Hemisphere Asymmetry & Complementary Constraint

It has been estimated that the human brain contains 80-100 billion neurons.

 Each of the 10¹¹ (one hundred billion) neurons has on average 7,000 synaptic connections to other neurons.

Each neuron can be excitatory or inhibatory.  The philosopher Dan Dennett decribes individual neurons:

"not as dutiful slaves or as simple machines but as agents that have to be kept in line and that have to be properly rewarded and that can form coalitions and cabals and organizations and alliances"

How does this massive number of potential signal transmissions form these alliances, and what are the underlying physical and organizational structures that allow for the sense of a single coherent mind to eventually emerge from this interconnected web of electrical signals ?

The brain can be conceptualy divided at many levels. A rational decision making process depends on the the frontal cortex and the limbic system mutually constraining each other. A primary (top-down) function of the  frontal cortex (often called the executive function) is thought to be inhibatory, helping to keep the more primitive and emotional limbic system under control.  Similarly the frontal cortex depends on its connection to the feelings and emotions that rise up from the limbic system to give its decisions a sense of proportion.

Much of the competition and coordination between these electrical signals occurs below the conscious surface within the autonomic nervous system. For example, the unconscious process of multiple negative feedback systems orchestrated primarily by the hypothalamus is responsible for maintaining a state of homeostasis in the body. These processes are also examples of mutally constraining systems.

At its most basic structural division the brain is divided into a left and right hemisphere separated by the corpus callosum which allows the two hemispheres to communicate.  An asymmetry in the functions of the two hemispheres will be the focus of this post. While there has been much over-simplification for the roles of the two hemispheres in the public arena there some fundamental and fairly consistent differences in the way they allow us to perceive the world. This basic structural and functional division very often takes the form of complementary constraint .

Cartoon Depicting Locally Compartmentalized Left Brain vs Globaly Holistic Right Brain.
Image from mattovermatter.com             

Thinking about Complementary Correspondences –
	Left Hemisphere Dominance	Right Hemisphere Dominance
General	Language (but RH for metaphors & prosody). Literal, certainty. Dopamine	Music, Spatial awareness. Sets the context for understanding. Uncertainty. Noradrenaline
Behavior	Approach Context dependent routines	Withdrawal Novelty
Emotions	Generally positive, also anger	Unconscious Emotions (&here) (in response to faces) Conscious Negative Emotions (in response to words)
Thinking	Convergent (knows what it knows) – Likes to analyze & interpret.	Divergent (Creativity) - Theory of Mind, Better at quickly perceiving images, faces, social meaning etc…
Attention	Focused	Sustained (Holistic) Visual-spatial
Neural Communication	Local connections (see pg 64)	Global connections (see pg 64)
Autonomic Nervous system	Parasympathetic	Sympathetic - Perceptions
Memory	Autobiographical Encoding	Retrieval

Keep in mind these are gross generalizations and tendencies. Within each hemisphere many regions (such as the older limbic system and more recent frontal cortex)  have different functions. For example this study on creativity found the following:

"Although the research reviewed indicates greater right hemisphere activity during creative tasks, the interaction between many varied, often distant, cortical regions across both the left and right hemispheres is also a crucial component of creativity. This interaction facilitates the integration of a variety of separate cognitive abilities, fostering creative thinking. As such, creativity is better conceptualised as a distributed, rather than a purely lateralised, function; more lateral thinkers have less lateralised brains."

Neuroscientist Michael Gazzaniga has done extensive work with split brain patients leading to greater understanding of how the left & right brain hemispheres function. The left hemisphere likes to analyze, interpret & develop narratives (stories). When data or information is lacking (not communicated from the right hemisphere) the left hemisphere will create a story based on its best guess from prior beliefs and this in turn can lead to false beliefs (See this video).  The right hemisphere is also thought to be an important source of inhibition. This paper points to a specific right frontal area as primary.

Interesting disease states such as anosognosia, and capras syndrome among others which can be caused by brain lesions are also informative (see the work of Ramachandran & Sacks). Anosognosia presents as an inability for patient to recognize their own illness. Extreme examples include a (right hemisphere) stroke patient who could not recognize her own left arm as paralyzed.  Capgras syndrome can cause a patient to believe a family as been replaced by an imposter who looks just like the family member. Collectively these results suggest that severe right hemisphere damage is more likely to result in the denial of reality as compared to left hemisphere damage.

In this animated video Psychiatrist Iain McGilchrist, the author of The Master and his Emissary describes differences between the left and right halves of the human brain. He feels our modern society has become to reliant on the left hemisphere.

When we practice Tai Chi we attempt to develop a particular type of sustained global spatial attention. This type of attention is often referred to as mindfulness in meditation circles. A person practicing Tai Chi is in continual motion yet finding a stillness from within orient oneself to the outside world. If McGilchrist is correct then Tai Chi practice may help encourage a type of hemisphere integration that is under utilized in our compartmentalized modern world.