The evolution of vocal imitation. 

Dr. John R. Skoyles
6 Denning Road

'Children acquire a good deal of their verbal and non-verbal behavior by causal observation and imitation of adults and older children', Noam Chomsky (1959, p. 49). 

What might explain Chomsky's observation? According to Gisela Speidel and Keith Neilson it is not farfetched to suggest that we possess, 'a biological language acquisition device in our ability to imitate speech sounds' (1989, p. 11). But how could such a device 'mirror' speech sounds into the very different modality of articulation -- particularly given that the vocal tracts of children are of different shape and size to those of adults? In 1992, mirror neurons (premotor neurons that not only instruct motor actions but detect and observe them in others) were discovered in the Broca area (Di  Pellegrino, Fadiga, Fogassi, Gallase, & Rizzolatti, 1992). Vilayanur Ramachandran, a former president of the Society for Neuroscience, has claimed: 'mirror neurons will do for psychology what DNA did for biology' (1999). Rizzolatti, the discoverer of mirror neurons (together with Arbib, 1998) has already linked them to the evolution of speech, (though not to the above problem of imitation). Here following earlier formulations (Skoyles, 1997), I link mirror neurons and imitation to the nature and evolution of speech. 

Imitation is an overlooked but logical key issue in linguistics. Without a child's ability to imitate tens of thousands of words, spoken vocabulary could not be passed on to new generations preventing the continued existence of language. Children must therefore necessarily possess a biological means to imitate. 

They do: children as young as 12 weeks imitate overheard vowels (Kuhl & Meltzoff, 1996). At 18 weeks of age infants spontaneously copy vocal expressions (Kuhl & Meltzoff, 1982). At around two years of age children imitate between 5% and 45% of their words (Bloom, Hood, Lichtbown, 1974). In addition, an unknown number of delayed imitations exist that are reused several days or weeks later (Miller, 1977). This enables vocabulary expansion: children that are more imitative of new words (but not ones already in their vocabulary) at 13 months have larger noun vocabularies, four months later, than those that do not (Masur, 1995). In older children, the ability to repeat nonword phone sequences predicts vocabulary increase (Gathercole &  Baddeley, 1989). Conversely, defects in word imitation link to problems in language acquisition (Bishop, North & Donlan, 1996). 

Empirical evidence exists for a special imitation pathway. Research upon speech shadowing finds a 'privileged' input/output speech loop that is separate to the rest of the speech system (McLeod & Posner, 1984). And research upon brain impaired and injured individuals finds a direct link between phonological analysis input and motor programming output (McCarthy & Warrington, 1984). 

How might we build an information processing system that could imitate vocalisations? First, a clue exists in that children can imitate adults: vocal imitation therefore cannot directly imitate physical movements. Vocalisations, however, are made in regard to motor targets (Shaffer, 1984): for example, if a lip is blocked while articulating a consonant, the vocal apparatus makes a target-related correction (Gracco & Löfqvist, 1994). This suggests that imitation can be based upon copying motor goals (or at least goals inferred from motor trajectories). This is doubly convenient since not only are these independent of the particular shape and size of a vocal tract but they might be directly detectable in spoken sound. 

Such goals must be ones that a child can readily perceive and copy from overheard vocalisations. We must thus ask what kind of properties would motor goals  theoretically need to have if they were to be easily picked up by children. 

First, goals must wherever possible exploit categoricalness. Along some sensory continua percepts are noncategorical [like colours], and along other categorical and separate in discrete categories. If such categorical percepts were used for defining goals, this would make them robust against any slight imperfection in achieving them (or hearing them in another).  

Second, it is easier to imitate the contrast between opponent properties than an intermediate degree since all that needs to be imitated consistently is the opposed contrast. Thus, motor goals should be set up in contrasts with each other. 

Third, the parameter percepts that define goals should be innate. The newborn faces the problem that speech is one 'all one great blooming, buzzing confusion'. How will it avoid attempting to use the wrong speech information for motor targets? Innate percepts, however, would innately bias a child to the right information. 

