On the tip of my tongue
Say these words out loud.
‘The tip of the tongue, the lips and the teeth.’
Whilst you were speaking, what were your tongue and lips doing? How were you breathing? Can you breathe in and still speak?
Now try reciting this (rather peculiar) poem. It contains every sound (phoneme) used in spoken English.
The pleasure of Shawn’s company
Is what I most enjoy.
He put a tack on Ms. Yancey’s chair
When she called him a horrible boy.
At the end of the month he was flinging two kittens
Across the width of the room.
I count on his schemes to show me a way now
Of getting away from my gloom.
This ‘panphonic’ poem was written by linguist Neal Whitman, and used in the film Mission: Impossible 3 (2006).
Did you notice that whilst you were talking, your tongue and lips never moved sideways?
Talking connects us across all cultural boundaries, and sets us apart from other animals. Our diverse vocal repertoire, contributing to nearly 7,000 languages worldwide, has no equivalent anywhere in the animal kingdom. We arrange complex sounds into phrases with rhythm, stress and intonation (prosody), and deliver them with visual emphasis using facial expressions and gestures.
Traditionally, linguists have considered that our speech is too complex to have arisen by natural selection, suggesting instead that it results from a sudden event such as a ‘freak’ genetic mutation.
This view is at odds with what we understand about the rest of our biology. Evolution works by selecting from existing variation in forms and behaviours. Adaptations are not a ‘best-design’ solution to a survival problem, but a balance of innovations that arrive with inherited constraints.
During our first seven years of life we learn to articulate the sounds unique to the language(s) we hear. We structure these sounds into syllables, and use them to build words, phrases and sentences that convey meaning. We also use these sounds to coin new words and invent new meanings for these sounds. For our ancestors to begin to do this, however, requires that the physical ability to make this diversity of sounds already to have been in place.
How do we physically produce speech sounds?
Speech is musical; whilst we produce precisely articulated sounds, our voices also use pitch, tone and timbre to emphasise words and give rhythm and shape to our phrases. This requires precise coordination of multiple muscles in the chest, larynx, throat, mouth and face. These movements literally do not ‘come to mind’. Instead we focus on what we are saying, and how the listener responds.Sound generation (phonation) in almost all mammals involves coordinating their breathing with the control of tension in the vocal folds of the larynx. We selectively process the harmonics from these basic sounds, and articulate precise sound sequences using rapid and rhythmic movements of the tongue, lips and associated structures.
At the simplest level, speaking involves alternating open-closed movements of the jaw at the same time as generating sound in the larynx. This produces an alternating stream of open (resonant) and closed (muted) sounds. We build words and phrases from these alternating ‘segments’, using the lips and tongue to produce precisely articulated consonants and control pitch, timbre, tone and stress.
We vocalise as other mammals do, using our feeding and breathing apparatus. The muscle movements that operate both chewing and speaking are controlled by rhythmic nerve impulses from ‘Central Pattern Generators’. These are autonomous nerve ‘modules’ in the lower brain and spinal cord. They co-ordinate all our repetitive movements, from walking to vomiting.
Which aspects of our speaking abilities are found in other animals?
There is no animal equivalent to the combined movements that make up our vocal cycle, although other animals make all of these movements. Most mammals call with their mouths open, using coupled Central Pattern Generators that link the out-breath with sound production (phonation) in the larynx. A few animals such as dogs make occasional calls using a partial open-close oscillating jaw movement, although they typically repeat the same sound (bow-wow-wow). We coordinate the circuits for breath control and phonation with another set of pattern generators that operate the rhythmic movements of our jaw, lips and tongue.
These movements have other functions, as in the suckling of newborns. This ability defines us as mammals. But these movements may also be linked to talking. James Lund and co-workers suggest that the human Central Pattern Generators controlling chewing, licking and sucking also participate in speech.
Peter MacNeilage goes on to suggest that the rhythmic repetitive movements used for eating have been coordinated in mammals since the clade arose some 200 Ma ago, and that they lie at the root of our articulations skills. As we speak, our tongue moves up and down, and front to back in the mouth. Chewing also includes sideways motions of the tongue and jaw. These are not included in our vocal movements, indeed they would leave us more prone to biting our tongue. MacNeilage proposes that coupling our pre-existing capability for making vocal calls with this subset of movements used during eating, gave our more immediate ancestors the capacity to articulate simple ‘proto-syllables’.
