Summary : This article is subdivided
into two parts. In the first part we review the
properties of a particular class of premotor
neurons, the "mirror" neurons. With this
term we define neurons that discharge both when
the monkey makes a particular action and when it
observes another individual (monkey or human)
making a similar action. The second part is an
attempt to give a neurophysiological account of
the mechanisms underlying behaviors where an
individual reproduces, overtly or internally,
movenients or actions made by another
individual. We will refer to these behaviors as
"resonance behaviors". We distinguish two
types of resonance behavior. The first type is
characterized by imitation,
immediate or with delay, of movements made by
other individuals. Examples of resonance
behavior of this type are the "imitative"
beliaviors observed in birds, young infants and
patients with frontal lesions. The second type
of resonance behavior is characterized by the
occurrence, at the observation of an action, of
a neural pattern, which, when internally
generated. determines the making of the observed
action. In this type of resonance behavior the
observed action is, typically, not repeated
(overtly). We argue that resonance beliavior of
the second type is at the basis of the
understanding of actions made by others. At the
end of the article we review evidence of mirror
mechanisms in humans and discuss their
anatomical localizations.
[...] RESONANCE BEHAVIOR
There are many behaviors for which a "mirror"
mechanism, similar to that described for F5
neurons, could represent the simplest (and most
plausible) neural mechanism. We will refer to
these behaviors as "resonance behaviors". We
will posit that in resonance behavior a neural
activity that is spontaneously generated during
movements, gestures, or actions is also elicited
when the individual observes another individual
making similar movements, gestures and
actions.
We will discuss two types of resonance
behavior. One is that in which an individual
repeats overtly, in a quasi automatic way, a
movement made by another individual. The second
is that in which an individual uses an internal
motor template to repeat internally the actions
made by others. This internal resonance may be
used for an overt action, but its fundamental
role is that of recognizing the observed
action.
The two resonance behaviors may interact. For
example, an individual may understand the goal
of an action and try to achieve that goal. This
can be obtained by repeating (or trying to
repeat) the same movements that the actor of the
action made or by making other movements
different from those employed by the actor. In
the present article we will not deal with these
more complex cases, but we will focus on the two
resonance behaviors defined above. We will refer
to them as resonance behavior of the first type
and resonance behavior of second type,
respectively.
Resonance behavior of the first
type
We define as resonance behavior of the first
type the tendency that individuals have to
reproduce, immediately or with some delay,
movements, gestures or actions made by another
individual. The repeated actions and the
conditions in which they are repeated vary very
much. Yet, in spite of this heterogeneity, we
propose that in all of them the basic mechanism
is an activation of neurons that generate motor
actions identical to those observed.
A typical example of a resonance behavior of
the first type is the "imitative" behavior
observed in many species of animals when one or
a few of them start an action. One of the best
studied examples is, probably, the behavior
displayed by shore birds when alarmed.
Typically, one or few birds start wing flapping,
then others repeat it and, eventually, the whole
flock turns in flight (42, 43). This
"contagious" behavior does not require,
necessarily, an "understanding" of the action.
What is important here is only that the action
emitted by the first bird could act as a
"release" signal (43). A "resonance" of the
motor system of the observing individual would
be a simple and very advantageous mechanism for
implementing this behavior.
Another example of the resonance behavior of
the first type is represented by the capacity
that some birds have to repeat the songs of
conspecifics. There is convincing evidence that
the neural mechanism at the basis of it is
represented by neurons that discharge both when
the bird produces a song and when it hears it
(see 17 for review).
Resonance behavior of the first type appears
to be present also in humans, in infants where
it plays a fundamental role in establishing a
communication with adults, and in adults as
well.
An example of resonance behavior in infants
is the capacity that even very young infants
have of imitating buccal and manual gestures
(25). It is hard to think that, at this early
age, there is an understanding of the meaning of
the observed gesture and a subsequent conscious
desire to repeat it. A "resonance" mechanism of
the first type appears to be the mechanism most
likely underlying the phenomenon. This
explanation appears particularly convincing in
the case of facial gestures that the infant is
able to imitate, in spite of the tact that it
has never seen its own face.
There is, however, an important difference
between the infant behavior and the "released"
behavior of birds described above. As shown by
Meltzoff and Moore
(25; see also 26), when the infant response is
artificially delayed, the behavior does not
disappear as it should if it was simply a matter
of response release, but is emitted subsequently
when the response becomes possible. This
difference is probably related to the presence
in humans (as well as in most evolved species of
animals) of mechanisms (see below) storing the
externally evoked response and controlling its
emission. Although these control mechanisms are
not mature in infants - typically adults do not
repeat overtly the observed gestures - still
they appear to be already present in infants and
allow a storage of the response and its delayed
repetition.
As far as the adults are concerned, one has
to distinguish actions related to emotional
and vegetative life ("hot actions") from
actions in which these components are limited or
absent ("cold actions"). While adults usually do
not repeat cold actions, an "imitation"
frequently occurs in the case of hot actions.
Smiling produces a tendency to respond with
smiling. Similarly, adults and children alike
respond to the sight of an individual yawning by
yawning themselves. Laughing is contagious.
For all these actions there is no need to
postulate a comprehension of the observed
actions. The observed action simply releases in
the observers the seen action. The term
"response facilitation", proposed by Byrne (3)
describes this behavior very well.
In contrast to normal adults, the repetition
of "cold" action occurs in some severely
demented patients. This behavior was named
echopraxia. It is described as follows:
(echopraxia) "is an impulsive or automatic
imitation of other's people gestures, an
imitation which is performed immediately with
abruptness and speed of a reflex action.
