The early studies done in this area were by Broca (1861) and Wernicke (1874). Broca's work included the patient, now known as “tan", who could only say the word “tan", although his speech comprehension stayed in tact. The patient died shortly afterwards and autopsy revealed that he had been damaged in the lower part of the left frontal lobe. Broca collected eight more cases and concluded that the identified area was essential in the production and comprehension of speech. The area is called Broca's area and the condition of damage to it is called Broca's aphasia.
Wernicke reported a different aphasia. Patients could speak in organized and grammatical sentences, although what they said seemed to have little to do with the on going conversation. On the other hand they seemed to have no understanding of what was spoken to them. On autopsy, they all had damage to an area at the top of the left temporal lobe, now known as Wernicke's area.
These speech zones also tally with the sensory and motor cortical mechanisms. For example, Wernicke's aphasia represents a problem with speech processing. The spoken word is a sound stimulus which enters our ear and in transferred as an electrical signal through our nerves until it reaches the primary auditory cortex in the temporal lobe. This area is close to Wernicke's area which contains the “word analyzer". The word analyzer contains the sound patterns of words that are essential in converting speech into words. If Wernicke's are
damaged the sounds cannot be identified as speech and comprehended.
We humans have a wonderful opportunity to communicate with each other both spoken and written languages, unlike our “smaller brothers” from animal world who can only use some specific signals which are recognized by them only. Language is a very powerful tool that enables us not only to communicate, but also plays a vital part in our brain’s development and simply “working’.
Broca's area is in the frontal lobe and contains the motor cortex. Speech is a motor process requiring sophisticated control over muscles of the throat, lips and mouth. Broca's area contains the motor plans for words. When Broca's damaged the plans cannot be activated even though the motor cortex is intact.
Speech, reading and writing are also language functions that have a role in the human brain. Aphasia study showed that the condition was more common in left hemisphere damage than in right. In contrast to the symmetry of sensory and motor functions, language functions are asymmetrical, being more dominant in the left hemisphere. Global aphasia is when both Broca's and Wernicke's areas are damaged and “shadowing" experiments in which words are spoken to a subject quickly and they must repeat them back quickly suggest that a global aphasic will be unable to “shadow". The accurate fasciculus’s the direct pathway from Wernicke's to Broca's area. If this is damaged then conduction aphasia results, which is severe impairment of repetition or “shadowing". However, normal speech production and comprehension are less affected. This implies other less direct pathways exist apart from the direct pathway of accurate fascicules.
Writing, as a motor process is produced from the motor cortex under the control of the planning centre in Broca's area. The input system, reading, involves the visual system and a region that contains the visual pattern of words and is capable of converting visual input into words. The key region for reading and writing is the angular gyros, on the borders of the temporal and parietal lobes.
This region contains visual word patterns, the visual pattern is transmitted to Wernicke's area, where it arouses the auditory form of the word and comprehension occurs. Pure word blindness occurs when we have someone able to write a page of coherent and fluent prose, but who cannot read it back. This syndrome is known as alexia without agraphia.
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