In most people, the language functions of Broca’s and Wernicke’s Areas are associated with activity in the left hemisphere of the cerebral cortex. In some people, however, this is not the case: the location of the language areas are shifted towards the right hemisphere, either completely or partially. Rasmussen and Milner (1977) showed that handedness is one variable correlated with these differences in the location of the language areas. They did this by using a technique known as the Wada Test. It involves anesthetizing one hemisphere at a time, and then observing people to see if they are able to continue speaking (see video below). For example, if a person is able to continue speaking when her right hemisphere is anesthetized, but is unable to speak when her left hemisphere is anesthetized, we can conclude that her language areas are located in the left hemisphere.
Rasmussen and Milner (1977) used the Wada Test with 134 right-handed participants and 122 non-right-handed (left-handed and mixed-handed) participants. Their results appear in Table 1. In the Right-Handed Group, almost all of the participants (96%) experienced problems in language production when the left sides of their brains were anesthetized, whereas only 4% experienced problems in language production when the right sides of their brains were anesthetized. The column labeled Both Hemispheres refers to those able to speak when their right hemispheres were anesthetized and then again when their left hemispheres were anesthetized, which means that, for them, language functions exist in both hemispheres. As you can see, none of the right-handed people fit into this category.
Table 1. The association between handedness and hemispheric dominance for language based on results from the Wada Test (Source: Rasmussen & Milner, 1977)
The Non-Right-Handed Group consisted of both left-handed and mixed-handed participants (the term mixed-handedness applied to people who use their right hands for some tasks and their left hands for other tasks). In this group, 70% experienced problems in language production when their left hemispheres were anesthetized, 15% experienced problems when their right hemispheres were anesthetized, and 15% experienced no problems regardless of which hemisphere was anesthetized. In the last group of people, the language areas are located in both hemispheres.
No one knows for certain the reason or reasons for the shifting of language areas towards the right hemisphere in some left-handed and mixed-handed people. One hypothesis is that portions of their left hemispheres typically associated with language have either been damaged or did not develop normally (Coren, 1993). According to this hypothesis, the shifting of the language areas to the right hemisphere is due to a reorganization of the brain. In other words, language areas must have developed to some extent in the right hemisphere. Some studies suggest that this can happen in young children who suffer damage to their left hemispheres, especially in those who have most of their left hemispheres removed because of neurological illness (Boatman, et al., 1999). Other studies have demonstrated that, in people with epilepsy caused by damage to the left hemisphere, the right hemisphere takes over some language functions (Voets, et al. 2006). A likely explanation for these findings is that, when a language area in the left cerebral cortex is damaged or removed, the corresponding area in the right hemisphere reorganizes itself, thereby allowing people to regain at least some of their ability to use language.
The ability of the nervous system to respond to environmental events and physical damage by forming new connections between brain cells is referred to as neural plasticity. Because of plasticity, functions that are lost because of damage may begin to re-emerge over time. Plasticity and the regaining of lost functions are greatest in young children; but they also occur to a limited extent in adults. For example, when a stroke causes damage to a language area in an adult, at least some of the lost language abilities may return within several months after the stroke. It is highly unlikely, however, that a new language area will develop in the opposite hemisphere in adults because such plasticity decreases rapidly after childhood. Plasticity also is necessary for our ability to learn new information: if new neural connections couldn’t form (and if we couldn’t eliminate unused ones), it would be impossible for us to remember our experiences and to accumulate knowledge.
Phillips (2006) described the case study of a man, Terry Wallis, who recovered from a coma after 19 years, apparently due to an extensive reorganization of pathways in his brain. In 1984, he was in an automobile accident in which he suffered “massive brain injuries” after being thrown from his vehicle. When Wallis was discovered 24 hours later, he already was in a coma. After several weeks, he was classified as being in what now is called a “minimally conscious state,” which means that he showed some awareness of himself and his surroundings, but the degree of his awareness was significantly reduced from and less consistent than that occurring during the normal waking state (Giacino, et al., 2002). For example, people in a minimally conscious state may follow a moving object with their eyes, respond “yes” or “no” to a question (although not necessarily accurately), reach for an object, or hold an object. But these people are incapable of engaging in complex social interactions or knowing what is going on around them in anything more than a simple and partial manner.
