We review the question on the origin of the differences observed on various properties when we scan the femtosecond laser beam in an isotropic media (i.e. a glass) in two orientations of a given direction. Publications on refractive index changes, birefringence, nanogratings, stress, bubbles formation and on quill writing effects are analyzed. A new interpretation based on space-charge built from ponderomotive force and stored in the dielectric inducing an asymmetric stress field is proposed.