Madan, Christopher R. and Kensinger, Elizabeth A. (2016) Cortical complexity as a measure of age-related brain

26 The structure of the human brain changes in a variety of ways as we age. While a sizeable 27 literature has examined age-related differences in cortical thickness, and to a lesser 28 degree, gyrification, here we examined differences in cortical complexity, as indexed by 29 fractal dimensionality in a sample of over 400 individuals across the adult lifespan. While 30 prior studies have shown differences in fractal dimensionality between patient 31 populations and age-matched, healthy controls, it is unclear how well this measure would 32 relate to age-related cortical atrophy. Initially computing a single measure for the entire 33 cortical ribbon, i.e., unparcellated gray matter, we found fractal dimensionality to be 34 more sensitive to age-related differences than either cortical thickness or gyrification 35 index. We additionally observed regional differences in age-related atrophy between the 36 three measures, suggesting that they may index distinct differences in cortical structure. 37 We also provide a freely available MATLAB toolbox for calculating fractal 38 dimensionality. 39 40