The Myth Of Mirror Neurons Essay
The Myth Of Mirror Neurons Essay
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The sensory grounding speeches generally observe the motor speech behavior. I think this is significant because the difficulty in learning and speaking it has the courage to develop language that can be recognized easily. I personally trust that sensory problems in the grounding speech can be justified with a larger framework based on outlining the key solutions (Weyhenmeyer & Gallman, 2007). I have observed that the frameworks can be divided into two parts. The first one
The discovery of mirror neurons in macaque frontal cortex has sparked a resurgence of interest in motor/embodied theories of cognition. This critical review examines the evidence in support of one of these theories, namely that the mirror neurons provide the basis of action understanding. It is argued that there is no evidence from monkey data that directly tests this theory, and evidence from humans makes a strong case against the position.
“… we understand action because the motor representation of that action is activated in our brain.”
-Rizzolatti, Fogassi, and Gallese (2001) p. 661
“The [motor] theory is so simple and so easy to present that every one is glad to believe it. The only question that any one cares to raise is how much of it will the known facts permit one to accept.” The Myth Of Mirror Neurons Essay.
-Walter B. Pillsbury (1911) p. 84
Motor theories of cognition have a long history in psychology (Scheerer, 1984), dating back at least to Berkeley’s motor interpretation of depth perception (Berkeley, 1709), and have been proposed as explanations for a wide range of mental processes. For example, in the early part of the 20th century, Margaret Floy Washburn proposed a motor theory of mental imagery (Washburn, 1914, 1916), and John B. Watson explained thought as nothing more than speech-related sensory-motor processes: “according to my view, thought processes are really motor habits in the larynx” (p. 174) (Watson, 1913). As early as 1910, in the Presidential Address at the American Psychological Association meeting in Minneapolis, Walter B. Pillsbury summarized the prevalence of motor theories in the history of psychology succinctly, “… there is nothing in the mind that has not been explained in terms of movement” (Pillsbury, 1911) (p. 84). He also highlighted the widespread popularity of motor theories in his own time, commenting that, “A reader of some of the texts lately published would be inclined to believe that there was nothing in consciousness but movement, and that the presence of sense organs, or of sensory and associatory tracts in the cortex was at the least a mistake on the part of the Creator” (Pillsbury, 1911) (p. 83). The Myth Of Mirror Neurons Essay.
The mirror neuron theory of action understanding (di Pellegrino, Fadiga, Fogassi, Gallese, & Rizzolatti, 1992; Gallese, Fadiga, Fogassi, & Rizzolatti, 1996; Rizzolatti & Craighero, 2004; Rizzolatti, Fogassi, & Gallese, 2001) is the latest in this long line of motor theories — the motor theory of speech perception (Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967) being a prominent mid-century representative — and as with motor theories of the past, seems to have a firm grasp on the field. In fact, judging from the frequency of appearance of mirror neuron-related publications in prominent journals, and the range of abilities and disorders to which the theory has been extended (e.g., speech perception, music perception, empathy, altruism, emotion, theory of mind, imitation, autism spectrum disorder, among others), the comments of Pillsbury, appropriately updated, are equally applicable today as they were a century ago.
Pillsbury’s goal in his address was “to attempt a critical if sympathetic survey of the different formulations of the theory and to compare it with the facts” (p. 84). My goal here with respect to mirror neuron theory is the same. Mirror neurons are an interesting class of cells that deserve to be thoroughly investigated and their function fully understood. My view is that the intense focus on one interpretation of mirror neuron function, that of action understanding, has impeded progress on mirror neuron research. Although the action understanding hypothesis is interesting and worthy of investigation, I will argue that it fails dramatically on empirical examination (Mahon & Caramazza, 2005; Negri et al., 2007). The Myth Of Mirror Neurons Essay. I will start by providing a brief review of the properties of mirror neurons, followed a discussion of eight problems for the mirror neuron theory of action understanding.