Now we have the beginnings of a theory. Science progresses through deconstruction and construction: taking apart familiar phenomena to show that they can be built from other ones. If imitation is based upon motor goals then we should be able to find in speech entities that are  characterised by articulatory information with the above properties. 

Two and half thousand years ago, Panini found that the smallest unit of speech, the phone, was not characterised by auditory features but articulatory ones. Since then all phones have been characterised in this way. For example, the International Phonetic Alphabet defines them using 11 places of articulation and ten manners (plus voicing). Phonetics courses and textbooks teach phone identification and production in terms of movements in a student's own vocal tract. Indeed, research finds that instruction in the 'silent making' of phones leads to better recognition than auditory based instruction (Catford & Pisoni, 1970). Research finds, moreover, that lesions which impair phone production impair their perception (Baso, Casati, & Vignolo, 1977), and that the repeated listening to a phone primes the pronunciation of phones containing the same articulatory features (Cooper & Lauritsen, 1974). 

The articulatory information of phones link to the above  properties required for a target to be readily imitated by children since they are (i), contrastive (i.e., [b] vs [d]; [t] vs [d]), (ii) categorical (Harnad, 1987), and (iii) innate -- infants can initially detect categorical distinctions in unfamiliar foreign languages, though this ability is usually lost by their first year (Eimas, Miller & Jusczyk, 1987).

Is this coincidence? I suggest not: the existence of language is necessarily linked with the existence of a vocabulary acquisition device. Therefore presently unknown means must exist to do this. Mirror neurons look like a possible candidate but they need complimentary specialised information – motor targets to imitate that overcome the problem that vocal traits vary in size and shape, are hidden, and must be copied by children. Theoretically we can predict what they will look like in speech. Conceivably, speech could contain both such motor targets and phones as separate phenomena. William of Occam however has taught us not to multiply entities beyond necessity. We should therefore look closely at the possibility that they concern one and the same phenomena. 

If doubts remain then neurology helps remove them: 81% of areas upon the brain which when simulated electrically disrupt phone identification also disrupt the imitating of oral movements, and vice versa (Ojemann, 1983). Brain injuries in the speech areas also have a 0.9 correlation between impairing the duplication of oral movements, and the production and perception of phones (Kimura & Watson, 1989). Mirror neurons for nonoral actions have been found in these areas in humans (Gallese, Fadiga, Fogassi,, & Rizzolatti, 1996), and in homologous ones in nonhuman primates (Di  Pellegrino, Fadiga, Fogassi,  Gallase & Rizzolatti, 1992). Imitation and phone processing are thus intimately linked in the brain; the above account parsimoniously explains why. 

The vocabulary acquisition device theory offered here is consistent with the claims of the motor theory of speech perception. For example, that 'the objects of speech perception are the intended phonetic gestures of the speaker' (Liberman & Mattingly,  1985, p. 2). And that speech perception 'recovers the coarticulated gestures that produced it. These gestures are the primitives that the mechanisms of speech production translate into actual articulator movements, and they are also the primitives that the specialized mechanisms of speech perception recover from the signal' (Liberman & Mattingly, 1989, p. 491). This theory differs, of course, in being a motor theory of speech imitation.

The vocabulary acquisition device I suggest evolved from mirror neurons in the Broca area. Human language could not begin until patterns of vocalisations (words) were being passed down generations. Such vocal replication would put different kinds of motor goals into a selective competition. Those that were most easily copied (and so made easily transmitted words) would be selected, propagated and so end up forming the phonetic/vocalisation basis of speech. Due to the advantage individuals with a good ability to acquire vocabulary had over those that did not, mirror neuron circuits themselves would have also become selected thus expanding the Broca's area. This would have feed into the further development of speech. Ramachandran compares mirror neurons in importance to DNA: this theory suggests his analogy might not be far wrong. 

References.

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