Cortical Complexity Estimation Using Fractal Dimension: A Systematic Review of the Literature on Clinical and Nonclinical Samples

Fractal geometry has recently been proposed as a useful tool for characterizingthe complexity of the brain cortex, which is likely to derive from the recur-rence of sulci–gyri convolution patterns. The index used to describe the corti-cal complexity is called fractal dimensional (FD) and was employed bydifferent research exploring the neurobiological correlates of distinct patholog-ical and nonpathological conditions. This review aims to describe the literatureon the application of this index, summarize the heterogeneities between stud-ies and inform future research on this topic. Sixty-two studies were included inthe systematic review. The main research lines concern neurodevelopment,aging and the neurobiology of specific psychiatric and neurological disorders.Overall, the included papers indicate that cortical complexity is likely toreduce during aging and in various pathological processes affecting the brain.Nevertheless, the high heterogeneity between studies strongly prevents thepossibility of drawing conclusions. Further research considering this indexbesides other morphological values is needed to better clarify the role of FD incharacterizing the cortical structure.