2.1. Behavior

Imitative learning is a sophisticated form of social cognition. It requires copying in a generic sense: Perception of behavior causes similar behavior by an observer, and the similarity plays a role – not necessarily consciously – in generating the observer's behavior. True imitation, restrictively understood, requires novel action learned by observing another do it, plus instrumental or means/ends structure: the other's means of achieving her goal is copied, not just her goal or just her movements. The concept of true imitation is contested, given the different aims and methodologies of imitation researchers (Byrne Reference Byrne, Hurley and Chater2005; Heyes Reference Heyes, Heyes and Galef1996; Reference Heyes2001; Rizzolatti Reference Rizzolatti, Hurley and Chater2005).

Other forms of social learning can seem similar to imitation, but should be distinguished. In stimulus enhancement, another's action draws your attention to a stimulus, which triggers an innate or previously learned response; but a novel action isn't learned directly from observation. Bird A's pecking may draw bird B's attention to a food, which evokes pecking in bird B. In goal emulation,Footnote ³ you observe another achieving a goal by certain means, find that goal attractive, and try to achieve it yourself. Monkey A may use a tool in a certain way to obtain an attractive object, leading monkey B to acquire the goal of obtaining a similar object. Through his own trials and errors, monkey B may arrive at the same type of tool use to obtain the object. Emulation is found in macaques, who have not shown imitative learning. In movement priming, bodily movements are copied, but not as learned means to a goal. Primed movements can be innate, as in contagious yawning.

Goal emulation and movement priming provide the ends-and-means components of full-fledged imitation. Ends and means can be relatively distal or proximal; the distinction is relative, not absolute. Misunderstandings can result concerning whether ends, means, or both are copied and hence whether imitation or emulation is present (Voelkl & Huber Reference Voelkl and Huber2000, pp. 196, 201). A movement can be the proximal means to a bodily posture, which is regarded as the proximal end of the movement (Graziano et al. Reference Graziano, Taylor, Moore and Cooke2002, pp. 354–55), but posture can also be a means to more distal ends – effects on objects or others in social groups. Complex imitation can involve structured sequences or hierarchies of ends–means relations: acquiring a goal, learning to achieve it via subgoals, and so on.

How are these forms of copying distributed across animals, children, and adults? Stimulus enhancement, goal emulation, and movement priming are certainly found in nonhuman animals. But careful experiments are needed to distinguish these from imitation proper and obtain evidence of the latter. The two-action paradigm has been the tool of choice. Suppose two demonstrators obtain an attractive result by two different means: One group of animals observes one demonstrator, and the other observes the other demonstrator. Will the observer animals tend differentially to copy the specific method they have seen demonstrated? If not – if the animals' choices of method do not reflect the specific method they have observed, say, because both groups converge on one method – they may be displaying stimulus enhancement or goal emulation plus trial-and-error learning, but not imitative learning. Even if they do differentially display the behavior demonstrated, this may be merely movement priming if the behavior is already in their repertoire. But if the behavior is differentially used in a new way to achieve a result, it expresses imitative learning (Call & Tomasello Reference Call and Tomasello1994; Nagell et al. Reference Nagell, Olguin and Tomasello1993; Voelkl & Huber Reference Voelkl and Huber2000).

The difference between copying ends and copying means is important for theorizing the phylogeny of imitation and action understanding. (Action understanding is short for understanding observed behavior as goal-directed action.) Is action understanding phylogenetically prior to imitation? This view seems to face an objection: Some animals copy movements (schooling fish), though we don't think they understand the others' actions. A response to this objection distinguishes movement copying from mirroring of goals (Rizzolatti Reference Rizzolatti, Hurley and Chater2005) and both from imitation. Movement copying may precede action understanding, whereas action understanding may require goal mirroring, but precede imitation. True imitation involves something phylogenetically rare: the flexible interplay of copying ends and copying means; a given movement can be used for different ends and a given end pursued by various means (Barkley Reference Barkley2001, p. 8; Tomasello 1999). This is something humans are distinctively good at.

It is difficult to find evidence of true imitation in nonhuman animals (Byrne Reference Byrne1995; Galef Reference Galef, Zentall and Galef1988; Reference Galef, Sabourin, Craik and Roberts1998; Reference Galef, Hurley and Chater2005; Heyes & Galef Reference Heyes and Galef1996; Tomasello Reference Tomasello, Heyes and Galef1996; Tomasello & Call Reference Tomasello and Call1997; Voelkl & Huber Reference Voelkl and Huber2000; Zentall Reference Zentall2001). Early work with chimps seems to reveal imitation, but critics have challenged this interpretation effectively; the results of subsequent experiments were negative for chimp imitation. Skeptics about nonhuman imitation long had the upper hand; for example, Tomasello et al. (Reference Tomasello, Kruger and Ratner1993) found no convincing evidence of nonhuman imitative learning. They proposed that understanding behavior as intentional distinguishes human from nonhuman social learning. On this view, humans can imitate observed means or choose other means to emulate observed goals. Other animals don't distinguish means and goals this way; rather, they copy movements without understanding their relevance to goals, or learn about the affordances of objects by observing action on them. In neither case, it was claimed, do other animals learn about the intentional, means/end structure of observed action.

Many skeptics have now been won over by work on imitation in great apes and monkeys (Voelkl & Huber Reference Voelkl and Huber2000; Whiten et al. Reference Whiten, Horner, Marshall-Pescini, Hurley and Chater2005b), dolphins (Herman Reference Herman, Dautenhahn and Nehaniv2002), and birds (Akins & Zentall Reference Akins and Zentall1996; Reference Akins and Zentall1998; Akins et al. Reference Akins, Klein and Zentall2002; Hunt & Gray Reference Hunt and Gray2003; Pepperberg Reference Pepperberg1999; Reference Pepperberg, Dautenhahn and Nehaniv2002; Reference Pepperberg, Hurley and Chater2005; Weir et al. Reference Weir, Chappell and Kacelnik2002). Continuities are described along a spectrum from the capacities of other social animals to human socio-cognitive capacities (Arbib Reference Arbib2005; Tomasello 1999). For example, innovative experiments extend the two-action method by using “artificial fruits” that can be opened in different ways to obtain a treat: Chimps tend to imitate for one aspect of a demonstrated task and emulate for another aspect, whereas children tend to imitate both aspects, even when the method imitated is inefficient. These and other experiments suggest that chimps imitate more selectively than children (Whiten Reference Whiten, Dautenhahn and Nehaniv2002; Whiten et al. Reference Whiten, Custance, Gomez, Teixidor and Bard1996; Reference Whiten, Horner, Marshall-Pescini, Hurley and Chater2005b; see also Call & Tomasello Reference Call and Tomasello1994; Galef Reference Galef, Hurley and Chater2005; Harris & Want Reference Harris, Want, Hurley and Chater2005; Heyes Reference Heyes1998; Nagell et al. Reference Nagell, Olguin and Tomasello1993; Tomasello & Carpenter Reference Tomasello, Carpenter, Hurley and Chater2005).

Children have been called “imitation machines” (Tomasello 1999, p. 159). They don't always imitate unselectively and sometimes emulate goals instead (Gergely et al. Reference Gergely, Bekkering and Király2002), but they have a greater tendency than chimps to imitate rather than emulate when the method demonstrated is transparently inefficient or futile (Tomasello 1999, pp. 29–30). After seeing a demonstrator use a rake inefficiently, prongs down, to pull in a treat, 2-year-old children do the same; they almost never turn the rake over to use more efficiently, edge down. By contrast, chimps given a parallel demonstration, tend to try turning the rake over (Nagell et al. Reference Nagell, Olguin and Tomasello1993).Footnote ⁴ The differential tendency of children and chimps to imitate suggests an interplay of biological and cultural influences, with a role for innate endowment enabling human imitation (perhaps a matter of articulated relations among multiple mirror subsystems, enabling recombinant structure in social learning, rather than the presence versus absence of a mirror system at all; see discussion of mechanisms in sect. 2.2).

