Fractal fluctuations and complexity: Current debates and future challenges

Complexity is perhaps one of the less properly understood concepts, even within the scientific community. Recent theoretical and experimental advances, however, based on the close relationship between the complexity of the system and the presence of 1/f fluctuations in its macroscopic behavior, have opened new domains of investigation, which consider fundamental questions as well as more applied perspectives. These approaches allow a better understanding of how essential macroscopic functions could emerge from complex interactive networks. In this review we present the current state of the theoretical debate about the origins of 1/f fluctuations, with special focus on recent hypotheses that establish a direct link between complexity and fractal fluctuations. Further, we clarify some lines of opposition, especially between idiosyncratic versus nomothetic conceptions, and global versus componential approaches. Finally, we discuss the deep questioning that this approach can generate with regard to current theories of motor control and psychological processes, as well as some future developments which may be evoked, especially in the domain of physical medicine and rehabilitation.