Copyright © 1999 Massachusetts Institute of Technology
The modern era of neuroscientific investigation into the asymmetry of the cerebral hemispheres began in the 1860s when localization of function within the cerebral cortex was thrust into the forefront of scientific thought by Paul BROCA. Broca etched out his place in history by announcing that language resided in the frontal lobes and that the left hemisphere played the predominant role. Although neither of these ideas originated with Broca, the recognition that the brain may be functionally asymmetric opened up new avenues of cognitive and neurobiological investigation that have persisted for well over a century. This summary paper will briefly describe a number of lateralized cognitive functions, including language, FACE RECOGNITION, fine MOTOR CONTROL, visuospatial skills, and EMOTIONS, and will examine whether structural asymmetries in the organization of cerebral cortex are related to these functional specializations. The interested reader is referred to several thorough reviews on the topic of lateralization (see further readings).
Language Lateralization
Language is perhaps the most notable and strongly lateralized function in the human brain. Much of our knowledge of the organization of language in the brain is based on the correlation of behavioral deficits with the location of lesions in the neocortex of patient populations. Several language areas are found to be located within the left hemisphere and the behavioral outcome of injury to these particular cerebral locations is generally predictable (e.g., Broca's APHASIA, Wernicke's aphasia, conduction aphasia). In other cases uniquely specific linguistic deficits can result. For example, one case has been reported in which the subject showed an unusual disability at naming fruits and vegetables despite normal performance on a variety of other lexical/semantic tasks following injury to the frontal lobe and BASAL GANGLIA (Hart, Berndt, and Caramazza 1985). Recent reports describe two more patients who are able to produce a normal complement of verbs, but are extremely deficient in noun production, while a third case shows exactly the reverse deficit. Despite the variety of deficits and lesion locations, all are associated with the left hemisphere (Damasio and Tranel 1993).
Modern research techniques including regional cerebral blood flow, POSITRON EMISSION TOMOGRAPHY (PET), functional MAGNETIC RESONANCE IMAGING (fMRI), and intraoperative cortical stimulation, have continued to localize cortical regions that are activated during language tasks and further support the left hemisphere"s special role in language functions.
Although it is true that individuals can be right hemisphere, or bilaterally, dominant for language, 90 percent of the adult population (both left- and right-handed) have language functions that are predominantly located within the left hemisphere. Even in seemingly anomalous situations the left hemisphere maintains its "specialized" role in language functions. Studies of bilingual subjects indicate that both languages are located in the same hemisphere, but may be dif-ferentially distributed (Mendelsohn 1988). In addition, language lateralization is not dependent on the vocal-auditory modality. Disturbances of SIGN LANGUAGE in deaf subjects are also consistently associated with left hemisphere damage, and signing deficits are typically analogous to the language deficits one observes in hearing subjects with the same lesion location (Bellugi, Poizner, and Klima 1989).
In early decorticate patients with one hemisphere missing, language development proceeds relatively normally in either hemisphere (Carlson et al. 1968). Language development in the right hemisphere, while retaining its phonemic and semantic abilities, has deficient syntactic competence that is revealed when meaning is conveyed by syntactic diversity, such as repeating stylistically permuted sentences and determining sentence implication (Dennis and Whitaker 1976). These results suggest that although the right hemisphere is capable of supporting language, language usage does not reach a fully normal state.
In adults who have had language develop in the dominant hemisphere, but later became available for the testing of language in their right hemisphere due to commissurotomy or hemispherectomy, the right hemisphere appears capable of understanding a limited amount of vocabulary, but is usually unable to produce speech. In recent years speech production by the right hemisphere of commissurotomy patients has also been reported, albeit in an extremely limited context (Baynes, Tramo, and Gazzaniga 1992).
Motor Control and the Left Hemisphere
Nine out of ten individuals demonstrate a clear preference for using the right hand. Broca inferred that the hemisphere dominant for language would also control the dominant hand; however, it soon became clear that this was not universally true. Most studies suggest that over 95 percent of right-handers are left-hemisphere dominant for language; however, only 15 percent of left-handers show the expected right-hemisphere dominance. Of left-handers, a full 70 percent are left-hemisphere dominant, while the remaining 15 percent have bilateral language abilities.
Disorders of skilled movement are referred to as apraxia. These disorders are characterized by a loss of the ability to carry out familiar purposeful movements in the absence of sensory or motor impairment. The preponderance of apraxia following left hemisphere damage has led many researchers to suggest that this hemisphere may be specialized for complex motor programming. Although lesion studies argue for the left hemisphere"s dominance of complex motor control, the lateralization of this function is not nearly as strong as that seen for language. In addition, studies of commissurotomy patients suggest that the right hemisphere is capable of independently directing motor function in response to visual nonverbal stimuli without the help of the left hemisphere (Geschwind 1976).