Mirror neurons: the data
Mirror neurons, which famously respond both when the monkey makes active movements and when it observes the experimenter making meaningful movements, were discovered in frontal area F5 of the macaque monkey (macaca nemestrina) (di Pellegrino et al., 1992; Gallese et al., 1996). Studies of F5 before the discovery of mirror neurons revealed that most cells in that region respond during the execution of motor acts such as grasping, holding, and tearing, and a fraction of these also respond to passive somatosensory (~40%) or visual (~17%) stimulation in the absence of action (Rizzolatti et al., 1988). Accordingly, the region’s function was interpreted as supporting a motor “vocabulary where proximal and distal movement necessary for reaching, grasping, holding and bringing food to the mouth are represented” (Rizzolatti et al., 1988)(p. 506). In this context, responses to visual objects or somatosensory stimulation were interpreted not as the neural basis of object or tactile understanding, but as a mechanism for sensory stimulation to access various motor acts (Rizzolatti et al., 1988). Since the discovery of mirror neurons, interpretation of non-mirror neurons in F5 has not changed among most F5 experts. The Myth Of Mirror Neurons Essay.For example, with respect to “canonical” (i.e., non-mirror) object-responsive neurons in F5, Nelissen et al. (Nelissen, Luppino, Vanduffel, Rizzolatti, & Orban, 2005) state, “These neurons are known to play an important role in the visuomotor transformation for grasping, but they do not appear to have any role in objects’ identification” (p. 334).1
According to the most detailed early study (Gallese et al., 1996) mirror neurons comprised 17% of sampled cells in the portion of F5 that was examined, and exhibit the following properties. The cells were activated when the monkey observed hand and/or mouth movements that were directed toward objects (“goal-directed” actions). Roughly half (55%) were selective for one type of action, with grasping the most frequently represented movement across the population of cells (75% of cells). The majority of cells were either strictly or broadly congruent with their action execution response properties. The cells did not respond to visually presented objects or food items, faces, non-goal-directed body movements, goal-directed actions made using tools (although see (Ferrari, Rozzi, & Fogassi, 2005)), mimicking of grasping in the absence of an object (pantomime), or gestures having emotional meaning. The cells do not exhibit movement preparation activity: they discharge when the monkey observes an action, stop firing when the action terminates, and remain quiet even if the object is moved toward the monkey, firing again only when the monkey initiates its own action. This is an important fact as this property distinguishes mirror neurons from well-known “set-related” neurons in nearby monkey area 6 that discharge before movement onset (Weinrich, Wise, & Mauritz, 1984; Wise & Mauritz, 1985). As important controls for the possibility that “mirror activity” reflected some form of covert movement, Gallese et al. (1996) recorded from the hand area of primary motor cortex (F1 or M1), and recorded EMG activity from several hand and mouth muscles during action observation. No M1 cells fired, and no EMG activity was elicited in response to action observation. On the basis of this evidence, mirror neurons were hypothesized to support “action understanding.” The Myth Of Mirror Neurons Essay.
Since these early studies, mirror neurons have also been found in monkey parietal cortex (Gallese, Fogassi, Fadiga, & Rizzolati, 2002), and problematically (see below), in M1 (Tkach, Reimer, & Hatsopoulos, 2007).
Mirror neurons: The theory
Unlike the majority of the (non-mirror) neurons in macaque area F5, which are argued to support a “motor vocabulary” (Rizzolatti et al., 1988), mirror neurons are claimed to support “action understanding” (di Pellegrino et al., 1992; Gallese et al., 1996; Rizzolatti & Craighero, 2004; Rizzolatti et al., 2001). “Action understanding” is defined somewhat differently in various papers. Gallese et al. (1996) define it as “the capacity to recognize that an individual is performing an action, to differentiate this action from others analogous to it, and to use this information in order to act appropriately” (p. 606). Rizzolatti et al. (2001) propose that action understanding is, “the capacity to achieve the internal description of an action and to use it to organize appropriate future behaviour” (p. 661). Rizzolatti and Craighero (2004) claim, “Each time an individual sees an action done by another individual, neurons that represent that action are activated in the observer’s premotor cortex. This automatically induced, motor representation of the observed action corresponds to that which is spontaneously generated during active action and whose outcome is known to the acting individual. Thus, the mirror system transforms visual information into knowledge” (p. 172). Nelissen et al. (Nelissen et al., 2005) state that “A mere visual representation [of an action], without involvement of the motor system, provides a description of the visible aspects of the movement of the agent, but does not give information critical for understanding action semantics, i.e., what the action is about, what its goal is, and how it is related to other actions” (p. 332).The Myth Of Mirror Neurons Essay. The notion “action understanding” has been generalized in humans to include speech perception (Gallese et al., 1996; Rizzolatti & Arbib, 1998; Wilson, Saygin, Sereno, & Iacoboni, 2004).
