User:Quallsk/Transcortical sensory aphasia

Wernicke-Lichtheim Connectionist Model
In 1874, Carl Wernicke claimed that thought and language were supported by two distinct regions in the brain. He believed that disturbances in language due to brain damage resulted from damage to psycholinguistic functions that were represented by these specific areas. These areas are the center for acoustic images, found in the temporal lobe cortex, and the center for motor images, located in the inferior frontal region, which are connected by subcortical fiber tracts. According to Wernicke, sounds were sent to the center for acoustic images via the acoustic nerve. Wernicke also proposed the idea that these two centers, along with the commissure linking the acoustic nerve with the center for motor images, were the first structures used when a child is beginning to acquire language abilities through imitation of what he hears.

Ludwig Lichtheim, another German physician, was influenced by the work of Wernicke and Broca and is responsible for developing the localizationalist concept of aphasia. He had a great interest in aphasia, especially those that he believed could not be explained by Wernicke’s model. He proposed that these types of aphasia were due to disturbances in the pathways that connect major speech centers instead of the speech centers themselves. According to Lichtheim, there were specific centers in the brain for auditory images and motor images, and these centers were connected by a commissure that passes through the insula, similar to Wernicke’s proposal. However, Lichtheim postulates the existence of a third center where concepts are elaborated because he believed that other parts of the brain had to be accessed when less automatic characteristics of language, such as comprehension, were involved. Pathways that link the auditory center to non-language areas where concepts are determined accomplish this. Lichtheim also presents the idea that the phonological information that is used in verbal output is controlled by not only the direct connections linking the center for auditory images and the center of motor images, but also by an indirect connection linking these two centers with the non-language concept center. According to his model, the commissural pathways that link the concept center with the sensory and motor speech centers were include two separate commissures and a set of converging fiber tracts that come from various regions of the cerebral cortex to the sensory and motor centers.

Lichtheim suggested that a lesion interrupting the commissures between the auditory image center and the concept center would cause a sensory aphasia with fluent paraphasic speech and difficulty in understanding both spoken and written language, similar to Wernicke’s aphasia. However, unlike Wernicke’s aphasia, the lesion would not disrupt the capacity to repeat, read aloud, or write to dictation, although there is a loss of intelligence regarding what the patient comprehends. Lichtheim discusses a patient in an 1885 article who could repeat what was spoken to him and had fluent expression of language but had poor comprehension of what was spoken to him. Lichtheim first refers to this as inner-commissural word-deafness. In 1908, Wernicke recognizes aphasias in which repetition remains intact and refers to them as transcortial aphasias, using transcortical sensory aphasia to refer to Lichtheim’s “inner-commissural word-deafness.”