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Fractal brain dynamics: from Mandelbrot to marmosets

Certain aspects of brain function operate with a well-defined timescale. For instance, the timescale associated with a single action potential is reliably around a millisecond; a spike never takes 10 ms or 100 μs. Another reliable well-defined timescale is the day-long period of circadian rhythms. What is the typical timescale of cortical and thalamic neural firing rate dynamics? In this issue of The Journal of Physiology, Munn et al. (2020) show that this might be an ill-posed question. Indeed, they found that many neurons exhibit fluctuations in spike rates composed of many timescales simultaneously and that these diverse timescales are related in a quite peculiar way – like a fractal. As popularized by the visually stunning fractals of Mandelbrot, if one zooms in on a small part of a fractal, the structure looks the same as the structure at larger scales. A cartoon of how such self-similar structure could manifest in temporal fluctuations is illustrated in