Monday, August 05, 2013

That most insular of cortices

Public domain, courtesy of Wikipedia
I've developed a fascination with the insula - a small cortex wedged between areas of the recently evolved outer portion of the brain. It talks to just about all the other interesting parts of our wetware. Specifically, it processes a lot of traffic going back and forth between the decision-making, rational-ish frontal cortex and the emotion-mediating structures of the limbic system. It's an astoundingly multifunctional bit of gray matter heavy tissue, but what most intrigues me about the insula is that it's a part of the brain that sits astride one of the functional marriage points of instinct and culture.

OK, that's a little misleading. We human beings don't appear to have much in the way of instincts - that is to say innate behaviors. Rather we're born with drives: emotional propensities, desires, and some general fears that bias us towards modes of behavior, as well as honing in on certain features of our environment. Most of our behavior-specific instincts appear to be an early life array of reflexes such as, turning towards the source of a soft brush on the cheek (a nipple-seeking feeding behavior), and that automatic wiggly leg attempt to walk when an infant is held in the standing position with its feet on the floor. That, and lots of language acquisition behaviors in the form of all the cooing, babbled consonants, and an amazingly powerful drive to imitate adult speech. All things that completely deaf toddlers do.

More Accurately Put

So it's fairer to say that the insula sits at one of the marriage points between biological drives and cultural outlooks, as well as the emotions that underlie them. It's also an exemplar of how evolution bootstrapped an early neurological function from a simple reptilian representation into a complex and dynamic component of our present day sapient consciousness. Namely, the transformation of bodily sensations into emotions - reactions that describe to us the urgency of everything from the changes to the body (horror at mutilation, satisfaction at weight loss or muscle gain) to abstracts such as success at solving a math problem or the results of a political election.

Reactions we continue to feel in our bodies. We experience emotions in our viscera and muscles and on our skin because those are the collective birthplace of our feelings.

Our emotions are descended from simple descriptors of bodily states. The conditions of organs and muscles as read by the embedded autonomic nervous system system, and then routed to early versions of the brain, which in turn generated responses that altered the functioning of those organs and the states of skeletal muscles. As brains grew more adept at generating representations of the outside world based on sensory data and modeling functions, the mind triggered reactions on its own, feeding them into the autonomic nervous system from the top down. For example: The flight or fight response sent through the sympathetic sub-system upon sighting a crouched predator, or encroaching wildfire, or a nearby cliff edge, readying organs and other bodily systems to participate in the ramped up metabolic and cardiovascular modes that enable peak physical performance.

Disgustingly So

One of the bodily state descriptors that the insula helped bootstrap is that of disgust. Originally this feeling started out as a representation generated by the viscera in response to the detected presence of the toxic byproducts of problematic bacteria in the digestive tract. Since these kind of bacterial pathogens are invisible and often lethal - particularly for organisms living out in the stressful environment of the natural world, subject to heat, cold, and sometimes yearly periods of near starvation - it was a good idea for the brain be born with an in-built aversion to some of the most dangerous vectors and formites that carry pathogenic microorganisms. Hence, human beings come into life with a deep-seated disgust for pus, most other excretions, open wounds, and rotting flesh, among other things.

Lesions or other forms of damage to the insula sometimes remove the ability to feel that disgust. An individual with a damaged insula might lose all sense of revulsion at sights and smells that  would have provoked nausea and reflexive gagging or vomiting, pre-injury. That's not to say such affected persons forget that these things were once disgusting. Only that the emotional reaction is largely gone. However, disgust is evolutionarily old enough that some of its processing still resides in the guts. Toxins in the machinery of digestion can still generate a negative bodily experience in some individuals with compromised insulas.

Getting Social 

More fascinating for me is the insula's important role in generating complex social emotions in humans, including revulsion triggered by cultural rather than biological cues. That previously mentioned marriage-point of biology and culture in the brain-mind entity.

Where most emotions appear to be heavily mediated by the limbic system, we, and to a degree our cousins, the great apes, appear to process several dualistic social emotions such as gratitude and resentment, atonement and guilt, in our cerebral cortex. More specifically, we do it through the insula. That said, it should be noted these processes are also linked to parts of the frontal cortex that appear to be heavily involved in processing social norms, and even gender roles.  More on the latter in a bit*.

Thanks to the insula we can learn to experience revulsion at the violation of deeply held mores. Sometimes these take the form of strong reactions to atrocities and crimes that we've heard about secondhand, but not actually witnessed.

While revulsion against such wrongs is useful in building and sustaining a society in a often hostile world filled with individuals willing to shatter social bonds and trust thresholds for selfish gain, there is a clear dark side to the processes. The insula also allows us to learn culturally induced revulsion towards people of the wrong caste, the wrong orientation, and disgust at the thought of having sex with a person of the wrong ethnicity or race. In that sense, neurobiology and biology in general are like technology. They're amoral systems whose uses can be good or bad, depending on things like context, intention, and outcome.

*Gender roles and the brain: Gender roles are one of those interesting universals in human societies. All our cultures have them, but they can vary greatly. It's a bit like a how a smile means happiness among all peoples, though the degree of width matches up differently to how much happiness is being expressed. Then there are differing conditions in which it's appropriate to show joy or any emotion at all. And having an associative frontal cortex that can fuse concepts and generate novel behaviors, we can choose to subvert the meaning of a smile, turning it into an ironic grimace. 

Not too surprisingly, given the universal cultural existence of gender roles, there appears to be an area of the brain that plays an important role in processing them. Insults to the ventromedial prefrontal cortex correlate with specific difficulties in recognizing gender-specific cultural cues. That's not say that any one set of cultural roles is hardwired into our brains. Rather, it looks like the brain is softwired to broadly support and possibly help generate broad gender roles. 

As mentioned above, such roles manifest a fair amount of variety around the world and throughout history and varying economies. Many societies, from seal-hunting Inuits to medieval European agrarian societies, allowed widows to assume their deceased husband's profession or even a man's cultural role in order to continue providing for her family. Some, such as Classical Sparta, institutionalized homosexuality as a means of strengthening social bonds, other societies have worked to suppress it. Some societies generate stark differences between genders, and others weaker. Such a flexibility helps with the creation and maintenance of durable bodies of group behaviors suited to specific environments and modes of production, and then helping to adapt to moments of historic change, when new technologies alter what is possible or when massive ecological shifts from climactic disruptions or local resource collapses take place. As has happened to our species on regional scales many times over the past ten-thousand years. 

No comments: