Fig 1. A three component model for single cell movement.

In the case of purely diffusive motion (A), cell displacement is modelled as a random vector of radius r. Persistent and biased movement (B and C), respectively add a persistence (p) or bias (b) vector to the random one. In the general case of combined movement (D), displacements are a combination of the three vectors and the directional component (d_b) consists of the whole bias module with the addition of random and persistence contributions (respectively r_b and p_b). The persistence contribution depends on the difference between bias and persistence angle (α angle); its length increases, according to the cosine function, when the α angles decreases.