Brain Mind Center

Two Hands, Two Hemispheres

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 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.

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 .

Linear and Spatial

The term "linear' has become popular to describe behavior that is goal oriented and sequential. The shortest journey between two points is a line. Linear thinking is supposed to be analytical and some even claim it occurs on one side of the brain and not on the other. The opposite of linear thinking is supposed to be spatial, somehow multidimensional and freer than linear thinking. Speculations about hemispheric specialization in terms of linear-analytic and spatial-artistic are misleading.

Everyone who goes to the store to buy groceries has a goal and a plan. The plan is implemented by following a sequence of steps. If you have gone to get groceries many times the sequence is stored in memory and you do not have to think about it. We can describe your plan as intuitive, spatial and holistic; you just go to the store and get groceries. If you are new to town and this is your first trip, your strategy is quite different. Going shopping for the first time is different from going the tenth time. A good idea is to get directions from someone who has gone to the store many times. They tell you how to get there and they may draw you a map showing the waypoints along your path. Males prefer a map and females prefer verbal instructions that emphasize landmarks that act as waypoints. Waypoints, as every navigator knows, are intermediate destinations along a travel path that allow you to confirm that you are heading in the right direction. Some waypoints are decision points. You have to identify a landmark and decide to turn left or right. Some describe implementing the first trip to the store as “linear and analytic” because you need more conscious effort; the directions and the sequence you follow may be written down in words and recorded in a diagram that shows lines connecting A to B to C.

Most human sequences follow the same pattern of transforming from “linear and analytic” as you learn into “synthetic after you have practiced a great deal. Practice and experience transform "linear analytic” tasks into “spatial synthetic” tasks. The brain works harder and operates differently when the task is new. Modules in both sides of the brain are active with every new sequence that has been observed, but when the sequence is practiced repeatedly, brain activation becomes more efficient, more localized and sometimes more lateralized. Another observation is that younger brains are more efficient and older brains are less efficient, using more diffuse and bilateral processing.

Further reading: Neuroscience Notes, Intelligence and Learning, Language & Thinking

Persona Digital

Persona Digital Books is the publisher of The Human Brain, Neuroscience Notes, Intelligence & Learning and related books. Three books are available as print editions for mail delivery. All books are available as eBooks for download (PDF files).

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