develop systems, that are able to act and react properly to the world; And some day, are able to act
intelligent, perhaps.
Hemispheric Distribution
After having dealt with how knowledge is stored in the brain we now turn to the question of
whether the brain is specialized and if it is which functions can be located where. These questions can
be subsumed under the topic “hemispheric specialisation” or “lateralization of processing” which looks
at the differences in processing between the two hemispheres of the human brain. Differences between
the hemispheres can be traced back to as long as 3.5 million years ago. Evidence for this are fossils of
australopithecines (which is an extinct ancestor of homo sapiens). Because differences have been
present for so long and survived the selective pressure they must be useful in some way for our
cognitive processes.
Differences in Anatomy and Chemistry
Although at first glance the two hemispheres look identically they differ in fact quite a lot in
various ways.
Concerning the anatomy, some areas are larger and the tissue contains more dendritic spines in one
hemisphere than in the other. An example of this is what used to be called “Broca’s area” in the left
hemisphere. This area which is –among other things- important for speech production shows greater
branching in the left hemisphere than in the respective right hemisphere area. Because of the left
hemisphere’s importance for language, with which we will deal later, one can conclude that anatomical
differences have consequences for lateralization in function.
Neurochemistry is another domain the hemispheres differ in: The left hemisphere is dominated by
the neurotransmitter dopamine, whereas the right hemisphere shows higher concentrations of
norepinephrine. Theories suggest that modules specialized on cognitive processes are distributed over
the brain according to the neurotransmitter needed. Thus, a cognitive function relying on dopamine
would be located in the left hemisphere.
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Historic Approaches
Hemispheric specialisation has been of interest since the days of Paul Broca and Karl Wernicke,
who discovered the importance of the left hemisphere for speech in the 1860s. Broca examined a
number of patients who could not produce speech but whose understanding of language was not
severed, whereas Wernicke examined patients who suffered the opposite symptoms (i.e. who could
produce speech but did not understand anything). Both Broca and Wernicke found that their patients’
brains had damage to distinct areas of the left hemisphere.
Because in these days language was seen as the cognitive process superior to all other processes,
the left hemisphere was believed to be superior to the right which was expressed in the “cerebral
dominance theory” developed by J.H. Jackson. The right hemisphere was seen as a “spare tire […]
having few functions of its own” (Banich, S.94). This view was not challenged until the 1930s. In this
decade and the following, research dramatically changed this picture. Of special importance for
showing the role of the right hemisphere was Sperry, who conducted several experiments in 1974 for
which he won the Nobel Prize in Medicine and Physiology in 1981.
Experiments with Split-Brain-Patients
Sperry’s experiments were held with people who suffered a condition called “split brain syndrom”
because they underwent a commissurotomy. In a commissurotomy the corpus callosum which is the
major cortical connection between the two hemispheres is cut so that communication between the
hemispheres becomes severed in these patients. Before dealing with the role of the corpus callosum, we
will talk about Sperry’s pioneering experiments with which he wanted to find out whether the left
hemisphere really played such an important role in speech processing as was suggested by Broca and
Wernicke.
Sperry used different experimental designs in his studies, but the basic assumption behind all
experiments of this type was that perceptual information received at one side of the body is processed
in the contralateral hemisphere of the brain.
In one of the experiments the subjects had to recognize objects by touching it with merely one
hand, while being blindfolded. He then asked the patients to name the object they felt and found that
people could not name it when touching it with the left hand (which is linked to the right hemisphere).
The question that arose was whether this inability was due to a possible function of the right
hemisphere as “spare tire” or due to something else. Sperry now changed the design of his experiment
so that patients now had to show that they recognized the objects by using it the right way. For
example, if they recognized a pencil they would use it to write. With this changed design, no difference
in performance between both hands were found.
Another experiment conducted by Sperry involved chimeric pictures. A chimeric picture is a
picture that is made up of two things that are each cut in half and then put together as one.
