What We Know Currently about Mirror Neurons

Abstract

Mirror neurons were discovered over twenty years ago in the ventral premotor region F5 of the macaque monkey. Since their discovery much has been written about these neurons, both in the scientific literature and in the popular press. They have been proposed to be the neuronal substrate underlying a vast array of different functions. Indeed so much has been written about mirror neurons that last year they were referred to, rightly or wrongly, as "The most hyped concept in neuroscience". Here we try to cut through some of this hyperbole and review what is currently known (and not known) about mirror neurons.

Full text

MinireviewWhat We Know Currently about Mirror
Neurons
J.M. Kilner and R.N. Lemon
Mirror neurons were discovered over twenty years ago in
the ventral premotor region F5 of the macaque monkey.
Since their discovery much has been written about these
neurons, both in the scientific literature and in the popular
press. They have been proposed to be the neuronal
substrate underlying a vast array of different functions.
Indeed so much has been written about mirror neurons
that last year they were referred to, rightly or wrongly, as
‘‘The most hyped concept in neuroscience’’. Here we try
to cut through some of this hyperbole and review what is
currently known (and not known) about mirror neurons.
Introduction
Mirror neurons are a class of neuron that modulate their
activity both when an individual executes a specific motor
act and when they observe the same or similar act performed
by another individual. They were first reported in the
macaque monkey ventral premotor area F5 [1] and were
named mirror neurons in a subsequent publication from the
same group [2]. Ever since their discovery, there has been
great interest in mirror neurons and much speculation about
their possible functional role with a particular focus on their
proposed role in social cognition. As Heyes [3] wrote
‘‘[mirror neurons] intrigue both specialists and non-special-
ists, celebrated as a ‘revolution’ in understanding social
behaviour . and ‘the driving force’ behind ‘the great leap
forward’ in human evolution.’’. Indeed so much has been
written in both peer-review literature and elsewhere about
mirror neurons and their proposed functional role(s) that
they have recently been given the moniker ‘‘The most hyped
concept in neuroscience’’ [4].
For us, the discovery of mirror neurons was exciting
because it has led to a new way of thinking about how we
generate our own actions and how we monitor and interpret
the actions of others. This discovery prompted the notion
that, from a functional viewpoint, action execution and
observation are closely-related processes, and indeed that
our ability to interpret the actions of others requires the
involvement of our own motor system.
The aim of this article is not to add to this literature on the
putative functional role(s) of mirror neurons, but instead to
provide a review of the studies that have directly recorded
mirror neuron activity. To date, there have been over 800
published papers on mirror neurons (from a PubMed search
using: ‘‘mirror neuron’’ OR ‘‘mirror neurons’’). Here, we
restrict our attention to only the primary literature on mirror
neurons. Mirror neurons were originally defined as neurons
which ‘‘discharged both during monkey’s active movements
and when the monkey observed meaningful hand move-
ments made by the experimenter’’ [2]. Thus, the key charac-
teristics of mirror neurons are that their activity is modulated
both by action execution and action observation, and that
this activity shows a degree of action specificity. This distin-
guishes mirror neurons from other ‘motor’ or ‘sensory’ neu-
rons whose discharge is associated with either execution
or observation, but not both. It also distinguishes mirror
neuron responses from other types of response to vision of
objects or other non-action stimuli. As the activity of mirror
neurons cannot yet be unambiguously detected using
neuroimaging techniques, we have excluded human and
non-human primate imaging studies from this review. We
therefore focus on the 25 papers [1,2,5–27] that have
reported quantitative results of recording mirror neurons
or mirror-like neurons in macaque monkeys since 1992
(Table 1).
Mirror neurons were first described in the rostral division of
the ventral premotor cortex (area F5) of the macaque brain,
and have subsequently been reported in the inferior parietal
lobule, including the lateral and ventral intraparietal areas,
and in the dorsal premotor and primary motor cortex. But
despite the large array of areas in which mirror neurons
have been reported, the majority of mirror neuron research
has studied the activity of mirror neurons in area F5 (15/25
papers; Figure 1A).
Mirror Neurons in Ventral Premotor Region F5
Of the 15 papers reporting mirror neuron activity in area F5,
11 provide details of the number of mirror neurons recorded
when observing the experimenter (not a video) reaching and
grasping objects with their hand. On average, 33.6% of
neurons recorded in F5 have been described as mirror
neurons when the monkey observed hand actions performed
by a human experimenter in front of them (ranging from
9.8–49.8%; Figure 1A,B). It is of note that the percentage of
mirror neurons reported appears to increase as a function
of time. This most likely reflects a sampling bias during
data collection.
