Synonyms

Hemispheric specialization

Definition

Asymmetry is the discordance between the right and left sides of the brain in respect to structure and/or function.

Current Knowledge

Although not initially linked to brain asymmetry, the first behavioral asymmetry that was likely noted was the superiority of motor skills exhibited by one hand, most commonly the right, over the other. The next real breakthrough with regard to asymmetry is generally thought to have occurred in the nineteenth century with the discovery that acquired language deficits (aphasia) were typically associated with lesions of the left hemisphere. Since then, other asymmetries, both functional and structural, have been identified with regard to the two cerebral hemispheres.

Structural Asymmetries

Structural asymmetries of the brain were first noted around the beginning of the twentieth century, but it was not until the late 1960s that these were first strongly correlated with functional differences between the hemispheres. In a study of 100 postmortem brains, Geschwind and Levitsky (1968) noticed that the planum temporale, located in the temporal operculum, was larger in 65% of the brains studied as compared with only 11% in which the right was larger. They concluded that this difference was likely related to the left hemisphere’s association with the production of language in most individuals. Subsequent studies have demonstrated that this asymmetry can be shown to present even prior to birth, reinforcing the genetic predisposition to left-hemispheric dominance for language.

Since the advent of more sophisticated imaging techniques that allow for large-scale in vivo studies of the brain, other structural differences have been documented. The inferior frontal gyrus in the left hemisphere, corresponding to Broca’s area, has been shown to be more highly developed on the left side for most individuals. The gyri and sulci associated with the motor strip (Brodmann’s area 4) are more prominent in the left hemisphere of right-handers. Fairly consistent differences in the lateral fissure have been found, with the posterior ascending ramus of this sulcus making a more abrupt upward turn in the right hemisphere as compared with the left. This would suggest likely differences in the distribution of the supramarginal and angular gyri in the inferior parietal lobules of the two hemispheres. Even on a more microlevel, differences in the size and organization of individual cells or cell columns have been identified in the two hemispheres.

It is reasonable to speculate that some structural differences likely relate to functional differences between the two hemispheres, particularly behaviors such as language and handedness. However, functional asymmetries have either been demonstrated or are suspected well beyond those which can currently be explained by structural differences. The following represent a sampling of some of the functional differences that have been observed.

Functional Asymmetries

It has been well established that language expression and comprehension are normally mediated primarily, if not exclusively, by the left hemisphere, even among left-handers. However, the right hemisphere has also been shown to play an important role in communication. Verbal communication is not just about using words in sentences or paragraphs; emotional tone or nuances of the speaker often convey important meaning. In some communications, such as those with a sarcastic intent, the real message is carried by the tone rather than by the words, which, if interpreted literally, might actually convey a very different message. The ability to use as well as interpret these emotional components of speech, known as prosody, is primarily mediated by the right hemisphere; damage to this side of the brain may produce various forms of aprosodia. With regard to using or interpreting the language of others, the right hemisphere is also believed to play an important role in identifying the central theme or point of the discourse of others and being able to stay on point when speaking or writing. It appears to be important in appreciating verbal (as well as nonverbal) humor and in detecting meaning from the differential inflections given to individual words in speech.

In addition to words, numbers also have their own symbolic meaning. Hence, as might be expected, the ability to use numbers is thought to be a function normally carried out by the left hemisphere, the disturbance of which following a lesion to the left hemisphere may be defined as acalculia (dyscalculia). However, most complex arithmetical operations also have a spatial component. For example, precise alignment of rows and columns of numbers is critical in mathematical operations, whether completed mentally or on paper. These spatial relations can be disturbed following right-hemispheric lesions, resulting in what has been termed spatial dyscalculia.

It is known that the hemisphere contralateral to the hand being used to carry out some motor tasks is immediately responsible for the execution of these movements. However, the motor programs or engrams for overlearned motor skills are believed to reside in the left hemisphere, certainly for the vast majority of right-handers, as well as many left-handers. Thus, any lesion that either directly interferes with those engrams or the ability of that information to reach the premotor cortex of either hemisphere can result in an impaired performance, especially if the individual is asked to demonstrate the action in the absence of the actual object. This latter condition is referred to as an ideomotor apraxia.

Perceptual abilities appear to be differentially distributed between the two hemispheres. It has already been noted that the left hemisphere is normally the leading hemisphere in interpreting verbal (semantic) information, while the right appears to be better adapted to processing certain types of emotional cues. It seems that the right hemisphere is also the more proficient in processing many types of visual-spatial or visual-gestalt information. Thus, the right hemisphere has been found to be generally superior in carrying out certain constructional tasks, making judgments regarding the orientation of lines in space, in making discriminations regarding unfamiliar faces, and in recognizing familiar tunes or environmental sounds. On the other hand, the left hemisphere appears to be the leading hemisphere when it comes to perception of certain aspects of one’s own body. Problems of right-left orientation and difficulty recognizing individual fingers of one’s hands (finger agnosia) are typically associated with lesions of the left inferior parietal lobule. Functional MRI studies have demonstrated consistent activation of right hemisphere structures during tests of vigilance and directed attention. However, divided attention tasks have been shown to selectively activate left prefrontal cortex. PET imaging studies have demonstrated increased blood flow in the right prefrontal and superior parietal cortex during tasks requiring sustained attention, regardless of the type of stimulus (verbal, visual, etc.) or where it is introduced (left vs. right).

Differences between the two hemispheres have also been demonstrated in learning and memory tasks and other cognitive domains. Of the two, the left hemisphere has been more strongly associated with learning verbal information. While many studies have shown that the right hemisphere is perhaps better at learning certain “nonverbal” or “visual-spatial” type information, the findings are generally less robust compared to the left hemisphere and verbal memory. One frequent explanation for this is that when faced with any memory task, humans have a natural tendency to try to verbally encode the stimulus, thus bringing the left hemisphere into play.

It has also been suggested that the two hemispheres play different roles in attention. The much more frequent association of disorders such as unilateral neglect and anosognosia with right hemispheric lesions has led to the hypothesis that while, as might be expected, the left hemisphere attends to the right side of space (both personal and extrapersonal), the right hemisphere focuses on both right and left space.

Finally, the association of the right hemisphere and emotional expression would appear to go beyond the affective intonations of speech as described above. It is commonly observed, both by health-care professionals as well as the spouses and other family members of persons with brain injury that individuals with right hemisphere lesions often behave differently than those with left-sided lesions. While the latter seem to remain emotionally attached, even if that emotion is often one of anger, frustration, or sadness, right hemispherically damaged patients are more likely to be described as apathetic, indifferent, emotionally flat, both in terms of their verbal and facial expressions and their interpersonal relationships.

Cross-References