Which hemisphere is responsible for language and speech left or right?

Disorders of Spatial Localization

The right hemisphere plays a special role in the spatial localization of stimuli. This effect has been demonstrated for both visual and auditory stimuli. With regard to determination of spatial orientation of objects in space, the most severe deficits have been noted after posterior right hemisphere damage.368369 Short-term spatial memory (a skill analogous to the auditory short-term memory task of digit span) is a dominant function of the posterior right hemisphere.368 Auditory localization of sounds in space also depends on an intact posterior right hemisphere.349

Language and the Right Hemisphere

The right hemisphere, although dominant for language in only a small minority of people, plays an important role in communication. The elements of communication most affected by right hemisphere disease are prosody, or cadence and intonation of speech, and pragmatics, or practical, extralinguistic messages that normal speakers convey. Patients with right hemisphere disease sound flat in their intonation, and they may fail to comprehend emotional nuances, irony, sarcasm, and humor in the speech of others. They understand what is said but not how it is said. The communication deficit of right hemisphere lesions, although not strictly meeting the definition of aphasia, is socially disabling to patients, hindering readjustment to family and work environments.

Discourse and Pragmatics

RH deficits compromise complex language functions in a nonspecific manner that belies simple explanations (McDonald, 2000). This statement, although relevant to the study of language in general, is particularly true when one attempts to clarify issues regarding the RH and discourse and pragmatics. The two terms generally overlap but convey slightly different meanings depending on the author describing the terms. For the purposes of this article and in an effort to add clarity, discourse will be considered as a pragmatic function specifically concerned with cohesion and coherence (integrating meaning) and deriving the general idea from the whole message through complex and abstract linguistic interpretation. Thus defined, discourse is not limited to conversation but has various other forms – procedural, narrative, and expository (Hough and Pierce, 1993). Also, as in semantics, production and comprehension discourse capabilities are difficult to isolate and are best understood holistically, outside the usual dichotomous model of comprehension versus production.

Pragmatics can be thought of as the ‘hows’ of communication – how and how well we make our point. By definition, it encompasses the rules underlying conversational discourse, the function of language particular to participants, situational knowledge, the topic at hand, related ‘world’ knowledge or information, and social conventions. It includes both the context and the intent of a message and when applied to discourse includes additional rules to enable interactive communication (turn-taking, topic maintenance, etc.). Pragmatics integrates language functions mediated by the two hemispheres with major contributions from the RH in two areas discussed previously (prosody and semantics) and also refers to the nonverbal skills used in communication and the communication-related disturbances believed to be mediated by the RH.

Discourse deficits are the hallmark of the communication difficulties of RHD patients (Meyers, 1999). Results of numerous studies on RHD patients indicate RH involvement in discourse (integrating meaning and determining the general idea from the whole message). The characteristics of the discourse disturbances in the presence of RHD are wide ranging, with particular difficulty in the comprehension and production of narratives, generally considered the domain of the RH. In complex or lengthy narratives, the absence of a unifying theme or main idea causes particular problems in comprehension and retention. In discourse interpretation, main ideas serve to provide structure for identifying and organizing key concepts rather than the ‘details’ underlying those concepts, thus enhancing comprehension and recall. In support of this notion, a number of studies suggest that basic discourse form is retained within structured tasks (e.g., providing the order of a short story when given individual sentences presented in random order (Huber and Gleber, 1982), predicting story endings (Rehak et al., 1992), and drawing simple inferences in short stories and interpreting sarcasm in comments in forced choice format (Kaplan et al., 1990)). In contrast, it has been suggested that in RHD the comprehension of complex narratives poses particular problems that may be related to the need for reinterpretation (Brownell et al., 2000), a task that relies heavily on the identification of the main idea to define key concepts and aid recall. RHD subjects also have difficulty recognizing purpose or intent of the narrative (a skill related to comprehension of theme and main idea), drawing inferences, inferring motives in complex narratives (Wapner et al., 1981), predicting outcomes in narratives without explicit cues (Rehak et al., 1992), applying alternate meanings, and understanding implied and nonliteral meanings. In discourse production, RHD subjects have difficulty following conversational rules and a propensity for excessive detail with an absence of a unifying theme or idea. Similar problems occur in writing, which is also characterized by excess detail and an absence of unifying main ideas, transitions, and conclusions. Finally, with respect to pragmatic functions during conversational discourse, RHD patients have difficulty interpreting speaker intent and indirect requests and evaluating paralinguistic cues.

