The Brain's Two Hemispheres and the Corpus Callosum
The brain's two hemispheres intricately interconnect to unify our experience. This interconnection is accomplished via the recessed corpus callosum, also called a commissure, which means the place where two things are joined. The corpus callosum is a wide bundle of white-matter made up of axons that facilitate communication between the brain's hemispheres. You can see this white bundle in the image below left (links to source). The arrow on the MRI image below right points to the corpus callosum. Linda J. Richards produced the website from which these images were taken; Richards does research related to the corpus callosum.
Regarding our movements and our senses, the brain's hemispheres lateralize operations. Wikipedia provides a summation: "Motor connections from the brain to the spinal cord, and sensory connections from the spinal cord to the brain, both cross the midline at brainstem levels." This lateralization means that motor control and sensory data related to the left side of the body is processed in the right side of the brain and vice versa.
In The Modular Brain: How New Discoveries in Neuroscience are Answering Age-Old Questions about Memory, Free Will, Consciousness, and Personal Identity (1994), neurologist Richard M. Restak sums up how either the left or right hemisphere usually handles certain processes: "The right hemisphere is dominant for nonverbal, largely spatial tasks like copying designs, interpreting facial expression, mentally transforming or transferring visual images in ones' mind, intuitively appreciating geometrical designs. In addition, the right hemisphere is superior to its counterpart in expressing and appreciating emotions. The left hemisphere is the speech and language maven: it handles reading, writing, and understanding spoken language. The left hemisphere is also specialized for calculation. But more important than the parceling of function is the fact that each hemisphere has its own unique consciousness."
To illustrate the separate consciousness of each hemisphere, Restak points to experiments with patients who have had a procedure in which a surgeon severs the previously described corpus callosum, the thick fiber system that connects the two hemispheres. This procedure, called a commissurotomy, is sometimes used to control severe, crippling epileptic seizures. These so called split-brain subjects are shown a chimeric picture—two completely different photos joined together in the middle. Restak explains that one such chimeric picture "shows the right face of the actress Catherine Deneuve joined to the left face of an elderly Italian worker from Fisherman's Wharf in San Francisco." But before we go on with this story, we need to discuss visual processing.
The illustration to the right (links to source) clarifies the lateralization of visual field processing. Eran Zaidel, Dahlia W. Zaidel, and Joseph E. Bogen, California Institute of Technology, authored the webpage from which the illustration, titled "The Split Brain," is taken.
According to the MedlinePlus Dictionary, the optic chiasma is "the X-shaped partial decussation on the undersurface of the hypothalamus through which the optic nerves are continuous with the brain—called also optic chiasm." This definition is a bit redundant and confusing in that decussation is a Latin derivative meaning an instance of two things intersecting each other to form an X. I include it here because of the good description of anatomical location. A simpler way to interpret the term is to remember that chiasma comes from the Greek word for the Greek letter X and means "crossing." The optic chiasm, according to Wikipedia, allows "for parts of both eyes that attend to the right visual field to be processed in the left visual system in the brain, and vice versa." This is one aspect of the brain's lateralization. Thus, in the experiment Restak describes above, Ms. Deneuve's image will be routed to the split-brain subject's right brain for processing and the wharf worker's image will be routed to the subject's left brain for processing. Because the right hemisphere processes facial recognition, when the subject is instructed "Point to what you saw," he or she chooses the picture of the actress, explains Restak. But because the left hemisphere processes words, sentences, and verbal responses, when the subject is asked "What did you see?" the subject responds with: "A man with dark hair and heavy busy eyebrows." Restak goes on to explain that in recognizing Catherine Deneuve, subjects never report that they recognize her based on having seen only one-half of a face. Likewise, when asked about what they saw, subjects never respond that they saw only one-half of a man's face. Restak explains: "The brain here is performing a kind of 'filling-in process,' that enables it to guess a complete picture from only partial information."
The two hemispheres also function differently in processing emotions. The right hemisphere responds to negative emotions while the left hemisphere responds to positive emotions.
Regarding a stroke in the right hemisphere, Jaak Panksepp, in Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), explains in Affective Neuroscience that "often patients remain cheerful despite the severity of their problems." He points out that comparable damage to the left hemisphere "can cause catastrophic emotional distress, and such patients are more prone to become despondent and depressed."
In The Blank Slate: The Modern Denial of Human Nature (2002), Steven Pinker observes that "the evidence is overwhelming that every aspect of our mental lives depends entirely on physiological events in the tissues of the brain." Later he writes, "Each of us feels that there is a single 'I' in control. But that is an illusion that the brain works hard to produce." Regarding the astonishing results of commissurotomy, Pinker writes: "When surgeons cut the corpus callosum that joins the cerebral hemispheres, each hemisphere can exercise free will without the other one's advice or consent. Even more disconcertingly, the left hemisphere constantly weaves a coherent but false account of the behavior chosen without its knowledge by the right."
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