Language
Fall 2002

The cerebral cortex has two separate hemispheres connected by a large bundle of axons called the corpus callosum. These axons allow the two hemispheres to communicate.
 

As you should be aware by now, the right and left hemispheres do different things. The most obvious difference is the processing of sensory and motor information.

The right visual field goes to the left hemisphere
Touch from the right side of the body goes to the left hemisphere
Muscles on the right are controlled by the left hemisphere. Most humans have a preference for using their right hand indicating that they are left brain dominant. In contrast, the 10% of the population that is left handed tend to use right and left hemispheres more equally. Being left handed is associated with higher incidence of: dyslexia
stuttering
migraines
mathematical abilitiy
creative abilities Although it is common to describe someone as being either "right brain" or "left brain," no one uses just one half of their brain unless half of the brain has been damaged. The two hemispheres communicate continuously so as to integrate information. Nonetheless, there appear to be psychological differences between the right & left hemispheres. Typical Left brain traits are: verbal, logical, rational
Typical Right brain traits are: spatial, intuitive, emotional


An important difference between the hemispheres is the processing of language. Language is a combination of words and emotions.

The word aspect of language is processed in the left hemisphere
The emotional aspects of language are processed in the right hemisphere Damage to the right hemisphere can cause a deficit in understanding humor or irony in speech. The location of speech in the left and emotion in the right hemisphere is true of nearly all right handers (96%) and most left handers (70%). However, some lefties have word dominance in the right and some have words represented bilaterally.
 
 

A loss of language as a result of brain damage is called Aphasia. There are several different types of aphasia (Table 21.1).

Wernicke's aphasia is characterized by a lack of comprehension of written or spoken language. These patients can speak (often without stopping) and can hear sounds, but they can't comprehend speech (see page 584). Brain damage is on the input side of the brain (i.e., Temporal lobe) (Figure 21.1)

Broca's aphasia is characterized by a loss of meaningful speech. These patients can understand most of what is said, but they can not respond with meaningful speech. They have difficulty finding words and do not use pronouns and conjuctions (see page 582). Brain damage is on the output side of speech (i.e., Frontal lobe near the motor cortex).

Wernicke's and Broca's areas are part of the brain pathway necessary to carry on a conversation:

ear to
medulla to
Wernicke's area to
Broca's area to
motor cortex to
speech muscles
Although Wernicke's and Broca's areas are located in the left hemisphere in 93% of all people, sounds are processed in both hemispheres.
 

Many animals communicate, but this is not the same as language (see Box 21.1).

One famous example is Clever Hans, the counting horse. Clever Hans was a German horse that could answer number questions by stomping his foot. Could Clever Hans really answer questions? Caution must be used because what may appear to be language may just be sophisticated conditioning. Although there have been serious attempts to teach apes language (e.g., Washoe, Koko), these animals have limited language abilities if they can use language at all (this is a controversial issue).
 
 

The corpus callosum is a huge bundle of axons just above the lateral ventrical (Figure 21.6). The corpus callosum allows anything that happens in one hemisphere to be known by the other hemisphere.
 

The treatment for some types of epilepsy is to cut the corpus callosum (Figure 21.7). This greatly inhibits the ability of the right and left hemispheres to communicate. These split-brain patients show surprisingly few deficits following this surgery. In fact, the only way to uncover a change is to test the person in a controlled laboratory setting. Because language is located in the left hemisphere, patients can not talk about information that is in the right hemisphere. Information can be selectively sent to the right hemisphere by:

Putting objects in their left hand.
Presenting stimuli in the left visual field (Figure 21.8). Experiment example: Present the word "key" in the left visual field.
"Key" is processed in the right hemisphere
When asked what was seen, the subject will say "nothing."
When asked to reach in a bag with the left hand and grab an object, the key will be selected.
The right hand can not grab the key becaue the left brain is not aware of the stimulus. In contrast, anything presented to the right visual field goes to the left hemisphere. Because the left hemisphere has access to speech, the subject can talk about these objects. Without the corpus callosum the right and left hemisheres function independently and can even have conflicts where the right and left hands do opposite things.
 
 

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