Investigating Figurative Language

Right hemisphere involvement in figurative language (such as in the comprehension or production of metaphors, humor, and idioms, or the making of requests) has been claimed ever since it was observed that some people with damage to the right hemisphere show problems interpreting figurative language appropriately. The idea of right hemisphere involvement in the comprehension of figurative language also has its roots in psycholinguistic models of figurative language comprehension.

A lively debate in psycho- and neurolinguistics has surrounded questions such as whether metaphors can be understood as quickly and automatically as regular sentences, whether specific processing stages are necessary for their comprehension, whether the same processes underlie the comprehension of different types of metaphors, and whether such differences are represented in different parts and sides of the brain (for a summary see Glucksberg, 2003). Despite the numerous psycho- and neurolinguistic studies, only a few neuroimaging studies have directly investigated this question. In a positron emission tomography (PET) study, participants had to make plausibility judgments while listening to sentences (plausible metaphors, implausible metaphors) and to random word strings. Greater activation in the right hemisphere (right inferior frontal gyrus, the right premotor cortex, and the right posterior temporal cortex) was found when comparing plausibility judgments of metaphoric sentences with those of literal sentences (Bottini et al., 1994).

Understanding jokes is, in a way, similar to metaphoric or other figurative language in that what is meant is not said. Comprehending jokes, however, involves more than just understanding what is meant. Neuroimaging studies using fMRI have identified dissociated neural substrates for a cognitive and an affective element of humor (Goel and Dolan, 2001; Moran et al., 2004; see Figure 2). Unfortunately, the neural substrates identified by the two groups do not correspond. For the affective component, one study described the medial ventral prefrontal cortex and bilateral cerebellum (Goel and Dolan, 2001) whereas the other study found the bilateral amygdala and bilateral insula to be activated (Moran et al., 2004). The left insular region (BA44/4) was also activated in the Goel and Dolan study (2001), but it was related to phonological, not affective, joke processing. For the cognitive component, both studies identified the posterior middle temporal gyrus to be implicated, but in the right hemisphere in one study (Moran et al., 2004) and in the left hemisphere in the other (Goel and Dolan, 2001). The left posterior inferior temporal gyrus (Goel and Dolan, 2001) and the left inferior frontal gryus (Moran et al., 2004) were also involved.

Taken together, the right hemisphere contributes to keeping track of the topic or theme of a discourse and to drawing high-level inferences, including attributing mental states to others and interpreting figurative language, and to integrating meaning into a larger discourse or social context. The right hemisphere thus seems to be indispensable for successful pragmatic and social communication. Inconsistencies remain, however, concerning the specific neural substrates or networks involved. It is also possible that not all neural substrates and networks involved have yet been identified.

In addition to its role in low-level language processing, the left hemisphere is also involved in deductive and syllogistic reasoning processes and in establishing local coherence, that is, in low-level inferencing processes (for a summary see Bookheimer, 2002).

Perception

The right hemisphere is ‘dominant’ in tasks requiring an appreciation of spatial relationships. The syndrome of ‘neglect’ involves a failure to attend to or act towards the side of space contralateral to a brain lesion; as this is usually in the right hemisphere, it is usually the left side of space that is neglected. The right hemisphere is also dominant in other perceptual tasks. ‘Prosopagnosia’, for example, a selective difficulty in recognising familiar faces, is more common after right than left hemisphere damage. Agnosia is difficulty in recognising objects where basic sensory functions are intact. Agnosia can be ‘apperceptive’, if relatively ‘early’ processes of percept formation are involved, or ‘associative’ if the failure lies in perceptual memory. Associative agnosias merge into deficits of semantic memory. Perception is tested using naming tasks, which depend on both recognition and name finding, and by testing copying, which taps perceptual as well as motor processes.

Serotonergic modulation

Right hemisphere lesions are associated with negative affective symptoms and depression (Starkstein et al., 1989) thus patients with neglect may often be treated with antidepressants. These drugs may have overall positive effects in affective symptoms, but how they affect neglect has not been investigated. Physicians treating neglect patients should consider that serotonin reuptake inhibitors have multiple mechanisms of action involving different monoamines and, in some cases, anticholinergic effects. Serotonin may modulate dopaminergic activity by means of multiple mechanisms (see Alex and Pehek, 2007 for review) and produce extrapyramidal and behavioral symptoms including parkinsonism and apathy (Leo, 1996; Lane, 1998; Barnhart et al., 2004; Wongpakaran et al., 2007). Hypokinesia and apathy are likely to be overlooked as part of the depressive symptomatology or right hemisphere injury, therefore, physicians should be observant of possible “paradoxical” effects of these substances and consider the differences in pharmacodynamic profiles, favoring more activating agents with noradrenergic and dopaminergic properties.