It is not obvious from the definitions quoted above what “action understanding” means. For example, with respect to the first definition, upon seeing an individual producing meaningless, non-goal-directed actions (e.g., flailing the arms, which should yield no mirror neuron activity), one could presumably “recognize” that actions are being performed, “differentiate” such actions from other actions (e.g., swinging the arms rhythmically), and “act appropriately” in response (walk away, or call 911), all without “understanding” the meaning of the actions in the goal-directed sense. The nature of the “internal description” in the second definition is itself undefined and therefore adds little clarity to the nature of “action understanding.” In the third definition, the idea that understanding is achieved by knowing the “outcome” is also somewhat vague, because “outcome” is not defined. The fourth definition also includes concepts that are underspecified: what is the action of grasping a peanut “about”? What is the “goal” of such an action? And on what level of analysis is “relation” between actions defined?
The most reasonable interpretation (in my mind) is that what is being “understood” by mirror neurons while observing peanut-grasping is something closer to the concept, ‘grasping-with-the-hand.’ However, Nelissen et al. suggest that mirror neurons are coding more than the “essence of grasping” (p. 334) which they believe is coded in a more anterior region of F5 (Nelissen et al., 2005). In short, the concept of “action understanding” has been evolving, but at its core is the idea that self-generated actions have an inherent semantics and that observing the same action in others affords access to this action semantics. The Myth Of Mirror Neurons Essay.
The existence of mirror neurons has been inferred to exist in humans, beginning with the earliest mirror neuron reports (di Pellegrino et al., 1992; Gallese et al., 1996). These early claims (Gallese et al., 1996) were based on (i) the fact that pantomime recognition deficits exist in aphasia (Gainotti & Lemmo, 1976), (ii) a PET study in humans showing activation in Broca’s region during action observation, (Rizzolatti et al., 1996), and (iii) a transcranial magnetic stimulation (TMS) study that showed enhanced distal muscle motor-evoked potentials (MEPs) during action observation (Fadiga, Fogassi, Pavesi, & Rizzolatti, 1995). However, the empirical basis for the generalization of the mirror neurons to humans was dubious from the start based on the very data that was claimed to support it: (i′) Mirror neurons do not respond to pantomimed actions and so pantomime recognition should not rely on the mirror system. Further, pantomime recognition deficits were not associated with frontal lesions, but rather were predominantly associated with posterior lesions (Heilman, Rothi, & Valenstein, 1982). (ii′) The PET study showing Broca’s region activation during action observation failed to show overlapping activation during grasping production (Rizzolatti et al., 1996), in contrast to the central mirror neuron observation. And (iii′) the TMS finding of peripheral motor activation during action observation directly contradicted the early demonstration in monkeys that M1 and the peripheral motor system did not exhibit mirror properties (Gallese et al. 1996).The Myth Of Mirror Neurons Essay. Mirror neuron findings were also quickly generalized to speech (di Pellegrino et al., 1992; Gallese et al., 1996) on the basis of analogy to the motor theory of speech perception (Liberman et al., 1967). But despite its mirror neuron-led resurgence in popularity among non-speech scientists, the motor theory of speech perception “has few proponents within the field of speech perception” (p. 361) (Galantucci, Fowler, & Turvey, 2006). Thus, the theoretical grounding of mirror neuron theory in the speech domain was not particularly strong.
Mirror neurons have also been generalized to explain imitation (Rizzolatti & Craighero, 2004). This function of mirror neurons, however, has been restricted to humans because macaques (at least adult macaques (Ferrari et al., 2006)) do not imitate (Visalberghi & Fragaszy, 2001). This means that mirror neuron function, as it is studied in macaque monkeys, cannot be the basis of imitation. Rizzolatti and Craighero (2004) emphasize that, “the primary function of mirror neurons cannot be action imitation” (p. 172). Any evidence regarding the neural basis of imitation in humans, therefore, cannot be empirically linked to mirror neurons.
Although the “mirror system” has been used as the basis for understanding a range of behaviors, we will focus our attention on the core function supposed to hold across species, namely action understanding. If mirror neuron theory fails to stand up empirically with respect to its core claim, as I will argue, then linkage between mirror neurons and the many systems and disorders linked to their function is highly dubious.