Imitative and related behaviors appear throughout human development (Meltzoff Reference Meltzoff1988a; Reference Meltzoff, Cicchetti and Beeghly1990; Reference Meltzoff1995; Reference Meltzoff, Heyes and Galef1996; Reference Meltzoff, Meltzoff and Prinz2002a; Reference Meltzoff and Goswami2002b; Reference Meltzoff, Hurley and Chater2005; Meltzoff & Moore Reference Meltzoff and Moore1977; 1983a; 1983b; 1989; 1997; 1999; 2000). Infants younger than 1 month of age appear to copy facial gestures. By 14 months, infants imitate a novel act a week later: They turn on a light by touching a touch-sensitive panel with their forehead instead of their hand, differentially copying the novel means demonstrated, as well as the result (Meltzoff Reference Meltzoff1988a; Reference Meltzoff, Hurley and Chater2005; cf. Gergely et al. Reference Gergely, Bekkering and Király2002). They don't turn the light on in this odd way unless they have seen it demonstrated. By 15 to 18 months, infants recognize the underlying goal of an unsuccessful act they observe, and produce it: After seeing an adult try but fail to pull a dumbbell apart in her hands, they succeed in pulling it apart using knees and hands. But they don't pick up goals from failed “attempts” involving similar movements by inanimate devices, thus apparently discriminating agents from non-agents (Meltzoff Reference Meltzoff1988a; Reference Meltzoff1995; Reference Meltzoff, Heyes and Galef1996; Reference Meltzoff, Hurley and Chater2005; Meltzoff & Moore Reference Meltzoff and Moore1977; 1999; Tomasello & Carpenter Reference Tomasello, Carpenter, Hurley and Chater2005). Children's perception of behavior tends to be enacted automatically in similar behavior, unless actively inhibited; but frontal inhibitory functions are not well developed in young children (Barkley Reference Barkley2001, pp. 5, 22; Kinsbourne Reference Kinsbourne, Hurley and Chater2005; Preston & de Waal Reference Preston and de Waal2002, p. 5).

Adult “imitation syndrome” patients with frontal brain lesions also imitate uninhibitedly (Barkley Reference Barkley2001, p. 15; Frith Reference Frith1992, pp. 85–86; Lhermitte Reference Lhermitte1983; Reference Lhermitte1986; Lhermitte et al. Reference Lhermitte, Pillon and Serdaru1986, p. 330). They persistently copy the experimenter's gestures, though not asked to, even when these are socially unacceptable or odd, such as putting on eyeglasses when already wearing glasses.

But the human copying tendency isn't confined to the young or brain damaged! Normal adults can usually inhibit overt imitation selectively, which is evidently adaptive, but their underlying tendency to copy is readily revealed or released. Overt imitation is the disinhibited tip of the iceberg of continual covert imitation (Barkley Reference Barkley2001; Dijksterhuis Reference Dijksterhuis, Hurley and Chater2005). Experiments show how action is modulated or induced by perception of similar action (Brass et al. Reference Brass, Bekkering and Prinz2001; Prinz Reference Prinz, Meltzoff and Prinz2002). Imitative tasks have shorter reaction times than nonimitative tasks; gestures are faster when participants are primed by perceiving similar gestures or their results or goals – even when primes are logically irrelevant to the task (W. Prinz Reference Prinz, Hurley and Chater2005). Similarity between stimulus and response also affects which response is made. Normal adults, instructed to point to their nose when they hear “Nose!” and to a lamp when they hear “Lamp,” performed perfectly while watching the experimenter demonstrate the required performance, but made mistakes when watching the experimenter doing something else: they tended to copy what they saw done rather than to follow instructions (Eidelberg Reference Eidelberg1929; Prinz Reference Prinz, Neumann and Prinz1990). Movements can be induced by actions you actually perceive or by actions you would like to perceive – as when moviegoers or sports fans in their seats make movements they would like to see (W. Prinz Reference Prinz, Hurley and Chater2005). Visually or verbally represented, as well as observed, actions can induce similar actions.

It is helpful to distinguish copying of specific behaviors from chameleon effects, where complex patterns of behavior are induced – a relevant kind of copying, if not strict imitation. In an experiment involving specific behaviors, when normal adults interact in an unrelated task with someone rubbing her foot, they rub their own feet significantly more. Transferred to another partner who touches his face, they touch their own faces instead. Demonstrations of chameleon effects show that exposure to traits and stereotypes automatically elicits general patterns of behavior and attitude and influences how behavior is performed (Bargh Reference Bargh1999; Reference Bargh, Hassin, Uleman and Bargh2005; Bargh & Chartrand Reference Bargh and Chartrand1999; Bargh et al. Reference Bargh, Chen and Burrows1996; Reference Bargh, Gollwitzer, Lee-Chai, Barndollar and Trötschel2001; Chartrand & Bargh Reference Chartrand and Bargh1999; Chartrand et al. Reference Chartrand, Maddux, Lakin, Hassin, Uleman and Bargh2005; Dijksterhuis & Bargh Reference Dijksterhuis and Bargh2001). Normal adults primed with stimuli associated with traits (e.g., hostility, rudeness, politeness) or stereotypes (e.g., elderly persons, college professors, soccer hooligans) tend to behave in accordance with the primed traits or stereotypes. For example, hostility-primed participants deliver more intense “shocks” than control participants in subsequent, ostensibly unrelated experiments based on Milgram's (1963) classic shock experiments. Priming can also affect intellectual performance: College professor–primed participants perform better and soccer hooligan–primed participants perform worse than controls on a subsequent, ostensibly unrelated general knowledge test (Dijksterhuis Reference Dijksterhuis, Hurley and Chater2005; Dijksterhuis & van Knippenberg 1998).

Such priming results are robust across a wide range of verbal and visual primes and induced behavior, using dozens of stereotypes and general traits, and various priming methods, including primes perceived consciously and subliminally. Whether subjects perceive primes consciously or not, they are unaware of any influence or correlation between primes and their behavior. These influences are rapid, automatic, and unconscious, apply both to goals and means, and don't depend on subjects' volition or having independent goals that would rationalize their primed behavior.

Copying, at various levels of generality, is thus a default social behavior for normal human adults; it requires specific overriding or inhibition (Barkley Reference Barkley2001, p. 22; Dijksterhuis Reference Dijksterhuis, Hurley and Chater2005; Preston & de Waal Reference Preston and de Waal2002). Just thinking about or perceiving action automatically increases, in ways participants are unaware of, the likelihood that they will perform similar actions themselves. Nevertheless, these influences are often inhibited, as when goals make conflicting demands: elderly–primed participants tend to walk more slowly, but not if they have independent reasons to hurry.

2.2. Mechanisms

Copying perceived behavior seems to pose a correspondence problem (Nehaniv & Dautenhahn Reference Nehaniv, Dautenhahn, Dautenhahn and Nehaniv2002): How is another's observed action translated into the observer's similar performance? When I copy your hand movements, I can see my own hands, though my visual perspectives on the two movements are different. But when I copy your facial expressions, I cannot see my own face. What information and mechanisms are needed to map perception to similar behavior?

Evidence that newborns copy facial gestures, though they cannot see their own faces, suggests innate supramodal correspondences between action and perception of similar action (Meltzoff Reference Meltzoff1988a; Reference Meltzoff, Cicchetti and Beeghly1990; Reference Meltzoff1995; Reference Meltzoff, Heyes and Galef1996; Reference Meltzoff, Meltzoff and Prinz2002a; Reference Meltzoff and Goswami2002b; Reference Meltzoff, Hurley and Chater2005; Meltzoff & Moore Reference Meltzoff and Moore1977; 1983a; 1983b; 1989; 1997; 1999; 2000). Although further correspondences could be acquired as imitative abilities develop, skeptics about newborn copying can also be skeptical about the need to postulate any innate correspondences (Anisfeld Reference Anisfeld1979; Reference Anisfeld1984; Reference Anisfeld1991; Reference Anisfeld1996; Reference Anisfeld, Hurley and Chater2005; Anisfeld et al. Reference Anisfeld, Turkewitz, Rose, Rosenberg, Sheiber, Couturier-Fagan, Ger and Sommer2001; Heyes Reference Heyes, Hurley and Chater2005).

Heyes, who is one such skeptic, argues that sensorimotor associations subserving copying can be acquired through general-purpose associative learning mechanisms whereby neurons that fire together wire together. Direct sensorimotor associations between motor output and sensory feedback could result from watching one's own hand gestures. An indirect route is needed when the agent cannot perceive her own actions, as in facial expressions: The sensorimotor association could be mediated by environmental items such as mirrors, action words, or stimuli that evoke similar behavior in the actor and in other agents the actor observes. Moreover, adults commonly copy infants, performing the associative function of a mirror. When baby smiles and father smiles back, baby's motor output is associated with sensory input from father's smile (Heyes Reference Heyes, Hurley and Chater2005, p. 161; Preston & de Waal Reference Preston and de Waal2002, p. 8). Imitation can thus develop from interactions between organisms with associative learning mechanisms and certain cultural environments (see Heyes Reference Heyes2001; Reference Heyes, Hurley and Chater2005; see also SCM, Layer 3 in sect. 3.3 here).

