Appendix 2—video 5. Regions like eyelids, physical breaks in the skin can break homogenous wave spreading.

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The black region is ‘cut-off’ from the homogeneous outside region, i.e., no HF growth is allowed in this region and no molecules are allowed to diffuse into it. As the HF wave propagates, the black region breaks the wave spreading, creating a distortion-like effect. Also see Figure 7A.

DOI: http://dx.doi.org/10.7554/eLife.22772.088