I think this is do to the number of images the brain can process. As the wheel starts spinning forward it sees this, as the wheels revolutions begin to get faster. As the speed increases the wheels blur because the individual frames cannot be detected, so the brain merges them together, resulting in a blur that os revolving forward. The forward motion can be ascertained because every so often when the brain picks out an image, it shows the wheel further forward than the last image. However, eventually the speed will be reached whereby after seeing one image, the next one registered is when the wheel has rotated so far forwards that it has made *almost a full revolution*, that it the position of the wheel appears to be slightly further back that before. Because it hasn't made a full revolution, when you put several of these together, where in each frame the position of the wheel is slightly less than the image before, despite it still moving forwards, it looks like it is spinning backwards. In the same way that adding 1 + 1 + 1 + 1 + 1 etc will produce eventually 20. If the speed is increased to 1.9, you will reach 20 faster but the decimal part will decrease (3.8, 5.7, 7.6).