Common coding for perception and action has been postulated to explain human copying tendencies. On this view, perception and action share subpersonal processes carrying information about (“coding for”) what is perceived or intended, in which perception and action are not distinguished. The differentiation between perception and action is overlaid on those shared resources, so that they are informationally interdependent at a basic level. If capacities X and Y share an information space, their commonality is informationally prior to their differentiation.

Meltzoff and Moore (1997) postulate common coding of perception and action in explaining infant imitation: proprioceptive feedback is compared to an observed target act, where these are coded in common, supramodal terms. Innate common coding could initially be for relations among, say, lips and tongue; more dynamic, complex, and abstract common coding could develop with experience of body babbling. Common coding might also be acquired as in Heyes's (2005) model.

Wolfgang Prinz (Reference Prinz, Prinz and Sanders1984; Reference Prinz, Hurley and Chater2005; cf. Bekkering & Wohlschläger (Reference Bekkering, Wohlschläger, Prinz and Hommel2002); Preston & de Waal Reference Preston and de Waal2002, pp. 4ff., 9–10) appeals to common coding of perception and action to explain the normal adult tendency to imitate and the reaction-time advantage of imitative tasks. Common coding facilitates imitation, avoiding the correspondence problem and any need for translation between unrelated input and output codes to solve it. Prinz associates common coding with what William James called ideomotor theory, on which every representation of movement awakes in some degree the movement it represents (Brass Reference Brass1999; Prinz Reference Prinz, Heuer and Sanders1987). Perceiving another's observed movement tends inherently to produce similar movement by the observer, and primes similar movement even when it doesn't break through overtly. Regular concurrence of action with perceived effects allows prediction of an action's effects and selection of action, given an intention to produce certain effects (Greenwald Reference Greenwald1970; Reference Greenwald1972). Thus, representation of an action's regular result, whether proximal or distal, can evoke similar action, in the absence of inhibition.

Other sources also support the view that perception and action share processing resources. Observing an action primes the very muscles needed to perform the same action (Craighero et al. Reference Craighero, Buccino and Rizzolatti2002; Fadiga et al. Reference Fadiga, Fogassi, Pavesi and Rizzolatti1995; Reference Fadiga, Craighero, Buccino and Rizzolatti2002). Watching an action sequence speeds the observer's performance of that sequence; merely imagining a skilled performance, in sport or music, improves performance – is a way of practicing – as many athletes and musicians know (Jeannerod Reference Jeannerod1997, pp. 117, 119–22; Pascual-Leone Reference Pascual-Leone2001). Similar points concern perception and experience of emotion: Gordon argues that a special containing mechanism, which isn't fail-safe, is needed to keep emotion recognition from producing emotional contagion. On his simulationist theory, only a thin line separates one's own mental life from one's representation of another's; offline representations of others tend inherently to go online (Gordon Reference Gordon1995b; cf. Adolphs Reference Adolphs2002; Preston & de Waal Reference Preston and de Waal2002).

Common coding theories characterize subpersonal architectures for copying functionally. What neural processes might implement such functional architectures?

Certain neurons directly link perception and action: their firing correlates with specific perceptions and specific actions. Canonical neurons (Gallese Reference Gallese, Hurley and Chater2005; Rizzolatti Reference Rizzolatti, Hurley and Chater2005) reflect affordances (Iacoboni Reference Iacoboni, Hurley and Chater2005; Miall Reference Miall2003): They fire when an animal perceives an object that affords a certain type of action and when the animal performs the afforded action. Mirror neurons fire when an animal perceives another agent performing a type of action, and also when the animal performs that type of action itself; they don't distinguish own action from others' similar actions (see SCM, Layer 3 in sect. 3.3). Some fire, for example, when a monkey sees the experimenter bring food to her own mouth with her hand or when the monkey brings food to his own mouth with his hand (even in the dark, so the monkey cannot see his hand). Specificity of tuning varies.

How mirror neurons relate to imitation is of much current interest (e.g., see Frith & Wolpert Reference Frith and Wolpert2004; Rizzolatti et al. Reference Rizzolatti, Fadiga, Fogassi, Gallese, Meltzoff and Prinz2002; Williams et al. Reference Williams, Whiten, Suddendorf and Perrett2001). It may be tempting to think they avoid correspondence problems, thus facilitating imitation: If the same neurons code for perceived action and similar performance, no translation is needed. But things are not so simple. Rizzolatti, one of the discoverers of mirror neurons, holds that imitation requires both the ability to understand another's action and the ability to replicate it. On his view, recall, action understanding precedes imitation phylogenetically; action understanding is subserved by mirror systems, which might be necessary, but are not sufficient, for imitation. Rizzolatti (Reference Rizzolatti, Hurley and Chater2005) suggests that the motor resonance set up by mirror neurons makes action observation meaningful by linking it to the observer's own potential actions.

Mirror neurons were discovered by single-cell recording in macaques (Di Pellegrino et al. Reference Di Pellegrino, Fadiga, Fogassi, Gallese and Rizzolatti1992; Rizzolatti et al. Reference Rizzolatti, Camarda, Fogassi, Gentilucci, Luppino and Matelli1988; Reference Rizzolatti, Fadiga, Gallese and Fogassi1995), which can emulate but have not been shown to imitate in a strict sense (cf. Voelkl & Huber [2000] on marmoset imitation). Evidence for human mirror systems (Craighero et al. Reference Craighero, Buccino and Rizzolatti2002; Decety & Chaminade Reference Decety, Chaminade, Hurley and Chater2005; Decety et al. Reference Decety, Grèzes, Costes, Perani, Jeannerod, Procyk, Grassi and Fazio1997; Fadiga et al. Reference Fadiga, Craighero, Buccino and Rizzolatti2002; Hari et al. Reference Hari, Forss, Avikainen, Kirveskari, Salenius and Rizzolatti1998; Iacoboni et al. Reference Iacoboni, Molnar-Szakacs, Gallese, Buccino, Mazziotta and Rizzolatti2005; Rizzolatti et al. Reference Rizzolatti, Fadiga, Matelli, Bettinardi, Paulesu, Perani and Fazio1996; Ruby & Decety Reference Ruby and Decety2001) includes brain imaging and demonstrations that observing another person move primes the muscles needed to move in a similar manner (whether or not movements are goal directed; Fadiga et al. Reference Fadiga, Fogassi, Pavesi and Rizzolatti1995).

Rizzolatti (Reference Rizzolatti, Hurley and Chater2005) describes mirror neurons in monkey frontal brain area F5 as part of a circuit including parietal area PF and visual area STS (superior temporal sulcus). He regards a similar human brain circuit as a control system: Sensory results associated with certain movements are compared in PF to observed target movements, enabling imitative learning (cf. Iacoboni Reference Iacoboni, Hurley and Chater2005, with regard to locating the comparator in STS). Differently structured mirror systems may explain different copying capacities across species. In monkeys, mirror neurons appear to code for the goals or results of performed or observed actions.Footnote ⁵ By contrast, human mirror systems include specific movements that can be means to achieving goals (Fadiga et al. Reference Fadiga, Fogassi, Pavesi and Rizzolatti1995). Recall how the difference between mirroring ends versus means of action matters for the view that action understanding precedes imitation phylogenetically. If seeing someone reach for an apple produces motor activation associated with the same goal in the observer (though not necessarily with the same movements in the observer), that could provide information about the observed action's goal directness. But it wouldn't provide information about how to achieve the goal by means of the observed movements, as in imitation.

Brain imaging suggests a division of labor in the human mirror system: Its frontal regions tend to code for goals of action, whereas its parietal regions tend to code for means (i.e., movements; Iacoboni Reference Iacoboni, Hurley and Chater2005). (Monkey parietal mirror neurons seem to be goal–related; Fogassi et al. Reference Fogassi, Ferrari, Gesierich, Rozzi, Chersi and Rizzolatti2005; Nakahara & Miyashita Reference Nakahara and Miyashita2005.) One theory of how this division of labor enables imitation relates signals generated by these brain regions to comparator circuits for instrumental motor control combining inverse and forward models. Inverse models estimate the motor plan needed to achieve a goal in a given context. They can be adjusted by comparison with real motor feedback, but this is slow. It is often more efficient to use real feedback to train forward models, which anticipate the sensory effects of motor plans, associating action with its perceived results (as do mirror neurons). Forward models combine with inverse models to control goal-directed behavior more efficiently. Forward models can predict the results of imitative motor plans for comparison to observed action, and motor plans can be adjusted until a match obtains (Flanagan et al. Reference Flanagan, Vetter, Johansson and Wolpert2003; Iacoboni Reference Iacoboni, Hurley and Chater2005; Miall Reference Miall2003; Wolpert et al. Reference Wolpert, Doya and Kawato2003).

