The FD effect occurs when the contrast of low spatial frequency gratings is modulated quickly. Under those conditions the subject sees a stable percept of the grating having twice as many stripes. The effect may be produced by the rapid contrast gain control system of cell projecting to the magnocellular lateral denticulate nucleus (LGN). Recent work has revealed that there may be multiple sources of this effect. It is also possible that the effect is produced by Y-like ganglion cells a anatomical substrate for which has recently been discovered in primates. Interest in FD arises from the very successful FDT and Matrix perimeters invented by Ted. Projects can include psychophysical investigations of the basis FD or new perimeter variants. A related topic is so called illusory fineness of stationary patterns.