Does this then apply to our nearest relatives, the chimps? Philip Lieberman has shown that their vocal apparatus is anatomically suitable to produce a range of human syllables, and yet they do not speak. This suggests that they cannot coordinate their Central Pattern Generator signals for vocal sound production and chewing. The reason for this appears to be linked to differences in cognition. Recent work comparing the neurology underpinning chimp calls and human word-based speech has found that the neural circuits driving these respective vocalisations originate from different parts of the brain.
However chimps and other primates do make rhythmical face and jaw movements, producing lipsmacks, tongue smacks, teeth-chatters and other facial gestures. Lipsmacks involve moving the jaw without the teeth coming together, as in human speech, and are often made by juveniles as they approach their mother to suckle. Primate grooming is a 1:1 interaction using touch, eye contact and other positive one-to-one interactions which often involve ‘taking turns’.
We are able to precisely control the pitch, and resonance as well as the rhythm of speech and articulation of our sounds thanks to our flexible and dextrous tongue. The tongues of new-born babies lie flat in their mouths. The permanent descent of our larynx occurs early during our development; this raises the tongue in the mouth, allowing it to move freely.
The ability to learn and reproduce complex sounds in the form of song has arisen independently in whales, humans, and at least three times in birds. Only male songbirds make complex learned song calls; they sing to attract mates. In contrast our language ability is gender-balanced. No other primates have elaborated male mating calls. However the second descent of the larynx in boys during puberty suggests that sexual selection may have refined our control of the resonant qualities of our vocal tract.
How could natural selection have favoured our ancestors’ ability to produce these sounds?
Our vocal flexibility comes at a price; an increased individual risk of choking. Our ancestors’ ability to form ‘proto-words’ and ‘proto-song’ must have given their tribal group a selective advantage that outweighed this risk.
Strong bonds bring advantages to social groups including relative safety in numbers from predators, collective understanding of their environment, higher levels of parental care from the extended family (resulting in better juvenile survival), and coordination of hunting and foraging. Primates use grooming, which combines touch with emotionally-coded facial and vocal sound gestures, to make and maintain social bonds.
Robin Dunbar points out that the extent of primate grooming time can be predicted from their combined neocortex (‘thinking’ brain) size and social group size. It is thought our large-brained hominid ancestors lived in tribes of up to around 150 individuals, which would mean that they would need to spend up to 40% of their time manually grooming each other to maintain social bonds. Speech may have provided a time-saving alternative; a means of ‘vocally grooming’ others. A speaker may have been able to connect to and bond simultaneously with multiple individuals.
Human babies making new or unusual sounds quickly receive their parents’ attention. Ulrike Griebel and Kimbrough Oller suggest that our hominin ancestors’ babies may have produced sounds that provoked more parental attention and so more effective bonding. Their better survival would select for babies with vocal variation and flexibility.
Involuntary pleasure sounds encourage continued social interactions between primates of all ages. Perhaps our hominid ancestors learned to make diverse and pleasurable sounds as babies, and as a result were better equipped as adults to vocally groom their extended families.
Conclusions
- Fine motor control of the tongue, lips and jaw allow us to produce a huge repertoire of diverse sounds. This control likely comes from combining the movements used for eating with the production of vocal sounds.
- Our ancestors may have evolved this capability at a time when their social group size increased, with a more time-efficient form of grooming required if the cohesion of the ‘tribe’ was to be maintained.
- Selection for diverse and flexible speech sounds may have begun with babies using suckling and other sounds to gain more parental attention. These individuals would be more effective as adults at ‘vocally grooming’ their wider social group.
- Our ability to produce speech sounds is balanced between genders, suggesting that group selection rather than sexual selection has driven the evolution of this ability.
Text copyright © 2015 Mags Leighton. All rights reserved.
References
Arbib, M.A. (2005) From monkey-like action recognition to human language: an evolutionary framework for neurolinguistics. Behavioral and Brain Sciences 28, 105-124.
Bouchet, H. et al. (2013) Social complexity parallels vocal complexity: a comparison of three non-human primate species. Frontiers in Psychology 4, article 390.