The perception of a graspable object is sufficient to trigger the activation of cells in motor area F5 (Rizzolatti et al., 1988). Most mirror neuron theorists do not endow cells that respond to the perception of objects with an object semantics. Instead they propose that F5 contains a motor vocabulary, and that sensory (object) responses in F5 cells reflect a means for grasping-related sensory information to access that vocabulary.2When considering mirror neuron function, it is helpful to adopt this view of F5 function as the null hypothesis. The Myth Of Mirror Neurons Essay. Namely, that F5 is fundamentally a motor area that is capable of supporting sensory-motor associations. In order to make a serious case for mirror neurons as the basis of action understanding, one has to show that they are qualitatively different from other sensory-motor cells in F5, specifically, that they are coding more than just a sensory-motor association (they have a semantics that other sensory cells in F5 do not). In what follows, I will detail eight problems that undermine the claim that mirror neurons go beyond other sensory-motor cells in F5 and support action understanding.
1. There is no evidence in monkeys that mirror neurons support action understanding
The mirror neuron theory of action understanding predicts that disruption of motor areas in F5 should produce deficits in action perception. While functional disruption of macaque area F5 has been shown to disrupt grasping behavior (Fogassi et al., 2001), the predicted corresponding decrement in action perception has never been reported. Rizzolatti and Craighero (2004) argue that such studies are not feasible. This is because (i) the mirror system is bilateral, and involves parietal structures, (ii) there are other mechanisms that mediate action recognition, and (iii) if one lesioned the entire mirror neuron system, more general cognitive deficits would result, making interpretation difficult (Rizzolatti and Craighero, 2004). However, if the claim is that motor systems underlie action understanding, and if it is possible to impair motor behavior by disruption of motor systems in F5 (Fogassi et al., 2001), then it should follow that action understanding will be commensurately impaired. If, on the other hand, motor behavior and action understanding dissociate in macaque following F5 disruption, this would constitute evidence against a critical role for motor systems (and area F5) in action understanding, independently of whether the mirror system extends beyond F5 or not. The Myth Of Mirror Neurons Essay.
In place of the standard lesion method, three studies are held up as evidence that mirror neurons in monkeys support action understanding. One involves the demonstration that some mirror neurons (15%) respond to action-associated sounds presented in isolation (cracking peanut shell, ripping paper) (Kohler et al., 2002). The logic here is that “If mirror neurons mediate action understanding, their activity should reflect the meaning of the observed action, not its visual features” Rizzolatti and Craighero (2004) (p. 173). According to this logic, Kohler et al.’s findings indicate that 85% of mirror neurons do not mediate action understanding because their activity does not reflect the meaning of the perceived action3. But still this leaves a population of 15% of mirror neurons — the audiovisual type — that may code the meaning of actions. Does the existence of these audiovisual mirror neurons prove that they are coding meaning? No. A more straightforward interpretation of this result is that sounds can be associated with actions in F5 neurons, just as objects can be associated with actions in F5 neurons (Rizzolatti et al., 1988). Framed in terms of a priming explanation, we might argue that the animal has associated the action of breaking a peanut with the sound of breaking a peanut, and when hearing only the sound, the activation spreads to F5; a form of partial cue retrieval. Right or wrong, the point is that we don’t need to endow these cells with semantic properties to explain the finding.
The second experiment showed that while mirror neurons don’t respond to pantomimed actions (actions without the object present), they do respond if an action is directed toward an object that is hidden behind a screen such that the monkey knows it is there (Umiltà et al., 2001). In this scenario, more than half of the mirror neurons that were tested, also responded in the hidden condition. The Myth Of Mirror Neurons Essay.The logic here is the same, that it is not the physical features of the action that drives the response, but rather the knowledge of the “meaning” of the action. Again, following the logic, the results of the study indicate that half of all mirror neurons are not coding action meaning, and again there is a simpler explanation. The monkey can represent the object in working memory which, according to popular views, involves the same systems that represent the object when it is physically present (Fuster, 1995; Pasternak & Greenlee, 2005; Postle, 2006; Ruchkin, Grafman, Cameron, & Berndt, 2003). This information can then be used in the normal manner as if the object were visible.