Thus, mirror neurons are arguably part of the neural basis for true imitation, though not sufficient for it. Monkey mirror neurons code for ends rather than means. Human mirror systems, by contrast, have articulated structure: some regions code for goals, whereas others code for specific movements that are means to goals. It has been suggested that human mirror systems enable imitation (not just emulation) because they code for means as well as ends (unlike the macaque's system), and that mirror neurons contribute predictive forward models to subpersonal comparator control circuits.

2.3. Functions

Human brains differ most from chimp brains in expanded areas around the Sylvian fissure which subserve imitation, language, and action understanding – where many mirror neurons are found (Iacoboni Reference Iacoboni, Hurley and Chater2005). Can mirror systems illuminate the functions of imitation in relation to distinctively human capacities – for language, or for identifying with others and understanding the mental states motivating their actions? The relationships among capacities for imitation, language, and mindreading are important for understanding phylogeny and human development. Does development of either language or mindreading depend on imitation? If so, at what levels of description and in what senses of “depend”? Or does dependence run the other way? Or both ways, dynamically? Answers may differ for language and for mindreading. Issues about relations between imitation and mindreading entwine with issues about whether mindreading is best understood as theorizing about other minds or as simulating them.

I shall survey some hypothesized functions of imitation in language, cultural evolution, cooperation, and mindreading. The first three topics, discussed briefly, provide context for SCM and illustrate its broader relevance to understanding what is distinctive about human minds. Mindreading is directly related to SCM and so is discussed more fully.

3.1. Layer 1: Basic adaptive feedback control

SCM begins with specific comparator feedback control systems. A comparator system generates outputs that are means to a target, by establishing an instrumental association between outputs and their results. For example, a thermostat compares a target signal with an input signal. If they don't match, system output is adjusted and the resulting change in input signal or feedback is tracked. Input continues to be recompared with target and output readjusted, to minimize mismatch to target. The elements of such control are (Fig. 1):

Figure 1. Layer 1: Basic adaptive feedback control.

1. 1 A target or reference signal (e.g., target room temperature for a thermostat).

2. 2 An input signal (e.g., actual room temperature), the joint result of elements 3 and 5.

3. 3 Exogenous environmental events (e.g., nightfall).

4. 4 A comparator, which determines whether target and input signals match and the direction and degree of any mismatch or error (e.g., the room is still five degrees below target temperature).

5. 5 The output of the control system (e.g., the level of heat output), regulated by comparison between target and input signals (e.g., heat output is increased if measured room temperature is below target).

6. 6 A feedback loop by which output has effects on succeeding input signals (e.g., measured room temperature rises when heat output increases).

Feedback control is adaptive; output is adjusted to compensate for changing exogenous influences, keeping sensed input close to target. Under different exogenous influences, feedback calls for differing outputs to achieve the target; when the weather changes, a thermostat adjusts heat output to maintain the target temperature. Feedback at layer 1 operates in real space and time, and therefore can be slow (e.g., a room takes time to warm up after the heat is turned up). A control system implements a mapping from the target, in the context of actual input, to output, thus specifying the means for approaching the target in given circumstances. Inverse model is engineering terminology for this instrumental mapping.

Net sensed input results from the system's output plus independent environmental influences. In organisms, reafferent feedback carries input resulting from the organism's own activity, whereas exafferent input results from exogenous events. Reafference includes visual and proprioceptive inputs resulting from movements of one's hands, movement through space, manipulation of objects, and so on. Exafference includes visual inputs resulting from environmental events, such as movements by others in a social group. However, at layer 1, information distinguishing reafference from exafference is not available.

Feedback control is a cyclical and dynamic process, with no nonarbitrary start, finish, or discrete steps; input is as much an effect as a cause of output (Marken Reference Marken2002; Powers Reference Powers1973). Control depends on dynamic relations among inputs and outputs. Information about inputs is not segregated from information about outputs; this blending of information is preserved and extended in the informational dynamics of further layers. Perception and action arise from and share this fundamental informational dynamics (Hurley Reference Hurley1998; Reference Hurley2001).

Specific means/ends associations or instrumental mappings can be chained (output A is the means to controlled result B, while B in turn is the means to controlled result C, and so on) or organized into hierarchies. There are independently determined evolutionary, developmental, and individual differences in the grain and complexity of the possible control sequences and hierarchies of different creatures.

3.2. Layer 2: Simulative prediction of effects for improved control

Real-time feedback can be slow and produce overshooting. Control functions can be speeded and smoothed by adding simulative predictions to a comparator system (Grush Reference Grush2004; Miall Reference Miall2003): Instrumental output-result associations can then be activated predictively, simulating the effects of specific outputs for informational purposes. Over time, associations are established between output and subsequent input in certain contexts, so that copies of motor-output signals can evoke associated input signals. An inner loop maps copies of output signals directly onto “expected” input signals – means to results (Fig. 2; new aspects italicized). Forward model is engineering terminology for this mapping; copies of output signals in organisms are called efference copy. This subpersonal process simulates feedback – predicts the results of output on input. Prediction can occur during actual action, to smooth a behavioral trajectory by anticipating feedback, or prior to action, to provide information about alternative possible actions (layer 4).

Figure 2. Layer 2: Simulative prediction of effects for improved control.

A general improvement in instrumental control results. A control system with predictive simulation no longer need await actual feedback. A thermostat can turn the heat down before the target is reached, avoiding overshooting; hand movement can be initiated in accord with predictions of retinal signals based on eye movements. When real and simulated results don't match, a local switch can default back to actual feedback control while predictive simulations are fine-tuned to improve subsequent predictions.Footnote ¹³ For purposes of online instrumental control, the system need not monitor continuously or access globally whether it is using actual or simulated feedback.

Comparisons can now be made not just between targets and actual results of action, but between the latter and anticipated results. This permits reafference to be distinguished from exafference: Information about the organism's goal-directed behavior can be distinguished from information about environmental events. Consider the familiar ambiguity: When my train pulls out of the station, I register movement relative to the train on the next platform, but this does not itself provide information about whether my train or the neighboring train is moving. Comparison of predicted feedback from action (efference copy) with actual feedback provides resources to resolve an analogous subpersonal ambiguity (between reafference and exafference), and hence to distinguish the self 's activity from environmental events.Footnote ¹⁴ This subpersonal information can contribute to enabling the personal-level distinction between action of the self and perception of the world. The perception/action distinction emerges from subpersonal informational dynamics between world-to-animal inputs and animal-to-world outputs. However, perception and action don't map, respectively, onto input and output (Hurley Reference Hurley1998). Rather, layer 2 inherits unsegregated information about inputs and outputs from layer 1, and uses this blended information in enabling the perception/action distinction. Perception and action share these basic informational dynamics and processing resources. SCM thus provides a dynamic process version of a common coding view of perception and action (sect. 2.2).

However, the system does not yet provide information about similarities between the agent's actions and actions by others, nor information distinguishing the agent's actions from similar actions by other agents (as opposed to distinguishing the agent's actions from environmental events in general). This suggests that a basic distinction between action by self and perception of world, associated with instrumental control functions, can be available to creatures still lacking intersubjective information, a self/other distinction, or mindreading abilities. There are more and less fundamental layers of information about self (self in SCM is neutral between persons and other animals).

Some general predictions derive from layer 2. First, neural mechanisms that implement sensorimotor affordance associations (such as canonical neurons [sect. 2.2]) are predicted. Suppose an animal typically acts in ways afforded by certain kinds of object: for example, eating a particular food in a specific way. Copies of motor signals for eating movements will be associated with a multimodal class of exafferent and reafferent inputs deriving from such objects and the agent's eating of them. Cells mediating this sensorimotor association could thus have both sensory and motor fields and carry information about objects' affordances. Second, deficits in predictive simulation functions should be associated with deficits in distinguishing self from world and action from perception.Footnote ¹⁵ A specific first-order prediction relating to layer 2 might be that capacities requiring information that distinguishes self from world are phylogenetically prior to capacities for social learning and action understanding.