Experiments with Patients with other Brain-Lesions
Other experiments that were conducted with the aim to find out about hemispheric specialisation
was done with individuals who were about to receive surgery in which parts of one of their
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hemispheres was going to be removed due to epileptic seizures. Before the surgery was begun, it was
important to find out which hemisphere was responsible for speech in this individual. This was done
with the Wada-technique. Here, barbiturate was injected into one of the arteries supplying the brain
with blood. Shortly after the injection, the contralateral side of the body is paralysed. If the person was
now still able to speak, the doped hemisphere of the brain is not responsible for speech production in
this individual. With the results of this technique it could be estimated that 95% of all adult right-
handers use their left hemisphere for speech.
Drawbacks
Research with people who suffer brain lesions or even a commissurotomy has some major draw
backs: The reason why they had to undergo such surgery is usually epileptic seizures. Because of this,
it is possible that their brains are not typical or have received damage to other areas during the surgery.
Also, these studies have been performed with very limited numbers of subjects, so the statistical
reliability might not be high.
Experiments with Neurologically Intact Individuals
In addition to experiments with brain-severed patients,
studies with neurologically intact individuals have been
conducted to measure perceptual asymmetries. These are usually
performed with one of three methods: Namely the “divided visual
field technique”, “dichaptic presentation” and “dichotic
presentation”. Each of them again has as basic assumption the
fact that perceptual information received at one side of the body
is processed in the contralateral hemisphere.
The divided visual field technique is based on the fact that the
visual field can be divided into the right (RVF) and left visual
field (LVF). Each visual field is processed independently from
the other in the contralateral hemisphere. The divided visual field Highly simplified picture of the visual
technique includes two different experimental designs: The pathway.
experimenter can present one picture in just one of the visual fields and then let the subject respond to
this stimulus. The other possibility involves showing two different pictures in each visual field. A
problem that can occur using the visual field technique is that the stimulus must be presented for less
than 200 ms because this is how long the eyes can look at one point without shifting of the visual field.
In the dichaptic presentation technique the subject is presented two objects at the same time in
each hand. (c.f. Sperry’s experiments)
The dichotic presentation technique enables researchers to study the processing of auditory
information. Here, different information is presented simultaneously to each ear. Experiments with
these techniques found that a sensory stimulus is processed 20 to 100 ms faster when it is initially
directed to the specialized hemisphere for that task and the response is 10% more accurate.
Explanations for this include three hypotheses, namely the direct access theory, the callosal relay
model and the activating-orienting model. The direct access theory assumes that information is
processed in that hemisphere to which it is initially directed. This may result in less accurate responses,
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if the initial hemisphere is the unspecialised hemisphere. The Callosal relay model states that
information if initially directed to the wrong hemisphere is transferred to the specialized hemisphere
over the corpus callosum. This transfer is time-consuming and is the reason for loss of information
during transfer. The activating-orienting model assumes that a given input activates the specialized
hemisphere. This activation then places additional attention on the contralateral side of the activated
hemisphere, “making perceptual information on that side even more salient”. (Banich)
Results
All experiments had some basic findings in common: The left hemisphere is superior at verbal
tasks such as the processing of speech, speech production and recognition of letters whereas the right
hemisphere excels at non-verbal tasks such as face recognition or tasks that involve spatial skills such
as line orientation, or distinguishing different pitches of sound. This is evidence against the cerebral
dominance theory which appointed the right hemisphere to be a spare tire! In fact both hemispheres are
distinct and outclass at different tasks, and neither one can be omitted without this having high impact
on cognitive performance.
Although the hemispheres are so distinct and are experts at their assigned functions, they also have
limited abilities in performing the tasks for which the other hemisphere is specialized.
Do the Hemispheres Differ in What or How They Process?
There are two sets of approaches to the
whole question of hemispheric specialisation.
One set of theories goes about the topic by
asking the question “What tasks is each
hemisphere specialized for?”. Theories that
belong to this set, assign the different levels of
ability to process sensory information to the
different levels of abilities for higher cognitive
skills. One theory that belongs to this set is the
“spatial frequency hypothesis”. This hypothesis
states that the left hemisphere is important for
fine detail analysis and high spatial frequency
in visual images whereas the right hemisphere
is important for low spatial frequency. We have
pursued this approach above.
Experiment on local and global processing with patients with
left- or right-hemisphere damage
The other approach does not focus on what type of information is processed by each hemisphere
but rather on how each hemisphere processes information. This set of theories assumes that the left
hemisphere processes information in an analytic, detail- and function-focused way and that it places
more importance on temporal relations between information, whereas the right hemisphere is believed
to go about the processing of information in a holistic way, focusing on spatial relations and on
appearance rather than on function.