Rizzolatti and Craighero (2004) claim that these studies show that “the activity of mirror neurons correlates with action understanding” (p. 174). However, “action understanding” was never actually measured, and there is a simpler explanation of both results, one that fits well with the hypothesized function of the non-mirror neurons in F5, namely that perceptual information –including objects, tactile stimulation, sounds, and actions — can be associatively linked to, and can prime a “motor vocabulary” in F5 (Rizzolatti et al., 1988).
The third study by Fogassi et al. (Fogassi et al., 2005) uses a different approach to argue for abstract, action understanding properties of mirror neurons. These authors present very interesting data from the inferior parietal lobule (IPL) of monkeys, which also contains mirror neurons, as noted above. Monkeys were trained either to grasp a piece of food and put it in his (the monkey’s) mouth, or to pick up an object and put it in a container. In some conditions, the container was next to the monkey’s mouth such that the mechanics of the movement were very similar between grasping-to-eat and grasping-to-place. The Myth Of Mirror Neurons Essay.In addition, a condition was also implemented in which the monkey grasped and placed a piece of food in the container to control for differences between food items and objects, both visually and tactilely. In all variants of the experiment, the authors report that some IPL cells preferentially responded to the goal of the action: grasping-to-eat vs. grasping-to-place. Again, this was true even when the placing-action terminated in close proximity to the mouth and involved grasping a piece of food. Some of these cells also responded selectively and congruently during the observation of grasping-to-eat and grasping-to-place. So both in perception and action, there are IPL cells that seem to be selective for the specific goal of an action rather than the sensory or motor features of an action — a very intriguing result. Fogassi et al. discuss their motor findings in the context of “intentional chains” in which different motor acts forming the entire action are linked in such a way that each act is facilitated in a predictive and goal-oriented fashion by the previous ones. They give an example of IPL neurons observed in another unpublished study that respond to flexion of the forearm, have tactile receptive fields around the mouth, and respond during grasping actions of the mouth and suggest that, “these neurons appear to facilitate the mouth opening when an object is touched or grasped” (p. 665). Regarding the action perception response properties of the IPL neurons in their study, Fogassi et al. all conclude, “that IPL mirror neurons, in addition to recognizing the goal of the observed motor act, discriminate identical motor acts according to the action in which these acts are embedded. Because the discriminated motor act is part of a chain leading to the final goal of the action, this neuronal property allows the monkey to predict the goal of the observed action and, thus, to ‘read’ the intention of the acting individual” (p. 666). The Myth Of Mirror Neurons Essay.
According to Fogassi et al., IPL mirror neurons code action goals and can “read the intention” of the acting individual. Perhaps Fogassi et al.’s notion of predictive coding and their example of the IPL neuron with receptive fields on the face can provide a simpler explanation. Suppose the abstract goal of an action and/or it’s meaning is coded outside of the motor system. And suppose that Fogassi et al. are correct in that a complex motor act leads to some form of predictive coding (anticipatory opening of the mouth, salivation, perhaps even forward modeling of the expected somatosensory consequences of the action). The predictive coding in the motor system is now going to be different for the grasping-to-eat versus grasping-to-place actions. For eating, there may be anticipatory opening of the mouth, salivation, perhaps even forward modeling of the expected somatosensory consequences of the action. For placing, there will be no mouth-related coding, but there may be other kinds of coding such as expectations about the size, shape or feel of the container, or the sound that will result if the object is placed in it. If cells in IPL differ in their sensitivity to feedback from these different systems, then it may look like the cells are coding goals, when in fact they are just getting differential feedback input from the forward models. Observing an action may activate this system with similar electrophysiological consequences, not because it is reading the intention of the actor, but simply because the sensory event is associated with particular motor acts.
2. Action understanding can be achieved via non-mirror neuron mechanisms
Rizzolatti and Craighero (2004) noted that the mirror neuron system may not be the only mechanism that can support action understanding.The Myth Of Mirror Neurons Essay. Rizzolatti et al. (2001) also emphasize that “these [mirror neuron] findings do not exclude the possibility that other areas are involved in the description of biological movement and the understanding of action” (p. 662). The existence of other mechanisms for action understanding is a problem for the mirror neuron theory of action understanding for two reasons. One, it places action understanding on par with “object understanding.” Object responses in F5 are not generally interpreted as the neural basis for object understanding (Rizzolatti et al., 1988), presumably because other neural systems in the ventral visual stream support object recognition/understanding. Object information, processed for “meaning” in the temporal lobe, can gain access to motor programs as appropriate for behaviors like grasping, thus explaining the object response properties of F5 cells, even though the meaning of the objects is not coded in these motor areas (Nelissen et al., 2005). If there is a neural network outside of the mirror system that can support action understanding, as Rizzolatti and colleagues suggest, then we can propose an identical form of interaction. Actions are processed for “meaning” in this other system, which via the same associative mechanisms can gain access to motor programs in F5, thus producing “mirror” responses, analogous to object responses.
