Introduction

The original Mirror System Hypothesis (Arbib & Rizzolatti, 1997; Rizzolatti & Arbib, 1998) states that:

H1. The parity requirement for language in humans is met because Broca's area evolved atop the mirror system for grasping with its capacity to generate and recognize a set of actions.

Arbib (2005) follows Arbib (2002) in distinguishing a language-ready brain (equipping the child to learn a language) from a brain that “has” language (in the sense of, e.g., an innate “principles and parameters” Universal Grammar). It also stresses the role of “complex imitation” in the evolution of the language ready brain, and offers an account based on the following stages. (When I speak of a “stage” in phylogeny, I do not have in mind a discontinuous change in the phenotype, but rather the coalescence of multiple changes that can be characterized as forming a global pattern that may emerge over the course of tens of millennia.)

The first three stages are pre-hominid:

S1: Grasping.

S2: A mirror system for grasping shared with the common ancestor of human and monkey.

S3: A simple imitation system for object-directed grasping through much repeated exposure. This is shared with common ancestor of human and chimpanzee.

The next three stages then distinguish the hominid line from that of the great apes:

S4: A complex imitation system for grasping.

S5: Protosign, a manual-based communication system, breaking through the fixed repertoire of primate vocalizations to yield an open repertoire.

S6: Proto-speech, resulting from the ability of control mechanisms evolved for protosign coming to control the vocal apparatus with increasing flexibility.

The final stage is claimed to involve little if any biological evolution, but instead to result from cultural evolution (historical change) in Homo sapiens:

S7: Language: the change from action-object frames to verb-argument structures to syntax and semantics; the co-evolution of cognitive and linguistic complexity.

The Mirror System Hypothesis is simply the assertion that the mechanisms which get us to the role of Broca’s area in language depend in a crucial way on the mechanisms established in Stage S2. The above seven stages provide just one set of hypotheses on how this dependence may have arisen.

Looking in more detail at Arbib (2005), we find that in addition to analyzing the above stages, it replaces the original MSH by four hypotheses:

H2. Language readiness evolved as a multi-modal manual/facial/vocal system with protosign providing the scaffolding for protospeech – these then co-evolved in an expanding spiral to provide “neural critical mass” for protolanguage.

H3. Protolanguage was holophrastic – “protowords” were semantically more akin to phrases or sentences of modern language than words “as we know them”.

H4. Biological evolution gave humans a language-ready brain, but the emergence of human languages from protolanguage was a matter of history, not biology.

H5. While the original MSH focused on macaque F5 and Broca’s area, F5 is part of a larger F5-PF-STS system in the macaque and this “lifts” to a larger frontal-parietal-temporal language-ready system in the human brain.

Between them, H2-H5 constitute an extended MSH. What needs stressing is that these four hypotheses are almost independent – and thus each must stand on its own. My Response to the commentaries in BBS addresses arguments pro and con each of these four hypotheses. In the present discussion article, I want to focus on those commentaries which, while generally endorsing the role of the mirror system in evolution of the language-ready brain, stress the need to further consider mechanisms supporting conversation, motivation, prosody and theory of mind.

Complex Imitation

I want to stress (as Arbib 2002 did not sufficiently do) that the ability for complex imitation has a perceptual side whose importance cannot be overemphasized. This is the ability for complex action analysis: recognizing another's performance as combining actions which can be approximated by variants of actions already in the repertoire a set of familiar actions and then repeat them. Complex imitation then rests on the ability to exploit that analysis to ground imitation of the observed action (which can be further tuned by experience).

When the child is acquiring language, the whole process of complex imitation comes into play as the child acquires phonology and lexicon, and learns which “sentential actions” may be deployed to achieve its goals. However, when adults talk to each other, it is only the complex action analysis (recognizing what the other said) that comes into explicit play. Thus Bickerton (2005) is quite right to observe that when someone addresses you, you do not just imitate what they said. The human mirror system creates a representation that can be used for feedback control, imitation (which monkeys do not exhibit) or generating some appropriate response while inhibiting mimicking. Only in pathology does this inhibition fail, yielding compulsive imitation (echopraxia; Podell et al., 2001).