The first three papers [1,2,18] described the basic pro-
perties of mirror neurons, and their percentages are low
compared with later studies. The more recent papers, in
general, have investigated modulations of mirror neuron
activity with some form of task manipulation. The metho-
dological approach of these later papers is to first select neu-
rons based on their motor properties (for example, selectivity
for grasping) and then investigate the responses of this
neuronal population to observed actions. This subtle change
in the experimental strategy might explain the apparent
increase in the percentage of mirror neurons in F5 as a func-
tion of time. Some investigators have avoided the sampling
bias based on mirror properties by studying identified pyra-
midal tract neurons in area F5, selected on the basis of their
antidromic response and not for their properties during
action execution or observation [18]. A large proportion of
pyramidal tract neurons in F5 and in M1 appear to show
mirror-like responses (Table 1).
The three early papers [1,2,18] provided details about the
relative selectivity of mirror neuron discharge during action
execution and observation. On average, 48.9% of mirror
neurons were classified as broadly congruent. Some mirror
neurons discharged for only one action type, such as
grasping, during both execution and observation, but
showed no specificity for the type of grasp, for example
Sobell Department of Motor Neuroscience and Movement Disorders,
UCL Institute of Neurology, London, UK, WC1N 3BG.
E-mail: j.kilner@ucl.ac.uk
Current Biology 23, R1057–R1062, December 2, 2013 ª2013 Elsevier Ltd. Open access under CC BY license. http://dx.doi.org/10.1016/j.cub.2013.10.051

precision grip or whole hand prehension. Others discharged
for more than one type of observed action, for example
grasping and holding. One of the three papers [2] describes
a further category of mirror neurons, strictly congruent mirror
neurons; these are defined as mirror neurons that respond
selectively to one action type, such as precision grip, during
both action execution and observation, and are reported as
constituting 31.5% of mirror neurons recorded. Two of the
three papers [2,18] report a further category of neuron in
F5 that discharged during action observation but not during
action execution; on average these neurons, which would
not be included as mirror neurons, have been reported as
making up 5.1% of the neurons in F5.
Further neuroanatomical studies of area F5 have revealed
three interconnected sub-divisions [28]. The sub-division in
which mirror neurons are located is suggested to be on the
convexity of the precentral gyrus, adjacent to the inferior
limb of the arcuate sulcus, and referred to as area F5c.
This is distinguished from area F5p (posterior), which is
reciprocally connected both with posterior parietal area
AIP and primary motor cortex M1, and from area F5a (ante-
rior) in the depth of the sulcus, which has prefrontal
connections [29].
Two studies [7,9] have been reported that have shown that
F5 mirror neurons discharged both to the observation of an
action performed in front of the monkey by the experimenter
and to videos of the same action. On average 26.9% of F5
neurons discharged when the monkey observed a video of
a grasping action. One of the two studies [7] reported the
relative number of mirror neurons that discharged to real
and to videoed actions: 46.4% of neurons in F5 that re-
sponded to an executed action also responded when
observing a real action, whereas only 22.3% responded
when observing a videoed action. Although fewer mirror neu-
rons responded when the monkey was observing the video
of an action, for those mirror neurons that did discharge,
there was no significant difference in the pattern or rate of
mirror neuron discharge between real and videoed actions.
Two of the early papers [2,18] on mirror neurons reported
that they could not find any neurons that discharged when
monkeys observed an object being grasped with a tool. Sub-
sequently, two studies [12,19] showed that mirror neurons
did respond to such a tool-based action. In both these latter
cases, however, the monkeys had received a high exposure
to tool use during the training period prior to the recordings.
One study [12] reported that 20% of F5 neurons were tool-
responding mirror neurons, whereas the other reported the
much higher percentage of 66.6% [20]. This high percentage
most likely reflects a combination of a small sample size
(n = 27) and strict inclusion criteria.
Two papers [15,16] have reported that neurons in F5 re-
sponded to the sound of an action: so-called auditory mirror
neurons. On average, 17% of F5 neurons have been reported
to have auditory properties (12.7% and 21.3%, respectively,
in the two papers). Four papers [6–8,23] have reported that
mirror neurons not only discharged during action observation
but that their firing is further modulated by different factors:
occlusion [23], relative distance of observed action [8],
reward value [6] and the view point of the observed action
[7]. Umilta et al. [23] showed that 19/37 mirror neurons dis-
charged even when the observed action was occluded or
hidden from the observer, demonstrating that direct vision
of the action was not necessary to elicit mirror neuron
discharge. Caggiano et al. [7] showed that 149/201 mirror
Table 1. Proportion of neurons recorded in macaque premotor cortex (area F5) and posterior parietal cortex that showed mirror neuron properties.