A candidate region for an action understanding alternative to mirror neurons is the superior temporal sulcus (STS). Cells in portions of the macaque STS respond to a wide range of actions in a manner that appears more sophisticated than that found in mirror neurons. STS neurons respond to actions such as walking towards or away, head turning, movement into or out of view, arm movements, and hand-object interaction where there is selectivity for specific actions including reaching, retrieving, manipulating, picking, tearing, presenting to the monkey, and holding (Perrett, Mistlin, Harries, & Chitty, 1990; Perrett et al., 1985). These cells do not have motor properties in that they don’t appear to fire during action execution (although this hasn’t been investigated thoroughly). Interestingly, the region of the inferior parietal cortex that contains mirror neurons (PF), and which projects to F5, receives input from the STS (Rizzolatti & Craighero, 2004). This would seem to be an ideal circuit for representing actions (STS) and coordinating their interaction (PF) with the motor system (F5). The Myth Of Mirror Neurons Essay.
3. M1 contains mirror neurons
It was recently observed that mirror neurons exist in primary motor cortex of macaque monkeys (Tkach et al., 2007). While this is consistent with the motor evoked potential work in humans (Fadiga et al., 1995), it undermines an important control observation in the original mirror neuron reports. Recall that the lack of mirror neurons in M1 was taken as evidence against the possibility that the monkeys were covertly generating movement responses during the perception of actions. In other words, it ruled out the possibility that “mirror” responses were merely some kind of unimplemented motor command, and opened the door to a more interesting, higher-level function. Now with the demonstration of “mirror” responses in low-level motor circuitry (M1 in macaque, and distal muscles in humans, as demonstrated with TMS), it is entirely possible that “mirror” responses are nothing more than the facilitation of the motor system via learned associations. Tkach et al. (2007) suggest a similar interpretation of their data, namely, “that the neural activity during observation is attributable to the covert generation of a motor command and that [the reason] we observe congruent neural activity during observation [is] because the visual goal, and thus the motor command generated, is the same as during active movement” (p. 13247).
4. The relation between macaque mirror neurons and the “mirror system” in humans is either non-parallel or undetermined
As noted above, mirror neuron function has been generalized to a wide range of human behaviors. Indeed, much of the excitement over mirror neurons is directly related to their potential to explain complex human capacities and disorders. A statement by Oberman et al. (Oberman et al., 2005) illustrates both the extent of the generalization and the excitement: “Mirror neurons are primarily thought to be involved in perception and comprehension of motor actions, but they may also play a critical role in higher order cognitive processes such as imitation, theory of mind, language, and empathy, all of which are known to be impaired in individuals with autism spectrum disorders” (p. 190–191, citation numbers omitted).
The problem with statements such as this, and many like it, is that the species that has been shown to possess mirror neurons does not, to our knowledge, possess any of these higher order cognitive processes, and the species that possesses the higher order cognitive processes, has not been shown conclusively to possess mirror neurons (Chong, Cunnington, Williams, Kanwisher, & Mattingley, 2008; Dinstein, 2008; Dinstein, Hasson, Rubin, & Heeger, 2007; Dinstein, Thomas, Behrmann, & Heeger, 2008). To be sure, there have been a host of studies aimed at investigating the “mirror system” in humans, but much of this work has investigated behaviors that mirror neurons could not possibly support given their response properties in monkeys4, and therefore the connection between these behaviors and mirror neurons is tenuously based on a chain of assumptions: mirror neurons exist in humans (there are individual cells that respond both during action execution and action perception), mirror neurons have evolved to support functions in humans that they do not support in monkeys, this evolution has conserved the functional properties found in monkeys, and mirror neurons are responsible for the behavior in question. The Myth Of Mirror Neurons Essay. There is nothing wrong with using animal models to generate testable hypotheses in humans — indeed, this is a productive and important research strategy. The problem in the case of mirror neurons is that the system has been generalized to humans without systematic validation, and with the wholesale adoption of the mirror neuron doctrine concerning action understanding. When a human study starts with the assumption that mirror neurons support action understanding (see above quote from Oberman et al.), and that a homologous and functionally enriched system exists in humans, it is then an easy and prima facie logical inference that the human mirror system can support higher-order functions such as language and empathy. But this inference falls apart if any of the assumptions about mirror neurons are incorrect. Thus, my caution here is not that we can’t or even shouldn’t use mirror neurons to guide human research, but that we have to first validate our assumptions before making inferences regarding human behaviors, especially those that don’t exist in monkeys.
