The effect of such an illusion can be compared to the endless staircase
of the visual artist M. C. Esher. For one who is ascending the staircase, each
step is indeed moving up: there is the sensation, locally, of movement with respect to height.
However, globally, the sensation of upward movement begins to break down. Hence, the principle of height is
localized: as you move from step to
step you know that you are going up. Globally, however, you are not moving up (or
down) at all.
The following example is an endless ascending sequence of Shepard tones. As you listen, you will notice that at first each succesive tone seems indeed to be ascending. However, after awhile the sensation of ascending tone begins to weaken until you perceive that indeed the tone has not risen at all. The longer you listen, the stronger the effect becomes.