Conversation

The developing child must learn both affordances (opportunities for action as presented in the sensory stream) and effectivities (what the body can do; Shaw & Turvey, 1981) as two sides of the mirror system. By directing the child's attention to its own effectivities in relation to affordances, the caregiver narrows the search space for learning, and thus enhances that learning (Zukow-Goldring, 1996). These practices may pave the way to early word learning (Zukow-Goldring, Rader, & Cain, 2001). The prolonged period of infant dependency in humans combines with caregiving to provide conditions for complex social learning.

But in some sense the success of the caregiver depends on both the attention of the caregiver to the child and of the child to the caregiver. Nagy and Molnar (2004) demonstrate that newborn infants communicate by using “imitation” right after birth. The cycle of turn taking in “imitating” a small repertoire of  “almost innate” gestures is crucial in establishing the social pattern of turn taking (cf. “motherese”; Falk 2004). I agree, but suggest that this neonatal imitation is based on moving single effectors and thus differs from goal-directed imitation. (Studdert-Kennedy, 2002 discusses data consistent with the view that the infant at first imitates sounds by moving one articulator and only later coordinates articulators.) Social reciprocity in neonatal imitation may be a necessary precursor for complex imitation, establishing that “I am like the other” (Zukow-Goldring, 2005). This suggests an innate basis for “conversation” that precedes its pragmatic function; Nagy (2004) suggests that language develops from these early intersubjective “conversations” (Trevarthen, 2001). Biological evolution may have selected for neonatal imitation as a basis for complex imitation.

Motivation

Prudkov (2005) downplays the key role I hypothesize for complex imitation in the evolution of the language-ready brain. Instead, he argues that the complexity of languages builds on the ability of the human brain to construct diverse goals. He suggests that animals can only form learned motivations when basic drives are activated. However, animals can acquire secondary reinforcers, etc. Chimpanzees have the ability to develop non-innate subgoals (e.g., cracking nuts). The mirror system is well-linked to the motivational system in the macaque. Arbib (2005) shows that the F5 mirror system for grasping is best understood within the larger F5-PF-STSa mirror system for manual and oro-facial actions. Rizzolatti et al. (2001) observe that STSa is also part of a circuit that includes the amygdala and the orbitofrontal cortex and so may be involved in the elaboration of affective aspects of social behavior. Thus Prudkov’s transition to “non-innate motivation” may be less proximate for the evolution of the language-ready brain per se than complex imitation, which made possible the rapid acquisition of new skills.

Theory of Mind

Let me first state my prejudice that “Theory of Mind” (ToM) is a slippery concept, and that I think it is better looked at as an assemblage of diverse skills for relating one’s own action to those of others. Thus different creatures may have different potentials for ToM, and the particular bunch of ToM skills exhibited by a group of humans may reflect their history perhaps even more than their biology.

Arbib (2002) argues that pantomime plays a key role in the transition from complex imitation of praxis (using the hands for manipulation) to complex imitation for communication (protosign as scaffolding for protospeech – see Fogassi & Ferrari (2004), Arbib (2004) and MacNeilage & Davis (2004) for further debate on this notion). However, Fabrega (2005) stresses that successful pantomime presupposes social cognition, awareness of self, and goal setting. In other words, if we take mirror neurons “seriously” as a tool for recognizing the actions of others (I claim that was an exaptation of their earlier role in providing visual feedback for dexterous hand movements) – whether for imitation or communication then the social role (“I am like the other”) must be an explicit part of our account. In this respect, it is worth noting, with Arbib & Mundhenk (2004) that mirror neurons per se only recognize an action – it is the task of other parts of the brain to recognize the agent of the action and to bind the neural representations of agent and action appropriately.

Thinking this through returns us to the topic of conversation: utterances are usually part of an ongoing interaction between 2 or more speakers or signers. In line with this, Kotchoubey (2005) emphasizes pragmatics, e.g., what we say depends on the mental state of our “hearer”: However, his claim “We do not use language to transmit information, but to persuade and motivate.” seems a false dichotomy. “Look at this beautiful flower” combines information “This flower is beautiful” and persuasion “Look at this flower”. Kotchoubey (p.c.) stresses that his starting point is cooperation between two or more humans, reinforcing the claims of MSH for relating praxic and communicative actions.