Reference Recording area No. neurons No. mirror % mirror
1
Action specificity Observed effector
Bonini et al. [5] F5 154 36 23.4% y Hand
Caggiano et al. [6] F5 299 149 49.8% n Hand
Caggiano et al. [8] F5 219 105 48% n Hand
Caggiano et al. [7] F5 224 123 54.9% n Hand (video)
Caggiano et al. [9] F5 785 247 31.5% n Hand (video)
Ferrari et al. [11] F5 485 130 26.8% y Mouth
Ferrari et al. [12] F5 209 52 24.9% y Hand
Gallese et al. [2] F5 532 92 17.3% y Hand
Kohler et al. [16]
2
F5 497 63 12.7% y Auditory
Kraskov et al. [17] F5 64 31 48.4% y Hand (PTNs)
di Pellegrino et al. [1] F5 184 18 9.8% y Hand
Rizzolatti et al. [18] F5 300 60 20% y Hand
Rochat et al. [19] F5 282 92 32.6% y Hand
Umilta et al. [23] F5 220 103 46.8% y Hand
Bonini et al. [5] IPL 120 28 23.3% y Hand
Fogassi et al. [13] IPL 165 41 24.8% y Hand
Rozzi et al. [20] IPL 423 51 12% y Hand
Shepherd et al. [21] LIP 153 30 19.6% n Eye-gaze
Dushanova and Donoghue [10] M1 303 105 34.6% y Reaching
Tkach et al. [22] M1 829 581 70.1% y Tracking arm
Vigneswaran et al. [24] M1 132 77 58.3% n Hand (PTNs)
Tkach et al. [22] PMd 128 77 60.1% y Tracking arm
Ishida et al. [14] VIP 541 48 8.9% y Bimodal tactile/visual
Fujii et al. [27] PM
3
148 _ 3–14%
4
n Hand
IPS
5
148 - 10–42%
4
n
1
This column indicates if mirror neurons were tested for any form of action specificity.
2
These data were further analysed by Keysers et al. [15].
3
Including area F5.
4
See text.
5
Included anterior bank of the intraparietal sulcus (IPS). PTN, pyramidal tract neuron.
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neurons discharged preferentially for one or more of three
different views of the same action (at 0, 90 and 180 degrees).
Sixty of these neurons showed a preference for only one
view point.
Caggiano et al. [8] also found that F5 mirror neurons have a
preference for whether an observed action occurred in peri-
personal or extrapersonal space: 27/105 mirror neurons dis-
charged preferentially when the observed action occurred in
the monkeys extra-personal space, whereas 28/105 mirror
neurons discharged preferentially when the observed action
occurred in the monkey’s peri-personal space. The remain-
ing 50 mirror neurons showed no preference. Caggiano
et al. [6] reported that mirror neuron discharge is modulated
by the value of the reward associated with the action: they
showed that 40/87 mirror neurons responded more when a
rewarded object was grasped, while 11/87 responded
more when observing an action to a non-rewarded action.
The remaining mirror neurons showed no preference.
One study [17] recorded from 64 neurons in F5 that were
identified as pyramidal tract neurons. Thirty-one of these
neurons were classified as mirror neurons, with 14/31 mirror
neurons showing the ‘classic’ facilitation response during
the action observation condition. Compared with baseline,
the activity of the remaining 17 mirror neurons was signifi-
cantly suppressed during action observation. The inclusion
of these ‘suppression mirror neurons’ [8,17,24,25] clearly
changes the overall proportion of neurons responsive during
action observation.
In a recent study, Maranesi et al. [30] compared multiunit
activity responses in areas F5, F4 (premotor regions) and
F1 (primary motor cortex, M1). They reported a higher pro-
portion of recording sites showing mirror type responses in
area F5 (particularly in area F5c), compared with area F4
(caudal part of the ventral premotor cortex) and with F1. In
addition, they reported that in penetration sites where they
identified mirror responses, they were rarely able to evoke
movement using intracortical microsimulation and argued
that this might be due to presence of suppression mirror
neurons, as first identified by Kraskov et al. [17].
One interesting study [27] looked at activity in premotor
and parietal cortex neurons of the left hemisphere of a
Japanese macaque monkey, either while it observed another
monkey sitting opposite making reach-to-grasp movements
for food rewards, or when it performed similar actions itself.