3.3. Layer 3: Mirroring for priming, emulation, and imitation

SCM next postulates that instrumental output-result associations can be activated bilaterally, from effect to cause, as well as from cause to effect. Not only do copies of motor signals predictively simulate input signals (layer 2), but input signals can evoke corresponding motor signals.Footnote ¹⁶ To put it more technically, not only does efference copy produce simulated reafferent input in forward models, but input signals can evoke mirroring efference or motor output. Mirroring in effect runs the predictive simulations of forward models in reverse (Fig. 3).

Figure 3. Layer 3: Mirroring for priming, emulation, and imitation.

Observed actions are thus mirrored in the observer; if mirroring is sufficiently strong and not inhibited, overt copying results. (Mirroring is here a functional subpersonal rather than neural description of behavioral priming produced by observing action, and may be implemented in neural mirror systems.) Mirroring within specific control structures of differing grain and structure would enable different copying capacities, observed across various social species: mirroring of basic movements in priming (Rizzolatti's “low-level resonance”), mirroring of goal-directed action or emulation (Rizzolatti's “high-level resonance”), and even full-fledged imitation (if mirrored elements of control structures are sufficiently articulated and flexibly linked to provide the information needed for social learning of novel means to a goal [see further on in this section]).

Note the intimate relationship between the sharing of circuits for self and other and for action and perception: Layer 3's shared informational dynamics for intersubjectivity presupposes layer 2's shared informational dynamics for perception and action, which builds on layer 1's generic informational dynamics for sensorimotor control. SCM explicitly builds shared resources for self and other on those for action and perception. It thus integrates W. Prinz's shared information for perception and action (though in terms of functional dynamics rather than coding) with Gallese's primitive intersubjective “we”-centric information. By bringing together information about motor causes of one's own and others' similar observed actions, mirroring enables simulation of means/ends associations from either direction: Observed action retrodicts motor activation in the observer via mirroring of causes, which are associated with further results via simulative prediction of effects. But although mirroring makes information about action's instrumental structure accessible bilaterally (from acting and from observing others act), mirroring does not yet distinguish own action from observed action. At layer 3, self and other share informational resources: intersubjective information is subpersonally prior to the self/other distinction (as Gallese holds [sect. 2.3]).

Instrumental mapping and mirroring both map input to output; SCM distinguishes them functionally (neural implementations may overlap). Instrumental mappings have control functions: given certain inputs, they select motor outputs that in turn produce inputs matching a target. Although mirroring exploits instrumental control structures and also produces motor outputs, given certain inputs, it does not itself select outputs as means to inputs that match observed action – or any other target (cf. Peterson & Trapold Reference Peterson and Trapold1982). Rather, SCM postulates, mirroring is a by-product (via reversal) of predictive simulations, which do have instrumental control functions. However, the resulting automatic copying tendency has evolutionary functions, and copying can be exapted for cognitive functions associated with imitation, action understanding, or signaling; these in turn can enable advanced social (“Machiavellian”) forms of instrumental control (see sects. 2.1 and 2.3). Whether specific mirroring capacities are adaptive depends on their potential functions for different social species under different evolutionary pressures.

How could mirroring arise? Consider first movements that produce visual reafference. When creature A sees her own hand movements, associations form between copies of motor signals for these movements and visual reafference from these movements. Cells mediating this association can acquire congruent sensory and motor fields. These cells would also fire if creature A receives similar visual inputs from creature B's similar hand movements; the cells wouldn't distinguish observer's action from observed action producing similar inputs. So, like mirror neurons, they would fire both when A acts and when she observes such similar action by B. They mediate associations between copies of motor signals and a class of inputs including both characteristic reafference from the agent's movement and similar exafference from others' similar movements.

How could mirroring arise for movements not seen by their agents? This requires a correspondence between one's own and others' similar acts, without reafferent feedback from own acts in the same modality as observations of others' acts. Facial movements normally produce proprioceptive rather than visual feedback: How can one's own facial movements be associated with visual information about similar observed facial movements, to enable copying of facial expressions?

Several answers are possible (sect. 2.2), all compatible with SCM. Some supramodal correspondences may be innate (as in newborn copying; Meltzoff Reference Meltzoff, Hurley and Chater2005; Meltzoff & Moore 1997). Some may be acquired through experience with mirrors, or with being imitated (Heyes Reference Heyes, Hurley and Chater2005). Some could be established via stimulus enhancement, as follows. Suppose a creature repeatedly sees conspecifics act a certain way; their actions draw its attention to the typical objects of their actions, which evoke in the observer an innate or previously acquired response. An indirect association results between visual observations of others' actions and one's own similar action. This isn't copying initially; the object independently evokes others' and own similar acts. But the indirect, object-mediated association between own and others' similar acts may become direct with repetition. Cells mediating it could thus acquire similar sensory and motor fields, so that observing another's act primes similar action by the observer. Therefore, stimulus enhancement could develop into copying, and an indirect link via an enhanced stimulus into a direct sensorimotor mirroring link.Footnote ¹⁷ SCM is neutral about whether such correspondences are innate, acquired, or both.

SCM does not describe one all-inclusive structure, but has multiple instances for specific movements and results, at various points along different means/ends chains (cf. Fogassi et al. Reference Fogassi, Ferrari, Gesierich, Rozzi, Chersi and Rizzolatti2005; Wolpert et al. Reference Wolpert, Doya and Kawato2003). The ends/means distinction is relative and applies along spectra of means/ends links in which basic movements are means to proximal results that are means to more distal results. Such control spectra can vary in grain. SCM could apply at successive points along a control spectrum, or between spectra (with the right neural connections); one circuit's target could be the means to next circuit's target. A network of control spectra could support hierarchical control and flexible recombination of means and ends. With mirroring added, it could enable movement priming, emulation, or imitation.

SCM predicts that capacities for specific kinds of copying and social learning will vary across species and development with

1. (A) the grain and complexity of instrumental control capacities, and

2. (B) which of these have associated mirroring functions and how richly and flexibly they are linked (see discussion following; Csibra Reference Csibra2005).

How do these two influences operate?

(A) Different animals are equipped to different degrees with capacities for instrumental control and associated predictive simulation. This variation reflects potential means/ends chains of differing grains and lengths and with differing degrees of lateral connectivity to enable novel combination of means and ends as instrumentally appropriate. Animals suitably equipped with control functions by evolution and development could form chains of simulative predictions, resulting in information such as: this tail movement has that effect on weight over legs, facilitating this movement trajectory in relation to that gazelle, and so on – with eating the prey later in the chain. A similar chain could lead from knee movement to winning a slalom race.

Mirroring operates such instrumental associations in reverse. Mirroring the cause of another's movement, or resulting relationship to an object, could enable movement priming, goal emulation, or even full-fledged imitation (if instrumental control and associated mirroring functions are sufficiently articulated and flexible). Combined with further information distinguishing self from other (layer 4), simulative mirroring can provide information to enable understanding of others' observed movements as instrumental actions with intentional, means/end structure. Simpler control and predictive simulation capacities, with shorter, coarser, means/ends chains, limit correspondingly an animal's potential for mirroring and related functions.

Whether observed movement primes copying or is recognized as goal-directed depends on, inter alia, the instrumental capacity potentially available for mirroring and simulative functions. Mirroring and simulation might provide information about the goals of certain observed movements, given fine-grained, complex means/ends associations, but not given coarser control capacities. No doubt a monkey can move her hand to grasp a piece of sushi and move it to her mouth to eat it. But I can move my hand to operate chopsticks to pick up sushi to dip it in soy sauce and then move it to my mouth to eat it, in order to impress my boss; given associated simulative mirroring functions, I may start to resent you for eating the last piece of sushi as soon as you reach for your chopsticks.

(B) While mirroring of instrumental associations can provide information about the instrumental structure of observed movements, mirroring does not itself determine which instrumental associations, or predictive simulations thereof, are in place and hence potentially exploitable by mirroring. Control processes can be neurally distributed, with components of an articulated mean/ends chain processed in different brain areas – control of fine movements versus gaze versus posture versus whole body movement versus external objects. Some such neural areas may have mirroring as well as predictive simulation capacities, whereas others do not. Whether mirroring is associated with particular control structures will vary across species and development, as a result of evolutionary and developmental processes, yielding different capacities for copying and for generating information about goals of observed actions.

