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When Do We Stop Calling Them Mirror Neurons?
Sebo Uithol (S.Uithol@nici.ru.nl)
Willem F. G. Haselager (W.Haselager@nici.ru.nl)
Harold Bekkering (H.Bekkering@nici.ru.nl)
Nijmegen Institute for Cognition and Information
Montessorilaan 3, 6525 HR Nijmegen, The Netherlands
An analysis of the characterizing properties of mirror
neurons and the representational roles they may and may
not play can prove helpful to distinguish between neurons
that could rightfully be called ‘mirror neurons’ and neurons
that have other – nevertheless interesting – properties.
Abstract
The discovery of mirror neurons in the 1990s has led to much
excitement in the cognitive neurosciences. After the initial
discovery more and more abilities have been attributed to
these neurons. As mirror neurons are commonly viewed as
vehicles of representation, we analyze the increasingly wider
representational role mirror neurons play and argue for a
principled distinction between mirror and non-mirror neurons.
Keywords: mirror neurons;
recognition; goal understanding.
representation;
Mirror Neurons as Representations
action
Introduction
In 1992 Di Pellegrino and his colleagues discovered that
neurons in the rostral part of the inferior premotor cortex of
the macaque brain fire both during the execution and the
observation of an action (1992). Because of the double
representational role these neurons play, they were later
dubbed ‘mirror neurons’ (Gallese et al., 1996; Rizzolatti et
al., 1996). The discovery of mirror neurons caused great
excitement in the cognitive neurosciences, as these neurons
seem to offer a solid, neuronal base for the coupling of
perception to action. Over time, more and more abilities
were attributed to these neurons. Mirror neurons were
deemed to be involved in the inference of intentions and
goals (Fogassi et al., 2005; Iacoboni et al., 2005; Rizzolatti,
Fogassi, & Gallese, 2001), imitation (Brass & Heyes, 2005;
Iacoboni et al., 1999; Wohlschläger & Bekkering, 2002),
emotion understanding (Keysers & Gazzola, 2006; Wicker
et al., 2003) and complementary action (Newman-Norlund
et al., 2007). Also mirror properties were connected to other
modalities such as hearing (Keysers et al., 2003) and touch
(Keysers et al., 2004) and found in other brain regions
(Gallese et al., 2002).
The attribution of increasingly general abilities to mirror
neurons has diminished the original clarity on what mirror
neurons are, what they do and how they could be capable of
performing the functions attributed to them. For example, a
debate has risen about whether mirror neurons show that
direct matching of low-level motor activity is sufficient for
describing the coupling of perception to action (Rizzolatti &
Craighero, 2004), or that it should still be accompanied by
the goal-directed hypothesis (Erlhagen, Mukovskiy &
Bicho, 2006; Koski et al., 2002). Also, the use of mirror
neurons as support for the simulation theory of mind
reading (Gallese & Goldman, 1998) has been questioned
(Csibra, 2005, 2007; Saxe, 2005).
When one wants to emphasize the special character of
mirror neurons, one naturally adopts a representational point
of view. For example, when Gallese et al. (1996, p. 606)
speculate upon the role of mirror neurons, they state that
“[a]nother possible function of mirror neuron movement
representation is that this representation is involved in the
‘understanding’ of motor events”. Likewise, Rizzolatti et al.
(1996, p. 131) propose that “that [mirror neuron’s] activity
‘represents’ the observed action.” The idea that mirror
neurons carry representational content is based on
covariance as measured, in particular by single cell
recordings. Every time the monkey executes a particular
movement or observes that particular movement being
executed by the experimenter, a neuron fires.
Representations based on covariance are ubiquitous in
daily life. For instance, we regard the gas meter a
representation of the amount of fuel in our tank precisely
because there is a reliable covariance between the angle of
the meter and the level of fuel in the tank. There have been
arguments showing that a reliable covariance is neither a
sufficient (Haugeland, 1991) nor a necessary (Millikan,
1984) condition for representation, but we want to jump
over these foundational difficulties, as we do not want to
argue for or against the representational view in general (see
e.g. Beer (2000), Clark (1997), Haselager, De Groot & Van
Rappard, (2003) and Markman & Dietrich (2000) for
various positions in this debate).