Right Hemisphere Specializations
The right hemisphere also plays a predominant role in several specialized tasks. Right hemisphere lesion patients have greater difficulties localizing points, judging figure from ground, and performing tasks that require stereoscopic depth discriminations than do patients suffering damage to the left hemisphere. Additionally, commissurotomy patients show a right hemisphere advantage for a number of visuoperceptual tasks (Gazzaniga 1995). Many investigators have also reported a right hemisphere advantage for visuoperceptual tasks in normal subjects, but these results are controversial. On the whole visuoperceptual abilities do not appear to be strongly lateralized as both hemispheres are capable of performing these types of low level perceptual tasks. Several suggestions have been made to account for the asymmetries that are present. One suggestion is that there is no right hemisphere advantage for visuoperception, but a left hemisphere disadvantage due to that hemisphere"s preoccupation with language functions (Corballis 1991; Gazzaniga 1985). Other authors have reported a difference in the ability of each hemisphere to process global versus local patterns or in terms of a hemispheric specialization for different spatial frequencies. The right hemisphere is typically much better at representing the whole object while the left hemisphere shows a slight advantage for recognizing the parts of an object (Hellige 1995).
One specific task that does show convincing evidence for a right hemisphere advantage is face perception. Prosopagnosia, the inability to recognize familiar faces, occurs more often following damage to the right hemisphere than the left (although most cases result from bilateral damage). In addition, commissurotomy patients have a right hemisphere advantage in their ability to recognize upright faces (Gazzaniga and Smylie 1983; Puce et al. 1996).
In support of a facial processing asymmetry, a number of cognitive studies have indicated that normal subjects attend more to the left side of a face than the right and that the information carried by the left side of the face is more likely to influence a subject"s response. Finally, numerous imaging studies have demonstrated right hemispheric activation using a variety of facial stimuli.
The right hemisphere may also be superior at tasks requiring spatial attention (Mangun et al. 1994). Hemineglect patients typically do not attend to one side of space and do not recognize the presence of individuals in the other hemifield. Additionally, they ignore one side of their body and copy drawings in a manner that entirely ignores half of the picture. This attentional deficit is more often observed following right hemisphere damage.
Studies of normal subjects, psychiatric patients, and lesion patients indicate that the right hemisphere is dominant in the recognition and expression of emotion and is preferentially activated during the experience of emotion. Lesions of the right hemisphere are also often associated with affective disorders. Many of the lesion results remain controversial, but experimental studies do demonstrate a left visual field/right hemisphere superiority for the recognition of emotions.
Structural Asymmetry
If the hemispheres are not symmetrical in their functioning then the physical structure of the brain may also be asymmetrical. Although many contradictory reports regarding the weight and volume of the two cerebral hemispheres were published following the discovery of the left hemisphere"s role in language, it was not long before the differences between the length of the left and right sylvian (lateral) fissures were described. Related to this difference in sylvian fissure length are the casual reports by von Economo and Horn in 1930 and later Pfeifer (1936) that the planum temporale, the dorsal surface of the temporal lobe, is typically larger in the left hemisphere than the right. This very specific size difference between the two hemispheres became a focus of research in the late 1960s after it was described that the left planum temporale (the dorsal surface of the temporal lobe) is significantly larger than the right in 65 percent of the population (GESCHWIND and Levitsky 1968). Based on these studies, it was commonly accepted that a difference in the size of cortical regions could account for the left hemisphere"s specialization for language.
A recent reanalysis of this question using computer- generated three-dimensional reconstruction techniques has revealed a different story. The right lateral fissure rises dramatically at its caudal extent which results in an apparent foreshortening of the planum in the right hemisphere when it is studied using the previously applied methods (i.e., photo graphic tracings and slice reconstruction). Three- dimensional measurements that accurately map the highly convoluted cortical surface reveal no size difference between the left and right planum temporale (Loftus et al. 1993). Thus these anatomical differences may not reflect size differences between the hemispheres, but rather differences in gross cortical folding.
Many modern authors have also continued to report the difference in the length of the sylvian fissure that borders the lateral aspect of the planum on the dorsal surface of the temporal lobes (Rubens et al. 1976). Subsequently these findings have been corroborated in certain primate species, human fossils, infants, and, interestingly enough, in the male cat (Tan 1992).
