(A) Average track curvature upon entry in to the pause state in wild type worms. Prior to computing curvature, tracks of individual worms were mirror-imaged as needed such that positive curvature corresponds to a ventral bend. Tracks in the vicinity of pause events were aligned according to the location of the tracking spot in the pause state, converted to curvature, then averaged over all FX transitions (solid blue line; n = 1907), and all RY transitions (red; n = 295) for which the track length was >1.5 mm; shading shows ± 1 S.D. The trace depicts the curvature of the worm posterior to the tracking spot at the end of forward movement (FX transitions) and anterior to the tracking spot at the end of reverse movement (RY transitions). The dashed blue line shows the average curvature at FXR transitions (i.e., excluding FXF stutters). (B) Locomotory phases at which FX transitions occurred, plotted as blue dots on the unit circle. The phase at each FX transition was computed as , where and are the positions of the two downward zero crossings of curvature preceding the pause as indicated in panel A, right. The uniform distribution of points around the circle, and therefore the small magnitude of the vector strength (; arrow), shows that there was only a small (but statistically significant) phase preference at the end of forward motion (; Rayleigh test). (C) Same as B, but for RY transitions. Vector strength is large (), indicating a strong tendency to end reverse runs at a particular phase (), with a ventral bend in the middle of the body. (D) Average posture at FXR transitions, calculated by integrating the average curvature, computed over all tracks that persisted for >1.5 mm in state F before the pause and >1 mm in state R after the pause. Arrows indicate direction of motion along the track (blue, forward; red, reverse). FXR transitions were typically a simple reversal along the same track. (E) Same as D but for RYF transitions that persisted for >1.5 mm in state R before the pause and >1 mm in state F after the pause. RYF transitions at the end of reverse runs that persisted for >1.5 mm were usually associated with a ventral bend that resulted in a ~180° change of direction as previously described (Gray et al., 2005).