Remembering what I will not forget

In my former life as a speech-language pathologist working in an early intervention/prevention project focused on the mental health of preschoolers, I enjoyed giving parent/teacher talks on language development, communication difficulties, how to recognize when help is needed and how to promote healthy development.  Lovely work, eh?  In many of these talks I presented a brief glimpse into the complexity of development that started something like this:  After fertilization, the egg begins to divide and multiply. When there are roughly 50-100 cells, one cell appears that becomes the mother of all neurons.  Slowly this cell line multiplies to form a neural tube and from within that tube more cells would be born that would then travel to the outer edges and form the brain. 10,000,000,000 cells would arise and find their place in this way in just a few months, so a few traffic jams and mis-directions might be expected along with some individual variability.  Wow!  I would also talk about some of the maturational/developmental differences between boys and girls and then get into the specifics of language development.

Now I am finishing up Georg Striedter’s text, Principles of Brain Evolution, and understand that my earlier rendition of complexity was more a 2 page Reader’s Digest version of the Encyclopedia Britannica (anyone else remember those?)  The task neuroanatomists take on is enormously complex and even knowing a little bit for sure requires diligent, rigorous, and assiduous study.  Understanding how brains increase in size and connectivity and then how brain functions change and increase in power is a humbling endeavor, one that I am glad those with such talents work on and one that I find spiritual in Monod’s sense of spirit (see post on 3/25/17).  So let me add some to my story above.

Those 1010 cells find their way along a variety of chemical trails and gradients and then when they arrive they send out dendrites and axons to connect with other cells and this connectivity is also developed through a variety of biochemical trails, and then synapses are formed and coordinated so that integrated intercellular communication can begin.  Striedter cites estimates that each mammalian neuron connects with around 500 other neurons through 8000 synapses.  Let’s see:  1010 x 500 x 8000 = a lot.  Also, remember that neurogenesis, that early embryonic stage when virtually all of our neurons appear, produces many cells that disappear in the first years after birth through apoptosis, i.e., cells die because they are not in the right place or connected in viable networks.  Streidter says that brain areas vary in how many cells are lost and cites evidence that different systems have 20% to 80% fewer neurons at maturity than at birth.  Finally, remember that neurons communicate with over 50 neurotransmitters that form the substrates of different systems processing information in their various ways, e.g., inhibitory, excitatory, etc.

White_Matter_Connections_Obtained_with_MRI_Tractography

Our connectome: If you get dizzy reflecting on the complexity of embryogenesis and subsequent functional development given the numbers cited here, please sit down and breathe slowly.

The individual brains of any one species are remarkably similar in terms of neuronal systems, etc.  The genetic controls and epigenetic forces are quite rigorous in their replication of each organism.  I especially like the story of C. elegans, a roundworm whose nervous system comprises 302 neurons that connect in very consistent ways.  Thank you, diligent researchers for finding that out through marvelously detailed work.

So I learn again and remember what I will not forget, that understanding enough to know what we do not know is the prime intellectual task, and good scholars and mystics look at our ignorance with excitement.  Travel on.

Evolutionary tidbits

To reiterate my understanding of the biological roots of our humanity, I see human empathy as something special and it laid the foundation for symbolization and that enables us to think and talk about everything and nothing and to create it if it is not already there in reality.  Through our empathy we humans are keenly aware of another’s mind, that they have subjective considerations, and how we can interact with each other mindfully.  Symbols carry this social effort forward with scope and power.

This empathic capability is centered in the right hemisphere that processes kinesic communication and maintains Empathy Central in the temporal-parietal junction where knowledge about our relationships contributes to what the academics call ToM (theory of mind).  Anyway, my thought is that this keen sensitivity to others’ minds became integrated with our mirroring capabilities, so that certain actions could be replicated readily upon observing them in another.  This replication of mirrored actions comprises the invariant forms of social communication, and when our mirroring system came to include vocal signals, so that we could hear a conspecific vocalize/verbalize and reproduce that sound and not just the objectively observable motoric behaviors, e.g., lifting a cup to drink.  This is the functional significance of the arcuate fasciculus on both the right and left sides, but especially on the left, where the af enables the repetition of what we just heard another say (see my post of 4/24/2014 on the arcuate fasciculus and mirroring).  Putting together, i.e., integrating, the awareness of another’s mind and the knowledge produced by the mirrored invariant behaviors led to symbolization, at first linguistic and then artistic (ask me to explain that sometime).  Symbols, if you remember, have a deep structure (what resides in our minds subjectively) and a surface structure (what we use to formulate and then communicate those subjective musings), and voila! language, art and the cultural wealth of our kind.

