LAUSR

As the brain ages, it changes shape. The gross changes in the brain are very obvious. The brain appears to shrink within the skull, and the ventricles enlarge, meaning that the space in the head occupied by brain tissue is smaller than in a younger person. These changes begin at roughly the age of 25 and continue for the remaining lifespan. Figure 1 shows a comparison between an older person's brain and that of someone in the middle years. As well as the largescale changes in the morphology of the brain, there are associated changes in the thickness of the cortical sheet (Fjell et al., 2015; Thambisetty et al., 2010). The most striking feature of the brain is its pattern of gyrification. The organization of the gyri of the brain is astonishingly consistent across people and even within and between animal species. This consistency hints at a developmental process, and it is reasonably well established that a combination of genetic and physical factors interacts during gestation to ‘crumple’ the brain into a consistent shape (see Ronan & Fletcher, 2015 for a review of the different models). While there is considerable interest in understanding the mechanisms of brain gyrification in earlylife development, it is also important to understand morphological change in the aging brain. These changes do not result from a reversal of the mechanisms that operate in the developing fetus so must relate to changes that occur during maturity. In understanding these changes, it is helpful to have measures that capture the changes with sufficient sensitivity that individual differences can be explored. Madan (2021) tackles this problem by looking at the gradients of gyrification across the brain. Madan uses a gyrification index (GI) that captures the smoothness of the brain at each point on its surface (Schaer et al., 2012; Zilles et al., 1988). The GI is a ratio of the surface area of the brain to the convex hull that encloses the brain volume. A more deeply folded brain surface will have a greater surface area compared to this outer surface so giving a higher GI than a less ‘crumpled’ brain. Previous studies have used crosssectional data to infer agerelated changes in GI, finding that gyrification decreases during aging. Here, Madan (2021) uses longitudinal data to examine withinsubject trajectories (LaMontagne et al., 2019). The main finding here is that the changes in gyrification are not uniform across the brain. Gyrification decreases in this longitudinal series, as demonstrated in previous, crosssectional, studies. However there is a more prominent decrease in GI in the parietal cortex, which is a relatively more folded region. Madan (2021) extends this analysis by showing that the change in GI is driven by changes in the morphology of the sulci, becoming shallower and wider as the brain ages (see also Madan, 2019). These changes to the sulci are obvious in the slices in Figure 1, which are taken roughly through the regions with the greatest GI change in Madan's analysis. What does this analysis add to our understanding of the aging brain? Madan shows that the agerelated gyrification changes are most obvious in the parietal cortex and the posterior parts of the frontal lobe. Gyrification changes in these areas correlate with cognitive abilities and with genetic factors (in crosssectional studies, Fjell et al., 2015; Gregory et al., 2016), suggesting causal relationships among these measures. These regions are also associated with complications of vascular disorders such as hypertension (Raz et al., 2007), suggesting a role for lifestyle factors in predicting structural and cognitive changes. The shape of the brain affects how neuroscientists interact with the brain in health and in disease. Localizing