The two hemispheres of the human brain are anatomically and functionally asymmetric.
Anatomical Asymmetry
Anatomical differences between the two hemispheres are observed in gross sulcal and gyral patterns and size. The left Sylvian fissure is often more branched, longer, and horizontal than the corresponding structure in the right hemisphere (RH). The planum temporale (PT), associated with language processing, is larger in the left hemisphere (LH) in two thirds of the population. In addition, the posterior portion of the superior temporal gyrus (area Tpt) is generally larger in the LH, and its size is correlated with that of the PT. Primary auditory cortex (Heschl’s gyrus) commonly contains a double gyrus in the RH, but not in the LH. There are also known neurochemical asymmetries, with greater abundance of dopamine receptors in the LH and that of the noradrenergic receptors in the RH. Such chemical asymmetries may play a role in lateralized functions.
Lateralized Functions
In the majority of right-handers, the LH is specialized for syntactic and semantic aspects of language, whereas the RH seems to be specialized for spatial and affective processing including associative learning to emotional stimuli. Lesions in Broca’s area in the left frontal cortex disrupt language production and syntax. Lesions in Wernicke’s area in the left temporal cortex disrupt semantic processing. The role of the RH in language processing lies in prosody and pragmatics. The RH is also specialized for spatial attention, vigilance, and arousal. Visuospatially, the LH is more sensitive to high spatial frequencies (e.g., fine-grained features) whereas the RH is sensitive to low spatial frequencies (global, holistic form) with an advantage in face recognition and discrimination.
Roles Of Development And Gender
Anatomical differences between the LH and RH are evident in the fetal Sylvian fissure and PT after the gestational midpoint, and rates of hemispheric growth alternate. Following the initial predominant growth of the RH in the first 6 months after birth, a period of predominant LH growth continues for the next 4 to 5 years, which includes the critical period for language acquisition. Handedness, a reliable behavioral signature of hemispheric dominance, is not completely established until about 7 to 12 years of age, but hand or side preference is usually consistent from infancy. Laterality in part is modulated by gender primarily through the effects of testosterone during gestation.
Both sexes are exposed to testosterone, but the exposure is greater in males. Testosterone may delay the development of the Sylvian fissure and Wernicke’s area, thereby favoring earlier development of the RH in males. Some studies have reported increased right-handedness and spatial abilities and reduced verbal abilities in males than in females, and these functional sexual dimorphisms may be subsumed by differential gestational exposure to sex hormones. RH cortical thickness is also greater in males than in females.
Other Species
Brain lateralization may not be specific to humans but is more extreme in our species. Larger LH areas, especially the regions homologous to the human PT and the left inferior frontal gyrus operculum (Broca’s region), were observed in the majority of chimpanzees studied. Additionally, captive chimpanzees show LH dominance in approximately two thirds of behaviors studied. LH dominance is also pronounced in the orangutan. Some species of birds and fish also show evidence of asymmetry, often biased toward the LH.
References:
- Davidson, , & Hugdahl, K. (Eds.). (1998). Brain asymmetry.Cambridge: MIT Press.
- Toga, W., & Thompson, P. M. (2003). Mapping brain asymmetry. Nature Reviews Neuroscience, 4, 37–48.