On Making Faces

How do babies learn to make faces? With arm or leg movements, it seems plausible that, as William James suggested, one might gain insight from simply associating observed appendage position with concurrent muscle activation patterns¹. However, in contrast to an attempted stirring of limbs, the infant cannot see what results from the activation of his facial muscles. Thus, the learning mechanism cannot rely on sensory confirmation that the indented action was successfull. That new humans very rapidly learn to express their emotional state through a smile, frown or furrowing of brow hints that there is some implicit path to the acquisition of this skill. It has been suggested by some that the Mirror Neuron (MN) system constitutes this road, or at least a map of it².

In the adult, MNs are cells that respond when an animal performs some act – picking up a piece of fruit, say - and when it observes another individual doing the same thing, even if the observed actor is a member of another species or stranger, a robot. Beyond this, their activation potentiates the pathways that would be involved in the execution of such a movement by the observer: producing measurable sub-threshold responses in the muscles involved. The suggestion that this system is involved in motor learning and imitation from birth amounts to the assumption that MNs are prenatally wired to function as they do in the mature brain. This is a very attractive idea as it removes the need for some external form of reinforcement - like a visual confirmation of the completed movement - to inform the motor-learning process. Instead, the responsibility for being both carrot and stick is shifted internally, to the MNs. The proposition is that the genetically defined circuitry imbues the MNs with a "knowledge" of the pattern of muscle activity associated with both an observed or executed behavior.

This, unsurprisingly, presents a further question: if the MN system is present from birth and possesses such information as described above, why do infants need to learn how to move at all? This is where we must tread a bit into the realm of informed speculation. First, the MN system cannot know how to execute every movement possible: for instance it certainly cannot know at birth the set of motor commands associated with performing some complex gymnastic move, say a double backflip. If the MN is an artifact of evolution, then it is likely that there is a continuum of innate interpretability, from simple acts like smiles that are well known to the MN system to more rare or contemporary behaviors like figure skating or fixing a bicycle. Thus, using the MN system as a template, of sorts, can only be effective for behaviors on the oft-encountered end of the spectrum. Second, since babies sadly do not leap from the the womb as masters of muscular control, the wiring of the MN system must itself develop at a pace commensurate with the time-course of an individual's behavioral procurement.

How is it then, that this internal electro-cultivation proceeds in lock-step with the infant's newfound agility? It has been well documented that humans lose half the total number of neurons in their central nervous system by the time they've reached six months of age. This pruning is a synaptic refinement process, also termed neural darwinism⁴. It is quite beneficial to the newborn animal, since his neural connectivity is far too manifest, too spatially noisy, and must be cleaned up. This happens in all areas of cerebral cortex. For example, in the visual system, molecular cues guide the axons of nearby retinal ganglion cells to adjacent targets in the thalamus while the animal is still in the womb in a gross way, but it is the activity arising from visual stimulation which pares down the connections to the state we see them in the adult. the exquisite spatial precision of connections between the retina and thalamus cells in the retina are connected to cells in the thalamus with exquisite spatial precision because of visual experience. It was Hebb who pointed out that cells that "fire together wire together.⁵" This means that if two retinal cells fire at the same time, they will tend to be connected to the same post-synaptic target. That is not to say that any two cells that fire at the same time anywhere in the brain will inevitably be connected to each-other, but rather that in deciding which of the molecularly defined crude connections to keep, a post synaptic cell will retain those which tend to fire at the same time in response to stimulation. The stimuli that cause the retinal cells to fire are not simply sets of independently fluctuating pixels; rather they are full of spatial correlations. If a single vertical line passed across your field of vision, a single line of cells in the retina would be stimulated at once as the line moved by. It is almost never the case that two abutting photoreceptors see completely uncorrelated (in time) patterns of illumination. In this way, the spatial relationships in the image translate into spatial relationships in the connections in the brain.

The goings-on I've outlined above might generally be termed activity dependent synaptic refinement (ADSR). What I'm hypothesizing is that, as with vision, some form of ADSR is at work in those cortical-areas involved in learning how to move: that the very act of generating motor output leads to more stereotyped action through application of the Hebbian "fire together wire together" motif. The commonality between vision and movement, and indeed the unifying principle behind ADSR, is that the systems are exploiting the presence of underlying statistical content. While the MN system is biasing motor output towards certain configurations, the motor cortex is learning about the possible relationships between muscle tensions, lengths and contractile velocities. It is thought that the brain might use such a mechanism universally as an attempt at maximizing efficiency. For example: if you always listen to symphonic classical music, you might set your stereo to boost bass & treble, but if you're more the piano concerto type, you'd want a touch more mid-range; then again, if your tastes are more varied, it might make sense to emphasize all three frequency bands equally so as not to aurally marginalize any particular genre. Another for instance more relevant to motor output: when you drive in the city, you rarely ever get out of 2nd gear, but on the highway, you're almost always in the upper gears. In the same sense, the brain attempts to use the Hebbian rule to optimize its (sensory) inputs and (motor) outputs to the sensory stimuli impinging on it and the motor programs it generates.

You may have noticed in all this that I've skipped over one important point: how (genetic wiring notwithstanding), do mirror neurons extract the information about a movement being performed by another agent? It is not at all understood how this occurs in the adult, so how it could be happening in babies is even more mysterious, especially in light of the messiness of infant brains that I've spoken of. It simply must be the case that visual stimuli are translated into data concerning the movement of bodies. It is possible that specialized structures for recognizing arms and hands, faces and feet, become sophisticated very early on, but nothing like this has been observed to-date. The needed course of research is clear, but developing experiments to elucidate what might be at work is not. We can only wait and watch from the wings while the scientific players act, and perhaps deliver soto voce direction from time to time.

References

1. James, W. The Principles of Psychology Vol. 1. Henry Holt & Company (1890)
2. Lepage JF, Théoret H. (2007) The mirror neuron system: grasping others' actions from birth? Dev Sci. 10(5):513-23.
3. Rizzolatti, G., & Craighero, L., (2004) The Mirror Neuron System. Annu. Rev. Neurosci. 27, 169-192.
4. Hebb, D.O. The Organization of Behavior. John H. Wiley & Sons (1967)
5. Edelman, G.M. Neural Darwinism. Oxford Paperbacks (1990)