  The elastic coupling between plastic events is generally invoked to interpret
plastic properties and failure of amorphous soft glassy materials. We report an
experiment where the emergence of a self-organized plastic flow is observed
well before the failure. For this we impose an homogeneous stress on a granular
material, and measure local deformations for very small strain increments using
a light scattering setup. We observe a non-homogeneous strain that appears as
transient bands of mesoscopic size and well defined orientation, different from
the angle of the macroscopic frictional shear band that appears at failure. The
presence and the orientation of those micro-bands may be understood by
considering how localized plastic reorganizations redistribute stresses in a
surrounding continuous elastic medium. We characterize the lengthscale and
persistence of the structure. The presence of plastic events and the
mesostructure of the plastic flow are compared to numerical simulations.
