Heterogeneous granular mixtures tend to segregate when tumbled in a partially filled, horizontal rotating drum. The dynamical evolution of segregation can, under certain conditions, be oscillatory. Continuum, order parameter-style models of this process posit two coupled fields which oscillate out of phase with one another. Here we examine three candidate fields, the surface concentration, the local streaming angle and the projected concentration of the subsurface core. We find that all these quantities are in phase with one another, in contradiction to a recent order parameter model.