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
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.