Charlton, B. et al. (2011) Perception of male caller identity in koalas (Phascolarctos cinereus): acoustic analysis and playback experiments. PLoS ONE 6, e20329.
Fitch, W.T. (2010) The Evolution of Language. Cambridge University Press.
Green, S. and Marler, P. (1979) The analysis of animal communication. In Social Behavior and Communication (P. Marler, ed.) pp. 73-158. Springer.
Hauser, M. et al. (2002) The faculty of language: what is it, who has it, and how did it evolve? Science298, 1569-1579.
Kimbrough Oller, D. and Griebel, U. (eds) (2008) Evolution of Communicative Flexibility: Complexity, Creativity, and Adaptability in Human and Animal Communication. Vienna Series in Theoretical Biology, MIT Press.
Lehman, J., Korstjens, A.H. and Dunbar, R.I.M. (2007) Group size, grooming and social cohesion in primates. Animal Behaviour 74, 1617-1629.
Lieberman, P. (2006) Toward an Evolutionary Biology of Language. Harvard University Press.
Lund, J.P. and Kolta, A. (2006) Brainstem circuits that control mastication: do they have anything to say during speech? Journal of Communication Disorders 39, 381-390.
MacNeilage, P. (2008) The Origin of Speech. Oxford University Press.
MacNeilage, P.F. and Davis, B.L. (2000) On the origin of internal structure of word forms. Science288, 527-531.
Parr. L.A. et al. (2007) Classifying chimpanzee facial expressions using muscle action. Emotion 7, 172-181.
Pearson, K.G. (2000) Neural adaptation in the generation of rhythmic behaviour. Annual Review of Physiology 62, 723-753.
Redican, W.K. and Rosenblum, L.A. (1975) Facial expressions in nonhuman primates. Stanford Research Institute.
Titze, I. R. (1989) Physiologic and acoustic differences between male and female voices. The Journal of the Acoustical Society of America 85, 1699-1707.
Traxler, M.J. et al. (2012) What's special about human language? The contents of the "Narrow Language Faculty" revisited. Linguistics and Language Compass 6, 611-621.
van Wassenhove, V. (2013) Speech through ears and eyes: interfacing the senses with the supramodal brain. Frontiers in Psychology 4, article 388.
Weusthoff, S. et al. (2013) The siren song of vocal fundamental frequency for romantic relationships. Frontiers in Psychology 4, article 439.
Willemet, R. (2013) Reconsidering the evolution of brain, cognition, and behaviour in birds and mammals. Frontiers in Psychology, 4, article 396.
What’s so different about human speech?
Upon leaving the island, Odysseus is warned that storms lie ahead. His route home to Ithaca passes the sirens; monsters whose beautiful, haunting voices lure sailors to their deaths.
He sets a course and explains his intention to the crew. At his command they fasten him to the mast, seal their ears with wax, and prepare for their encounter.
They reach treacherous waters. The siren song reaches into Odysseus’ mind, resonating with his deepest longings. The storm rages within. He struggles, but his bindings, the result of his clear intention, secure him tightly to the mast.
Forced to stand still and listen, he finds that he starts to hear the voices for what they really are; the empty fears of his own soul. He relinquishes his fight and hears the voice at his own still centre. The storm calms.
The crew notice that he has returned to his senses. They cut him loose.
He is indeed a wise and worthy captain.
Speaking involves transmitting and interpreting intentional signs, some of which are also used in the instinctual communications of animals. These signals are of three kinds: 1. An index physically shows the presence of something, e.g. wolves tracking their prey by scent.
2. An ‘icon’ resembles the thing it stands for, like a photograph or a painting. Dolphins, apes and elephants recognise their own reflection; we assume that they interpret this two-dimensional image as representing their three-dimensional physical selves.
3. A symbol associates an unrelated form with a meaning. Our words are symbols, linking an idea with unique sound-and-movement sequences. They do not resemble the things they represent.
Symbolism is almost unknown amongst animals, with a few rare exceptions. A stereotypical form of symbol is the ‘waggle dance’ of honeybees.
Although chimpanzees can be taught to use some sign gestures, they do not naturally communicate using symbols. In contrast, we use our symbolic language intentionally.
Our uses of speech are unique. We revisit our memories, order our thoughts and make future plans. With a destination in mind, we can listen for our ‘inner voice’, map out our route, take a stand against the storm of inner and outer distractions, and find our way home.
