The old Jewish Cemetery; Venice, 1790. 

Goethe loosens the earth from the skull, and holds it up to the sun. 

Turning the fractured bone back and forth, he gasps.  A series of marks appear inside the cavity, reminding him of the vertebrae.  He has looked at this pattern many times, but without seeing what lights up before him today. 

Here is the shadow of a blueprint; the ‘primal repeating units’ of animal bodies, from which their many variations form. 

Puzzled, Götze watches his master’s eyes shine with delight. 

---

Watch this baby babbling; her limbs move, often in time with her sounds.  the coupling of gestures and vocal calls are widespread amongst social vertebrates.  And the story began with fish.

The rhythm of our breath keeps us alive.  Conscious muscle movements are made through spinal nerve reflexes, but like our heart beats, the repeating sequences of muscle actions which fill and empty our lungs are outside our conscious awareness.

The movements behind repetitive activities like breathing are driven by rhythmic nerve impulses from ‘neural oscillators’.  These pattern-generating circuits, located in the central nervous system, are known as ‘Central Pattern Generators’.

To breathe, to speak and to swallow we use the same internal tube; that is our throat (the pharynx).  These activities are necessarily exclusive; consider what happens when a crumb ‘goes down the wrong way’.  Speech therefore needs to be coordinated with our breathing.

We make vocal sounds by passing air through the larynx as we breathe out, at the same time as vibrating our vocal folds (vocal cords).  These actions involve coordinating a sequence of repetitive movements inside the throat with the repeating muscle actions that drive our breath.

Communicating with sound evolved long before animals emerged from the sea onto land and began to breathe air.  Many fish use pectoral fin movements as communication gestures; some also generate sounds by fin waving.

In species of vocal fish, the calls are coordinated with these pectoral fin signals.  Significantly, the muscles operating these social communication cues are controlled using the same neural oscillator ‘module’.

‘Central Pattern Generators’ are neural oscillators that generate a rhythmic output, used to control repeating muscle movements.  These ‘neural metronomes’ were first discovered in insects, and produce their steady pulse without any sensory stimulus.  In contrast, our other nerves operate on a ‘stimulus-response’ basis.

 

Central Pattern Generators reveal what can be called our ‘deep homology’.  First discovered in insects, all vertebrates, including ourselves, have these ancient neural circuits.  They links vocal calls with gestures, and coordinate our ‘fins’ with our speech.

How do Central Pattern Generators work? 

We consciously control our limbs through spinal nerve reflex arcs.  In contrast, rhythmic movements controlling oscillating cycles are driven by Central Pattern Generators (CPGs).  These autonomous modules in the central nervous system produce a rhythmic output (like a neural ‘black box’).  CPG modules are comprised of a dense interconnected local network of neurons; a neural ‘node’.

These nodes are organised into three levels, each with a different function, and in each case, the parts of the circuit ‘higher’ in this organisation regulate the outputs of those below.

In the developing embryo there are functional units, ‘segments’ which give rise to our vertebrae and their associated nerves and muscles.  The nerves from each of these ‘segments’ form our local sensory spinal reflexes and also the CPG modules.  As needed, higher brain centres trigger these ‘neural motors’ to produce their rhythmical nerve impulses and drive all of our rhythmical movements from walking to chewing.

CPGs controlling rhythmic movements of the tongue, throat and breathing (including the vocal neural oscillator module) are in the lower brainstem and neck.  The CPGs that drive the rhythm of our walking are low down in the spinal cord, in the thoracic and lumbar regions.

Why don’t we sound like fish? 

Toadfish vocalise with either a sequence of repetitive grunts during aggressive encounters or the prolonged drone of their mating call.  In both cases, each nerve impulse from the vocal pattern generator produces a single synchronised contraction in their sonic muscles; this muscle pair flexes the rigid walls of the swimbladder, producing a ‘grunt’.  This sound receives no further processing.  As a result, its tone is rather mechanical.

