  We examine the stimulated light scattering onto the propagation modes of a
dissipative optical lattice. We show that two different pump-probe
configurations may lead to the excitation, via different mechanisms, of the
same mode. We found that in one configuration the scattering on the propagation
mode results in a resonance in the probe transmission spectrum while in the
other configuration no modification of the scattering spectrum occurs, i.e. the
mode is dark. A theoretical explanation of this behaviour is provided.