How is our speech unique?
Aspects of our language ability are found in other animals, but the way we have combined and developed these traits is uniquely human.
1. We use any available channel.
Most human languages use vocal speech. Under circumstances where speaking is not possible, we find other ways, e.g. sign languages and Morse code.
2. We build our words from parts that gain meaning as they are combined
Most of the syllables we use to build words lack meaning on their own. Combining them together (as in English) or adding tonal shifts (as in Chinese) creates words.
3. We code our words with meanings, making them into symbols
Symbols are ‘displaced’, i.e. they do not need to resemble the thing they represent. Our words symbolise ideas, experiences and things.
4. We combine these symbols to make new meanings.
We build words into phrases and stories, use these to revisit and share our memories, combine them into new forms, and communicate this information to others in various ways. Combining different symbols brings us a new understanding, which changes how we respond.
Look at this painting. As you do, consider what feelings it provokes.
It is, of course, by Vincent Van Gogh. As you may know, his choices of colour and subject material were a personal symbolic code. He often used vibrant yellows, considering this colour to represent happiness.
His doctor noted that during his many attacks of epilepsy, anxiety and depression, Van Gogh tried to poison himself by swallowing paint and other substances.
As a consequence, he may have ingested significant amounts of toxic ‘chrome yellow’, which contains lead(II) chromate (PbCrO4).
Now consider this statement.
“This is the last picture that Van Gogh painted before he killed himself” (John Berger 1972, p28)
Look again at the picture.
What do you feel this time?
Certainly our response has changed, though it is difficult to articulate precisely what is different. The image now seems to illustrate this sentence. Its symbolic content has altered for us. This example shows how combining two types of information –an image and text- can change the meaning it symbolises.
Some animals can be trained to recognise simple symbols. The psychologist Irene Pepperberg taught her African Grey parrot ‘Alex’ to count; he learned to use numbers as symbols, and could identify quantities of up to 6 items.
5. The order in which we combine symbols defines their meaning
We put word symbols together into phrases, sentences, descriptions, sayings, stories, poems, documents, manuals, plays, oaths, promises, parodies, plays, pantomimes….
The ordering of words follow rules (grammar and syntax). Animals such as dogs and dolphins show some form of syntactical ability, but there is no evidence that they are on the verge of using what we understand as language. The order of words shows us their relationship, allowing us to understand how they are interacting. We change the order of our words and phrases to change the meaning we wish to communicate.
For instance; this makes sense.
‘Jane asked Simon to give these flowers to you.’
This doesn’t quite fit our normal understanding of reality…
‘These flowers asked Simon to give Jane to you.’
This works, but the meaning has changed.
‘Simon asked you to give these flowers to Jane’
However grammar is not enough . The words in combination need to ‘make sense’ for us to understand the meaning the speaker wishes to communicate.
What does this enable us to say?
When we make new combinations of words, or add words to a visual signal such as a gesture, we create a new meaning.
We can add adjectives to a description, add qualifiers, combine phrases into a sentence, and make statements one after the other so that our listener associates these ideas. This process is known as ‘recursion’, a linguistic term borrowed from mathematics.
Our ideas about time vary between cultures, but we all mentally ‘time travel’ by revisiting our memories. For instance, the scent of something can evoke a memory that transports us back into an earlier event; suddenly we experience again the emotions and sensations we felt at that time. Putting our current selves into the past memory, or imagining a future scenario and inserting ourselves into that story, is a form of recursion.
Memory allows us to link speaking and listening with the meanings of our words. Our language is well structured to easily express recursive ideas. This shows us that our thinking uses recursion.
Why are we able to do this?
Our thinking capacity, through which we learn and remember, means that we can copy and learn to use language. Although some brain regions appear specialised for roles in memory and language, our ‘language function’ uses our entire brain, and cannot be dissociated from our minds.
Our ‘language brain’ includes the ‘basal ganglia’; these are neurons which connect the outer cortex and thalamus with lower brain regions.
We need this connectedness to coordinate movements in our fingers, to understand the relationships between words that are inferred by their order in our phrases, and to solve abstracted (theoretical) problems. This network interacts with ‘mirror neurons’ which allow us to relate to and decode the posture, speech and emotional cues of others.