That said, I have been reading Georg Striedter’s Principles of Brain Evolutionand find a couple of evolutionary tidbits that help to carry my speculative imaginings forward (and I find nothing so far contrary to this path). Consider that human eyes are almond shaped and that our irises are surrounded by white sclera while the eyes of other primates are round and the irises surrounded by dark sclera (though the sclera hidden within the eye socket is white.  Striedter interprets this to show that we humans monitor each other’s gaze and so gather more information about the other’s subjective musings; further that our eyes’ structure facilitates this with its almond shape and white sclera shows that such kinesic communication is important evolutionarily. I see this as an example of our keen awareness of the other’s mind.

Think of some examples of this.  Parents follow the gaze of pre-verbal infants and move to facilitate their exploratory activity.  As Michael Tomasello explains, joint action is a critical advance in our social coordination and eye gaze is an important means by which we cooperate, e.g., one holds something still while another performs a more intricate action such as a nurse clamping a wound while another stitches it up, or one hunter with a bow shifting gaze to match another’s and finding prey.  Finally in this regard, in my early career I learned about the challenge of hearing impaired children (and adults) who must watch the other’s hands to communicate about a task that needs to be seen to be learned. Eye gaze is important in juggling these gaze shifts and we humans have extra talent for this.

Father child

joint gaze and joint action

Streidter also discusses the size of our brains in absolute terms, compared to our body mass, relative to other animals, the amount of cortex relative to the medulla, etc.  He points out that large brains are ‘expensive’, e.g., they require high protein diets, they pose problems for live births due to mismatch between skull size and birth canal, and they pose challenges to communication between neural areas.  This last comes about because areas farther away take longer to communicate with each other and that poses a problem for timing.  Much of our neural processing depends upon the simultaneity or temporal match of parallel processes.  Our brains have evolved with some work-arounds such as long, thicker nerve tracts that nerve impulses travel along faster than thin fibers.  Our brains have many more modules and these connect especially to those nearby with some longer fasciculi, e.g., the arcuate fasciculus, the superior longitudinal fasciculus, the claustrum and the corpus callosum, bearing the burden of longer range communication.

Sobo_1909_670_-_Uncinate_fasciculus

The arcuate fasciculus is part of the superior longitudinal fasciculus. Thicker axons help nerve impulses travel long distances faster.

Now here is another interesting tidbit.  Our corpus callosum is relatively smaller than those in other primate species, i.e., our cerebral hemispheres are less connected than might be expected.  Streidter says the data show that the human brain is more asymmetrical than other species’ brains; this works because our two hemispheres specialize in different functions (yes, even as they perform much of the same functions, one leads, and while brain damage when young can be compensated for, damage when older is less so because the specialization has become at least partially irreversible). Again this difference in connectivity is relative; I have posted here before that studies of our connectomes show females generally have more bilateral connections, i.e., they make more use of their corpus callosum, while males have more connections within each hemisphere than between.

corpuscallosum

corpus callosum with part of right hemisphere cut away

Now this bit of information speaks to two issues.  First is that females and males (please remember that I use the terms in a relative manner and appreciate all manner of androgeny in our variations) approach interactions differently.  This is especially noticeable in preschoolers where girls are both more verbal and tuned into relationships and boys are somewhat less verbal and their attunement to others is, shall we say, less robust.  Actually, talking with my 30 something daughter and others, this difference may even be accentuated in mature humans (maturity, again, is a relative term, guys).  In any event, the functioning of the connectome when emphasizing social and linguistic information together would use the corpus callosum more fully and that would correlate with a female sort of pattern.

The second issue here goes back to my thesis that symbolization arose from, first, the integration between the keen empathic apprehension of another’s subjectivity and the invariant behavioral forms that operate in mirroring, and then, second, once the connections are formed, their separation into the surface and deep structures of our symbols.  Human brains are more asymmetrical and this I associate with the differentiation of function between Empathy Central on the right side and linguistic functions on the left, e.g., one side is pragmatic and the other syntactic/semantic.

The last tidbit comes from Streidter’s analysis of the human brain’s enlarged lateral prefrontal cortex (adjacent to motor and premotor areas) primarily on the left side.  This relatively species-specific area serves, Streidter hypothesizes, our abilities to use our hands and words in very flexible, facile, novel and unconventional ways.  We are able to do things hitherto unseen, un-imitated and even unimagined until we do them.  This includes our words as well as our hands.  This highlights one of the great paradoxical strengths of our language. We use words, conventional symbols with socially established meanings, to say many things that have never been said before, i.e., they are novel and unconventional.  We do this day in and day out in small and large ways for mundane and profound topics.  Back in the day Noam Chomsky focused on this generative capacity to demonstrate the theoretical poverty of behaviorism, and we are still learning about this today.

Lobes_of_the_brain

lateral prefrontal is in lower blue area towards the front

So a long post.  Funny how tidbits expand when I am (you are too hopefully) having fun and learning about our humanity, eh?  Travel on.