Slow modulation of the contraction patterns in Physarum polycephalum.

The slime mould Physarum polycephalum has emerged as a model system for self-organization and coordination of contractile activity at large spatial scales. This organization results from cytoplasmic flows generated by propagating contractile waves of the actomyosin cortex. In addition to these relatively fast travelling waves, slower modulations of contractile activity occur on timescales much longer than the primary oscillation period, yet remain poorly characterized. Here, we investigate these slow modulations by confining organisms inside annular geometries, and quantifying contractile activity across the organism over long recordings. We reveal correlations between contractile wave direction, amplitude modulation and slow variations of the mean vein diameter. Using an automatic Fourier-based classification, we identify three families of slow modulation modes: fundamental rotating modes, higher order modes and standing modes. Fundamental rotating modes dominate and exhibit periods compatible with linear scaling with system size. Remarkably, measured periods cluster near integer multiples of an intrinsic modulation timescale obtained independently from statistical analysis. These observations are consistent with a transport-mediated scenario in which a slowly advected regulator biases local excitability and mechanical properties, coordinating activity across the organism on long timescales.