A representation consists of a vehicle and a content and
relates to a user and an object. These elements have been
visualized in figure 1. The vehicle of a representation is the
physical carrier (e.g. neural state) that represents. The
information that is carried by the vehicle is called its
content. Content is not the same as the object that is
represented. An object or event in the outside world can be
misrepresented or the content can be of a more general or
more abstract nature than the object represented (e.g. “a
sparrow” can get represented as “a bird”). The fourth and
final element of a representation is a user. The user is the
system or process that uses the representation to guide its
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behavior. As the user is mostly unspecified in case of mirror
neuron representations, we will pay little attention to this
aspect of representation here.
Vehicle
User
Object
Content
Representation Proper
Figure 1: The four aspects of a representation.
In their influential study, Gallese et al. (1996) recorded the
activity of single cells in the brain of a monkey that was
performing actions or observing actions made by the
experimenter. Because certain neurons appeared to fire both
during the observation and execution of an action, these
neurons were dubbed mirror neurons. Not all neurons
responded similar in terms of congruence to the actions,
which led Gallese et al. to discriminate three categories of
mirror neurons: strictly congruent, broadly congruent and
non-congruent. Mirror neurons of the strictly congruent
category get triggered by observed and executed movements
that correspond both in terms of general action (e.g.
grasping) and in terms of the way in which that action was
executed (e.g. precision grip). These neurons can be seen as
the archetypical mirror neurons.
During action observation, the object of the representation
of the strictly congruent mirror neuron is the movement of
the experimenter or the movement of another monkey.
During action execution, the object of this representation is
the movement of the monkey. In both cases the content of
the representation is the particular action (the means
towards an end, for example grasping through a precision
grip). The content is abstracted from the performer of the
movement, as the neurons fire equally in response to
movements made by the monkey or the experimenter, so no
information on the executer is included here. The neuron
itself is the vehicle of the representation.
We can now reformulate what is special about this type of
neurons: one neuronal vehicle covaries its activities with
two objects or events of apparently different domains
(action and perception) that share the same representational
content. Neurons ‘mirror’ when different objects share a
common property that gets reflected in the activity of the
vehicle.
Representational Content and Levels of
Abstraction
With the broadly congruent mirror neurons things are not as
straightforward. These neurons display a connection, but not
identity, between the observed and executed action and
appear to be more specific on the motor side than on the
perceptual side. Gallese et al. (1996) discern three groups of
broadly congruent mirror neurons. Neurons of group 1 are
highly specific for motor activity in terms of action and
specific type of grip, but respond to the observation of
various types of grips. An example of a neuron of this group
is a neuron that fires only when the monkey grasps an object
using a precision grip, and not with any other type of grip,
but also when the experimenter grasps the object with
various kinds of grips, unlike strictly congruent neurons,
that fire only at the observation of one specific grip type.
So, when speaking at the level of grips, it is not possible to
specify the shared property, and hence the representational
content of these neurons cannot be formulated. However,
congruence can be found one level up, i.e. the level of
actions, because from this perspective the response profile is
equally specific on the motor and perception side, namely
actions, e.g. grasping. The key property that mirror neurons
of the strictly congruent type owe their name to – the fact
that the common property of two different events gets
reflected in the activity of one vehicle – can be preserved,
but only by moving the description of the shared property
from the level of grips on to the level of actions.
Neurons of group 2 become active during one motor
action with a hand, but visually respond to two or more
different hand actions. Like the neurons of group 1, the
content is more detailed on the motor side than on the
observation side. Here the congruence is only preserved for
categories of actions. This demands climbing yet another
level of generality, i.e. by distinguishing hand actions and
non-hand actions. So the representational content of these
neurons can only be described at this level of broad action
categories.
The activation of neurons of group 3 is dependent on the
goal of an action, regardless of how it was achieved. These
action-dependent neurons are neither specific on the motor
side, nor on the perception side. The level on which
congruence can be found is even higher than that of neurons
of group 2, as hand actions (for example grasping as well as
other actions (grasping with the mouth) can serve a common
goal (grasping to eat)).