The basal ganglia that influence our speech also regulate the muscles controlling our posture. Standing is therefore more than just balancing on two legs; it is a whole body activity and requires much finer muscle control than walking on all fours. It also frees the hands, which allows us to manipulate tools. Lieberman suggests that it is the fine motor control required to maintain our upright posture which pre-adapted our ancestors for manipulating hand tools as well as the tongue, lips and other structures that make speech possible. This upright posture is linked with a remodelling of our breathing apparatus, giving us more control over our larynx.
Philip Lieberman’s work with people suffering from Parkinson’s’ disease suggests that it is the ability to remember that makes speaking possible. Parkinson’s patients have degraded nerve circuits in their basal ganglia, so these patients have short term memory problems and difficulties with balancing and making precise finger movements. They also struggle with understanding and using metaphors and longer word sequences. This suggests that when we speak we are using the circuitry for sorting and remembering movement sequences, irrespective of whether these are producing words or actions.
The basal ganglia that influence our speech also regulate the muscles controlling our posture. Standing is therefore more than just balancing on two legs; it is a whole body activity and requires finer muscle control that walking on all fours. It also frees the hands, which allows us to manipulate tools.
Lieberman suggests that it is the fine motor control required to maintain our upright posture which pre-adapted our ancestors for manipulating hand tools as well as the tongue, lips and other structures that make speech possible. This upright posture is linked with a remodelling of our breathing apparatus, giving us more control over our larynx.
The nerve networks that control our limbs and voices are linked across all vertebrates. Our basic ‘walking instinct’ initially activates Central Pattern Generator circuits driving movement in all four limbs. These are the same neural outputs that control our lips, tongue and throat.
Conclusions: What does this say about our language?
- Our hominin ancestors evolved to use symbolic words and stories as a code to store and share memories, develop new skills and ideas, and coordinate their intentions and actions with their tribe.
- When we revisit our memories or ‘reword’ our experiences into new sequences, we remodel the past, and project our thoughts into the future.
- The control we have over our vocal sounds is linked with our neural circuits for movement. The ability to balance ideas and manipulate our tongues is linked to our ability to stand upright, balance on two feet and manipulate tools with our hands.
- Language, then, is a cultural tool that allows us to order our thoughts, go beyond our instincts, share our intentions, and choose our own story.
Text copyright © 2015 Mags Leighton. All rights reserved.
References
Berger J (1972) ‘Ways of Seeing’ Penguin books Ltd, London, UK
BickertonD and Szathmáry E (2011) ‘Confrontational scavenging as a possible source for language and cooperation’ BMC Evolutionary Biology 11:261 doi:10.1186/1471-2148-11-261
Corballis MC (2007) ‘The uniqueness of human recursive thinking’ American Scientist Volume 95 (3), May 2007, Pages 240-248
Corballis, M.C.(2007) ‘Recursion, language, and starlings’ Cognitive Science 31(4) 697-704
Everett D (2008) ‘Don’t sleep, there are snakes: Life and language in the Amazonian jungle’ Pantheon Books, New York, NY (2008)
Everett, D (2012) ‘Language: the cultural tool’ Profile Books Ltd, London, UK
Gentner TQ et al (2006) ‘Recursive syntactic pattern learning by song birds’ Nature, 440;1204–1207
Upon reflection; what can we really see in mirror neurons?
“Mirror, mirror on the wall,
who is the fairest of them all?”
“Fair as pretty, right or true;
what means this word ‘fair’ to you?
Fair in manner, moods and ways,
fair as beauty ‘neath a gaze…
Meaning is a given thing.
I cannot my opinion bring
to validate your plain reflection!
You must make your own inspection.”
Mirrors shift our perspective, enabling us to see ourselves directly, and reflect ideas back to us symbolically. But how do we really see ourselves?
Quite recently, neuroscientists discovered a new type of nerve cell in the brains of macaques which form a network across the primary motor cortex, the brain region controlling body movements. These nerves are intriguing; they become active not only when the monkey makes purposeful movements such as grasping food, but also when watching others do the same. As a result, these cells were named ‘mirror neurons’.
