(A) Alternation of polarization at various ATP concentrations. The excitation intensity was 6.2 mW at 200 nM ATP, 3.2 mW at 60 nM ATP, and 1.8 mW at 20 nM ATP over a sample area of 24 μm in diameter (the frequency of alternation did not depend on excitation intensity). The polarizations at 60 and 20 nM ATP were calculated after V and H were median-filtered over four and eight video frames, respectively. Vertical red lines indicate termination of one revolution, identified by eye as the crossing of polarization through zero in a unique direction. All records were terminated by photobleaching. (B) Distribution of times for a turn at various ATP concentrations. The average values were 0.84 s at 200 nM ATP, 2.4 s at 60 nM ATP, and 5.5 s at 20 nM ATP. Solid lines show fits with the equation in the text.