Williams (2005) sees the greatest evolutionary advantage conferred by spoken language as its ability to communicate mentalistic concepts (Theory of Mind, ToM). He stresses selection pressure for social maneuvering where I have emphasized physical activities. However, I think he would agree that this is not a simple case of either/or. Surely the two “domains of discourse” complement each other. Williams (2005) notes the possible role of the mirror neuron system in mentalizing and this is indeed a theme that has been developed by Gallese (e.g., 2003) and others (e.g., Meltzoff & Decety, 2003). We need to investigate whether an account can be given of a shared evolution of  “mirror systems” suiting both ToM and complex imitation. I hypothesize that the ancestral mirror system for manual praxis was distinct from the putative mirror system for facial expression of emotion. The former would support pantomime and thence on to multi-modal symbols; and then the availability of symbols could enrich the latter to yield rudiments of ToM.

Williams (2005) and Théoret & Fecteau (2005) see autism as providing a window on the role of the mirror system in ToM and language. (Théoret & Fecteau add analysis of blindness). Deficits in autism are prominent in speech associated with social communication but praxic aspects of language are fairly well preserved. Perhaps what is affected is not so much language per se as the integration of this with affect and ToM. Interestingly, autistics may exhibit stereotypic mimicking (which monkey’s don’t have). Thus it must be reiterated that a fully functional human mirror system can inhibit mere repetition (echopraxia and echolalia) and instead relate the perception of perceived actions to the planning of an appropriate course of action.

Prosody

Kotchoubey (2005) and Fitch (2005) note that my emphasis on cognitive-symbolic aspects of language ignores prosody. Kotchoubey notes that prosody subserves both affective prosody (emotional expression) and linguistic prosody (as in distinguishing between an assertion and a question) and that both forms of prosodic information are processed mainly in the right temporal lobe. In similar vein, Gilissen (2005) notes that human vocal behavior resembles monkey calls in the emotional intonations superimposed on the verbal component though I would stress [sic] that such calls lack the semantic openness that is the hallmark of protolanguage and, a fortiori, language. Kotchoubey (p.c.) observes that in many languages,intonation is the only distinction between question and declaration. He thus suggests that linguistic prosody is a part of the right hemisphere so closely controlled by the left that they cannot work without each other. This is reminiscent of the coupling of gesticulations to the syntax and semantics of a specific language (Kita & Özyürek, 2003).

Gilissen (2005) cites Falk’s (2004) evolutionary perspective on the hypothesis that, as human infants develop, a special form of infant-directed speech (motherese) provides a scaffold for their eventual acquisition of language. This enriches our discussion of the role of the caregiver in neonatal “conversation”. Gilissen says that the special vocalizations of human motherese are in marked contrast to the relatively silent mother/infant interactions that characterize chimpanzees, yet suggests a possible link between monkey calls and motherese. This apparent contradiction suggests that the affective content of motherese (and protolanguage) builds upon the monkey vocalization system, but the information content of motherese (and protolanguage) has a complementary evolutionary history. Kotchoubey (2005) suggests that the left hemispheric subsystem develops as described by MSH to subserve the cognitive-symbolic function, whereas the right hemispheric subsystem is a direct successor of monkey vocalization mechanisms and gives language its intonational color. It is a long standing observation (Hughlings Jackson 1879-80) that imprecations survive damage to the human brain that blocks normal speech. Arbib (2002) thus suggested that the language-ready brain integrates action-oriented and affect-oriented systems in a pattern of cooperative computation.

Fitch (2005) adopts Darwin’s hypothesis that our prelinguistic ancestors possessed an intermediate "protolanguage" that was musical and that music scaffolds the early structural and imitative aspects of language (prosody). He sees the semantic stage as coming later. However, even if we accept the importance of “musicality”, it does not follow that the co-evolution of vocal and manual gesture is tied more closely to music than pantomime and linguistic communication – but it does encourage us to investigate how dance and music might enrich MSH.

References

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