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Let me illustrate the problem with an abstract argument. Suppose that Rizzolatti and colleagues are correct, namely that mirror neurons in monkeys are the basis for action understanding, but not imitation (because adult macaques don’t imitate). In humans, the “mirror system” behaves differently than in monkeys such that it appears to support imitation (Rizzolatti & Craighero, 2004): it activates during the perception and execution of even meaningless movements (Iacoboni et al., 1999). This observation has led some mirror neuron theorists to argue that the mirror system in humans has evolved to support not only action understanding (based on inferences from monkey data), but also imitation (based on human data) (Rizzolatti & Craighero, 2004). The assumption made by these authors is that in the evolution of this system, old properties of mirror neurons are fully conserved. But what if the mirror system evolved in humans such that it now supports imitation but no longer supports action understanding? Perhaps humans evolved a more sophisticated semantic system, distinct from the motor system, that freed the mirror system to support imitation. Possibilities such as this are not considered in mainstream mirror neuron theorizing. Instead, monkey data and theories are typically imported to human work without empirical validation of the assumptions. The Myth Of Mirror Neurons Essay.
Here is a concrete example of how monkey data are assumed to hold, problematically, in human work. In the context of studying the human “mirror system,” a number of functional imaging experiments have investigated the perception of meaningless gestures, pantomimed gestures, and imitation (Decety et al., 1997; Grezes, Costes, & Decety, 1998; Iacoboni et al., 1999; Koski, Iacoboni, Dubeau, Woods, & Mazziotta, 2003; Koski et al., 2002). These studies, which often implicate portions of the inferior frontal gyrus and the inferior precentral gyrus, are cited as evidence for the existence of a human “mirror system” that has evolved to support imitation (Rizzolatti & Craighero, 2004). But this is not the only interpretation. There are at least three logical possibilities. (i) Mirror neurons do not exist in humans, and the activation in these studies results from the function of some other system. (ii) Mirror neurons exist in humans exactly as they do in monkeys (with the same properties), and the activations in these studies result from the function of some other system. (iii) Mirror neurons exist in humans, but have evolved such that they now support pantomime recognition and imitation. The third interpretation is typically adopted while the other possibilities are not even considered. Why? In monkey mirror neuron research other possibilities were considered. Gallese et al. (1996) considered both the possibility that mirror neurons reflected “set-related” responses and the possibility that mirror neurons were reflecting a non-implemented motor plan (see above). Because these possibilities were ruled out empirically in monkeys, it is assumed (probably implicitly) that there is no need to rule them out in humans. But this is faulty logic. If mirror neurons exist in humans as is claimed, the system is demonstrably different from that in the monkey. One therefore cannot assume that monkey data will hold in the human system. The alternative possibilities have to be ruled out empirically again. Consider in this respect a highly cited study of imitation in the human mirror system (Iacoboni et al., 1999), which found equivalent activation during the passive perception of an action (a moving hand), a static hand, and a rectangle with a spatial cue (to which subjects were previously trained to make a hand movement). The authors explain the activation to the latter non-action stimulus is this way: “During all scans the participants knew that the task was either to move a finger or to refrain from moving it. Thus mental imagery of their finger (or of the finger movement) should have been present even during simple observation.” (Iacoboni, et al. 1999, p. 2526–2527). This would suggest that it is not action perception that is driving these “mirror activations,” but simply the internal activation of a motor act. Indeed, there is evidence that human area 44, a presumed component of the human mirror system, is involved in movement preparation (Krams, Rushworth, Deiber, Frackowiak, & Passingham, 1998). The Myth Of Mirror Neurons Essay.
