Role of twin boundaries on the vortex dynamics in YBa${}_2$Cu${}_3$O${}_7$

By means of a novel technique of rotating the applied current we have directly measured the influence of twin boundaries on the vortex motion in a YBa${}_2$Cu${}_3$O${}_7$ single crystal. The results indicate that the effect of twin planes on the vortex dynamics starts to develop below a certain temperature, being responsible for an anisotropic viscosity in the vortex liquid state and a guided motion in the solid state.