Figure 6. Relative tumbling frequencies (at the mesoscopic scale) obtained from the numerical experiment described in Fig. 3 : the tumbling probability is higher when moving to the left (upper red line) at the back of the pulse, whereas the tumbling probability when moving to the right is lower (lower green line), resulting in a net ux towards the right, as the pulse travels (see Fig. 3 ).

Notice that these two curves are not symmetric w.r.t. to the basal rate 1, but the symmetry defect is of lower order. The peak location is also shown for the sake of completeness (blue line).