Autopsy Studies
Autopsy is done to the dead people. Careful observations were made of a patient’s behavior, and subsequent to his or her death, the brain was examined to determine which areas were damaged. By comparing the area of brain damage and the type of disorder the patient displayed while alive, neurologists could develop theories about the role of the damaged brain parts in normal brain parts in normal brain functioning.
Paul Broca, a nineteenth-century French neurologist observer a patient who had been hospitalized for twenty years in Paris. The patient was unable to speak but he can understand everything that was said to him. He was also developed paralysis of the right arm and leg. The autopsy showed severe damage (called lesion) in the lower area of the frontal lobe. Broca concluded that the area must be responsible for speech production. Now, the area is called Broca’s area.
Language processes in the left hemisphere. Damage to Broca’s area is usually associated with non-fluent speech and difficulty in processing complex syntactic patterns. Damage to Wernicke’s area is usually associated with comprehension disturbances. Damage to the area around the angular gyrus results in reading impairment.
Images of the Living Brain
Autopsy analysis has been and continues to be an important tool in the understanding of the brain. But an autopsy can only be carried out after the patient’s death.
Computerized Axial tomography (also called CT scanning) is a relatively new technique which uses a narrow beam of X-rays to create brain images that take the form of a series of brain slices. CT scans have offered neuroscientists their first opportunity to look inside a living brain. CT scanning provides a static image of the brain that mostly useful in identifying brain lesions and tumors.
Another technique is positron Emission Tomography (PET). PET capitalizes on one of the brain’s many interesting properties, it is extremely hungry for oxygen. This technique shows that the brain consumes 20 per cent of the oxygen the body uses. What is important for this technique is the fact that the areas of the brain that are most active during any particular activity require more oxygen to be transported to them.
In a PET experiment, a subject inhales a minute quantity of some radioactive gas, or is injected with a safe dose of radioactive fluid which soon renders the blood radioactive. Cerebral blood flow studies have greatly increased our knowledge of where language processing takes place in the brain. When subjects speak, much blood flows to the left hemisphere of the cortex and to Broca’s area in particular. When subjects read, much blood flows to the occipital lobe to the angular gyrus and to other areas of the left hemisphere. These observations support the view that the left hemisphere is primarily responsible for language and that there are specific language areas within the left hemisphere.
Learning from Hemispheric Connections and Disconnetions
Information about language representation in the brain is gained through an investigation of the brain itself.
Dichotic Listening Studies
Dichotic listening studies have been extremely important in the accumulation of the knowledge we posses about the specialization of the cerebral hemispheres. Most of the input to your right ear goes to the left hemisphere of your brain. When the receiver is held to the right ear, it will appear that the speech is louder and clearer. This phenomenon is called Right Ear Advantage (REA).
In general, the right ear shows an advantage for words, numbers, and Morse code. The left ear shows an advantage for the perception of melodies and environmental sounds such as bird songs.
Split Brain Studies
It is impossible to understand speech presented to the left ear because there are two reasons. The first is that the auditory pathways to the brain are not completely crossed, the second is that after the right hemisphere receives information from the left ear, that information can be transferred to the left hemisphere via the corpus callosum- the bundle of fibres that connects the two hemispheres.
Studies that have investigated the effect of this surgery on cognition are referred to as split brain experiments. They have provided dramatic illustrations of what happens when the hemisphere cannot communicate with one another. It appears from the behavior of split brain patients that although the right hemisphere does show some language understanding, it is mute. The right hemisphere which receives information from the left hand, knows what is there, but it can neither put this into words nor transfer the information across the severe corps callosum to the left brain.
Split brain experiments have presented new and important knowledge about the functioning of the brain. In fact, the logic of split brain experiments is identical to the logic employed by Broca in 1860. In the case of split brain studies, the damage is surgically induced. In this section, we return to these experiments in nature and examine what they reveal about language representation in the brain.