We can understand the enabling of movement priming, goal emulation, or imitation in terms of mirroring that exploits different control structures. Mirroring associated with more basic means/ends links (Rizzolatti's ‘low-level resonance’) predicts priming of basic movements; mirroring associated with less basic means/ends links predicts priming of less basic movements or results of more basic movements. Finger movements are means to chopstick deployment; when I can control chopsticks, chopstick deployment is a means to sushi eating, which could be a means to some further social result. Suppose I eat sushi by deploying chopsticks by moving my fingers a certain way. You watch. If seeing me move my fingers generates mirroring motor activation, then you are primed to move your fingers similarly. Such movement priming predicts interference if you watch me doing X while you are doing Y.

If you can already control chopsticks, less basic mirroring and prime chopstick deployment can occur. Such priming could be goal-mediated: Your chopstick deployment could mirror mine when sushi-eating results (even if no sushi is visible), but not when the results are unrelated to sushi (cf. Umiltà et al. Reference Umiltà, Kohler, Gallese, Fogassi, Fadiga, Keysers and Rizzolatti2001). Goal emulation could be enabled by mirroring midway along a control spectrum of proximal results of movements (rather than basic movements themselves) that are in turn means to more distal results (cf. Rizzolatti's “high-level resonance”). Such midway mirroring would generate motor activation associated with a corresponding midway goal for the observer (rather than with similar basic movements in the observer). As well as enabling emulation, this would contribute information for understanding observed action as goal directed (layer 4).

The phylogenetically rare capacity for imitative learning requires flexible means/ends associations; priming and emulation, respectively, provide its ends and means components (sect. 2.1). Articulation within and linkages between mirror circuits determine whether mirroring enables imitative learning. An animal with mirror circuits along a chain of means/ends associations may never have used a certain means to a given goal. But if an observed novel means to that goal is mirrored, and neural links permit specific mirroring activations to be flexibly combined with targets, that goal and those mirrored means may be newly associated, capturing information about novel instrumental structure in observed action and enabling imitative learning. You might learn to use chopsticks by watching me; emulation plus trial and error would be bettered by mirroring that primes your finger movements towards those you see me make that are associated with the target of chopstick deployment.

Children's greater imitative tendencies, compared with chimps, may depend not on the presence versus absence of a mirror system, but on articulated relationships among multiple mirror circuits, permitting recombinant structure in social learning (sect. 2.1; cf. Barkley [2001] on recombinant structure in executive functions and their relation to imitation). SCM predicts correctly that imitation should be found in fewer species than is movement priming or emulation separately, because imitation additionally requires linkages supporting flexible instrumental associations between mirrored means and ends at a relatively fine grain. Flexibly linked mirror circuits could also generate behavioral building blocks combined in program-level imitation of sequential or hierarchical structure. And they could allow infants to form three-way associations among observed behavior by parents (who have survived to reproduce, so may have adaptive behaviors, not all of which are heritable), observed circumstances of such parental behavior, and infants' own similar behavior, enabling contextual imitation: act like that, when the environment is like this. A network of linked mirror circuits could also permit mirroring activation to spread and generalize automatically, as in chameleon effects (see sects. 2.1 and 2.3).

Some general predictions derive from layer 3. First, neural systems are predicted that implement mirroring functions and don't distinguish own and others' actions. Mirror neurons need not individually implement mirroring and simulation functions. Specific hypotheses might concern how mirror neurons with varying sensorimotor congruence contribute to the functions of mirror systems (cf. Csibra Reference Csibra2005). Second, functional associations are predicted between deficits in mirroring and in predictive simulation for control, which reflects shared circuits for these functions. Third, implementational associations are predicted between neural mirror systems and neural systems for comparator control and predictive simulation (sect. 2.2). Fourth, automatic behavioral priming and copying tendencies are predicted at varying grains, reflecting the articulation and complexity of control functions across species and development (as in movement priming, emulation, human infant copying, human perceptual induction effects, imitative interference and reaction-time effects, and chameleon effects). Overt automatic copying tendencies should be greater where inhibition is weaker, that is, in young children or imitation syndrome patients (sect. 2.1). Fifth, the phylogenetic rarity of imitation as opposed to movement priming and emulation is predicted (sect. 2.1).

Another thought: Recall that canonical neurons mediate sensorimotor affordance associations; for example, between copies of motor signals for eating and a class of inputs associated with objects that afford eating and the eating of them. As a result of such associations, observing an object may prime action it affords and produce a tendency to automatic action on affordances where inhibitory function is reduced – in young children, utilization syndrome patients, or subjects with attention deficit hyperactivity disorder (Barkley Reference Barkley2001, p. 15; Lhermitte Reference Lhermitte1983; Reference Lhermitte1986; Rietveld, in preparation). Whether inhibitory functions are specific to imitation or action on affordances is a further question (Brass et al. Reference Brass, Derrfuss, Matthew-von Cramon and von Cramon2003).

3.4. Layer 4: Monitored output inhibition combined with simulative prediction and/or simulative mirroring

Information about the instrumental structure of observed action provided by a flexibly articulated network of mirror circuits can not only enable imitative learning, but also contribute to enabling the understanding of another's actions as instrumentally structured, including in novel or complex ways. SCM's layer 4 introduces the capacity to inhibit actual output and monitor this inhibition while instrumental associations are activated. This capacity for monitored inhibition could combine with layer 3's mirroring to enable action understanding. Or, it could combine with layer 2's online predictive simulations to enable offline instrumental deliberation. Or both.

Take instrumental deliberation first. Layers 2 and 4 could combine functionally to distinguish actual from possible actions. At layer 2, simulative predictions improve online control of ongoing action. This online function does not require the control system to monitor whether it is currently using actual or simulated feedback, as long as it can switch between them as needed to achieve the target. However, simulative predictions of results could also function offline, with actual motor output inhibited. Multiple simulative predictions could provide information about results of alternative possible actions, rather than anticipating results for ongoing action. Simulated results of alternative possible actions could be compared for the closest match to a target prior to actual action. Layers 2 plus 4 could thus provide information for “trials and errors in the head” (Millikan Reference Millikan, Hurley and Nudds2006) prior to actual trials and possibly fatal errors, allowing simulations to die in a chooser's stead. They could thus enable counterfactual instrumental deliberation and choice among alternative possible actions.

Enabling these capacities requires more than comparing simulated results of different acts with a target. It also requires monitoring whether motor output is inhibited, to track the distinction between actual and possible actions. The predicted results of actual actions will call for a very different response to the predicted results of possible actions. A creature unable to distinguish these two types of predictions wouldn't be long for this world. (Barkley Reference Barkley2001). Layer 4's monitored inhibition provides a basis for this distinction: Simulated results with output inhibition provide information about possible actions, whereas simulated results without output inhibition provide information about actual actions. Multiple predictive simulations provide information about the consequences of various actions by the agent, whereas monitoring of output inhibition provides information that these are possible actions, not actual ones.

Whether such a subpersonal informational structure corresponds directly to the personal-level sense of ability to do otherwise – of having alternative possible actions open to choice – is a further question. The point here is to explain how information for an actual/possible distinction and counterfactual practical reasoning emerges in SCM. Although this information concerns agency, it may provide a basis, when combined with language, for counterfactual reasoning in theoretical contexts.

Now consider action understanding. Layers 3 and 4 could combine functionally to distinguish self from other – more precisely, to distinguish one's own action from another's. At layer 3, observing another's action primes similar action by the observer, through mirroring. At layer 4, the observer's similar action is inhibited; observed behavior isn't actually copied. Copying behavior can be beneficial (especially for young organisms), but unselective overt copying would often have disastrous results for copiers; a prey that chases predators won't survive for long. The capacity to inhibit copying is adaptive and should be expected to evolve (Barkley Reference Barkley2001). Offline mirroring simulates in the observer the causes of observed action, reversing the direction of simulative prediction: Instead of simulating feedback that would result from motor activations, mirroring simulates motor activation that would produce results similar to those observed, with actual motor output inhibited. Simulative mirroring can thus provide information for understanding the instrumental structure of observed actions. In effect, offline copying enables action understanding (Fig. 4).

Figure 4. Layer 4: Simulative mirroring (or prediction) combined with monitored output inhibition, enabling action understanding (or instrumental deliberation).

