The sample is a nematic liquid crystal (PAA) in a Hele-Shaw cell. This cell is submitted to a vertical temperature difference Delta T and a magnetic field H. The temperature difference destabilizes the system, producing the well-known Rayleigh-Benard rolls. The magnetic field stabilizes the system. The combination of both produces a phase diagram. T. Riste et al. showed that, for moderate temperature difference and fields, periodic fluctuations of the neutron scattered intensity appear. Moreover the squared frequencies are linear versus Delta T over adjacent Delta T intervals (frequency quantification). The aim of this experiment was to determine the origin of these fluctuations by simultaneous neutron scattering time series measurements and optic observation thanks to a transparent cell. In our first experiment with a cell similar to Riste’s we have been able to reproduce the first part of his observations; our conclusion is that the fluctuations do not arise from the rolls themselves but from oscillators which periodically perturb the rolls structure.