In 1961, a 41-year-old policeman complained to his family of frequent headaches, nausea, and vomiting. They already had noticed that he had become forgetful, confused, and apathetic; and his odd behaviors often provoked anger in others. When examined by a neurologist, a brain tumor was discovered in the middle of his brain and immediately removed. Nevertheless, he continued to show a variety of behavioral, cognitive, and emotional problems. For example:
Asked to write with his right hand, he performed quite normally. Letters, words, numbers, and sentences were correctly written to dictation, and he was able to copy printed material, converting it (as do most individuals) to script in the process. His performance with his left hand, however, offered the sharpest possible contrast: His letters were either incorrect or completely unrecognizable; when requested to write particular words or sentences, he would produce an altogether illegible scrawl. He could copy printed materials, but instead of converting to script, he would faithfully imitate each detail of the block letters. When allowed to view these error-laden productions, he was quite astonished. Yet he could not spontaneously correct his errors as long as he was using his left hand. (Gardner, 1974, p. 260)
Although the man was right-handed and, therefore, found it difficult to write with his left hand, most right-handed people still can write legibly with their left hands if they try. Why was he unable to do so?
In addition to his left-handed copying difficulties, the man could not speak or write (with either hand) the names of objects, such as a toothbrush, that had been placed into his left hand, but only when he was not allowed to look at them: if he was allowed to see the objects, he had no problems (except when writing with his left hand, as already noted). On the other hand (no pun intended), he was able to draw with his left hand objects placed into that hand, even when he was not allowed to look at them. And he also could demonstrate with his left hand how to use these objects. Thus, he identified with his sense of touch any object placed into his left hand but was unable to express with written or spoken language what he was holding (as long as he was not allowed to look at it). How can we understand this strange pattern of abilities and disabilities?
In order to make sense of this case, we need to remember that each hemisphere of the brain:
- receives tactile and visual information from the opposite side of the body,
- transmits messages to move skeletal muscles to the opposite side of the body.
Furthermore, Broca’s and Wernicke’s Areas, two major language areas in the cerebral cortex, are located in the left hemispheres of most people. Although such laterality (“sidedness”) of functioning might suggest that, in essence, we have two brains (and, hence, two minds) working independently of one another, one in each hemisphere of the cerebral cortex, this is not the case: information processed in one hemisphere is shared almost immediately with the other hemisphere. This is accomplished by transmitting the information across the corpus callosum (see Figure 1), which is a structure consisting of millions of neuronal connections between the two cortical hemispheres — connections that allow the two hemispheres to communicate and, therefore, to coordinate their functions.
When the corpus callosum is extensively damaged, as was the case for the man described above, information processed in one hemisphere is unable to travel to the other hemisphere, thereby making it difficult for the two hemispheres to coordinate their functions and, hence, to respond as one unit, as they do in people without damage. I made a point of stating above that the man showed impairments only when he was not allowed to see the objects. The reason for not allowing him to look at the objects was that, if the man saw them, the visual information would go to both hemispheres — each eye has connections to each hemisphere — and the left hemisphere with its language areas would have allowed him to respond correctly, regardless of which hand was holding the object.
But let’s look in detail at why the man could verbally name objects only when they were placed in his right hand (it may help you to draw the following so that you can visualize the pathways from the hands to the brain). When, for instance, a key was placed into the man’s right hand and he was asked to name it, the tactile information from the key would travel up his right arm and into the spinal cord, then enter the brain stem where it would cross over (within the medulla oblongata) to the left side of his brain, and finally be routed through his left thalamus into the left parietal lobe of his cerebral cortex, which contains the left somatosensory cortex. The man identified the key by touch with his left somatosensory cortex and, because Broca’s and Wernicke’s Areas also are in the left hemisphere, he was able to name it.
When the key was placed into the man’s left hand, the tactile information from the key would travel up his left arm and into the spinal cord, then enter the brain stem where it would cross over to the right side of his brain, and finally be routed through his right thalamus into the right parietal lobe of his cerebral cortex, where the object was identified by his right somatosensory cortex. Because this information was unable to travel across his damaged corpus callosum to the language areas in the left hemisphere, the man was unable to speak or write the name of the object. But when asked to communicate nonverbally what the object was — by drawing it, pointing to it, or demonstrating how it was used — the man was easily able to do so.
Because the man’s cerebral hemispheres no longer could communicate due to his tumor-damaged corpus callosum, he showed peculiarities in behavior and cognitions that made it seem as if he had two distinct and independently acting minds, at least with respect to some actions:
This [conclusion] was strengthened by a demonstration that objects placed in either hand could not be identified by the other hand, and that a task learned by one hand (such as a simple maze) had to be completely relearned by the other…. Carrying out [spoken] commands with the right hand [controlled by the left hemisphere], such as drawing a square, waving goodbye, or pretending to brush his teeth, posed no problems; these same commands, however, could not be executed with the left hand [because the right hemisphere had little or no ability to understand the spoken commands]. Asked to draw a square with that hand, he drew a circle. Instructed to point to the examiner with his left index finger, he pointed to his own eye with his left index and middle finger. Commanded to pretend to brush his teeth with his left hand, he on one occasion pretended to lather his face, and on another, mimed combing his hair. (Gardner, 1974, p. 261)
There is a problem, however, with concluding that the man’s damaged corpus callosum was the primary cause of his symptoms: the tumor also damaged areas of his brain near the corpus callosum. It’s possible that damage to one or more of these other areas caused his peculiar behaviors and cognitions, or at least contributed to them. This is always a problem with case studies: it’s impossible, at least when considering only one or a few case studies, to control adequately for the effects of extraneous variables — in this case, the possible influence of nearby brain areas on the man’s behaviors and cognitions.
Well-designed experimental studies, however, allow us to make cause-and-effect conclusions directly. But experimental studies in which the brains of humans are purposely damaged generally are not possible except when neurosurgery is performed to reduce the symptoms of serious neurological disorders — disorders that have not responded to other treatments. Neurosurgical experiments performed since 1961 on the corpus callosum have shown conclusively that extensive damage to that structure alone produces symptoms similar to those observed in the case study described in this section. I will summarize some of these findings in the next section.
Study Questions for Section 3-13
- Which behaviors expressed by the policeman led his family to seek out help?
- What problems did his doctors find when they looked more closely?
- Where is the corpus callosum found in the brain?
- What does the corpus callosum do?
- Will a person with a severely damaged corpus callosum typically be able to NAME (through spoken language) an unseen object placed into her LEFT hand? (NOTE: In this and the following questions, we will assume that the primary language areas are in the left hemisphere.)
- Will a person with a severely damaged corpus callosum typically be able to NAME (through spoken language) an unseen object placed into his RIGHT hand? Why or why not?
- Will a person with a severely damaged corpus callosum typically be able to DRAW with the LEFT hand an unseen object placed into her LEFT hand? Why or why not?
- Will a person with a severely damaged corpus callosum typically be able to DRAW with the LEFT hand an unseen object placed into his RIGHT hand? Why or why not?
- Will a person with a severely damaged corpus callosum typically be able to DRAW with the RIGHT hand an unseen object placed into her LEFT hand? Why or why not?
- Will a person with a severely damaged corpus callosum typically be able to draw with the RIGHT hand an unseen object placed into his RIGHT hand? Why or why not?
- What is a major problem with using the case study described above to answer conclusively the previous six questions?
Gardner, H. (1974). The shattered mind: The person after brain damage. New York: Vintage.