In monkeys and other non-human primates, these nerves fire only in response to movements with an obvious ‘goal’ such as grabbing food. In contrast, our mirror network is active when we observe any human movement, whether it is purposeful or not. Our brains ‘mirror’ the actions of ourselves and others’ actions, from speaking to dancing.
However it is to interpret what function these cells are performing. Different researchers suggest that mirror neurons enable us to:
– assign meaning to actions;
– copy and store information in our short term memory (allowing us to learn gestures including speech);
– read other people’s emotions (empathy);
– be aware of ourselves relative to others (giving us a ‘theory of mind’, i.e. we have a mind, and the contents of other people’s minds are similar to our own).
Whilst these opinions are not necessarily exclusive, they do seem to reflect the different priorities of these experts. Their varied interpretations highlight how difficult it is to be aware of how our beliefs and assumptions affect what our observations can and cannot tell us.
What we can say, is that the behaviour of these nerve cells shows that our mirror neuron responses are very different from those of our closest relatives, the primates.
What do we know about mirror neurons from animals?
Researchers at the University of Parma first discovered mirror neurons in an area of the macaque brain which is equivalent to Broca’s area in humans. This brain region assembles actions into ordered sequences, e.g. operating a tool or arranging our words into a phrase. Later studies show these neurons connect right across the monkey motor cortex, and respond to many intentional movements including facial gestures.
The macaque mirror system is activated when they watch other monkeys seize and crack open some nuts, grab some nuts for themselves, or even if they just hear the sound of this happening. Their neurons make no responses to ‘pantomime’ (i.e. a grabbing action made without food present), casual movements, or vocal calls.
Song-learning birds also have mirror-like neurons in the motor control areas in their brains. Male swamp sparrows’ mirror network becomes active when they hear and repeat their mating call. Their complex song is learned by imitating other calls, suggesting a possible role for mirror neurons in learning. This is tantalising, as we do not yet know the extent to which mirror neurons are present in other animals.
The primate research team at Parma suggest the mirror system’s role is in action recognition, i.e. tagging ‘meaning’ to deliberate and purposeful gestures by activating an ‘in-body’ experience of the observed gesture. The mirror network runs across the sensori-motor cortex of the brain, ‘mapping’ the gesture movement onto the brain areas that would operate the muscles needed to make the same movement.
An alternative interpretation is that mirror neurons allow us to understand the intention of another’s action. However as monkey mirror neurons are not triggered by mimed gestures, the intention of the observed action presumably must be assessed at a higher brain centre before activating the mirror network.
How is the human mirror system different?
Watching another human or animal grabbing some food creates a similar active neural circuit in our mirror network.
The difference is that our nerves are activated by us observing any kind of movement. Unlike monkeys, when we see a mimed movement, we can infer what this gesture means. Even when we stay still we cannot avoid communicating; the emotional content of our posture is readable by others. In particular we readily imitate other’s facial expressions.
As we return a smile, our face ‘gestures’. Marco Iacoboni and co-workers have shown that as this happens, our mirror system activates along with our insula and amygdala. This shows that our mirror neurons connect with the limbic system which handles our emotional responses and memories. This suggests that emotion (empathy) is part of our reading of others’ actions. As we see someone smile and smile back, we feel what they feel.
Spoken words deliver more articulated information than can be resolved by hearing alone. Our ability to read and copy the movements of others as they speak may be how we really distinguish and understand these sounds. This ‘motor theory of speech perception’ is an old idea. The discovery of mirror-like responses provides physical evidence of our ability to relate to other people’s movements, suggesting a possible mechanism for this hypothesis.
Further studies suggest that these mirror neurons are part of a brain-wide network made of various cell types. Alongside the mirror cells are so-called ‘canonical neurons’, which fire only when we move. In addition, ‘anti-mirrors’ activate only when observing others’ movements. Brain imaging techniques show that frontal and parietal brain regions (beyond the ‘classic’ mirror network) are also active during action imitation. It is not clear how the system operates, but in combination we relate to others’ actions through the same nerve and muscle circuits we would use to make the observed movements. We relate in this way to what is happening in someone else’s mind.
Are mirror neurons our mechanism of language in the brain?
Mirror-like neurons activate whether we are dancing or speaking. Patients with brain damage that disrupts these circuits have difficulties understanding all types of observed movements, including speech. This suggests that we use our extended mirror network to understand complex social cues.
