The pattern formation of nonlinear light propagation in periodic media (fiber grating) is typically described using the so-called nonlinear coupled mode equations (NLCME) that represent the nonlinear interaction of two slowly modulated electromagnetic wave trains that propagate (backward and forward) along the fiber and are reflected by the grating. The NLCME formulation does not take into account the effect of the (always present) material dispersion. I will discuss in this talk the effect of adding small dispersion terms to the NLCME that produce new spatio-temporal complex states that are not captured by the NLCME. I will show also that this is a universal phenomenon in extended pattern forming systems, in the generic case of non-vanishing group velocity, which is due to the competition of transport and dispersion/diffusion.