The picture above shows an exemplary response to different target stimuli in an experiment on
global and local processing with patients who suffer right- or left-hemisphere damage. Patients with
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damage to the right hemisphere often suffer a lack of attention to the global form, but recognize details
with no problem. For patients with left-hemisphere-damage this is true the other way around. This
experiment supports the assumption that the hemispheres differ in the way they process information.
Communication Between the Hemispheres via the Corpus Callosum
After we have looked at the different functions of each hemisphere and how researchers went
about finding this out, we will now look at the role of the corpus callosum. With its 250 million nerve
fibres the corpus callosum is like an Autobahn for neural data connecting the two hemispheres. There
are in fact smaller connections between the hemispheres but these are little paths in comparison to the
corpus callosum. All detailed higher order information must pass through the corpus callosum when
being transferred from one hemisphere to the other. The transfer time which can be measured with ERP
takes between 5 to 20 ms.
So why is this transfer needed at all if the hemispheres are so distinct concerning functioning,
anatomy, chemistry and the transfer results in degrading of quality of information and takes time? The
reason is that the hemispheres, although so different, do interact. This interaction has important
advantages because as studies by Banich and Belger have shown it may “enhance the overall
processing capacity under high demand conditions” (Banich). (Under low demand conditions the
transfer does not make as much sense because the cost of transferring the information to the other
hemisphere are higher than the advantages of parallel processing.)
The two hemispheres can interact over the corpus callosum in different ways. This is measured by
first computing performance of each hemisphere individually and then measuring the overall
performance of the whole brain.
In some tasks one hemisphere may dominate the other in the overall performance, so the overall
performance is as good or bad as the performance of one of the single hemispheres. What’s surprising
is that the dominating hemisphere may very well be the one that is less specialized, so here is another
example of a situation where parallel processing is less effective than processing in just one half of the
brain.
Another way of how the hemispheres interact is that overall processing is an average of
performance of the two individual hemispheres.
The third, most surprising way the hemispheres can interact is that when performing a task
together the hemispheres behave totally different than when performing the same task individually.
This can be compared to social behavior of people: Individuals behave different in groups than they
would when being by themselves.
Individual Factors may Influence Lateralization
After having looked at hemispheric specialization from a general point of view, we now want to
focus on differences between individuals concerning hemispheric specialization. Aspects that may have
an impact on lateralization might be age, gender or handed-ness.
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Age
Let’s first look at whether the age of an individual decides in how far each hemisphere is used at
specific tasks. Researchers have suggested that lateralization develops with age until puberty. Thus
infants should not have functionally-lateralized brains. Here are four pieces of evidence that speak
against this hypothesis:
Infants already show the same brain anatomy as adults. This means the brain of a new born is
already lateralized. Following the hypothesis that anatomy is linked to function this means that
lateralization is not developed at a later period in life.
Differences in perceptual asymmetries that means superior performance at processing verbal vs.
non-verbal material in the different hemispheres cannot be observed in children aged 5 to 13, i.e.
children aged 5 process the material the same way 13 year olds do.
Experiments with 1-week-old infants showed that they responded with increased interest to verbal
material when this was presented to the right ear than when presented to the left ear and increased
interest to non-verbal material when presented to the left ear. The infants’ interest was hereby measured
by the frequency of soother sucking.
Although children who underwent hemispherectomy (the surgical removal of one hemisphere) do
develop the cognitive skills of the missing hemisphere (in contrast to adults or adolescents who cannot
compensate for missing brain parts), they do not develop these skills to the same extent as a child with
hemispherectomy of the other hemisphere. For example: A child whose right hemisphere has been
removed will develop spatial skills but not to the extent that a child whose left hemisphere has been
removed, and thus still possesses the right hemisphere.
