Using the technique of photoluminescence imaging, self-organized patterns of high-electron density in homogeneous n-GaAs layers under homogeneous microwave irradiation are studied. The structures are shown to be analogous to current filaments in a static electric field. The symmetry of the microwave induced patterns is not constrained by the current feeding electrodes. It is, however, concluded that a feedback mechanism exists between the formation of high-conducting structures and the homogeneity of the incident microwave irradiation.