The viscoelastic response to a strong externally imposed shear flow is generally non-linear. This behaviour is the result of shear induced microstructural distortion. Such non-linear microstructural response is particularly pronounced close to a gas-liquid critical point, since there the dynamics is very slow and microstructural order extends over large distances. In order to investigate the frequency dependence of the microstructural response, we perform time resolved small angle light scattering experiments on a polymer-colloid mixture under oscillatory shear flow. The slow dynamics near the gas-liquid critical point allows to perform experiments at frequencies that are much larger then inverse microstructural relaxation times, but still small enough to avoid surface loading effects.