Lateralized Cortical Circuitry
Although many studies have examined gross size differences between the two hemispheres, relatively few have directly examined whether connectional or organizational specializations underlie lateralized functions. Not surprisingly, both neurochemical and structural differences have been found between the hemispheres.
Columnar organization also varies between the left and right posterior temporal areas. The left hemisphere has been reported to be organized into clear columnar units, while columns in the right hemisphere appear to be much less distinct (Ong and Garey 1990; cf. COLUMNS AND MODULES). This difference may be related to previous reports that the left temporal lobe has greater columnar widths and intercolumnar distances. Sex differences in the density of neurons within cortical lamina have also been documented in posterior temporal regions (Witelson, Glezer, and Kigar 1995), and these results are beginning to support cognitive data suggesting that language functions in women are less lateralized than those in men (Strauss, Wada, and Goldwater 1992).
Differences in the fine dendritic structure of pyramidal cells in each hemisphere have also been reported within the frontal lobes (Scheibel 1984), and it has been suggested that the total dendritic length of left hemisphere pyramidal cells is greater than that of right hemisphere pyramidal cells and that this asymmetry may decrease with age (Jacobs and Scheibel 1993).
Cell size asymmetries have also been documented in these same areas. The cell size differences appear to be restricted to the largest of the large pyramidal cells within layer III of Broca"s area and are not apparent in adjacent cortical regions (Hayes and Lewis 1995). This same size difference also exists in posterior language regions, but is spread throughout auditory areas, including the primary auditory cortex (Hutsler and Gazzaniga 1995). What is the functional meaning of larger cell sizes? The answer is unclear, but differences in cell body size may indicate differences in the length of a cell"s axon or degree of bifur cation. Thus, pyramidal cell size may be related to connectivity differences between the two hemispheres. Recent studies of temporal lobe connectivity using newly-developed tract-tracing methods may support this notion. These studies demonstrate patchy connectivity within the posterior segment of Brodmann"s area 22 (Wernicke"s area) of both the left and right hemisphere. Additionally, the size of individual patches is quite symmetric, but the distance between individual patches of the left hemisphere is consistently greater than that found in the right (Schmidt et al. 1997). These connectional differences may play a role in the anatomical underpinnings of temporal processing differences between the two hemispheres that could be critical in asymmetric cognitive functions such as language analysis.
Although one might expect that symmetrical structure should be the norm in the human brain, symmetrical organization of the body may largely be due to the requirements of locomotion (Corballis 1991). In addition to the symmetrical placement of the limbs, sense organs may be placed symmetrically so that an organism can attend and respond equally to both sides of the world. Brain organization for these functions might mirror the body organization, but the hemispheric distribution of many cognitive functions may not be constrained in this way. Although there could be some advantage to having dual representations of functions not involved with locomotion (for instance, in the case of damage to one side of the brain), these benefits are likely outweighed by the disadvantages of delayed transmission across long fibers of the corpus callosum. When viewed in this context, it makes sense that certain functions would become largely the domain of one cerebral hemisphere and that damage to the normal brain, either through unilateral lesions or commissurotomy, would reveal a remarkable array of behavioral results.
Baynes, K., M. J. Tramo, and M. S. Gazzaniga. (1992). Reading with a limited lexicon in the right hemisphere of a callosotomy patient. Neuropsychologia 30:187-200.
Bellugi, U., H. Poizner, and E. S. Klima. (1989). Language, modality and the brain. Trends in Neuroscience 12:380-388.
Carlson, J., C. Netley, E. B. Hendrick, and J. S. Prichard. (1968). A reexamination of intellectual disabilities in hemispherectomized patients. Transactions of the American Neurological Association 93:198-201.
Corballis, M. C. (1991). The Lopsided Ape. New York: Oxford University Press.
Damasio, A. R., and D. Tranel. (1993). Nouns and verbs are retrieved with differently distributed neural systems. Proceedings of the National Academy of Sciences, USA 90:4957-4960.
Davidson, R. J., Ed. (1995). Cerebral Asymmetry. Cambridge, MA: MIT Press.
Dennis, M., and H. A. Whitaker. (1976). Language acquisition following hemidecortication: linguistic superiority of the left over the right hemisphere. Brain and Language 3:404-433.
Gazzaniga, M. S., and C. Smylie. (1983). Facial recognition and brain asymmetries: clues to underlying mechanisms. Annals of Neurology 13:536-540.
