What’s the difference?
a: the rightward motion of a simple black line can be decomposed into 2 vectors, along and across the line.
b: The vector along the line is not seen, so the line is perceived as moving at right angles to its own length.
c: The rightward motion of a dotted line comprises the same two vectors. However, the black/white contrast along the line is much stronger than the black/gray or white/gray contrast across the line.
d: So now the vector along the line is perceptually weighted more than the vector across the line.
Move a pencil point slowly to left and right across the dots, and they will appear to move up and down. Reason: Each pair of black/white dots contains oblique components.