). Specifically,
human mirror activation occurs in both homologous areas and in areas
where mirror neurons have not been reported in monkeys (Dinstein et al.,
2008). Additionally the majority of mirror neurons found in monkeys are
responsive to actions on objects, while proposed human mirror neurons
often respond to gestures as well as actions on objects (Hickok, 2009).
POTENTIAL FUNCTIONS OF A HUMAN MIRROR MECHANISM
ACTION UNDERSTANDING
As mirror neurons in monkeys are thought to be the neural basis of action
understanding (Umilta et al., 2001), when researchers began to question
the existence mirror neurons in humans, they did so with the assumption
that if mirror neurons were observed, their function would involve action
perception (Hari et al., 1998; Grezes & Decety, 2001; Gazzola & Keysers,
2009; Mukamel et al., 2010). Further research questioned what other
functions mirror neurons might subserve, including higher social cognitive
abilities such as communication (Rizzolatti & Craighero, 2004), empathy
(Iacoboni, 2009), sensations (Keysers et al., 2004) and emotions (Wicker et
al., 2003). Gazzola and Keysers (2009) used fMRI to record shared neural
activity in individuals during action observation and execution. The areas
of the brain suggested to contain shared voxels include the dorsal
premotor cortex, the supplementary and cingulate motor areas, the
superior parietal lobe, the somatosensory cortices and the cerebellum
(Gazzola & Keysers, 2009) (see Figure 2). While Gazzola and Keysers
STUDENT PSYCHOLOGY JOURNAL VOLUME II
(2009) propose that their findings contribute to the body of evidence that
supports a human mirror neuron system, they do caution that further
research must be carried out as a voxel contains millions of neurons.
Stronger evidence from Mukamel et al.’s (2010) report suggests that
multiple systems in humans may have neural mirroring mechanisms for
both the integration and differentiation of execution and observation of
actions, supporting earlier indirect evidence.
Some recent reviews have argued that most of the current findings
offer little more than a minor, non-specialised contribution to action
understanding, rather than a major specialised one (Heyes, 2010; Hikock,
2009). It has been argued that this weakens the theory behind a human
mirror neuron system (Hikock, 2009). However, Heyes (2009) suggests
that this criticism against a human mirror mechanism is only valid if
mirror neurons are accepted to have evolved for the adaption of action
understanding. For this reason Heyes (2009) champions the alternative
suggestion that mirror neurons are a byproduct of associative learning.
IMITATION AND EMPATHY
A human mirror neuron mechanism has been suggested to mediate
imitation. While this assumption is widely accepted there is little
empirical evidence for mirror neurons’ role in imitation. (Catmur et al.,
2009). Mukamel et al. (2010) observed an opposing pattern of excitation
and inhibition when recording single cell activity during action
observation and execution which they suggested may contribute to an
individual’s sense of being the owner of their action, but may also exert
control on unwanted imitation during observation. Earlier studies also
support this proposition, such as Iacoboni et al.’s (1999) fMRI experiment
in which two areas of the brain; the inferior frontal gyrus and the rostral
part of the posterior parietal cortex showed activation both during
imitation and observation. In addition studies of neuropsychological
patients have offered evidence which suggests a role for mirror neurons in
imitation, although they cannot offer specific locations of mirror neurons
(Catmur et al., 2010). For example, lesions to the inferior parietal lobe (an
area thought to contain mirror neurons) often results in apraxia, a deficit
in imitation and mimicking (Wheaton & Hallett, 2007). Goldenberg and
Karnath (2006) also reported impaired imitation, specifically in hand
gestures after lesions to the left inferior parietal lobe. Accounts of
echopraxia, a condition in which patients suffering from frontal lobe
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damage repeat all observed movements (Luria, 1966), and ‘obstinant
imitation behaviour’, a severely increased urge to imitate observed action,
even when explicitly instructed not to (Lhermitte et al., 1986) support a
mirror neuron function for imitation as both conditions involve frontal
lobe damage and increased imitation (Bien et al., 2009).
Furthermore Iacoboni (2005) suggests that a core cortical circuitry
exists for imitation, and that this system’s interaction with the limbic
system allows individuals the ability for social mirroring and empathy.
