1.
Brain Structure and Function
Handedness
(left or right-handed) is directly related to the structure and development of
the brain. From the top of the spine upwards are the medulla oblongata, the
pons Varolii, the cerebellum and the cerebral cortex (cerebrum) in that order.
The cerebral cortex is characterized by a division into halves, termed hemispheres,
which are connected by tissue called the corpus collosum. It is not evident
exactly how important the actual size and weight of the brain are, perhaps the
ratio of brain to body size and weight which is important. Size of the brain
alone is not the crucial factor which would explain human language and
non-human lack of language. Structural differences must exist which account for
intelligence, language and other highly cognitive functioning.
2.
Hemispheric Dominance and Lateralization
The
left hemisphere controls the right side of the body, including the right hand,
the right arm and the right side of the face, while the right hemisphere
controls the left side of the body. For right-handed persons, the left
hemisphere generally dominate the right hemisphere, with the result that those
people tend to prefer their right hands. The majority have left-brain dominance
but their dominance is much less marked than in naturally right-handed persons.
This lack of strong dominance for left-handers is believed to be a factor
contributing to speaking problems and to various reading and writing
dysfunctions, such as reversal of letters and words when reading and writing.
Also, some studies suggest that there are differences between the brains of males
and females. Females normally have a thicker left hemisphere (one
specialization of which involves general sensory functions) and males have a
thicker right hemisphere (one specialization of which involves visual-spacial
functions).
Language,
logical and analytical operations, and higher mathematics, for example,
generally occur in the left hemisphere of the brain, while the right hemisphere
is superior at recognizing emotions, recognizing faces and taking in the
structures of things globally without analysis. This separation of structure
and function in the hemispheres is technically referred to as lateralization :
incoming experiences are received by the left or right hemisphere depending on
the nature of those experiences, be they speech, faces or sensations of touch.
Associated with lateralization is what might be termed ‘earedness’, where
right-handed persons with lateralization for language in the left hemisphere
will perceive more readily speech sounds through the right ear than the left.
The
main language centers in the left hemispheres are Broca’s area, in the front
part of the brain, Wernike’s area, towards the back, and the angular gyrus,
which is even further back. Broca’s area and Wernike’s area are connected by
tissue (the arcuate fasciculus). These areas are not found in the right
hemisphere. Generally, only the left hemisphere is used for speaking and
writing.
3.
Language Areas and Functioning
Language
areas :
1. The front part of
the parietal lobe, along the fissure of Rolando, is primarily involved in the
processing of sensation, and may be connected with the speech and auditory
areas at a deeper level.
2. The area in front of
the fissure of Rolando is mainly involved in motor functioning, and is thus
relevant to the study of speaking and writing.
3. An area in the upper
back part of the temporal lobe, extending upwards into the parietal lobe, plays
a major part in the comprehension of speech. This is ‘Wernicke’s area’.
4. In the upper part of
temporal lobe is the main area involved in auditory reception, known as
‘Heschl’s gyri’, after the Austrian pathologist R.L. Heschl (1824-1881).
5. The lower back part
of the frontal lobe is primarily involved in the encoding of speech. This is
‘Brocha’s area’.
6. Another area towards
the back of the frontal lobe may be involved in the motor control of writing.
It is known as ‘Exner’s centre’, after the German neurologist Sigmund Exner
(1846-1926).
7. Part of the left
parietal region, close to Wernicke’s area, is involved with the control of manual
singing.
8. The area at the back
of the occipital lobe is used mainly for the processing of visual input.
Wernicke
observed that Broca’s area was near that part of the brain which involves the
muscles which control speech while the area which he identified, later called
Wernicke’s area, was near the part of the brain which receives auditory
stimuli. Based on these observations, Wernicke hypothesized that the two areas
must in some way be connected. Later research showed that they are indeed
connected, being connected by the arcuate fasciculus.
Some of the neural
pathways that are considered to be involved in the processing of spoken
language.
1. Speech production.
The basic structure of the utterance is thought to be generated in Wernicle’s
area and is sent to Broca’s area for encoding. The motor programme is then
passed on to the adjacent motor area, which governs the articulatory organs.
2. Reading aloud. The
written form is first received by the visual cortex, then transmitted via the
angular gyrus to Wernicle’s area, where it is thought to be associated with the
auditory representation. The utterance structure is then sent on to Broca’s area.
3. Speech
comprehension. The signals arrive in the auditory cortex from the ear, and are
transferred to the adjacent Wernicle’s area, where they are interpreted.
Although
most language processes occur in Broca’s area, Wernicke’s area and the angular
gyrus, some language functioning does occur elsewhere in the left hemisphere,
and some even occurs in the right ‘non-language’ hemisphere.
4.