According to Gallese et al. (1996), mirror neurons of the
non-congruent group exhibit no clear-cut relationship
between the observed action and the movement of the
monkey. So their claim is that the response profile of these
neurons is too different on the perceptual and motor side to
find a level of analysis in which a shared property can be
found. At first sight, this makes it impossible to specify or
characterize the representational content of these individual
neurons, as it is difficult to find an informative description
of the property on motor and perception side that the neuron
covaries its activity with.
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Table 1: The various mirror neurons arranged according to their congruence at different levels of abstraction.
Type of mirror neuron
Response profile
(M=Motor, V=Visual)
Lowest common property in motor and
visual response profile
Non Congruent
M: Various actions
V: Various actions
Object-related actions
Broadly Congruent group 3
M: Specific action
V: Various actions
Specific goals (grasping to eating)
Broadly Congruent group 2
M: Specific hand action
V: various hand actions
Specific category of actions (e.g. hand
actions)
Broadly Congruent group 1
M: Specific grip
V: Various grips
Specific action (e.g. grasping with a hand)
Strictly Congruent
M: Specific grip
V: Specific grip
Specific grip (e.g. grasping with precision
grip)
Even here however, a higher level can be found to formulate
the shared property of the events involved, namely the fact
that the actions involve objects. Movements mimicking an
object related action evoke no response so the shared
property that gets reflected in the activity of the neuron
could be labeled as “object-related actions”.
More levels than the ones mentioned by Gallese et al.
could be postulated. Theoretically, below the level of grips
there can be thought to be a level of precise motor
execution. At this level no shared property can be found that
gets reflected in the activity of the neurons, as even the most
fine-grained mirror neurons – the strictly congruent mirror
neurons – allow for small variations in the execution of an
action.
The types of neurons and their lowest common property
in motor and visual response profile are shown in Table 1.
The tendency is obvious: when a shared property cannot be
specified at one level, one can go up one level and use a
new idiom in which commonalities can be found. It is
possible to formulate a level of abstraction on which all
neurons are incongruent, but also a level can be formulated
on which all neurons are congruent.
In all, neurons can be made to mirror, in the sense of
reflecting a common property of two events, by invoking
levels of description of an increasing abstractness. The
representational analysis has shown that, when moving
through the different categories of mirror neurons, the
representational content becomes of an ever more general
nature in order to be able to continue speaking of mirroring.
Vehicle-First Approach
The continuous search for higher levels of abstraction (from
action recognition to goal understanding) in which the
common factor in the observed and performed action can be
expressed is possibly due to the focus on response profiles
of individual neurons. This is a logical consequence of the
original approach taken in the monkey experiments, i.e.
single cell recordings. In this setup, one presumes a neuron
to be a vehicle of content and looks for the objects or events
that it might covary its activity with, whereupon a
conclusion is drawn regarding the potential content the
neuron’s firing might carry.
If one is looking for the representational content of a
single neuron, one assumes that this single neuron is a
vehicle by itself. This might be the case for some actions, as
the discovery of strictly congruent mirror neurons seem to
suggest, but certainly need not be the case for every action
and every neuron. There is always a higher level at which
there is a description of the behavior of the neurons
possible, but at one point, one may start to wonder whether
this continuously more abstract interpretation of what
supposedly gets mirrored is fruitful or warranted.
Things will become even more problematic when not only
local coding is considered, but also its contrasting coding
scheme, distributed coding. In a local coding scheme the
activity of a single neuron or group of neurons is sufficient
for representing a certain property (see Van Gelder (1999)
for an explanation of various types of coding). The
proverbial “grandmother-neuron” is the most famous
example of this type of coding. By contrast, in a distributed
coding scheme an item is represented by the pattern of
activity over a more than minimal extent of the resources
available for representing. In this case the pattern of activity
is the vehicle of the representation, not the individual nodes
that the vehicle consists of. This type of coding is often
discussed in the context of neural network models (e.g. Van
Gelder (1992; 1999)). In distributed representation, there
need not be a structure where a subpart of the vehicle
represents a subpart of the content. In that case, the vehicle
1785
as a whole carries the entire content and the subparts of the
vehicle do not carry identifiable content by themselves.
Locating a vehicle without knowledge of the content is
highly problematic in a distributed, unstructured coding
scheme.