Mirroring of means/ends associations for observed actions isn't enough to enable understanding action as another's. This also requires monitoring as to whether motor output from mirroring is inhibited, to separate information about others' actions from information about one's own. Layer 3 doesn't do this job. But use of information about actions to understand others has different consequences, and makes different demands on subsequent behavior, than does use of information about actions actually to copy them. Therefore, it would be adaptive to track the distinction between own and others' actions. Layer 4's monitored inhibition provides an informational basis for this distinction, which overlays the shared informational dynamics for own and others' actions at layer 3: simulative mirroring with monitored inhibition provides information about another's action, not one's own. Thus, simulative mirroring can provide information about the causes and instrumental structure of observed action, while monitoring of output inhibition can provide information that such actions are another's, not one's own. This is how information for a self/other distinction emerges in SCM.

The length and grain of chains of means/ends associations and the flexibility of linkages between them should affect not just the types of copying enabled by mirroring (as explained at layer 3), but also the types of action understanding enabled by simulative mirroring. Goal-mediated simulative mirroring would provide information about goals of others' movements, enabling an early stage in understanding others as acting on intentions (hence, in mindreading). If mirror circuits are sufficiently articulated and flexibly recombinant to enable imitative learning, then monitored inhibition of imitative mirroring would capture the means/end structure of novel observed action more fully and flexibly, enabling more sophisticated mindreading. Depending on the articulation of control structures with associated mirroring, adding layer 4 to layer 3 could enable different mindreading abilities.

Recall the discussion (sect. 2.3) of imitation-first versus understanding-first views. SCM reconciles them, by showing how different types of copying and action understanding can be built up, enabled by differently articulated mirroring structures combined with monitored inhibition. Mirroring can enable goal emulation without a capacity for imitation; with monitored inhibition, it can enable understanding the goals of another's action. Instrumentally articulated imitation may require understanding of goals plus mirroring of movements; covert imitation can then enable fuller understanding of the instrumental structure of observed actions.

Whether such informational structures correspond directly to personal-level empathic understanding of another's actions or to knowledge of other minds are further questions. Layer 3's “first person plural” is informationally prior to layer 4's self/other distinction. Subpersonally, the problem of “knowledge” of other minds is reconfigured: it is neither one of starting from information about self and constructing a bridge to information about others, nor one of starting from information about others and from these resources generating information about self. Rather, the similarity of own and others' acts comes first, with mirroring. Monitored inhibition then distinguishes subpersonally between instrumentally structured action “centered” on self versus “decentered” onto another; self-centering and other-centering of agency arrive together. SCM gives concrete subpersonal form to the interdependence and parity of information about self and other intentional agents. The subpersonal job that remains is not to bridge a gap between self and others, but to track distinctions among them, especially when multiple other agents are in play.

Is the subpersonal priority of intersubjective information reflected at the personal level, and if so, how? Is the personal-level problem of knowledge of other minds similarly reconfigured, to avoid both first-person to third-person and third-person to first-person inferences, that is, both the argument from analogy and from behaviorism? Not necessarily. Further questions arise, about phylogeny, development, the structure of mature capacities to understand others, and the epistemology of such understanding. The role of subpersonal information in the epistemology of other minds raises general philosophical issues about the roles of reliable information and justification in knowledge: for instance, can reliable subpersonal information support knowledge of other minds? SCM does not in itself answer these questions. Rather, it provides generic, adaptable tools for framing specific hypotheses. Care is needed: We shouldn't assume an isomorphic projection from the subpersonal level to the personal level. But we should allow that understanding subpersonal processes can contribute to understanding the personal level; this doesn't require interlevel isomorphism.

Gordon's simulation theory (sect. 2.3) suggests responses to such questions. It has affinities with SCM: both build mindreading on resources for perception and action. But there are also differences: In SCM, the priority of “first-person plural” information over the self/other distinction is subpersonal, whereas Gordon's account of how constitutive mirroring “multiplies the first person” links subpersonal mirroring directly to personal-level understanding of other minds: Observed behavior and mirroring are made sense of together, under a common scheme of reasons, and incoherent mental states assigned to different persons (Gordon Reference Gordon, Davies and Stone1995a, pp. 56–58, 68; 2002; 2005; cf. Hurley Reference Hurley1989; Reference Hurley1998).

Actual/possible and self/other distinctions are necessary for much explicit theorizing and for aspects of the normativity and intersubjectivity of the personal level. SCM thus helps to understand how features of the personal level could be informationally enabled by subpersonal processes. It suggests that these distinctions share an informational basis in monitoring of motor inhibition: theoretical informational resources arise from practical ones. But SCM does not specify a phylogenetic or developmental priority between the subpersonal actual/possible and self/other distinctions, which is left to specific first-order hypotheses: Layers 2 and 4 could combine even if layers 3 and 4 do not or if layer 3 is missing altogether; and layers 3 and 4 could combine even if layers 2 and 4 do not. Hence, in different species or stages of development, we might find creatures with action understanding but not instrumental deliberation, or vice versa.

Some general predictions derive from layer 4. First, there could be deficits in inhibitory capacities although capacities to copy, or to act on affordances, are intact (as in Lhermitte's imitation and utilization syndrome patients). Second, the priority of mirroring to inhibition of copying predicts that people with intact inhibitory capacities should nevertheless retain underlying default tendencies to copy (as in perceptual induction, imitation interference effects, priming, and chameleon effects; see sect. 2.1). In addition, SCM provides generic, adaptable tools for framing specific hypotheses. A specific first-order hypothesis might be that capacities for instrumental deliberation and for understanding others' actions are enabled by shared inhibitory resources at layer 4, predicting that deficits in these capacities should be associated (Frith Reference Frith1992, pp. 81–83, 93; cf. Barkley Reference Barkley2001). Another first-order hypothesis might predict pathological dissociations of capacities supported by layers 2 plus 4 from those supported by layers 3 plus 4. Other specific first-order predictions might concern the interleaving of various copying abilities with various types of understanding of others' actions, in an ontogenetic or phylogenetic progression leading to imitative learning and mindreading capacities. Copying without inhibition should come earlier in such progressions; capacities to inhibit copying and to distinguish others' intentional actions from one's own should develop together. Yet another hypothesis might be that adding layer 4's monitored inhibition of output to layer 3's mirroring enables a transition from behavior copying to mindreading that enables effective use of mirror heuristics by cooperators (sect. 2.3).

3.5. Layer 5: Counterfactual input simulation

Finally, the system can be taken offline for input as well as output (Fig. 5). Counterfactual inputs can simulate different possible acts by others and their results. Monitored simulation of inputs to control circuits with simulative prediction and mirroring functions can provide information distinguishing between others' actual and possible acts. This social extension of counterfactual information, combined with simulation of different possible acts by self and their results, provides information about how possible acts by self could result in possible acts by others with further results, and vice versa. These combined functions provide enabling information for strategic game-theoretic deliberation, coordination, and cooperation. To distinguish possible from actual acts by others, information is needed about whether inputs are simulated; so simulation of inputs must be monitored. Simulative informational resources for strategic in addition to instrumental deliberation enrich the practical foundations for more general capacities to manipulate counterfactual information and theorize counterfactually.

Figure 5. Fifth layer: Counterfactual input simulation enabling strategic deliberation.

Recall the discussion of TT versus ST in section 2.3. Again, SCM shows how they can be reconciled. Differentiating and tracking interacting means/ends relations for multiple possible acts by self and by multiple other agents make acute informational demands. Despite their foundational role, simulation mechanisms may be insufficient to provide information for such multi-agent, multi-possibility tracking with ramifying paths of decentering. Meeting these demands, and further demands in differentiating the epistemic states of multiple others, probably requires SCM's practical simulative informational basis for understanding other agents to be supplemented by language-dependent functions and theorizing. Mindreading, like social learning and instrumental control, is a graded phenomenon, not all or nothing (Tomasello 1999). Language can build on SCM's foundational actual/possible and self/other distinctions to enable interpretative understanding of multiple others with multiple alternatives and varying beliefs. SCM hypothesizes that mindreading has practical foundations, in simulative mirroring of means/ends relations, but allows that mature mindreading with all the bells and whistles (including understanding false beliefs) requires both simulation and language-based theorizing.

Specific predictions deriving from level 5 could concern ontogeny or phylogeny. First, since understanding goals and action is foundational for mindreading, it should be prior to understanding others' epistemic attitudes (on phylogeny, see Tomasello & Call Reference Tomasello and Call1997; on ontogeny, see Rakoczy et al. Reference Rakoczy, Warneken and Tomasello2007). Second, understanding the instrumental structure of observed action can precede, and may be a phylogenetic precursor of, understanding linguistic structure. The instrumental recombinant structure of imitative learning may combine with learned manipulation of external symbols to support the richer recombinant structure of language. Third, the fundamental transition from agents “playing against nature,” instrumentally, to agents playing against one another, strategically, can be supported by simulative mechanisms. Nevertheless, simulative mechanisms without linguistic capacities may not enable advanced mindreading, such as multi-person strategic deliberation or false-belief attribution.

