If a horse is on one side and a rider on the other, if a cage is on one and a bird on the other, we see the rider on the horse and the bird in the cage.

It cannot be otherwise.

It is simply the result of the positive afterimages.

If at dark we twirl a glowing joss stick in a circle, we do not see one point moving from place to place, but we see a continuous circular line.

It is nowhere broken because, if the movement is quick, the positive afterimage of the light in its first position is still effective in our eye when the glowing point has passed through the whole circle and has reached the first position again.
We speak of this effect as a positive afterimage, because it is a real continuation of the first impression and stands in contrast to the so-called negative afterimage in which the aftereffect is opposite to the original stimulus.