A team of researchers studied Wallis’s recovery from the minimally conscious state he had been in for almost two decades (Voss, et al., 2006). Their findings suggested that his recovery was due to the development of new connections between various brain areas:
The team’s findings suggest that Wallis’s brain had, very gradually, developed new pathways and completely novel anatomical structures to re-establish … connections, compensating for the brain pathways lost in the accident. …. Surprisingly, the circuits look nothing like normal brain anatomy. A lot of the damage had been to [connections] that passed from one side of the brain to the other, torn by the force of the accident. [The researchers concluded] that new connections … have grown around the back of the brain, forming structures that do not exist in normal brains (Phillips, 2006).
Plasticity and Phantom Limbs
When nerve connections to a part of the body are severed, usually through amputation of that body part, many people still experience sensations from that missing part. For example, a person whose left leg has been amputated may still feel as if the leg is there. This is referred to as a phantom limb. The following video explains why phantom limbs develop in many people after amputation of parts of their bodies.
Plasticity Across the Lifespan
The type of plasticity important for receovery from brain damage tends to be much reduced by adolescence because, during the early teenaged years, there is a general “streamlining” of neural connections that is associated with a loss of rarely used connections, as well as the cells that make up these connections. Once these brain cells and brain connections are lost, there are fewer ways for new connections to form after brain damage……
But plasticity associated with learning remains throughout life, although certain types of learning, such as language acquisition, are at their peak during childhood. For example, young children suffering from diseases in which one of their hemispheres becomes nonfunctional often have that hemisphere removed. The remaining hemisphere reorganizes itself, thereby allowing the functions associated with the removed hemisphere to reappear. The following video features such a case:
Neural plasticity after brain damage is greatest in early childhood, probably because of the abundance of surplus connections between cells in the cortex. Beginning sometime during grade school, these excess connections begin to be eliminated (REFERENCE). This process of the paring (“trimming”) of connections accelerates around puberty. This paring of brain connections is correlated with a greatly decreased ability of the brain to reorganize itself in ways that allow lost functions to reappear after brain damage.
The importance of neuroplasticity for learning will be discussed in Chapter 4.
Study Questions for Section 3-12
- How would you define “neural plasticity” in your own words?
- How would you describe the Wada Test in your own words?
- According to the results of Rasmussen and Milner (1977), activity in which side of your brain is necessary for the production of language?
- Why were you unable to give a single answer to the previous question?
- How is the development of new language areas in the right hemisphere after damage to the left-hemisphere language areas an example of plasticity?
- What is a phantom limb?
- According to the video on phantom limbs, how does neural plasticity lead to the development of phantom limbs? [HINT: It involves the somatosensory cortex]
- What evidence from brain scans supports the theory that phantom limbs develop because of plasticity in the somatosensory cortex?
- At which time of life is the ability to regain cognitive functions lost due to brain damage greatest? Why is plasticity greatest at that time of life?
- How can plasticity help us to understand why people regain intellectual functions lost by brain damage?
Boatman, D., Freeman, J., Vining, E., et al. (1999). Language recovery after left hemispherectomy in children with late-onset seizures. Annals of Neurology, 46, 579-586.
Coren, S. (1993). The left-hander syndrome: The causes and consequences of left-handedness. New York: Vintage.
Giacino, J. T., Ashwal, S., Childs, N., et al. (2002). The minimally conscious state: Definition and diagnostic criteria. Neurology, 58, 349–353. Retrieved September 22, 2011, from http://www.neurology.org/content/58/3/349.full.pdf
Phillips, (2006, July). ‘Rewired brain’ revives patient after 19 years. New Scientist.com. Retrieved September 22. 2011, from http://www.newscientist.com/article/dn9474-rewired-brain-revives-patient-after-19-years.html
Rasmussen, T., & Milner, B. (1977). The role of early left-brain injury in determining lateralization of cerebral speech functions. Annals of the New York Academy of Sciences, 299, 355-369. doi: 10.1111/j.1749-6632.1977.tb41921.x
Voets, N. L., Adcock, J. E., Flitney, D. E., et al. (2006). Distinct right frontal lobe activation in language processing following left hemisphere injury. Brain, 129, 754-766. doi: 10.1093/brain/awh679
Voss, H. U., Uluç, A. M., Dyke, J. P., et al. (2006). Possible axonal regrowth in late recovery from the minimally conscious state. Journal of Clinical Investigations. 116, 2005–2011. doi:10.1172/JCI27021.