Irrespective of whether the gesture is natural
or bizarre, helpful or dangerous, it is
invariably reproduced" (7, 40).
It is likely that echopractic behavior
represent a "release" of a covert resonance
phenomenon of the first type present also in
normal subjects, but inhibited in its
expression by frontal and mesial cortical areas
(21). Evidence that resonance phenomenon of
the first type is present in normal subjects is
provided by experiments in which evoked
potentials were recorded from various arm
muscles in normal subjects while they were
observing hand and arm movements performed by an
experimenter in front of them (8). The results
showed a selective increase of motor evoked
potentials in those muscles that the subjects
normally use for producing the observed
movements. The resonance phenomenon was present
not only during observation of goal directed
hand movements, but also during the observation
of meaningless arm movements. These findings
clearly show that the motor system "resonate"
also in adult normal subjects, although the
resonance is not sufficient to produce overt
movements.
Another phenomenon due to the release of a
cortical inhibition is the imitation behavior
(21). This phenomenon, which is observed in
frontal patients, especially in the case of
fronto-orbital lesions, does not appear,
however, to belong to resonance behavior of the
first type. Unlike in echopraxia, patients with
imitation behavior do not imitate the movements
of the acting individual, but rather perform an
action identical to the observed one. It is the
goal rather than movements that is imitated in
this pathology.
Resonance behavior of the second
type
We define resonance behavior of the second
type the activation of neurons coding motor
actions during observation of similar actions
made by others. While resonance behavior of the
first type may be easily observed in humans and
animals, the existence of a resonance phenomenon
of the second type was the unexpected result of
neurophysiological studies of area F5 of the
monkey. The resonance behavior of the second
type, although based as the first type on the
activation of motor system in response to an
observed action, differs from it for many
important aspects.
First, unlike in the resonance behavior of
the first type, the effect of the neural
activity elicited by the observation of an
action is not that of generating an overt motor
response. A monkey looks at the action, and
while looking at it, in its brain there is a
motor replica of it. Yet the monkey does not
repeat the seen action.
In an experiment we placed a second monkey in
front of that from which action potentials were
recorded. We then gave food to the newcomer
taking it from a container. In this condition
there was no obvious reason for the observing
monkey (that from which we recorded) to repeat
the gestures of the newcomer. The appropriate
response was to jump on it and chase it away.
Yet, the F5 mirror neurons fired any time the
newcomer grasped the food (34).
Second, if the aim of the resonance
phenomenon in F5 were that of allowing the
observing individual to repeat the observed
gesture, there ought to be a good motor matching
between the observed action and the one to be
repeated. The infants studied by Meltzoff and
Moore protrude their tongue in the same
direction as the experimenter does (25). The
same is true for other instances of resonance
behavior of the first type. In contrast, in F5
action generalization characterizes the visual
responses of most neurons. Neurons respond
regardless of whether the hand that grasps food
is seen moving toward the monkey or away to it,
of whether the movement is made from above the
object or from below. In some neurons even
grasping with the mouth is effective. From a
motor point of view all these movements are
different, but in terms of meaning they all
represent the same action, "grasping".
Third, in many neurons the congruence between
the effective observed action and the effective
executed action is broad. A neuron that
discharges during a specific hand action made by
the monkey, e.g. a precision grip, fires not
only when the monkey observes the experimenter
grasping an object using a precision grip, but
also when it observes the experimenter grasping
a larger object using ail fingers (11). Neurons
with these properties hardly could bc the basis
for imitation behavior.
On the basis of these considerations we
suggested that F5 mirror neurons are involved
not in "imitation" but in action understanding
(11, 34). The logic is the following. An
individual that emits a movement typically
"knows" (predicts) its consequences. This
knowledge probably results from an association
between the representation of the motor action,
coded in F5 and in other motor centers, and the
consequences of the action. The "resonance"
mechanism in F5 does not determine the
appearance of a motor response, but evokes a
neural activity that corresponds to that which,
when internally generated, represents a certain
action. The meaning of an action can be
therefore recognized, because of the similarity
between the two representations.
This interpretation implies that, unlike the
resonance activity of the first type the purpose
of which is to determine overt movements, the
purpose of the "resonance" in the mirror system
is to generate a representation of the goal of
an action. The properties of F5, or at least of
some neurons of F5 (sec above), have precisely
these characteristics. Note that the capacity to
generate a goal-directed representation of
illovement is present not only in F5 mirror
neurons, but also in F5 canonical neurons. This
indicates an evolution of the monkey motor
cortex from a purely executive system. in which
sensory input is hooked up directly to the
output systems, to a systern in which part of it
acts as a buffer storing the possible actions
evoked by the external stimuli.
It is important to make it clear that we do
not claim that F5 mirror neurons are exclusively
involved in a resonance behavior of the second
type. It may well be that in monkey the two
resonance levels are not anatomically
segregated. It could be, for example, that the
mirror neurons that we classified as highly
congruent, i.e. those that resonate only when
the observed action coincide with the emitted
one, underlie resonance activity of the first
type, while those broadly tuned are responsible
for action comprchension. Alternatively, il may
bc that motor areas differeni from F5 are
responsible for resonance activity of the first
type. Finally, one cannot exclude that resonance
activity of the first type concerns only
socially relevant behavior. Since monkeys do not
communicate using hands, it is possible that
this type of behavior is limited in this species
to facial or body movements and therefore does
not concern hand movements.
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