The advantage of the popular right and left-brain speculations is that most people know they have two cerebral hemispheres. The left hemisphere controls the right half of the body and visa versa.  The crossed innervation of the body is one of those curious facts that has no particular explanation. It just happens to be the case.

Damage or disease in the left hemisphere shows up in the right side of the body and visa versa. The left hemisphere tends to be dominant in terms of hand use and language storage in about 92% of humans. You determine dominance by watching which hand holds a pen and does more of the fine motor skills. The dominant side of the body also tends to be larger than the non-dominant side. About 4% of humans have right hemisphere dominance and another 4% are in the middle with more or less symmetrical hemispheric function.

The human hand is remarkably adaptable and the brain systems that control hand movements are more remarkable. Human hands hold tools, gesture, express feelings and meanings. Two hands work together in most tasks. This means that the two hemispheres work together by sending signals back and forth through a massive bundle of wires, the corpus callosum. In normal people, the left and right hemispheres form integrated operating systems that are often tightly coordinated as in walking, running, and tool use. Clumsy people are less coordinated and some have distinct difficulty achieving left and right cooperation.

The dominant hand leads the nondominant hand by 15 to 30 milli-seconds when coordinated movements are performed. This suggests that the left hemisphere initiates the movement and sends signals to the right. This asymmetric activation of the hemispheres may come from below the cerebral cortex (from the thalamus, for example) and may have implications about how all volitional activity is organized.

A popular notion, that the dominant left hemisphere is “analytic” and the right hemisphere is “synthetic or artistic,” makes little sense and is not a good way to try to understand how the human brain works. Roger Sperry and other surgeons launched the right-left theories by cutting the corpus callosum in patients with epilepsy. Studies of cognitive function revealed some interesting features of these “split-brain” patients who could not send signals back and forth between their hemispheres. These were distinctly abnormal people and their peculiarities did not reveal how normal people work.

As one would expect, the split-brain patients had disconnected cognitive functions because their hemispheres could not share information. In contrived experiments, information could be supplied to only one hemisphere and would not be available to the other. Each hemisphere revealed a separate consciousness in terms of responses to stimuli and reportable contents. Usually, only the left hemisphere could speak and could only report on information received on the left. The right hemisphere could not speak, but communicated with nonverbal vocalizations and in other ways

The coordination of left and right hand and arm movements is critically important to human survival. Both hands are needed to perform most tasks and although the hands may do different tasks, both hands cooperate and work toward the same goal. The right-left linkage shows up clearly whenever you try to perform distinctly different tasks with each hand. Even with sustained practice, the hands want to do similar things or perform linked movements as you do when you play the bongo drums or knit sweaters. 

Human Brain in Health and Disease

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