Biology 202
1999 Final Web Reports
On Serendip


Caroline Choe

In this world, humans and animals alike have come to communicate by using various mechanisms. Humans have advanced themselves beyond other organisms by using language, or a set of codes and symbols, in order to express themselves to others. Language has brought about a means to create new thoughts, to explore, and to analyze our everyday surroundings. It has also enabled us to retain past memories and to look deep into the advances for the future. However, for some individuals, this tool for communication has been plagued by a language and speech disorders, such as aphasia. Aphasia is the loss of the ability to speak or understand speech or written language. It is often detected at an early age, and contributes to the general class of speech and language disorders affecting "5% of school aged children" (1) . Aphasia is classified into three categories. The main two are receptive or sensory aphasia and expressive or motor aphasia. Receptive aphasia affects the input side and "the ability to understand spoken or written language may be partially or totally lost" (1) . Those with expressive aphasia "can speak but not find certain words or names, or may be totally unable to communicate verbally or by writing" (1) . For a majority of affected individuals, there is a combination of the two. The third type is conduction aphasia. This "involves disruption of transmission between the sensory and motor ends of the circuit" (1) . Here, individuals are able to produce speech despite the lack of connections to the input side. It seems that the ability to speak has a lot to do with your surroundings and how much emphasis was placed on developing this skill during the first few years after birth. Afterall, it's known that the first few years are critical because this is the time when the brain is "plastic" and is rapidly changing and being molded. By the time that adolescence is reached, the brain has become "less plastic". In this paper, I would like to explore theories proposed to try to understand the origins of this impairment.

Ongoing research has tried to pinpoint exact reasons as to why there is speech impairment for those with aphasia and other language disorders. Most theories suggest genetic and environmental implications. Is the speech disability some sort of defect from within the brain, or does the disability develop as a result of influence from your surroundings and lack of nurture from others? Currently, it seems that researchers are leaning towards biological factors. Learning, in general, is comprised by a conjunction between "experience-independent and experience-dependent mechanisms to extract information" (2) from surroundings. Language development uses both types of mechanisms, but theorists place more emphasis on the significance of experience-independent mechanisms. Evidence over the years has associated damage to the brain as the main cause for aphasia. This brain damage may be a result of conditions like stroke, tumor, trauma, and infections.

Studies on aphasia and other speech disabilities have shed new light on how the brain is involved with language. For a long time, it has been known that the network of language is under the control of the left hemisphere of the brain. Three significant contributions have come from Marc Dax, Paul Broca, and Karl Wernicke. All three scientists investigated patients with speech problems and concluded the same findings: each patient had damage to the left side of the brain. Broca concluded damage to the left frontal cortex and Wernicke found damage to an area further back and lower in comparison to Broca's area (3) . Presently, aphasia located in Broca's region is associated with those who have problems in language production, whereas aphasia located in Wernicke's area is assigned to those who are able to produce speech, but have trouble with comprehension (2) . In 94% of normal persons, Wernicke's area is enlarged on the left, their dominant side for language (3) . Other techniques have also been used to study mechanisms of language in the brain. One is called the Wada test. One hemisphere of the brain is placed into sleep. When patients, whose left hemispheres were put to sleep, were asked to speak, they were unable to. But when the right side was placed into sleep, the ability to speak and respond were found. Tests with electroconvulsive shock treatment have also proven that language is predominant to the left half of the brain. The most recent technique is the use of PET (positron emission tomography) scans to monitor areas of the brain with increased blood flow during language tasks (4) . This has also shown activity in the left hemisphere.

One specific region of interest for researchers lies with a part of the brain called the planum temporale. This paired structure is known to be involved in language skills. It is connected to the temporal cortex, which is situated below the parietal cortex. In humans, the planum temporale is found to be enlarged in the left hemisphere, in comparison to the right hemisphere (5) . Some individuals with developmental language disorders may "show unusual symmetry or reversed asymmetry in this region" (4) . To what extent would damage to this structure produce in individuals? Would it contribute to the lack of speech found in aphasia individuals? Researchers have yet to say

Recently, there has been a discovery of a gene that might be vital to language development (6) . This may lead researchers to link speech deficiencies to this gene, ultimately proving that the ability to acquire language is related to genetics. Does that mean that defects or mutations of this gene also contribute to the speech deficiencies of those with aphasia? This has yet to be proven as well.

In the book, The Inheritance and Innateness of Grammers (7) , there have been hypotheses stating that language is innate to each individual. And because of this innateness, all children are able to "acquire language quickly and accurately" (7) . The author also brings up the idea that if language were not linked to biological factors and instead to environmental factors, then children who are exposed to nurturing of their language during early growing years should present increased language efficiency over children who receive normal or limited nurture. The following is evidence to her argument:

"within Inuit culture, as opposed to general North American culture, young children are rarely spoken to and are not encouraged to speak. Yet even though these children receive very limited language input, they show the same timing and pattern of language development as that of their more talkative neighbors to the South" (7) .

The author finally states that if language is linked to genetic factors, then mutations may "interfere with the ability to acquire language" (7) .

Overall, there seems to be increasing evidence pointing to the conclusion that the origins of speech and language impairment found in individuals with aphasia and other language disorders come from biological factors, rather than environmental factors. Evidence can be found from techniques that locate specific regions of the brain that have significant roles in language ability. Studies have shown that damage to the left hemisphere of the brain are contributing factors to the deficiency of speech in individuals. To add, the findings of a gene linked to language may soon help show that genetics may also be a key contributor to the problems in speech. Even though more evidence links the language and speech disorder to biological factors, I do not think that environmental influences can be entirely ruled out. I find it interesting that outside influences, most importantly treatment using speech therapy, is the most effective way to help cure those with language disorders.

WWW Sources

1) Children with communication disorders

2) Statistical Learning by 8 month old infants

3) InSCight


5) Similarities found in Human, Chimp brains

6) Autism

7) Gopnik, Myrna. The Inheritance and Innateness of Grammers . New York, Oxford University Press, 1997

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