Handedness
Another factor that might influence brain lateralization is handedness. There is statistical evidence
that left-handers have a different brain organization than right-handers. 10% of the population is left-
handed. Whereas 95% of the right-handed people process verbal material in a superior manner in the
left-hemisphere, there is no such a high figure for verbal superiority of one hemisphere in left-handers:
70% of the left-handers process verbal material in the left-hemisphere, 15% process verbal material in
the right hemisphere (so the functions of the hemispheres are simply switched around), and the
remaining 15% are not lateralized, meaning that they process language in both hemispheres. Thus as a
group, left-handers seem to be less lateralized. However a single left-handed-individual can be just as
lateralized as the average right-hander.
Gender
Gender is another aspect that is believed to have impact on the hemispheric specialization. In
animal studies, it was found that hormones create brain differences between the genders that are related
to reproductional functions. In humans it is hard to determine to which extent it is really hormones that
cause differences and to which extent it is culture and schooling that are responsible.
One brain area for which a difference between the genders was observed is the corpus callosum.
Chapter 12
Although one study found that the c.c. is larger in women than in men these results could not be
replicated. Instead it was found that the posterior part of the c.c. is more bulbous in women than in
men. This might however be related to the fact that the average woman has a smaller brain than the
average man and thus the bulbousness of the posterior section of the c.c. might be related to brain size
and not to gender.
In experiments that measure performance in various tasks between the genders the cultural aspect
is of great importance because men and women might use different problem solving strategies due to
schooling.
Summary
Although the two hemispheres look like each other’s mirror images at first glance, this impression
is misleading. Looking closer, the hemispheres not only differ in their conformation and chemistry, but
most importantly in their function. Although both hemispheres can perform all basic cognitive tasks,
there exists a specialization for specific cognitive demands. In most people, the left hemisphere is an
expert at verbal tasks, whereas the right hemisphere has superior abilities in non-verbal tasks. Despite
the functional distinctness the hemispheres communicate with each other via the corpus callosum.
This fact has been utilized by Sperry’s experiments with split-brain-patients. These are outstanding
among other experiments measuring perceptual asymmetries because they were the first experiments to
refute the hemispheric dominance theory and received recognition through the Nobel Prize for
Medicine and Physiology.
Individual factors such as age, gender or handed-ness have no or very little impact on hemispheric
functioning.
References
Editors: Robert A. Wilson and Frank C. Keil.(Eds.) (online version july 2006). The MIT
Encyclopedia of the Cognitive Sciences (MITECS), Bradford Books
Knowledge Representation
Goldstein, E. Bruce.(2005). Cognitive Psychology - Connecting, Mind Research, and Everyday
Experience. Thomson, Wadsworth. Ch 8 Knowledge, 265-308.
Sowa, John F.(2000). Knowledge Representation - Logical, Philosophical, and Computational
Foundations. Brooks/Cole.
Slides concerning Knowledge from: http://www.cogpsy.uos.de/ , Knowledge: Propositions and
images. Knowledge: Concepts and categories.
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Hemispheric Specialisation/Distribution
Banich, Marie T.(1997).Neuropsycology - The Neural Bases of Mental Function. Hougthon
Mifflin Company. Ch 3 Hemispheric Specialisation, 90-123.
Hutsler, J. J., Gillespie, M. E., and Gazzaniga (2002). The evolution of hemispheric specialization.
In Bizzi, E., Caliassano, P. and Volterra V. (Eds.) Frontiers of Life, Volume III: The Intelligent
Systems Academic Press: New York.
Birbaumer, Schmidt(1996). Biologische Psychologie. Springer Verlag Berlin-Heidelberg.
3.Auflage. Ch 24 Plastizität, Lernen, Gedächtnis. Ch 27 Kognitive Prozesse (Denken).
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Ivanov, Vjaceslav V.(1983). Gerade und Ungerade - Die Assymmetrie des Gehirns und der
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Links
Knowledge Representation
From Stanford Encyclopedia of Philosophy: knowledge analysis, knowledge by acquaintance and
Lecture on Knowledge and Reasoning, University of Erlangen Germany
Links to Knowledge-Base and Ontology Projects Worldwide
Links on Ontologies and Related Subjects
Knowledge Representation: Logical, Philosophical, and Computational Foundations, by Sowa,
Hemispheric Specialisation
Evolution of Hemispheric Specialisation, by Hutsler, Gillespie, Gazzaniga
Cerebral specialisation and interhemispheric communication, by Gazzaniga,in Oxford Journals