Lateralization of mental activity

The right hemisphere may dominate awareness and image of self and the relation of self, visuospatially and psychically, to the environment (Devinsky, 1997). Acute lesions of the right hemisphere more severely disrupt the sense of self than left-sided lesions. Right hemisphere lesions can result in failure to recognize profound deficits as noted above, such as cortical blindness or left-sided hemiplegia (anosognosia), respond appropriately to recognized deficits (anosodiaphoria), or attend to the left half of extrapersonal and personal space. They can also cause the delusional belief that duplicate persons are impersonating well-known persons or delusional reduplications (Ruff and Volpe, 1981; Malloy et al., 1992; Signer, 1992). The right hemisphere may dominate our sense of self and lesions in the right parietal lobe can impair our body image. Left-sided neglect, anosognosia, and anosodiaphoria can be explained by destruction of a module of cortex controlling body image and physical relations of self to the environment.

Although the left hemisphere has been most often implicated in language, the right hemisphere has a role in prosody or interpreting the meaning of words by the way in which they are stated (Bryden and Ley, 1983). Goldberg and Podell (1995) discuss some of the observations that lead to the suggestion that the right hemisphere is critical for exploratory processing of novel cognitive situations for which there are no pre-existing codes or strategies, whereas the left hemisphere is critical for processing based on pre-existing representations and routine cognitive strategies. As they point out, the novelty-routinization hypothesis emphasizes the importance of instructional biases that accompany a task. An ambiguous task is likely to depend on the right hemisphere.

The novelty-routinization hypothesis can be assessed with functional imaging studies that engage some part of the cingulofrontal confluence region. Cognitively challenging tasks can activate the right cingulofrontal confluence region (Corbetta et al., 1991; Bench et al., 1993; Raichle et al., 1994; Bush et al., 1998). Although the left region is required for inductive reasoning (Goel et al., 1997) and performance on the sad Stroop task (George et al., 1994), interpreting ‘theory of mind’ stories appears to involve both hemispheres to some extent (Fletcher et al., 1995). Reasoning, processing of cognitively challenging information, assessment of emotionally charged faces, and ‘mentalizing’ functions may require both hemispheres even though studies of cortical damage lead to intriguing hypotheses about lateralization of functions. The specific contributions of each hemisphere to mental activity are not fully understood.

10.2.2 Spatial Attention

Right hemisphere lesions frequently involve absence of awareness (or “inattention”) of stimuli in the left visual field. Patients may fail to eat food on the left side of their plate or may consistently bump into objects to their left (see chapter 5). A common way of investigating such hemispatial neglect is by asking a patient to either draw or copy an object or by means of cancelation tasks that require patients to cross out (either timed or untimed) certain targets in an array of stimuli (eg, lines, letters, digits). The nature and severity of hemispatial neglect differs between patients and it is therefore common practice to present patients with more than one test. The Schenkenberg Line Bisection Test (Schenkenberg, Bradford, & Ajax, 1980), for example, consists of 20 lines of different sizes that are centered to either the left, right, or middle of a page. The mean percentage of deviation from the middle is then calculated and compared with normative data. Patients with right hemisphere damage tend to miss shorter lines on the left and center of the page. When using their right hand a clear rightward deviation of the marks is generally observed. The Bells Test (Gauthier, Dehaut, & Joanette, 1989) and the Star cancelation test (Halligan, Cockburn, & Wilson, 1991) are two of the most well-known cancelation tests. The Bells test involves 315 silhouettes of objects distributed on a page. Patients have to circle 35 bells that are scattered among them. These bells are arranged in seven columns of five bells, which allows for documentation of a patient’s scanning strategy. The Star cancelation test was designed as a more difficult version of a cancelation task by including stars of different sizes, letters, and words. Targets to be crossed out in this array are 56 small stars (Fig. 10.3). The Behavioral Inattention Test (BIT) is a comprehensive battery that provides a naturalistic set of tests for hemispatial neglect (Wilson, Cockburn, & Halligan, 1988). It consists of different cancelation tests, such as the Star cancelation test, a line bisection test, and several drawing and copying tests. More naturalistic tests in this battery include, for example, reading a menu, copying an address, and navigating a simple map. The BIT thus not only allows for assessment of the presence of hemispatial neglect, but also measures the extent of the neglect on several everyday activities.

