Monday, February 4, 2013
The neural circuits and hormones that give rise to the functions of the human brain were optimized for the survival of our ancestors.
The efficacy of most of those attributes is obvious, but others seem to be at war with one another giving rise to conflicting and sometimes harmful behavior.
Philosophers have pondered for most of recorded history and no doubt before how it could be that the physical “brain” gives rise to the ethereal “mind”. As neuroscience advances, there is no evidence for it being otherwise.
Just as the rest of our body bares marks of our evolutionary history, the brain does as well. We share spines with all vertebral organisms because we once had common ancestors. We share characteristics with mammals for the same reason. Similarly, humans display attributes that define the primate line of descent.
Though we share many anatomic structures and functions with other creatures, their means of survival are different from ours. Cheetahs run fast, lizards are good at stealth, turtles have armor. The peculiarities of their physical bodies, both inside their cranium and outside, reflect these abilities.
One common need of all organisms, from the earliest times in the history of life, has been some variation of the fight-or-flight instinct. Animal brains had to react to perceived threatening situations, “Do I fight or do I run?” The emotion of fear and a physiological reaction are elicited below the level of consciousness in an instant.
There are a host of other survival strategies that increase chances for survival of ones lineage. So some species simply give birth to large numbers of offspring, but don’t invest much effort in rearing. Others, including humans, have fewer young but provide prolonged nurturing to adulthood.
Protracted immaturity of young required a unique set of tools. Mammals co-opted sweat glands for the purpose of making milk. Brains had to adapt to behaviors required for parenting.
Young had to be defended and fed. Some creatures evolved to make this solely the task of the mother or parents, while for others it became a cooperative effort of an extended family. Think of the musk ox herd defending its young from attack by a pack of wolves, or, the wolf pack sharing its kill among its members.
The human brain became the cleverest and most adaptable. Most of the skills our brain facilitates are wedded to other attributes such as our hands with their opposable thumbs. We became the most adept creatures at tool making.
Each of these developments is accompanied by our having learned to cooperate. Lion prides cooperate in the hunt, apes have societies reflecting their interdependence, but humans developed the ability to speak. This brought cooperation to a new level.
It enabled us to share every nuance of our thoughts. It allowed us to build civilizations. It allowed us to pass on knowledge to succeeding generations for them to build upon. None of this was possible without developments in the brain. New structures arose, or were co-opted from old, with unique neural connectivity.
This souped-up brain could experience things its distant ancestors could not. But, because, in some instances, old structures and functions have been borrowed for new purposes, potential conflicts arose. The ability to empathize and yet feel rivalry with our fellow human beings is one such contentious area.
There is evidence humans have had the capacity to imagine themselves standing in the moccasins of others for at least many hundreds of thousands of years. Yet rivalries for mates and conflicts over territory are only slightly different now than they have ever been.
This is only the barest outline of how the human brain evolved in parallel with the rest of its body. What has neurobiology learned in support of what anthropology tells us about how humans came to carve out their place on this planet?
The uniquely human parts of the brain arose from and, in a sense, overlaid earlier versions of the brain. Like layers of an onion, at the core, are remnants of the brains of our ancestors. Those remnants reflect the survival priorities of those ancestors.
In some instances, like the fight-or-flight instinct, certain reflexive reactions have persisted because they have continued to provide a useful function. But, it must function in an environment where other requirements must also be met.
When a situation with another person arouses a reaction to fight, it may be moderated by emotions of empathy with the perceived rival. Or the brain may unconsciously take into account the need to cooperate and foster harmony for the good of society.
The fight-or-flight reflex is identified with what is, perhaps misleadingly, called the reptilian brain. There are many structures and connections at the core of our brain that are not so terribly different from those of reptiles with which we had a common ancestor many hundreds of million years ago.
In fact, the structures of our mid-brain are hardly at all different from those of other mammals. These include the organs that house the circuits for aggression: rage, fear and dominance. It also includes the amygdala, hypothalamus, periaqueductal gray and the striatum, which facilitate setting goals for the entire brain.
The periaqueductal gray monitors those parts of the brain registering pain, balance, hunger, blood pressure, heart rate, temperature and hearing. Each of these has a role in prompting an animal’s sense of irritation, frustration or rage. The corresponding motor reflexes make an animal lunge, kick and bite.
The amygdala is connected to those parts of the brain dealing with memory and motivation. It provides an emotional dimension, particularly fear, to thoughts and memories. Anxiety, dread and our response to an angry face can be attributed to memories whose associations are flavored by the amygdala.
In the human brain these structures are enveloped by a disproportionately large cerebrum, the part of the brain housing higher level functions. This region enables our exercise of prudence, moralization and self-control. It serves to restrain or tame our wilder tendencies.
The frontal lobes give us the ability to regulate the fear and anger responses of the limbic system I sketched above. Some frontal lobe regions enable us to act on goals and plans we have formulated ahead of the need for action. They liberate our behavior from knee-jerk reaction to stimuli for which a fight-or-flight reaction is not best.
The dorsolateral cortex governs rational calculations of costs and benefits. This enables us to recognize how delayed rewards may trump immediate gratification. The frontal pole sits higher in the chain of command enabling us to multitask, explore alternatives, and navigate the competing demands of life.
Cognitive scientists have shown that, with proper childhood nurturing (and adult reinforcement), those parts of our brains with the more recent evolutionary history are capable of bridling our reactionary mid-brain. So, rather than be torn by a conflict between the cooler, rule-driven parts of our brains and our inner gut reactions, we have the ability to tip the scales to our advantage.
Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at firstname.lastname@example.org.