To illustrate this, imagine that we have a neural network
capable of representing various items. And suppose that it
does so by exhibiting a unique pattern of activation over all
the units in the network’s output layer. In this case the
vehicle consists of multiple neurons. There need not be a
structure in the sense that identifiable subparts of the
content correspond to identifiable subparts of the vehicle
(i.e. neurons), in which case a subpart of the vehicle makes
a contribution to the entire content (again, see Van Gelder
(1992; 1999) for elaboration on this topic). When we record
one node in the layer we will not find unequivocal
representational content because this node does not carry
any straightforwardly identifiable content by itself.
With single cell recordings problems might be similar.
When the interpretation of the representational content of a
neuron’s activity reaches a level of abstraction of a less
plausible height, this can suggest that the activity of the
neuron is part of a distributed representation. In such cases a
search for the common property to be reflected is neither
necessary nor likely to produce illuminating results. At
higher levels of action interpretation it seems more likely
that neuronal systems rather than individual neurons are
providing the basic processing elements, so the focus shifts
from mirror neurons to the mirror neuron system.
With the widening of the scope, from mirror neurons to
mirror neuron system, the attributed task grows accordingly.
More competencies are attributed to the system while, at the
same time, holding on to the characteristics of individual
neurons. The problems involved in this strategy will be
discussed in the next paragraph.
From Action Recognition to Intention
Understanding
As research on mirror neurons and the mirror neuron system
continued, ever more competencies were attributed to the
neurons and the system. For instance, mirror neurons are
generally appreciated as the solid neuronal basis for the
understanding of actions (Nakahara & Miyashita, 2005;
Rizzolatti & Craighero, 2004). Recognized specific actions
are directly related to the motor system which facilitates the
understanding of these actions or their underlying goals
(Fogassi et al., 2005; Gallese et al., 1996). According to
Rizzolatti et al. (2001) an action is “understood” when its
observation causes the motor system of the observer to
resonate. This resonance is supposed to lead to the same
activity in the motor system as would be the case when the
observer would perform the action, which facilitates the
understanding of the observed action.
Iacoboni et al. (2005) claim that not only is the mirror
neuron system involved in the understanding of actions, it is
also used for the detection of intentions of others. They base
their claim on the fact that activity in areas associated with
the mirror neuron system is dependent on whether intention
of an action can be inferred. When an intention has to be
inferred from a context (cup grasping for drinking versus
cup grasping for cleaning up), there is a significant increase
in signal in the parieto-frontal cortical circuit for grasping.
We have argued that when the representational content of
mirror neurons gets of an increased level of abstractness, the
activity of these neurons can no longer be rightfully
described as mirroring. As argued, the suggestion that
mirror neurons are basic to action recognition is intuitively
plausible when applied to the level of specific grips and
relatively straightforward actions such as grasping, but it is
quite another thing to suggest that the same basic neuronal
mechanism underlies such a high level process as intention
understanding. The goal of an action has to be inferred from
the recognized action using context (van Rooij, Haselager,
& Bekkering, in press), past experiences with the observed
actor (Ferrari, Rozzi, & Fogassi, 2005), and a lot of
background knowledge. That is an awful lot for a single
neuron to directly mirror onto the motor system. There
seems to be too much inference and knowledge consultation
involved in goal understanding in order to plausibly
characterize the underlying process as a case of pure
mirroring. When climbing to higher levels of abstraction,
the attributed function of the neurons shifts from resonating
with something readily observable to making inferences
about a hidden – or at least indirectly observable – feature.
With every step up, the distance between the attributed
content and the observed input becomes larger. This
provides a reason to be skeptical, as the task of the
individual neuron grows to a questionable size. A lot of
processing has to take place before these neurons can be
specific to a particular goal of an action. Also, it is far from
obvious how a notion as abstract as a goal can be mapped
directly to a motor system, causing the right resonance to
occur.
Of course, Iacoboni et al. do not claim that a single
neuron is capable of recognizing intentions. Hence they
speak of a mirror neuron system instead of mirror neurons.
And naturally, a system may be able to accomplish a task
that its parts cannot accomplish by themselves. Yet, they
depict the mirror neuron system as a system consisting of
mirror neurons and they explain the working of the system
by describing the working of mirror neurons.