All of these tests and procedures also involve other cognitive processes such as visual perception, semantic information or even language comprehension to some degree besides the core spatial functions. Two of the more “spatial” ways of investigating the neglect phenomenon are asking patients to describe a symmetrically organized picture where events on the right and left side of the picture have to be connected to fully understand what is happening in the scene (eg, Cookie theft picture from the Boston Diagnostic Aphasia Examination; Fig. 10.4). One may also ask patients to imagine and describe a familiar scene from two viewpoints directly opposite one another. Hemispatial neglect may then be noticed in the form of absence or scarce mentioning of features on the left as opposed to detailed description of details on the right.

Language and the Right Hemisphere

While the right hemisphere has traditionally been called the ‘minor’ hemisphere because of its lack of language dominance, research has shown that patients with right hemisphere disease have significant problems with verbal communication. While they can produce and understand appropriate words and sentences, patients with right hemisphere lesions speak monotonously, lacking the emotional tone or ‘prosody’ which makes normal language colorful. These patients lose the ability to understand emotional tone, connotation, irony, sarcasm, satire, and humor, responding only to the literal meaning of the words; they understand what is said, but not how it is said. The right hemisphere may have a similar organization to the left in terms of speech prosody, the emotional intonation, and emphasis present in spoken language; frontal lesions disturb production of prosodic speech, while temporal lesions disturb comprehension of emotional tone. Right hemisphere stroke patients have difficulty even with nonemotional aspects of prosody, such as placement of emphasis within sentences or inflections, as in the difference between a statement and a question. Despite their intact language skills, these patients are at a great disadvantage in normal human communication.

Aside from these effects of right hemisphere disease, the right hemisphere appears to have considerable linguistic ability. Patients with surgical commissurotomy, or section of the corpus callosum, are able to recognize words flashed in the left visual field, or presented in tactile fashion to the left hand. They can perform simple matching tests between words or short phrases which are semantically related. The mystery about these abilities is the failure of patients with left hemisphere lesions and global aphasia to perform similar tasks, despite the presence of an intact right hemisphere. It is possible that the corpus callosum contains fiber tracts which inhibit the right hemisphere from expressing its full language capability. If such systems could be therapeutically disrupted, the recovery of aphasia might be significantly enhanced.

Right hemisphere developmental disorder

The right-hemisphere developmental learning disability, also categorized clinically as social–emotional learning disability (Voeller, 1991), right hemisphere learning disability (Tranel et al., 1987), and nonverbal learning disability (Semrud-Clikeman and Hynd, 1990; Gross-Tsur et al., 1995), was first described in the neurologic literature by Weintraub and Mesulam (1983) in adolescents and adults (n = 14). The syndrome is characterized by poor eye contact, flattening of affect involving both prosody and facial expression, problems in comprehending and mimicking affective prosody, and difficulties processing visuospatial information. Despite overall flattening of affect, the patients appeared to experience emotions inwardly. Although academic performance and verbal skills were found to be intact, the individuals all suffered from chronic emotional problems related to poor interpersonal social skills, causing social isolation, loss of psychosocial well-being, and depression. About half the patients reported that other family members had similar problems. Two patients were born with left-sided infantile hemiplegia, three had histories of perinatal distress, and five had an early onset seizure disorder. Neuropsychological evaluation using the WAIS demonstrated deficits consistent with right hemisphere injury. Voeller (1986) confirmed the observations by Weintraub and Mesulam (1983) in children (n = 15) and noted that many also had an attention-deficit disorder. CT imaging (n = 14) showed a variety of abnormalities: four scans were normal, three showed dilation of the right lateral ventricle, three showed atypical right–left hemispheric asymmetries, two showed right parietal lesions, one showed a right temporal lobe hypodensity, and one showed an enlarged right Sylvian fissure. Tranel et al. (1987) extended and further defined the right hemisphere developmental syndrome in adolescents and adults (n = 11). Of particular interest, all the patients had normal CT brain scans, a finding that is very similar to the imaging results for patients with developmental dyslexia who rarely show focal brain lesions on either CT (Hynd and Semrud-Clikeman, 1989) or MRI (Habib, 2000; Ramus et al., 2018). Whether or not individuals with right hemisphere learning disability have the microscopic ectopias and polymicrogyria (lesions associated with aberrant neuronal migration during cortical development) that have been reported in individuals with developmental dyslexia (Galaburda and Kemper, 1979; Galaburda et al., 1985; Kaufmann and Galaburda, 1989; Humphreys et al., 1990) is unknown.