The Human Brain - Right and Left

Our human history is the story of the development of our individual mind and our species collective mind.  The collective human mind has grown amazingly during our history and, it also serves as the base for that history. We all share the aspects of the brain that are shown below.  We have mechanisms that enable collective thinking and agreement with one another (V.S. Ramachandran, 1999).   Our cognition has grown rapidly and exceeded any cognition in the animal world. Cognition is generally our sense of awareness and includes knowing, perception, reasoning, and judgment.

Our mind, of course, is located in our brain.  The primary characteristic of the human brain that differentiates it from animal brains is our significantly larger cerebral cortex, shown in the figure.  A casual view of the human brain reveals two halves to the cerebral cortex, the left and right cerebral hemispheres.  However, before discussing the cerebral hemispheres, let us have a quick view of the brainstem that lies beneath the hemispheres.

The Brainstem

From: http://www.face-and-emotion.com/dataface/anatomy/media/Cunninham_brainstem-front.jpg 

Below the cortex lies the brainstem; it is older than the cerebral hemispheres in evolutionary terms.  It has numerous named landmark features, but perhaps the most prominent feature of the brainstem is the collection of 12 cranial nerves that emanate from the brainstem.  Memorizing these nerves is a primary part of any human neuro-anatomy class, and usually involves a pneumonic that begins “On Old Olympus’ Towering Top, a….” Each of these cranial nerves is paired, i.e. there are right and left nerve pairs, and for each the brainstem contains a nerve center or nucleus whichthat plays a major role in controlling the signals conveyed by those cranial nerves.  The list of cranial nerves and the functions they serve is shown in the table below. 

 


Cranial Nerve

Major Functions

I Olfactory

Smell

II Optic

Vision

III Oculomotor

Eye and eyelid movement

IV Trochlear

Eye movement

V Trigeminal

Chewing

Face and mouth touch and pain

VI Abducens

Eye movement

VII Facial

Controls most facial muscles

Secretion of tears and saliva

Taste

VIII Vestibulocochlear

Hearing

Equilibrium sensation

IX Glossopharyngeal

Taste

Senses carotid blood pressure

X Vagus

Senses aortic blood pressure

Slows heart rate

Stimulates digestive organs

Taste

XI Spinal accessory

Swallowing movements

XII Hypoglossal

Controls tongue

 

The brainstem and its cranial nerves, which is an older part of the brain, serve many of the basic body functions, but not the higher levels of cognitive function that are the topic of this book.  Considering the survival nature of evolution, the cranial nerves serve functions critical to the basic survival of the organism.  The cranial nerves serve smell, vision, muscular movement of the eyes, hearing, taste and the initial stages of food processing.  Most of these functions have been important to life survival and evolution going back approximately 530 million years to the Cambrian Period, to be discussed later.

The Cortical Hemispheres

The most apparent feature of the human brain is the two cerebral hemispheres.  Parts of the two hemispheres control the basic sensory and motor functions of the body that are not already controlled by the brainstem. The skeletal muscles for our arms, legs, and most other portions of our body; and also the senses of touch, pain, cold, and heat are controlled by specific locations in the cortical hemispheres.  The neural locations for sensing and skeletal muscle control are in contra-lateral hemispheres, i.e., the right side of the body projects to the left hemisphere and vice versa.  This contra-laterality also applies to the right and left fields of vision and also the initial sense of hearing.   

From: http://www.mayoclinic.com/images/image_popup/bn7_hemispheres.jpg

Although portions of the cerebral hemispheres serve muscular and sensory functions on opposite sides of the body, the higher cognitive functions of the two hemispheres are different from one another in a very different way.  Evidence for this difference has been accumulating for centuries, but it was most profoundly demonstrated by the groundbreaking work of R.E. Myers, R.W. Sperry, and M.S. Gazzaniga in the 1950’s and 1960’s in studying epileptic patients in whom the corpus callosum and other connections between the two hemispheres were severed.

The corpus callosum is a thick bundle of nerve fibers that is the primary neural connection between the right and left hemispheres.  In the early 1960's, the above-mentioned researchers performed surgical section of the corpus callosum to reduce the brain-wide spread of abnormal electrical discharges in selected epileptic patients.  Study of those patients has directly led to our current understanding of the significant differences between right and left brain function.

The cognitive abilities of the right brain are based upon the sense of vision.  It processes information in parallel - just as the sense of vision receives and processes an entire picture at once. All of the information in the picture is received simultaneously – i.e. in parallel.  Because of this type of processing, the right brain can integrate information from several sources and arrive at an “understanding”.  The right brain has no ability to communicate or think in words – it understands concepts but cannot understand or communicate in words.  The right brain is often referred to as the “silent brain”.  Feelings are generated and experienced in the right brain.  Feelings are non-verbal expressions – poets and writers often try to attach words to feelings; but words always fall short of completely capturing the essence of a feeling. 

The cognitive abilities of the left brain, on the other hand, are based upon speech and hearing.  Speech and hearing are time-sequential processes – one event follows another.  Hence the processing abilities of the left brain are serial.  Our thoughts, which come to us as words, emanate from the left brain.  Language skills are based in the left brain.  Deduction and logic require sequential processing and hence are products of the left brain.  Human ability to reason is based upon left brain activity.

In normal daily functioning, the cognitive abilities of the left and right brain are coordinated quite seamlessly.  Each half of the brain has distinct abilities and strengths that may be uniquely used for particular mental activities.  However, most mentally-guided tasks draw upon the blended abilities of right and left brain functions. 
 

The corpus callosum is one of the last cortical pathways to develop, it continues development up to the age of 18 (Luders et al, 2010) and probably beyond. Development of the corpus callosum has been linked to learning.  Late development of the corpus callosum also provides plausible support for the hypothesis that coordination of the right and left brains was accomplished late in evolutionary terms.

Caveat: This and further discussions in this book center on the differences between the cognitive skills that have developed from the neural architectures that serve the senses of vision and hearing.  Visual and auditory perceptions are very different from one another and require very different neural architectures.  Likewise the cognitive skills derived from visual and auditory perception are very different from one another.  The fact that the left brain is better at processing auditory perception and the right brain better at visual perception has been established by scientific inquiry.  The human brain is very complex and it is too simplistic to assign all visual perception and cognition to the right hemisphere and all auditory to the left.  Some physiological exceptions can be found.  However, to a very large extent the right/left separation is true and I continue to use those words in this book as shorthand synonyms for vision/speech based cognitive skills.