The unique brain anatomy of meditation practitioners: alterations in cortical gyrification

Abstract

Several cortical regions are reported to vary in meditation practitioners. However, prior analyses have focused primarily on examining gray matter or cortical thickness. Thus, additional effects with respect to other cortical features might have remained undetected. Gyrification (the pattern and degree of cortical folding) is an important cerebral characteristic related to the geometry of the brain's surface. Thus, exploring cortical gyrification in long-term meditators may provide additional clues with respect to the underlying anatomical correlates of meditation. This study examined cortical gyrification in a large sample (n = 100) of meditators and controls, carefully matched for sex and age. Cortical gyrification was established by calculating mean curvature across thousands of vertices on individual cortical surface models. Pronounced group differences indicating larger gyrification in meditators were evident within the left precentral gyrus, right fusiform gyrus, right cuneus, as well as left and right anterior dorsal insula (the latter representing the global significance maximum). Positive correlations between gyrification and the number of meditation years were similarly pronounced in the right anterior dorsal insula. Although the exact functional implications of larger cortical gyrification remain to be established, these findings suggest the insula to be a key structure involved in aspects of meditation. For example, variations in insular complexity could affect the regulation of well-known distractions in the process of meditation, such as daydreaming, mind-wandering, and projections into past or future. Moreover, given that meditators are masters in introspection, awareness, and emotional control, increased insular gyrification may reflect an integration of autonomic, affective, and cognitive processes. Due to the cross-sectional nature of this study, further research is necessary to determine the relative contribution of nature and nurture to links between cortical gyrification and meditation.

Keywords: MRI; brain; cortical complexity; curvature; folding; insula; meditation; mindfulness.

Figure 1

Estimation of cortical gyrification. Illustrated is the computation of local gyrification using a simulated folded surface. The magnitude of the folding increases from proximal to distal, and the frequency increases from left to right. Left Panel: After establishing mean curvature (in degrees), sulci can be identified as regions with large negative values (displayed in blue), while gyri are characterized by large positive values (displayed in red). Middle Panel: After averaging mean curvature within distances of 3 mm, values are transformed into absolute mean curvature (i.e., all values become positive regardless of whether they represent gyri or sulci). Higher values indicate areas with larger gyrification. Right Panel: Curvature values are smoothed using a surface-based heat kernel filter with FWHM = 25 mm. As demonstrated, increases in the amplitude and wavelength of the simulated folding are reflected in increased values of smoothed absolute mean curvature.

Figure 2

Group differences in cortical gyrification. Shown are group differences at p ≤ 0.05 (upper panel) and p ≤ 0.01 (lower panel), uncorrected for multiple comparisons. The color bar encodes significance (T). Areas with larger gyrification in meditators (MED) are depicted in yellow/orange; areas with larger gyrification in controls (CTL) are depicted in cyan. Callosal, subcallosal, and midbrain regions have been excluded on the medial surface maps. Numeric clusters indicate larger gyrification in meditators at p ≤ 0.01 within (1) left precentral gyrus; (2) left insula; (3) right insula; (4) right fusiform gyrus; (5) right cuneus. The red circle indicates the global maximum. LH, left hemisphere; RH, right hemisphere.

Figure 3

Links between cortical gyrification and number of meditation years. Shown are correlations at p ≤ 0.05 (upper panel) and p ≤ 0.01 (lower panel), uncorrected for multiple comparisons. The color bar encodes significance (T). Areas with positive correlations (i.e., more meditation years link with more gyrification) are depicted in yellow/orange; negative correlations (i.e., more meditation years link with less gyrification) are depicted in cyan. Callosal, subcallosal, and midbrain regions have been excluded on the medial surface maps. The red circle indicates the global maximum. LH, left hemisphere; RH, right hemisphere.