12 मार्च 2010

Connectionist model

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Connectionist model is also termed as “faculty model” .Faculty models are those in which the principal language functions are represented in the brain and these functions are entirely task oriented processes. The views of Wernicke and Lichtheim created a frame work for the classification and understanding of aphasia, and simultaneously, a model of the way language was represented in the brain which attempted to characterize language structures or the psychological processes that make up the various acts of language use, such as speaking, understanding and etc. It also included all the major tasks which language is put and claim to provide a complete numeration possible aphasic syndrome. The faculties which are postulated in the classical connectionist model are ex classically the major “on line” tasks of the psychology of language, tasks which occur in real time, and in which a speaker or listener produces or recognizes sequences of acoustics or graphic elements under time constraints. These are the usual “task” to which language put.
In the connectionist model, each of the psycholinguistic faculties of reading, writing, speaking, and hearing was considered an individual entity. Each was connected to the other and the connectionist model provides some provisions for the interaction between the components. Wernicke opposed “localization” and proposed that many functions resulted from connecting
various components. He thought that the major psycholinguistic functions understanding, spoken speech, speaking, reading, writing could be considered to constitute psychological entities which could be represented in the “center” , he believed that two or more centers could participate in a single function. This was the “simple” psychological function and the approach came to be known as “connectionist” because complex functions were built by connecting simple
components. Lichtheim adopted wernicke’s views essentially with respect to two major areas involved in language-Braca’s(H) and wernicke’s area(A). He thought that the first was involved in speech production, and believed that it contained the articulatory representations necessary for utterances. As for the second, he argued with wernicke’s notion that it contained the memory traces of the auditory term of words, and that its function was primarily the perception of
speech and followed wernicke in postulating “connectionist”. He suggested that there was “concept area” (B) which he thought was diffusely represented in the brain.

see fig. 1

The type of aphasia that would be predicted from the model depended upon the nature of information flow between various components. A lesion of H will cause the Broca’s aphasia. A disorder of A will cause wernicke aphasia. A disorder of connecting pathway will cause the “conduction” aphasia predicted by wernicke. In addition Lichtheim argued that there were four other forms of aphasia, all due to interruption of connecting pathways.Ttranscortical sensory aphasia, due to disruption of the path between B and A. Transcortical motor aphasia,due to a lesion between H and B. Subcortical sensory aphasia, interrupting the pathways from the periphery to A which leads to pure word deafness and subcortical motor aphasia, due to lesion between H and the oral, musculature which produces dysarthria-a disturbance of articulation.

see fig.2

Lichtheim’s extended the diagram incorporating the center ‘O’ for the memory of the visual form of words. He argued for the existence a separate area for the representation of the motor sequences involved writing.This center was ‘E’ in the diagram and the co-occurrence of disorder of writing areas testified to the link between Broca’s area H, and the writing center E.
Lichtheim attempted to provide principled description for the functioning of the components of his model. He realized that this description of the different syndromes as a result different lesion could be considered for the analysis of written language too. He was committed to an anatomically based model.
The model was originally based on the approach of the representation of language in the brain.
Advantages: 1. Connectionist theories have played a very significant role in the development of language representation in the brain. The theory has tremendous clinical utility. The clinician, by assessing a patient’s ability in each of the major psycholinguistics tasks- speaking, understanding, spoken language, reading, writing and repetition is able to make reasonable hypothesis as to the location of the lesion producing the aphasia, and to locates the pathological cause of the lesion.Especially in the era of clinical neurology in which sophisticated radiological diagnosis was not available, these hypothesis were the best theory available.
2. Secondly, it is the connectionist model in which the notion of “center” emerges most- clearly, A center consists of a single psycholinguistics faculty, associated with era major type of storage for linguistics item located in a particular area of the brain.
3. Finally, connectionist theory defined a level of observation and description of both normal and abnormal language which strongly influenced many works.
The feature of language in connectionist model were the on –line tasks of language use and stressed the assessment of the relative quantitative impairment of each of the on –line tasks. On the neurological side it emphasized or the localization of functions in gyri and groups of adjacent gyri.
Objections to connectionism: There were several claims to connectionism and these were divided into three main groups-
1. It was claimed that the basic observation made by connectionism were incorrect, there were additional observation about aphasic performance cast the theories of brain language relationships into doubt.
2. The second group of objection is directed towards the logic of the connectionist inferences from aphasic symptoms to neurolinguistics theory. These objections focused on the theory without disputing the actual observation of aphasia made by the connectionist.
3. The third set of objections consists of the view that the neurolinguistics theory should be based upon a different set of observation about the behaviour of aphasic patients from which the connectionists made

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