Many neurons in both cortical areas were active during the
other monkey’s movements, with the proportion varying
across different actions (Table 1). Premotor cortex neurons
showed a distinct preference for movements involving the
observed monkey’s right arm and hand, and showed a
similar preference for the monkey’s own right-sided actions.
Mirror Neurons in the Inferior Parietal Lobule
Four papers [5,13,20,25] have reported neuronal activity re-
corded in the inferior parietal lobule that the authors have
described as that of mirror neurons (Figure 1B). None of these
papers explicitly specifies the percentage of neurons that
were classified as mirror neurons; for three of these papers,
however, we were able to estimate from the numbers in the
papers that the average percentage of sampled neurons
that were mirror neurons was 20% (41/165 Fogassi et al.
[13]; 28/120 Bonnini et al. [5]; 51/423 Rozzi et al. [20]).
Two papers [5,13] describe the modulation of mirror
neuron activity in the inferior parietal lobule by the overall
goal of the observed action. Here monkeys observed an
experimenter reaching for and grasping an object and either
placing it in the mouth (eating) or placing it in a container
(placing). On average 53% of mirror neurons had a signifi-
cantly greater firing rate when the monkey observed the
‘eating’ compared with the ‘placing’ condition, 17% had a
significantly greater firing rate for ‘placing’ compared with
‘eating’. The remaining 30% showed no difference between
the two conditions. Yamazaki et al. [25] reported examples
of mirror neuron activity in macaque area inferior parietal
lobe; these neurons responded to the same action carried
out in rather different contexts, suggesting that they are
involved in encoding the ‘semantic equivalence’ of actions
carried out by different agents in different contexts.
Rozzi et al. [20] investigated the properties of mirror
neurons in the IPL. They reported that 58% of mirror neurons
were responsive to only one type of hand action, for example
grasping, and 25% were responsive to two different hand
actions. The remaining 17% were responsive to either
observed mouth actions or mouth and hand actions. Further-
more, they reported that 29% of IPL mirror neurons were
strictly congruent and 54% were broadly congruent.
Mirror Neurons in the Primary Motor Cortex
The first few papers [2,18] that described mirror neurons in
area F5 also reported that the authors found no evidence
of mirror activity in M1. Indeed, Gallese et al. [2] argued
0
10
20
30
40
50
60
1992
2013
% Mirror neurons
Publication year
0
5
10
15
20
25
30
% Mirror neurons
35
F5 (N=15) IPL (N=3)
Average %
% of total neurons
1362/4740
120/708
Current Biology
A
B
Figure 1. Number of mirror neurons recorded in areas F5 and in
the IPL.
(A) The percentage of mirror neurons as a function of publication year
for studies reporting mirror neurons in F5 when observing hand
actions. The black line shows the line of best fit. (B) The percentage
of mirror neurons in premotor area F5 and in the inferior parietal lobule
(IPL). The average percentage of mirror neurons for each region is
shown in black and the percentage of total mirror neurons is shown
in grey with the total number of mirror neurons and neurons recorded
given above.
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that, because most neurons in M1 show activity during
self-movement, the absence of detectable mirror activity in
M1 was evidence against the idea that this activity might
actually represent monkey’s making small, covert move-
ments while they watched the experimenter. Similarly, a
recent multiunit recording study [29] found only a low level
of mirror activity within primary motor cortex. However,
three papers [10,22,24] have reported mirror neuron-like
responses in M1.
Tkach et al. [22] reported that when monkeys either per-
formed a visuomotor tracking task themselves, or watched
the same target and cursor being operated by an experi-
menter, 70% (581/829) of recorded neurons in M1 showed
stable preferred direction tuning during both execution and
observation. These authors also reported that 60% (77/128)
of neurons in dorsal premotor cortex were modulated in
the same way.
Dushanova and Donoghue [10] recorded from neurons in
M1 whilst the monkey either performed a point-to-point
arm-reaching task or observed a human experimenter per-
forming the same action. This study reported that 34.7%
(105/303) of the neurons recorded in M1 were directionally
tuned during both action execution and action observation.
The mean firing rate during the observation condition was
on average 46% of that during the execution condition. In
addition, 38% of neurons retained the same directional
tuning during both execution and observation conditions. It
should be noted that these studies differ from those
previously described that recorded from F5 and IPL.
