The Disappearance and Emergence of Cognitive Skills in Aphasia
Brain damage is generally considered in terms of the functions that are lost. Damage to the motor cortex causes loss of motor function. Damage to the visual cortex causes blindness. We rarely think about the possibility of new functions emerging when old ones are lost. There is an increasing amount of evidence, however, that suggests that the loss of one function may allow another one to emerge, and that the brain is capable of creating new functions to compensate for the loss of an old one. The study of aphasia, which is usually associated with the loss of a function, provides insight into how new functions may emerge in the damaged brain.
Language can be thought of as two basic operations: selecting words and putting them in a particular sequence. The two most common forms of aphasia usually affect one or the other. The left hemisphere of the brain contains two regions involved in these two operations. Wernicke’s area is involved in understanding language and using words properly. Damage to this area generally results in impaired word selection. Broca’s area is involved in speech production. Damage to this area results in lack of fluency and grammar, and difficulty producing words (Ardila 2009). The localization of these linguistic skills is not specific to verbal language. Sign language also appears to use these areas of the brain for similar purposes, as similar aphasias appear in signers as well. Conversely, signers with right hemisphere strokes are known to neglect the left side of space, but still use the usual full range of space when signing, indicating that sign language is mainly localized in the left hemisphere, just as verbal language is observed to be (Sacks 1989). The left hemisphere can then be thought of as dominant for language in nearly all people. Studies of aphasia have also indicated that the brain represents first and second languages in different places. An aphasiac may be able to regain some use of one language, but not the other (University of Haifa 2009), making the distribution of language skills in the brain even more complicated.
All of these descriptions of aphasia refer to the loss of a specific function. But a recent case study of a woman known as AA reveals that aphasia may unlock other cognitive skills. AA had primary progressive aphasia (PPA), which is caused by degeneration of the language network rather than traumatic brain damage. Some patients with PPA develop artistic or musical abilities, which are generally associated with the right hemisphere. In the case of AA, she began creating paintings of abstract concepts like numbers and music, most notably of Ravel’s Boléro (Seeley, et al. 2008). During her most creative period, she was not yet experiencing any language deficits, but she seemed drawn to representing auditory concepts visually. As her language skills deteriorated, she became more interested in photographic realism, painting with very close attention to detail. Seeley et al. believe that losing some of the functions of her left hemisphere allowed dormant functions of her right hemisphere to emerge. They found four areas of AA’s brain with increased grey matter, all in the right hemisphere, and suggest that posterior regions of her right hemisphere had undergone localized enhancements. Connectivity among these non-dominant areas may have led to transmodal creativity that was normally inhibited by the more dominant left hemisphere language activities. It seems that by losing her language abilities, AA was able to unlock greater visual creativity.
Understanding how new abilities may emerge as other are lost is an important part of the rehabilitation of aphasiacs. Even if new cognitive skills do appear, the loss of language is still a significant problem that must be addressed. Thinking about regaining language skills as more of a process of recreating the lost skills rather than recovering them leads to new ideas for treatment. A study by Yamadori et al. (1977) revealed that people with Broca’s aphasia might still have the ability to sing, even if they cannot speak. Of the twenty-four subjects in the study, twelve were able to sing correct and fluent words with good melody, despite being unable to speak with clarity and fluency. Five were able to start singing without any prompting at all, including two in the “severe” group. Eight others only required a lead-in of the first few notes of the song, and then could finish perfectly without help. Overall, 87.5% of the subjects could still sing with no or minimal help. These results suggest that singing is somewhat independent from speech, and that the right hemisphere may actually be dominant over the left for this ability. Because aphasia is caused by damage to the left hemisphere, using music may be a viable way to help aphasiacs regain their language abilities.
A number of recent studies have investigated the effects of music therapy for aphasia and have found it to be very effective. One such study (Schlaug, Marchina and Norton 2009) used an intense course of melodic intonation therapy (MIT) for patients with aphasia. The therapy consisted of 1.5hr/day sessions, five days a week for 75-80 sessions, and paired singing with rhythmic tapping of syllables with the left hand. Tapping is thought to prime the right hemisphere sensorimotor network for articulation, which is lost in Broca’s aphasia. All six patients had significantly improved speech by the end of the therapy. The amount of improvement was correlated with a greater amount of fibers observed in the arcuate fasciculus (AF) in the right hemisphere. In the left hemisphere, the AF is a bundle of fibers that connects Wernicke’s and Broca’s areas to each other. The AF of the right hemisphere is normally far less developed. Following this therapy, however, all patients had more fibers in their right AF. This is likely due to increased myelinization of the axons, or growth of the axons and axon collaterals. These changes suggest that MIT engages the right hemisphere more than regular speech therapy and strengths the neural connections that already exist for singing. This allows for language skills to emerge again, but it is not the recovery of the old skills that were lost. It is the emergence of similar, but new, skills, assembled by the brain from the singing ability already found in the right hemisphere.
Discoveries like this illuminate how the brain compensates for the loss of particular functions, but music therapy has been used to treat aphasia for years. Just as singing in a choir can help someone lose a stutter, it can also help people with aphasia produce words they could not produce otherwise (NewScientist 2006). Singing slows down the rate of word production, which may make fluent enunciation easier. The rhythm of singing limits the number of syllables pronounced per beat, which may make word retrieval easier. Songs also have predictable rhyme and rhythm patterns. What is especially interesting about the choir effect on aphasia is that singing with a group appears to be more beneficial than singing alone. MIT may help strengthen neurological connections just by the intensity of the program, but singing in a choir may actually be more beneficial over a shorter period of time. Language is a social process, and singing with other people may provide both the auditory feedback and the social interaction that is needed to use language fluently.
The loss of a function as integral to human society as language is devastating and needs to be addressed. But if aphasia is viewed only as a loss, it becomes difficult to understand how to approach rehabilitation. Realizing that the brain is capable of creating new functions is an important part of treatment. Rehabilitation should not be seen as the attempt to recover a function that has been lost. It should be seen as facilitating the brain’s ability to reconstruct functions out of preexisting ones, allowing new functions to emerge.
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