[…] the intentions behind the actions of others can
be recognized by the motor system using a mirror
mechanism. Mirror neurons are thought to recognize
the actions of others, by matching the observed
action onto its motor counterpart coded by the same
neurons. (p.533)
The findings of their fMRI based research (the activity in
the parieto-frontal cortical circuit for grasping is dependent
on the context) are put against the background of the
findings with single cell recordings. A claim is made about
the mirror neuron system, but the explanation is based on
1786
mirror neurons. However, the exact relation between mirror
neurons and a mirror neuron system remains implicit1. The
working of the mirror neuron system is in need of more
clarification than the mirroring function of neurons can
provide. We have argued that it is unlikely that individual
neurons are capable of mirroring highly abstract categories
of actions or action goals. So explaining how a mirror
neuron system is capable of detecting intentions remains an
open challenge.
To be sure, we certainly do not wish to dispute the
findings of Iacoboni et al. After all, people can infer
intentions from a context, so obviously this is a capacity the
brain has and Iacoboni et al. have shown that the motor
system is involved in this. However, we do object to the
depiction of this capacity as mirroring. Also, this is not to
say that, for example, broadly congruent mirror neurons of
type 3 are not involved in intention detection, but rather that
they are unlikely to provide a similar solid basis as in the
case of strictly congruent mirror neurons. Results on action
recognition are in need of further explanation in order to
support theories on action understanding. Single cell
recordings cannot carry by themselves the full weight of
support for theories on action understanding.
Conclusion
Debating something trivial as the name of neurons might at
a first glance come across as a futile matter, not worth the
entire representational analysis. But it is important to note
that this is more than just quibbling about the right name for
these neurons. The working of the neurons does naturally
not depend on the name we use for describing them.
However, grouping various kinds of neurons under the label
mirror neuron can obscure important differences regarding
their functional contributions to the recognition of actions
and understanding of goals. When, for example, some
mirror neurons are supposed to do more than just mirroring,
this has serious consequences for, for example, the idea that
the existence of mirror neurons supports the direct matching
hypothesis.
Mirror neurons are almost always regarded as carriers of
representations, but the wide range of representational
claims about mirror neurons gives rise to conceptual
difficulties. On the basis of the above analyses, one could
argue that there is hardly any problem in claiming that
strictly congruent neurons can be said to mirror, as their
representational content reflects a property on the relatively
concrete and observable level of movements. Also, the
representational content of group 1 broadly congruent
neurons can be deemed to be on a relatively low and
unproblematic level of abstraction. More debatable is the
proper interpretation of the activity of group 2 of the
broadly congruent neurons. The categories, hand action
versus non-hand action are of a rather abstract level, as
1
As Dinstein et al. (2008) point out, mirror neurons as defined
in the single cell studies are very difficult to establish in humans.
many different actions can be grouped together within such
categories.
The level of abstraction involving goals of actions utilized
to describe neurons in group 3 as being mirror neurons
seems to be of such a general nature that claims on direct
observation without inference become dubious. The
retrieving of a goal involves an evaluation of the context,
the actor, past experiences with similar situations etc. This
is a lot of processing to be described as merely mirroring.
This certainly does not mean that group 3 of the broadly
congruent neurons and the non-congruent neurons are less
interesting or less important in the coupling of perception to
action. On the contrary, as strictly congruent mirror neurons
represent the action observed or executed and a single
neuron is not capable of recognizing an action by itself, the
crucial steps in action recognition must already have been
taken. Neurons from the broadly congruent or noncongruent category may very well be part of this action
recognition mechanism, representing partial contributions to
unfinished results of the analysis.
When analyzing the human mirror neuron system,
researchers often use data from single cell studies in their
explanation of how the system is able to facilitate action
recognition, action understanding or goal recognition.
However, when it is problematic to state that individual
neurons mirror categories of actions or action goals, as we
have argued, single cell study findings cannot play the key
role in explaining the human mirror neuron system without
further clarification.
Acknowledgments
The authors wish to thank Iris van Rooij for her constructive
comments. The present study was supported by a NICIinternal graduation grant to the second and last author, and
the EU-Project Joint Action Science and Technology (ISTFP6-003747) and a NWO-VICI grant to the last author.
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