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Summary form only given. In the present work we investigate experimentally the influence of higher harmonics on the pattern selection process for a variety of structures. The system under study consists of sodium vapor that is irradiated by a linearly or circularly polarized Gaussian beam, and a flat mirror that reflects most of the light back into the sodium cell. The polarization state of the reflected light is modified by phase retardation plates within the feedback loop. This system is known to form diverse patterns. Using a spatial filter, we can suppress the higher harmonics of the fundamental wave vectors of the patterns.