Regarding mirror neurons and early facial expression imitation, another hypothesis. Continues from: Learned or Innate? Nature or Nurture? Speculations of how a mind can grasp on its own: animate/inanimate objects, face recognition, language...
Emotional facial expressions are innate, I remember a many decades old research on understanding facial expressions around the world, which shows that we're compatible at emotions expression level, no matter of culture and the level of society development. I think this was proven thousands of times by Cinema, and it matches with the hypothesis that emotions are driven by older parts of the brain: Cingulate Cortex --> Fornix --> Thalamus --> Hippocampus --> Neocortex.
The Neocortex eventually learns to control face as well, and good actors do it well, however, being an actor myself, I would say that good acting involves feeling the emotions of the character, "living a part", which goes below neocortex. Indeed, I believe this is easier, because it happens partially subconsciously and automatically.
Perhaps neocortex "calls" or better "recalls" and "reruns" complex functions from the lower parts - memories of already felt emotions or better - situations when emotions were felt. On the other hand, bad actors probably do not feel, because cannot recall emotional memories so well. Bad actors are trying consciously /mechanically to pull-up/pull-down facial muscles which makes their faces to look unrealistic and unconvincing (like if their characters felt nothing or something inappropriate). Perhaps, because conscious cannot do control precisely in parallel so many muscles, and maybe because these muscles are too strongly linked to their thalamic nuclei.
I would suggest here: it's better not to smile if you're not happy, rather than do a "false smile".
Our face has a dedicated cranial nerve like the other sensory and motor head-"interfaces", and like the rest, except olfaction, but including optic nerve, facial nerve passes through a thalamic nuclei in the thalamus:
Going up, Thalamus has projections to the neocortex and back, so eventually it can turn into "magic neurons" up there.
Boris Kazachenko recently suggested in a comment here: Meta Evolution: fitness beyond reproduction
BK: "I think there have been minor genetic changes in humans that produced a major increase in intelligence, mostly through the growth & folding of neocortex"
Making a digital networks analogy (see Physical layer)
Face - physical level interface
If brains and intelligence have evolved to cope with more complex social interactions in the high level of processing, face should have taken a part of the evolution of the "Physical Level Interface" and the thalamic nuclei are a middle layer, doing primary decoding of physical layer signals, sorting, redirecting. I suspect they might/should be correlated at DNA level, because, growing one thing requires growing a counter-part, all muscles or receptors etc. need to have appropriate nerves that eventually reach the Central Nervous System, and I guess that evolving parts in pathways inside the CNS may also involve all subregions, if they develop together prenatally. (I'm not yet that much deep into embryology, though.)Flexible physical layer interface allows telling others clearly and visually how do you feel, so they can react appropriately and/or learn using it as a sign/conditioned stimulus. I guess this should be related also to the grow of the importance of vision in human senses.
Basic Emotions and Mammals
All mammals have the basic emotions, lower animals also should have at least part of theirs (such as: Fear, Panic, Lust, Search; even the Octopus seems to have an elaborated brain and "Play" system on its own). There's a funny research showing that rats laugh when being tickled...
However, most mammals, e.g. cats or dogs display just a portion of their emotions using face. Expression of Rage (human also growls and clenches its teeth) and Fear/Panic? (eyes wide open) seem to be similar, and I think they both are amongst the evolutionary older emotions.
When we do say that a cat or a dog has "a sad face", though I suspect it's rather a blind visual similarity we spot (like finding faces in a fish or insect), a form of anthropomorphizing, than real. Cats and dogs faces are not that flexible and expressive as ours - as Alice says in Wonderland: "Cats can't smile". Well, I don't know. :)
Whitney was a sad little kitty, starring in a photo story of mine.
Photos - (C) Todor Arnaudov, 2006 - "The Ghost" photo story
I haven't studied well great apes facial expressions to speak seriously. Of course they do use faces for expressing emotions, but I suspect their face is not as versatile as human's, the visual contrast between features and background is worse and they lack eyebrows, which are clues for human emotions.
Overall, I guess that mirror neurons might be projections or related to projections from facial and optic thalamic nuclei, maybe related to a primary integration of both. I suspect that part of the integration between face and optic, dealing with imitation of facial expressions, can be done as early as the thalamus itself, by sort of low resolution processing; I haven't studied internal thalamus anatomy yet, though (is it studied/understood?).
I would conclude, that if one day I could do study myself how exactly a baby imitates faces, I would try to find spatial/contrast etc. resolution thresholds when these reactions first appear. E.g. a baby boy may put his tongue out if he sees a quick-enough change in contrast in a wide spot of the visual field, not only in a mouth - it might be a high-contrast line moving quickly vertically anywhere etc. I don't know if such tests had been done already.
Appendix 1: Talents
I think there's no doubt that there are innate predispositions/talents for arts - drawing, music, dancing, also acting. So far I thought that it's related to details about personal cortical architecture, maybe differences in the speed of learning - ease/speed/durability of synaptogenesis/neuroplasticity. Now I have an additional guess: Thalamic Nuclei may also play a significant part in talents. E.g. people who can't feel the rhythm can barely learn dancing, while the others start dancing even without being taught. At a low level dancing seems to be related to basic prediction of sound patterns, linked with syncrhonized motions. Cingulate Cortex, related to emotions, has projections into motor-pathway as well, which may add to explanation why dancing is an emotional activity. I suspect that acting talent may have some of its roots in Cingulate Cortex as well.
Finally, good dancers and any talented people may have an advantage in that lower-level preprocessing part. The cerebellum of course should be also a "suspect" whenever there is fine motor-coordination, and it has much more neurons than the neocortex and supposingly vast computing power. I can't speak about cerebellum with supporting data yet, but my first guess is that some people have faster "processors" - cerebellum has longer-lasting and faster neuroplasticy capabilities, maybe wider range connections.
Appendix 2: Lower parts of the brain and prediction - prediction mechanisms below neocortex
Many if not all AGI researchers would agree that prediction and compression are amongst the main basic keywords when defining the substance of intelligence. I assume that classical conditioning, which seems to exist even in fish*, can be assumed as a primitive form of prediction of one stimuli, associated with another. The emotions or the basic behavioral drives are also forms of prediction, predicted behavioral patterns, appropriate for particular situations. This is there in all animals, in insects as well - genetically predicted/precomputed patterns.
I think a nice example is the ants "conditioning" to care for their "farms" of greenflies, which ants keep to milk juice. Ants are not eating the greenflies, but are just milking them, and even further - they are protecting their animals from enemies such as lady-birds...
It is known that thalamus does make primary decisions about what sensory information should pass up to neocortex and what should be processed faster through lower pathways.
I suspect (it may be a well known fact), that Thalamus and Cingulate Cortex do have capabilities to predict or drive complex motor patterns like the Neocortex, but of course their models are much vague, short and specific - emotional patterns, face expressions, body language(?).
I suspect that it may be connected with part of the body-language as well, which seems to be uniform in many cultures. (Not all gestures, though - e.g. the middle finger evidently passes through a visual analogy... ;) )
...To be continued:
Beauty, music and more.