4.1. Levels versus layers

Unlike some of the work surveyed earlier, SCM distinguishes neural, functional subpersonal, and personal levels of description. Each of SCM's functionally described layers raises questions at the level of neural implementation, as well as providing information enabling personal level capacities (Table 2). Clarity and progress are served by distinguishing levels and framing issues explicitly at a given level, or as concerning interlevel relations. Sliding between levels on a priori assumptions of isomorphism is unjustified. Nevertheless, one level can shed light on another. We can look “down” a level, seeking neural implementations of aspects of SCM's functional architecture, or “up” a level, considering what SCM's functional architecture would enable persons to do.

Table 2. The shared circuits model: Layers and levels

*JK&AC: Why the question mark? We think that Susan meant to indicate that the search for the neural basis of layer 5 might begin with mechanisms that underpin imagery, but that this was just the beginning. Just how the brain might go about simulating inputs was one of the many open empirical questions that SCM raises, which Susan hoped might be taken up in the commentaries or in future work developing SCM.

The relationship between conscious phenomena and physical reality in behaviour control: The need for simplicity through phenomenological clarity Behrendt, Ralf-Peter MRCPsych, The Retreat Hospital, York YO10 5BN, United Kingdom. rp.behrendt@btinternet.comMirroring cannot account for understanding action Carpendale, Jeremy I. M. and Lewis, Charlie Department of Psychology, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada; Department of Psychology, Fylde College, Lancaster University, Bailrigg, Lancaster LA1 4YF, United Kingdom. jcarpend@sfu.cac.lewis@lancsater.ac.ukhttp://www.psyc.sfu.ca/people/faculty.php?topic=finf&id=67 http://www.psych.lancs.ac.uk/people/CharlieLewis.htmlCan the shared circuits model (SCM) explain joint attention or perception of discrete emotions? Chakrabarti, Bhismadev and Baron-Cohen, Simon Autism Research Centre, Cambridge CB2 8AH, United Kingdom. bhisma@cantab.nethttp://people.pwf.cam.ac.uk/bc249 sb205@cam.ac.ukhttp://www.autismresearchcentre.comThe neural underpinnings of self and other and layer 2 of the shared circuits model Furey, Linda and Keenan, Julian Paul Cognitive Neuroimaging Laboratory, Montclair State University, Upper Montclair, NJ 07043. fureyl1@mail.montclair.eduKeenanj@mail.montclair.eduhttp://www.cogneurolab.comShared circuits in language and communication Garrod, Simon and Pickering, Martin J. Department of Psychology, University of Glasgow, Glasgow G12 8QB, United Kingdom; Department of Psychology, University of Edinburgh, Edinburgh EH8 9JZ, United Kingdom. simon@psy.gla.ac.ukhttp://www.psy.gla.ac.uk/~simon/ martin.pickering@ed.ac.ukhttp://www.psy.ed.ac.uk/people/martinp/index_htmlDoes one size fit all? Hurley on shared circuits Goldman, Alvin I. Department of Philosophy and Center for Cognitive Science, Rutgers University, New Brunswick, NJ 08901-2992. goldman@philosophy.rutgers.eduhttp://fas-philosophy.rutgers.edu/goldmanImitation as a conjunction Heyes, Cecilia Department of Psychology, University College London, London WC1 6BT, United Kingdom. c.heyes@ucl.ac.ukhttp://www.psychol.ucl.ac.uk/celia.heyes/netintro.htmShared circuits, shared time, and interpersonal synchrony Hove, Michael J. Department of Psychology, Cornell University, Ithaca, NY 14853. mjh88@cornell.eduMesial frontal cortex and super mirror neurons Iacoboni, Marco Department of Psychiatry and Biobehavioral Science, Semel Institute for Neuroscience and Human Behavior, Brain Research Institute, David Geffen School of Medicine at UCLA; and Ahmanson-Lovelace Brain Mapping Center, Los Angeles, CA 90095. iacoboni@loni.ucla.eduhttp://iacoboni.bmap.ucla.eduFlexibility and development of mirroring mechanisms Longo, Matthew R. and Bertenthal, Bennett I. Institute of Cognitive Neuroscience and Department of Psychology, University College London, London WC1N 3AR, United Kingdom; Department of Psychological and Brain Sciences, College of Arts and Sciences, Indiana University, Bloomington, IN 47405. m.longo@ucl.ac.ukbbertent@indiana.eduhttp://www.homepages.ucl.ac.uk/~ucjtml0/Failure, instead of inhibition, should be monitored for the distinction of self/other and actual/possible actions Makino, Takaki Division of Project Coordination, Tokyo University, Chiba 277-8568, Japan. mak@scint.dpc.u-tokyo.ac.jphttp://www.scint.jp/~mak/The social motivation for social learning Nielsen, Mark School of Psychology, University of Queensland, Brisbane QLD 4072, Australia. nielsen@psy.uq.edu.auhttp://www.psy.uq.edu.au/people/personal.html?id=636What kind of neural coding and self does Hurley's shared circuit model presuppose? Northoff, Georg Laboratory of Neuroimaging and Neurophilosophy, Department of Psychiatry, Otto-von-Guericke University of Magdeburg, 39120 Magdeburg, Germany. georg.northoff@medizin.uni-magdeburg.dehttp://www.med.uni-magdeburg.de/fme/znh/kpsy/northoff/How do shared circuits develop? Oberman, Lindsay M. and Ramachandran, Vilayanur S. Department of Neurology, Beth Israel Deaconess Medical Center, Boston, MA 02215; Department of Psychology, University of California, San Diego, La Jolla, CA 92093-0109. loberman@bidmc.harvard.eduvramacha@ucsd.eduMore than control freaks: Evaluative and motivational functions of goals Paglieri, Fabio and Castelfranchi, Cristiano Istituto di Scienze e Tecnologie della Cognizione, Consiglio Nazionale delle Ricerche, 00185 Rome, Italy. fabio.paglieri@istc.cnr.ithttp://www.media.unisi.it/cirg/fp/paglieri.html cristiano.castelfranchi@istc.cnr.ithttp://www.istc.cnr.it/createhtml.php?nbr=62Putting the subjective back into intersubjective: The importance of person-specific, distributed, neural representations in perception-action mechanisms Preston, Stephanie D. Department of Psychology, University of Michigan, Ann Arbor, MI 48109. prestos@umich.eduhttp://www.umich.edu/~prestosIn search of a conceptual location to share cognition Semin, Gün R. and Cacioppo, John T. Faculty of Social Sciences, Utrecht University, 3584 CS Utrecht, The Netherlands; Department of Psychology, The University of Chicago, Chicago, IL 60637. GR.Semin@psy.vu.nlhttp://www.cratylus.org Cacioppo@uchicago.eduhttp://psychology.uchicago.edu/people/faculty/cacioppo/Goals are not implied by actions, but inferred from actions and contexts van Rooij, Iris, Haselager, Willem and Bekkering, Harold Nijmegen Institute for Cognition and Information, Radboud University Nijmegen, 6500 HE Nijmegen, The Netherlands. i.vanrooij@nici.ru.nlw.haselager@nici.ru.nlh.bekkering@nici.ru.nlImitation, emulation, and the transmission of culture Whiten, Andrew Centre for Social Learning and Cognitive Evolution, and Scottish Primate Research Group, School of Psychology, University of St Andrews, St Andrews KY16 9JP, United Kingdom. aw2@st-andrews.ac.ukhttp://psy.st-andrews.ac.uk/people/lect/aw2Imitation and the effort of learning Williams, Justin H. G. Department of Child Health, University of Aberdeen School of Medicine, Royal Aberdeen Children's Hospital, Aberdeen AB25 2ZD, United Kingdom. Justin.williams@abdn.ac.ukhttp://www.abdn.ac.uk/child_health/williams.shtmlBootstrapping the mind Kiverstein, Julian and Clark, Andy School of Philosophy, Psychology and Language Sciences, University of Edinburgh, Edinburgh, EH8 9JX, Scotland, United Kingdom. j.kiverstein@ed.ac.ukandy.clark@ed.ac.uk