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The role of the right cerebral hemisphere in language and communication represents a relatively young area of research that has grown rapidly since the late 1970s. Recent interest in the role of the right hemisphere reflects an emphasis on language as a tool for communication in natural contexts, and an awareness that normal language use is the product of many regions of the two hemispheres working in concert. Left hemisphere structures are routinely linked to the nuts and bolts of what might be termed basic language: phonology, lexical semantics, and syntax. In contrast, right hemisphere structures have been implicated in less tightly constrained domains, including some uses of prosody (the “melody” of speech), metaphor, discourse such as conversations, stories, indirect requests, and other forms of nonliteral language, and even the social-cognitive basis for discourse. These domains most closely associated with the right hemisphere are especially sensitive to context and are ideally suited to expressing nuance. This article will first present some general issues pertaining to research in these areas and then describe, in turn, representative findings relating to prosody, lexical processing and metaphor, and discourse.
Claims related to right hemisphere contributions to language and communication can be stated in a strong form: that a specific function is housed in some region of the right hemisphere that is necessary and sufficient to support that function. However, most claims are more general and also weaker: that normal task performance draws on intact right as well as left hemisphere structures. For example, understanding the point of an ironic comment rests on a listener's appreciation of phonology, word meaning, and grammatical relations, as well as a speaker's tone of voice, preceding context, and the speaker's mood. In simplistic terms, the right hemisphere's contributions to language and communication are typically layered on top of the foundation provided by the left hemisphere. Even weak claims for the right hemisphere's role are extremely important clinically because injury to the right hemisphere often results in impairments that significantly reduce a patient's ability to communicate effectively in natural settings.
Another general point concerns localization of function. Often, a group of patients with right hemisphere lesions is compared with a group of non-brain-injured controls. Although this type of comparison does not allow localization of a particular function to the right hemisphere, it still supports the weaker interpretation mentioned earlier. In addition, some studies provide strong support for right hemisphere localization by (1) directly comparing the effects of unilateral lesions of the right and left hemispheres, (2) using lateralized presentation to intact left or right hemispheres, or (3) using functional imaging (PET, fMRI) to examine “on-line” brain activation in non-brain-injured adults. There is growing support for the right hemisphere's unique contribution to language and communication.
Prosody
Prosody refers to variation in frequency, amplitude, duration, timbre, and rhythm. Prosodic contour can be used to convey linguistic distinctions, such as distinguishing between meanings of words or phrases (“yellow jacket” meaning a kind of bee or a brightly colored piece of clothing) and between speech acts (a question versus statement signaled by a rising pitch toward the end of an utterance). Research with right hemisphere-injured patients suggests that both expressive and receptive deficits can occur, although there is disagreement across studies. In terms of production, there is some loss of control, sometimes manifested as an increased variability in pitch (specifically, fundamental frequency) after temporal and rolandic area lesions (e.g., Colsher, Cooper, and Graff-Radford, 1987). Patients with right hemisphere lesions are also impaired on a variety of discrimination and production tasks (Behrens, 1989; Weintraub, Mesulam, and Kramer, 1981).
Prosody can also be used to convey a range of emotions, such as anger or sadness. Ross (1981) has proposed a taxonomy of aprosodias to mirror the classical taxonomy of aphasias: a motor aprosodia associated with right frontal lesions, a receptive aprosodia associated with right temporal lesions, and a global aprosodia associated with extensive frontal-temporal-parietal lesions. Other research has confirmed the separation of an affective deficit from a linguistic prosody comprehension deficit based on direct comparison between the effects of left-and right-sided lesions (Heilman et al., 1984; Pell, 1998).
Lexical and Phrasal Metaphor
Several studies have used a sentence-picture matching task. Patients with right hemisphere lesions, more so than aphasic patients with left-sided lesions, tend to be overly literal, thereby missing the conventional meaning of phrasal metaphors such as “he has a heavy heart” or idioms such as “turning over a new leaf” (Van Lancker and Kempler, 1987). The characteristic literalness has been extended to single-word stimuli such as “warm,” “cold,” “deep,” and “shallow” in a semantic similarity judgment task presented to left-and right-lesioned patient groups (Brownell et al., 1984). Functional imaging in normal adults confirms that regions in the right hemisphere (including the dorsolateral prefrontal cortex, middle temporal gyrus, and the precuneus in the medial parietal lobe) are differentially activated during metaphor processing compared to literal sentence processing (Bottini et al., 1994).
Studies of lexical semantic processing by the left and right hemispheres of normal adults, together with work on discourse, have been used to support a comprehensive model. Beeman (1998), as well as others, suggests that the left hemisphere is designed for focused processing of the closest (literal) associations to a word demanded by preceding context and for actively dampening activation of alternative meanings. The right hemisphere, in contrast, is sensitive to looser, more remote associations and allows them to persist over time. Extrapolating Beeman's model, one can imagine what happens when a potential metaphor is presented: “Microsoft Corporation is the tiger of the software industry.” The right hemisphere maintains the various associations emanating from “Microsoft,” the topic of the metaphor, and “tiger,” the vehicle; the overlapping or shared associations between “Microsoft” and “tiger” provide the ground for the metaphor.
Discourse
Discourse processing requires that a listener integrate meaning across sentences and from non-or paralinguistic sources to achieve an understanding of an entire story, joke, or conversation. Several studies have documented that right hemisphere lesions more than left hemisphere lesions result in decreased humor appreciation (Shammi and Stuss, 1999). While patients with right-sided lesions have no trouble appreciating that short story jokes require an incongruous punch line, they are deficient in apprehending exactly how a punch line fits with the body of a joke on a deeper level, and they have analogous problems with other types of discourse for which comprehension requires a reinterpretation (Brownell et al., 2000). Patients with right hemisphere injuries have trouble extracting gist from extended narrative even in the absence of an obvious need for reinterpretation (Hough, 1990), although there are highly constrained situations in which they are able to perform inferences that span sentence boundaries (Leonard, Waters, and Caplan, 1997). Another realm of impairment centers on nonliteral language. Several studies of sarcasm and irony comprehension suggest a problem using context (mood, sentence prosody) as a guide to uncovering a speaker's intended meaning (Tompkins and Flowers, 1985). Similarly, a host of studies show that right-sided lesions alter patient's production and comprehension of indirect requests, which also require consideration of the preceding context (Stemmer, Giroux, and Joanette, 1994).
An overlapping body of work examines whether an underlying social cognitive impairment affects discourse performance. The ability to explain behavior in terms of other people's mental states, referred to as theory of mind, has been examined in several populations, including people with autism and stroke patients. Comprehension of stories and cartoons that rely on theory of mind are relatively difficult for patients with right-sided lesions, but not for aphasic patients with left hemisphere lesions (Happé, Brownell, and Winner, 1999). Also, functional imaging studies in normal adults suggest greater activation linked to theory of mind in a variety of regions, including the right middle frontal gyrus and precuneus (Gallagher et al., 2000).
There are, of course, unresolved issues. The range of language and communication skills associated with right hemisphere injury is extensive. These skills seem to represent several domains that will need to be examined separately, even though powerful unifying constructs have been explored, such as coherence (Benowitz, Moya, and Levine, 1990) and working memory (Tompkins et al., 1994). Finally, our understanding of localization of function involving the right hemisphere is poorly developed. Functions often associated with the right hemisphere may be as appropriately tied to prefrontal regions in either hemisphere (McDonald, 1993; Stuss, Gallup, and Alexander, 2001).
See also discourse; discourse impairments.
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