Carr et al. (2003) propose that empathy is enabled by a large-scale neural
network composed of the mirror neuron system connected by the insula to
the limbic system. In such a system mirror neurons would support the
simulation of facial expressions observed in others, which would then
activate the limbic system, allowing the observer to experience others’
emotions (Carr et al., 2003). Further research has used fMRI study on
typically developing preadolescents asked to observe and imitate
emotional expressions, and has found that activity in mirror neuron areas
is positively correlated with interpersonal competence and empathetic
concern (Pfeifer et al., 2008).
EMOTIONS AND SENSATION
Higher social cognitive functions such as understanding others’ emotions
and sensations have also been attributed to a neural mirroring mechanism
(Hutchison et al., 1999). Wicker et al. (2003) conducted a study using
fMRI when participants both inhaled odours that induced a strong feeling
of disgust, and observed others’ emotional expressions of disgust. Wicker
et al.’s (2003) findings suggested that observing an emotion activates a
similar neural representation of that emotion. Keysers et al. (2004)
questioned whether watching a movie that depicts touch would activate
the viewer’s somatosensory cortices and found that while the primary
somatosensory cortex was not activated, the secondary somatosensory
cortex was, indicating that the neural mechanisms which subserve the
sensation of touch may also be the basis of humans’ understanding of
touch. While such studies suggest that mirror-like neurons may exist for
emotions and sensations, without direct single cell recording, this
suggestion remains tentative.
STUDENT PSYCHOLOGY JOURNAL VOLUME II
SOCIAL IMPAIRMENTS
Because of the large number of social cognitive functions that have been
associated with human mirror neurons, researchers have proposed that a
dysfunction of the mirror neuron system may result in symptoms of social
impairment, particularly those typical of ASD (Ramachandran &
Oberman, 2006). ASD is characterised by abnormal social development,
poor language capacity, difficulty mimicking others’ actions and strong
obsessional interests from a young age. Many of these have been
associated, albeit weakly, with mirror neurons (Fan et al., 2010).
Conflicting evidence exists regarding mirror neurons’ potential
contribution to ASD, and what little empirical evidence exists is indirect
(Fan et al., 2010). Oberman et al.’s (2005) study suggests that mirror
neurons appear faulty in those with ASD. However, further study has
indicated that mirror neuron function appears to be retained to some
degree in individuals with ASD (Hamilton et al., 2007; Fan et al., 2010).
Ultimately, further research into the neurocognitive models of social
behaviour both within and beyond the dysfunctional mirror neuron
account is necessary (Fan et al., 2010).
CONCLUSION
Evidently this controversial field of research has greatly evolved in the
past decade. Researchers have been critical of some of the inferences made
regarding mirror neurons and higher cognitive functions, which are often
based on little empirical evidence but largely based on assumptions
(Hikock, 2009). Due to these inferences, some have become skeptical of
the existence of a human mirror neuron system. Currently, evidence
suggests that mirror neurons do exist in animals. However, implications
that mirror neurons play a causal role in cognitive functions such as
‘theory of mind’ (Gallese & Goldman, 1998), empathy (Carr et al., 2003),
sensations (Hutchison et al., 1999), language (Rizzolatti & Craighero,
2004) and autism (Oberman et al., 2005) are weak and are far from being
widely accepted by researchers. These proposed connections between
mirror neurons and social functioning is in no doubt fascinating, and may
greatly expand our current understanding of the human brain. Without
repeated, sound empirical evidence, the suggestion that human mirror
neurons cause social cognitive functions remains speculative. Further
knowledge of mirror neurons can also be gained through understanding
their evolution (Heyes, 2009). Mirror neurons can offer further insight into
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the social brain, but to achieve this, speculation and controversy must be
replaced by careful experimentation in order to reach a solid scientific
understanding.
Figure 1. Lateral view of the left hemisphere of the macaque brain. Highlighted areas
contain neurons that discharge during both observation and exectuion of bodily movements
(created from data recorded by Gallese et al., 1996, and Rozzi et al., 2008).
Figure 2. Lateral view of the left hemisphere of the human brain. Highlighted areas indicate
regions in which voxels are shared between action observation and execution (created from
data recorded by Gazzola and Keysers, 2009).
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