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Phototactic turnover after a change of light incidence. Gonium colonies are initially swimming (t < 0 s) toward a light of constant intensity on the right. At t = 0 s, this light is switched off while another of controlled adjustable intensity s0 is shone from the left. Trajectories are reported for t > 0 s and have been shifted to the origin at t = 0 s. Colors along the lines show the time for t > 0 s.
