It is shown that the effects of large de bias and ac driving fields on the dielectric permittivity of single crystals of PbMg1/3Nb2/3O3 (PMN) relaxer ferroelectric exhibit very different behavior, including anisotropy and sign of the effect, which is traced up to the high-temperature interval in the ergodic phase. The observed phenomena strongly suggest that in the ergodic phase of PMN the dielectric response to the ac field is controlled by a side-way motion of the interphase boundaries of the polar regions, rather than by the thermally activated reorientations of the local spontaneous polarization.