Gazzaniga, M. S. (1985). The Social Brain: Discovering the Networks of the Mind. New York: Basic Books.
Gazzaniga, M. S. (1995). Principles of human brain organization derived from split-brain studies. Neuron 14:217-228.
Geschwind, N. (1976). The apraxias. American Scientist 63:188-195.
Geschwind, N., and W. Levitsky. (1968). Human brain: left-right asymmetries in temporal speech region. Science 162:186-187.
Hart, J., R. S. Berndt, and A. Caramazza. (1985). Category- specific naming deficit following cerebral infarction. Nature 316:439-440.
Hayes, T. L., and D. A. Lewis. (1995). Anatomical specilization of the anterior motor speech area: Hemispheric differences in magnopyramidal neurons. Brain and Language 49:289-308.
Hellige, J. B. (1993). Hemispheric Asymmetry. Cambridge, MA: Harvard University Press.
Hellige, J. B. (1995). Hemispheric asymmetry for components of visual information processing. In R. J. Davidson and K. Hugdahl, Eds., Brain Asymmetry. Cambridge, MA: MIT Press, pp. 99-121.
Hutsler, J. J., and M. S. Gazzaniga. (1995). Hemispheric differences in layer III pyramidal cell sizes -- a critical evaluation of asymmetries within auditory and language cortices. Society for Neuroscience Abstracts 21: 180.1.
Jacobs, B., and A. B. Scheibel. (1993). A quantitative dendritic analysis of Wernicke"s area in humans. I. Lifespan changes. Journal of Comparative Neurology 327:83-96.
Loftus, W. C., M. J. Tramo, C. E. Thomas, R. L. Green, R. A. Nordgren, and M. S. Gazzaniga. (1993). Three-dimensional analysis of hemispheric asymmetry in the human superior temporal region. Cerebral Cortex 3:348-355.
Mangun, G. R., R. Plager, W. Loftus, S. A. Hillyard, S. J. Luck, V. Clark, T. Handy, and M. S. Gazzaniga. (1994). Monitoring the visual world: hemispheric asymmetries and subcortical processes in attention. Journal of Cognitive Neuroscience 6:265-273.
Mendelsohn, S. (1988). Language lateralization in bilinguals: facts and fantasy. Journal of Neurolinguistics 3:261-292.
Nass, R. D., and M. S. Gazzaniga. (1985). Cerebral lateralization and specialization in human central nervous system. In F. Plum, Ed., Handbook of Physiology. Bethesda, MD: The American Physiological Society, pp. 701-761.
Ong, Y., and L. J. Garey. (1990). Neuronal architecture of the human temporal cortex. Anatomy and Embryology 181:351-364.
Pfeifer, R. A. (1936). Pathologie der Horstrahlung und der Corticalen Horsphare. In O. Bumke, Ed., Foerster, O, vol. 6. Berlin: Springer.
Puce, A., T. Allison, M. Asgari, J. C. Gore, and G. McCarthy. (1996). Differential sensitivity of human visual cortex to faces, letterstrings and textures: a functional magnetic resonance imaging study. Journal of Neuroscience 16:5205-5215.
Rubens, A. B., M. W. Mahowald, and T. Hutton. (1976). Asymmetry of the lateral (Sylvian) fissures in man. Neurology 26:620-624.
Scheibel, A. B. (1984). A dendritic correlate of human speech. In N. Geschwind and A. M. Galaburda, Eds., Cerebral Dominance: The Biological Foundations. Cambridge, MA: Harvard University Press, pp. 43-52.
Schmidt, K. E., W. Schlote, H. Bgratzke, T. Rauen, W. Singer, and R. A. W. Galuske. (1997). Patterns of long range intrinsic connectivity in auditory and language areas of the human temporal cortex. Society for Neuroscience Abstracts 23: 415.13.
Strauss, E., J. Wada, and B. Goldwater. (1992). Sex differences in interhemispheric reorganization of speech. Neuropsychologia 30:353-359.
Tan, Ü. (1992). Similarities between sylvian fissure asymmetries in cat brain and planum temporale asymmetries in human brain. International Journal of Neuroscience 66:163-175.
von Economo, C., and L. Horn. (1930). Uber windungsrelief, masse, und rindenarchtektonik der supratemporalfalche. Z Gest Neurol. Psychiat. 130:678-755.
Witelson, S. F., I. I. Glezer, and D. L. Kigar. (1995). Women have greater density of neurons in posterior temporal cortex. Journal of Neuroscience 15:3418-3428 .
Copyright © 1999 Massachusetts Institute of Technology