Via Fourier analysis, applied to broad-band images of a sample of spiral galaxies, the relative contribution of each spiral mode to the spiral arms, as a function of the galactocentric radius, is evaluated and compared with the position of the corotation radius, as determined from independent methods by the present authors. The relative contribution of each mode before and after corotation should lead to a better understanding of the physical role played by the corotation radius in propagating the spiral waves throughout it.