Our mirror neuron responses to words map onto the same brain circuits as other primates use for gestures. However signals producing our speech and monkey vocal calls arise from different brain areas. This suggests that our speech sounds are coded in the brain not as ‘calls’ but as ‘vocal gestures’. This highlights the possible origins of speaking as a form of ‘vocal grooming’, which socially bonded the tribe.
When we think of or hear words, our mirror network activates the sensory, motor and emotional areas of the brain. We thus embody what we think and say. Michael Corballis and others consider that mirror neurons are part of the means by which we have evolved to understand words and melodic sounds as ‘gestures’.
What is unclear is how we put meaning into these words. Some researchers have suggested that mirror neurons anchor our understanding of a word into sensory information and emotions related to our physical experience of its meaning. This would predict that our ‘grasp’ of the meaning of our experiences arises from our bodily interactions with the world.
Vocal gestures would have provided our ancestors with an expanded repertoire of movements to encode with this embodied understanding. Selection could then have elaborated these gestures to include visual, melodic, rhythmical and emotional information, giving us a route to the symbolic coding of our modern multi-modal speech.
We produce different patterns of mirror neuron activity in relation to different vowel and consonant sounds, as well as to different sound combinations. Also, the same mirror neuron patterns appear when we watch someone moving their hands, feet and mouth, or when we read word phrases that mention these movements.
We process word sequences in higher brain centres at the same time as lower brain circuits coordinate the movements required for speech production and non-verbal cues. Greg Hicock suggests that our speech function operates by integrating these different levels of thinking into the same multi-modal gesture.
Mirror neurons connecting the brain cortex and limbic system may allow us to synchronously process our understanding of an experience with our emotional responses to it. This allows us to consciously control our behaviour, adapt flexibly to our world, and communicate our understanding to others and to ourselves.
Smoke and mirrors; what do these nerves really show and tell?
The word ‘mirror’ conjures up strong images in our minds. This choice of name may have influenced what we are looking for in our data on mirror neurons. However they appear crucial for language. This and other evidence suggests that our ability to speak and to read meaning into movement is a property of our whole brain and body.
Single nerve measurements show that the mirror neuron network is a population of individual cells with distinct firing thresholds. Different subsets of these neurons are active when we see similar movements made for different purposes. This suggests that the network responds flexibly to our experience.
Cecilia Heyes’ research shows that our mirror network is a dynamic population of cells, modified by the sensory stimulus our brain receives throughout life. She suggests that these mirror cells are ‘normal’ neurons that have been ‘recruited’ to mirroring, i.e. adopted for a specialised role; to correlate our experience of observing and performing the same action.
This gives us a possible evolutionary route for the appearance of these mirror neurons. Recruitment of brain motor cortex cells to networks used for learning by imitation would create a population of mirror cells. This predicts that;
i. Mirror-like networks will be found in animals which learn complex behaviour patterns, such as whales. (They are already known in songbirds.)
ii. It should be possible to generate a mirror-like network in other animals by training them to associate a stimulus with a meaning, perhaps a symbolic meaning as in Pavlov’s famous ‘conditioned reflex’ experiments with dogs.
Mirror neurons then, show us that something unusual is going on in our brain. They reveal that we use all of our senses to relate physically to movement and emotion in others, and to understand our world. They are part of the system we use to learn and imitate words and actions, communicate through language, and interact with our world as an embodied activity.
However beyond this, we cannot yet see what else they reveal. Until we do, our conclusions about these neurons must remain ‘as dim reflections in a mirror’.
Conclusions
- Monkey mirror neurons relate the observations of intentional movements to a sense of meaning.
- The human mirror network activates in response to all types of human movements, including the largely ‘hidden’ movements of our vocal apparatus when we speak.
- These neurons are a component of the neural network that allows us to internally code meaning into our words, and ‘embody’ our memory of the idea they symbolise.
- The mirror network neurons seem to be part of an expanded empathy mechanism that connects higher and lower brain areas, allowing us to understand our diverse experiences from objects to ideas.
- These cells are recruited into the mechanism by which we learn symbolic associations between items (such as words and their meaning). This shows that it is our thinking process, rather than the cells of our brain, that makes us uniquely human.
Text copyright © 2015 Mags Leighton. All rights reserved.
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