All the studies on mirror neurons in F5 and IPL have em-
ployed tasks where the macaque monkey observed either
a video or the experimenter performing simple reach and
grasp actions. The two studies [10,22] described above on
mirror-like responses in M1 differed in that they used tasks
in which the monkey had been extensively trained on the
motor execution task. It is unclear whether the relatively
high percentage of these mirror-like responses, compared
with those in F5 and IPL, reflects differences between the
task or real differences in the number of mirror neurons.
The final paper [24] on M1 mirror neurons recorded from
132 neurons that were identified as pyramidal tract neurons;
58% of these neurons (77/132) were classified as mirror
neurons. As in F5, these authors found that these pyramidal
tract neurons were either facilitation mirror neurons (58.5%)
or suppression mirror neurons (41.5%) during the action
observation condition. In contrast to F5, facilitation mirror
neurons in M1 fired at significantly lower rates during action
observation vs execution, with the former reported as ‘‘less
than half of that when the monkey performed the grip’’. It is
noteworthy that these authors made simultaneous EMG
recordings from up to 11 different arm, hand and digit
muscles and confirmed complete absence of activity during
action observation.
Mirror Neurons in Other Regions
Above, we have described the results of studies reporting
mirror neurons in ventral premotor cortex, dorsal premotor
cortex, primary motor cortex and inferior parietal lobule.
Three further papers [14,21,26] have reported mirror
neuron-like responses in two further areas. The first [14] re-
corded visuotactile bimodal neurons in the ventral intraparie-
tal area (VIP). These are neurons that exhibit tactile receptive
fields for a particular body part (for example, face or head)
and also exhibit visual receptive fields in the congruent
location. This study demonstrated that 48/541 bimodal
neurons also exhibited visual receptive fields when
observing the congruent area being touched on the experi-
menter. These neurons were not called mirror neurons but
‘body-matching bimodal neurons’.
Shepherd et al. [21] reported mirror neuron-like responses
in the lateral intraparietal (LIP) area. These authors reported
that 30/153 neurons in LIP responded not only when mon-
keys oriented attention towards the receptive field of those
neurons, but also when they observed other monkeys orient-
ing in the same direction.
Yoshida et al. [26] recently recorded from neurons in the
medial frontal cortex, some of which selectively responded
to self or observed actions within a social context. The
neurons were recorded in one of two monkeys who, on
alternate trials, chose a movement in order to earn a reward.
Correct (or incorrect) choices rewarded (or punished: no
reward) both monkeys. ‘Partner-type’ neurons were selec-
tively responsive to the choices made by the other monkey,
signalling the correct or incorrect choice made; interestingly
around 19% of these ‘partner neurons’ showed decreased
activity during self-movement.
Relating Human Neuroimaging Data to Mirror Neuron
Activity
Of the over 800 papers returned when searching PubMed
for ‘mirror neurons’ or ‘mirror neuron’, the vast majority
report the results of experiments on human subjects. Of
these, the results of human neuroimaging experiments, spe-
cifically fMRI [31], confirm a broad overlap between cortical
areas active in humans during action observation and areas
where mirror neurons have been reported in macaque
monkeys (see above). Thus, changes in the BOLD signal
during action observation seem to be consistent with the
existence of a mirror neuron system in humans, but they
cannot yet furnish conclusive proof. There has, however,
also been a report of single neuron activity recorded from
human neurosurgical patients that has demonstrated mirror
neuron activity [32]. Recordings were focused on medial
frontal cortex and temporal lobe structures, and show the
extensive nature of the mirror neuron system. Unfortunately,
neither of the premotor or posterior parietal areas so heavily
investigated in monkeys were available for study in these
patients.
Central to being able to interpret human fMRI studies of
the mirror neuron system is understanding the relationship
between the BOLD signal in human and mirror neuron
activity in macaque monkey. To this end, monkey fMRI
studies have now demonstrated significant activity during
action observation in regions where mirror neurons have
been previously reported [33,34]. These monkey imaging
studies have taken advantage of enhancing the neurovas-
cular responses with an iron-based (MION) contrast agent.
As with the vast majority of human fMRI studies, however,
there is difficulty in relating these results to mirror neurons,
in that they only employ an action observation condition
and have no action execution condition. This makes it diffi-
cult to calibrate the activity changes in observation to those
in execution, and also raises the possibility that sensory
responses other than mirror responses contribute to the
neurovascular changes (see Introduction).
One possible way of attributing the fMRI response to a
single neuronal population, such as mirror neurons, is to
use fMRI adaptation, or repetition suppression. This is a
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neuroimaging tool that has been adopted to identify neural
populations that encode a particular stimulus feature [35].