Brain Maturation and Critical Age for Learning Language
Five
years could not be a critical age because it is a commonplace observation that
children learn a second language easily until about 12 years of age and that
almost everyone can learn some or most of a second language after that age.
Children are generally better than adults at acquiring native-speaker pronunciation
in a second language.
5.
Language Disorders
Language
disorders, known as aphasias, are presumed to have as their cause some form of
damage to some specific site in the hemisphere where language is located. Such
damage causes characteristic problems in spontaneous speech, as well as in the
understanding of speech and writing.
5.1. Broca’s Aphasia
Broca’s
aphasia is characterized by meaningful but shortened speech and also occurs in
writing. There is a loss of syntactic knowledge in both speech production and
understanding for those with Broca’s aphasia. Interestingly, people with
Broca’s aphasia can often sing very well, even using the same words and
structures which they are unable to utter in conversation.
5.2. Wernicke’s Aphasia
This
condition is characterized by speech with often resembles what is called
nonsense speech or double-talk. It sounds right and is grammatical but it is
meaningless. It can seem so normal that the listener thinks that he or she has,
as is often the case in ordinary conversation, somehow misheard what was said
and therefore did not understand it. Patients with Wernicke’s aphasia also
commonly provide substitute words for the proper ones on the basis of similar
sounds, associations or other features.
5.3. Other Speech-related
Aphasias
Damage to the area
which leads into wernicke’s area from the auditory cortex may result in pure
word deafness, where one cannot recognize the sounds of words as speech but can
hear other types of sounds. A condition known as conduction aphasia is
characterized by a poor ability to repeat words despite relatively good
comprehension. Anomic aphasia involves problem in finding the proper words for
spontaneous speech, even though language comprehension and repetition are good.
Apraxia is inability to respond appropriately to verbal commands. Global
aphasia is a terrible condition in which many or all aspects of language are
severely affected, presumably due to massive damage at numerous sites in the
left hemisphere or to critical connections between language areas.
5.4. Reading and
Writing Aphasias : Dyslexias
The
type of aphasia which involves disorders in reading and writing is called
dyslexia. It may be subdivided into two basic categories: alexia, which
involves disorders in reading, and agraphia, which involves disorders in
writing.
5.5. Localism and Holism
The
localist model is the particular model of looking at the structure and function
of language by relating specific aspects of language to certain localized areas
of the brain. Although it is true that certain areas of the brain are involved
in language, it is also necessary to take into account holistic or global brain
phenomena in order to understand the effect on language of broader
psychological factors, such as attention span, motivation, alertness, the rate
at which auditory and visual memory traces dissipate, etc. A holistic type of
model does just this.
5.6. Sign Language
Aphasia
Native
signers of American Sign Language who have suffered trauma such as a stroke
(cerebral infarction) to the left hemisphere will produce sign language
equivalents of Broca’s aphasia or Wernicke’s aphasia. Signers who have suffered
damage to the right hemisphere generally do not display aphasia symptoms in the
production of signs.
6.
Methods of Investigating Brain and Language
6.1. Established
Methods: Post-mortem, Injured People, Electrical Stimulation
The
oldest method, that used by Broca himself, is the post-mortem examination of
the brain of patients who had displayed language disorders while they were
alive. A person might require—because of an accident or a tumour for
example—the removal of a lobe of the brain (lobectomy) or even of an entire
hemisphere (hemispherectomy). A more recent method, pioneered by Penfield in
the 1950s, involves the electrical stimulation of the cerebral cortex in
patients who are conscious during brain surgery (electroencephalography).
6.2. New High-Tech
Methods: CAT and PET
CT
(best known as CAT, Computerized Axial Tomography) and PET (Positron Emission
Tomography) are the most widely used in this regard. A CAT scan involves using an
X-ray source so as to make numerous slice scans, the images of which are
integrated by computer to construct an image of the whole brain or portion of
it. The PET procedure involves injecting a mildly radioactive substance into
the blood and then tracing the blood flow patterns within the brain by means of
special detectors surrounding the person’s head.
The New York Times News
Service reported that, by means of PET:
1. The brain
distributes language processing over a few or many cerebral areas. According to
researcher George Ojemann, many additional areas of the brain, besides Broca’s
and Wernicke’s areas, are involved in language.
2. Second languages are
rather loosely organized in the brain. A second language can even be localized
in the right hemisphere.
7.
Mind and Brain
Without
brain, there would be no mind. Is there a perfect correlation between a
person’s experiences and the events which take place in the brain? While there
is strong evidence of a general dependence of mental occurrences on the
functioning of the brain, it has never been shown that the correspondence is so
exact that from observation of a person’s brain one could arrive at a knowledge
of the person’s experiences in every detail.
Are
events in the mind wholly determined by other events? The determinism of events
can be physical or it can be mental, or both, depending on whether one takes a
dualist or monist view of the universe.
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