Brain asymmetry in cortical thickness is correlated with cognitive function

A brain is considered to be asymmetrical if the two hemispheres are different from each other, structurally, and/or functionally. The most well established brain asymmetry is related to language, in which the specialized left hemisphere is markedly expanded in most people, especially in Broca’s speech area, the planum temporale (Toga and Thompson, 2003). Brain asymmetry is believed to be evolutionally adaptive, since unilateral computation and control might be more efficient than bilateral. It also reduces possible interference between hemispheres, freeing up the opposite hemisphere for other specialized functions (Toga and Thompson, 2003). Thanks to recent advances in neuroimaging, more and more structural asymmetries—in terms of cortical volume, surface area size, or thickness—are being discovered (for review, see Toga and Thompson, 2003). However, unlike the well-established relationship between planum temporale asymmetry and language dominance, whether those newly found structural asymmetries are related to the cognitive functions and to what extent remains largely unknown. In a recent paper published in the Journal of Neuroscience, Plessen et al. (2014) set out to address these questions. Using high-resolution MRI, they measured brain asymmetry in terms of cortical thickness in a large sample (n = 215) with a wide age range (7–59 years old). Plessen et al. (2014) made three major findings regarding the relationship between brain asymmetry and cognitive function. Firstly, they found that two asymmetries, a left > right asymmetry in posterior regions and a right > left asymmetry along the medial wall, are more pronounced in older adults. The left > right asymmetry in posterior regions, including inferior sensorimotor, inferior parietal, posterior temporal, and inferior occipital cortices, was primarily the result of the decrease in thickness with aging in the right hemisphere, and the increase in thickness with aging in the left hemisphere. This decrease in thickness with aging in posterior right hemisphere is consistent with the observation that tasks involving posterior right hemisphere are more susceptible to decline with aging (Gerhardstein et al., 1998). The increase in thickness with aging in posterior left hemisphere found by Plessen et al. (2014) is particularly enlightening. It revealed that the developmental thickness of the lefthemispheric posterior temporo-occipital region, first discovered in young children through to older adolescents (Shaw et al., 2009), may well persist into older adulthood. Moreover, the phenomenon that people generally gain more life knowledge, better emotion regulation and life satisfaction, become increasingly empathic and engage in more prosocial activities as we grow older, has long been acknowledged as aging wisdom (Charles and Carstensen, 2009). However, the specific structural basis remains unclear (Charles and Carstensen, 2009; Meeks and Jeste, 2009). Since the posterior areas of left hemisphere thickening with aging found by Plessen et al. (2014) are related to the attributes comprising wisdom (Meeks and Jeste, 2009), whether these areas are the structural substrate of aging wisdom deserves further study. The right > left asymmetry along the medial wall, including the dorsal and posterior cingulate and the medial parietal and sensorimotor cortices, primarily resulted from the decrease in thickness with aging in the left hemisphere. This finding added greatly to our current understanding of brain aging loss. It suggested that the dominant right hemiaging hypothesis that the right hemisphere shows greater age-related decline than the left is incomplete and needs to be revisited (Dolcos et al., 2002). However, as the participants in this study were comparatively young (≤59 years old), whether or not these findings would hold true for older people remains subject to further investigation. Secondly, Plessen et al. (2014) found a significant gender difference, in which