The logic behind fMRI adaptation is that neurons decrease
their firing rate with repeated presentations of the stimulus
feature that those neurons encode. By extension it has
been argued that the BOLD signal will also decrease with
repeated presentations. It has been argued that areas of
the cortex that contain mirror neurons should show fMRI
adaptation both when an action is executed and subse-
quently observed, and when an action is observed and sub-
sequently executed. This is because the stimulus feature
encoded in mirror neurons is repeated irrespective of
whether the action is observed or executed [36].
The results of such studies have produced mixed results.
Of the five studies using this technique published to date
[36–40], only three have demonstrated significant fMRI adap-
tation consistent with the presence of mirror neurons in the
human brain [38–40]. One possible explanation for the mixed
results is that humans do have mirror neurons, but that they
do not alter their pattern of activation when stimuli that evoke
their response are repeated. Indeed a recent study [9] has
shown some evidence that mirror neurons may not alter their
firing rate during repetitions of the same action; however, in
this work the neuronal activity represented in the local field
potential (LFP) did modulate with repetition. Further work is
clearly required to determine why the BOLD signal in humans
and the LFP in monkeys do adapt with repetition, while the
evidence to date suggests that mirror neurons may not.
Great care must be taken when comparing the results from
human and monkey studies. Specifically, readers must pay
careful attention to the difference in the level of inference be-
tween the different modalities. The majority of human neuro-
imaging studies report significant results at the population
level where the variance is estimated across subjects. This
is in contrast to the studies reporting mirror neurons in ma-
caque monkeys, where the aim is to test whether individual
neurons show a consistent modulation of firing rate during
periods of action observation and execution. Here the infer-
ence is closer to the analysis of fMRI at the single subject
level. Therefore, when it is reported that 30% of neurons in
any region were significantly modulated during both action
observation and execution this does not mean that the
remaining 70% do not modulate at all. Rather, it means there
was not sufficient statistical evidence that these neurons dis-
played mirror activity. Indeed it is quite possible that when
tested at the population level, the neurons that are non-
significant at the single neuron level could be significantly
modulated when observing an action.
The point here is that care must be taken when arguing that
‘only’ X% of neurons in any brain region are mirror neurons.
The ‘only’ implies that the remaining neurons are not signifi-
cantly modulated in any way during action observation. This
is not a valid inference as to do so would be to accept the null
hypothesis. This may be particularly problematic for cortical
regions where responses in individual mirror neurons are
relatively weak, such as in M1.
It is often assumed that mirror neuron activity during
action observation is driven, bottom-up, by the visual (or
auditory) input. The review of mirror neuron discharge pre-
sented here provides evidence that this is, at best, an incom-
plete description of mirror neuron firing. We now know that
mirror neuron firing rates are modulated by view point [7],
value [6] and that they even discharge in the absence of
any visual input [23]. This suggests that mirror neurons can
be driven or modulated top-down by backward con-
nections from other neuronal populations. Indeed, the
requirement for such top-down input to regions containing
mirror neurons was realized by Jacob and Jeannerod [41],
who argued that it was impossible for a mirror neuron system
driven uniquely by the visual input to correctly infer an
intention from an observed action if two or more different
intentions would generate the same action. The fact that
mirror neurons can be driven by backward connections is
consistent with recent predictive coding models of mirror
neuron function [42–44]. Within this framework, mirror neu-
rons discharge during action observation not because they
are driven by the visual input but because they are part of
a generative model that is predicting the sensory input.
This framework provides a theoretical account of mirror
neuron activity that resolves the one-to-many mapping
problem described by Jacob and Jeannerod [41] and is
consistent with top-down modulation of mirror neuron firing
rates.
Concluding Remarks
The discovery of mirror neurons has had a profound effect on
the field of social cognition. Here we have reviewed what is
currently known about mirror neurons in the different cortical
areas in which they have been described. There is now evi-
dence that mirror neurons are present throughout the motor
system, including ventral and dorsal premotor cortices and
primary motor cortex, as well as being present in different re-
gions of the parietal cortex. The functional role(s) of mirror
neurons and whether mirror neurons arise as a result of a
functional adaptation and/or of associative learning during
development are important questions that still remain to be
solved. In answering these questions we will need to know
more about the connectivity of mirror neurons and their
comparative biology across different species.
Acknowledgements
J.K. and R.N.L. were both funded by the Wellcome Trust, London, UK.
We would like to thank Alexander Kraskov for helpful comments on
an earlier version.
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