Our voice, like that of frogs, birds and mammals, also begins with this simple rhythmic sound pulse.  This initial sound is then processed into croaks, calls songs and speech.  Our neck  allows us to create resonant areas in the throat which amplify certain frequencies.  Pitch is affected by vocal fold (vocal cord) tension, and manipulation of our tongue and lips produces precisely articulated words.

Why is ‘talking with our hands’ still a part of our language?

Many vocal fish make synchronised gestures with their front (pectoral) fins during mating calls.  These motor nerve connections from our ancient common ancestor are retained in other vertebrates.

People blind from birth move their hands when they talk.  Our vocal Pattern Generator circuits connects with both our larynx and pectoral muscles, coordinating our speech with our ‘body language’.  We subconsciously move our hands as we communicate thanks to these rhythmic central circuits.  As in all other vertebrates, we have inherited these from (as Palaeontologist Neil Shubin puts it) our ancestral ‘inner fish’.

Text copyright © 2015 Mags Leighton. All rights reserved.

References

Aboitiz, F. (2012)  Gestures, vocalizations, and memory in language origins.  Frontiers in Evolutionary Neuroscience 4, e2.

Bass, A.H. and Chagnaud, B.P. (2012)  Shared developmental and evolutionary origins for neural basis of vocal-acoustic and pectoral-gestural signalling.  Proceedings of the National Academy of Sciences, USA 109 (Suppl.1), 10677-10684.

Bass, A.H. et al. (2008)  Evolutionary origins for social vocalization in a vertebrate hindbrain-spinal compartment.  Science 321, 417-421.

Bostwick, K.S. (2000) Display behaviors, mechanical sounds, and evolutionary relationships of the Club-winged Manakin (Machaeropterus deliciosus). Auk 117, 465-478.

Bostwick, K.S. et al. (2010)  Resonating feathers produce courtship song. Proceedings of the Royal Society, B 277, 835-841.

Chagnaud, B.P. et al. (2012)  Innovations in motoneuron synchrony drive rapid temporal modulations in vertebrate acoustic signalling.   Journal of Neurophysiology 107, 3528-3542.

Dick, A.S. et al. (2012)  Gesture in the developing brain.  Developmental Science 15, 165–180.

Ghazanfar, A.A. (2013) Multisensory vocal communication in primates and the evolution of rhythmic speech. Behavioral Ecology and Sociobiology 67, 1441-1448.

Goethe, J.W. (1820).  Zur Naturwissenschaften überhaupt, besonders zur Morphologie; cited (p. 7) in G.R. de Beer The Development of the Vertebrate Skull.  Clarendon (1937).

Guthrie, S. (1996)  Patterning the hindbrain.  Current Opinion in Neurobiology 6,41-48.

Hanneman, E. et al. (1988) Segmental pattern of development of the hindbrain and spinal cord of the zebrafish embryo. Development 103, 49-58.

Iverson, J.M. and Thelen, E. (1999)  Hand, mouth and brain: The dynamic emergence of speech and gesture.  Journal of Consciousness Studies 6, 19-40.

Kelley, D.B. and Bass, A.H. (2010)  Neurobiology of vocal communication: mechanisms for sensorimotor integration and vocal patterning. Current Opinion in Neurobiology 20,748-53.

Marder, E. and Bucher, D. (2001)  Central pattern generators and the control of rhythmic movements.  Current Biology 11, R986-R996.

Shubin, N (2008)  Your inner fish; a journey into the 3.5 Billion year history of the human body.  Penguin Books Ltd.

Shubin, N. et al. (2009)  Deep homology and the origins of evolutionary novelty.  Nature 457, 818-823.

“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.
  

  Double rainbow. The second rainbow results from a double reflection of sunlight inside the raindrops; the raindrops act like a mirror as well as a prism.  The colours of this extra bow are in reverse order to the prima... morery bow, and the unlit sky between the bows is called Alexander’s band, after Alexander of Aphrodisias who first described it. (Image: Wikimedia commons)

• 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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