Try to change the values, to stretch the rectangle. Try with 180,110 and 180,130 and you will see an inversion in the pattern: from horizontal to vertical! With 180,120 you have a perplexing situation...
[from: http://pages.slc.edu/~ebj/sight_mind/motion/Nakayama/1.35.html ]
<<The stimulus Nakayama et al used to measure the extent of amodally completed surfaces. The motion is perceived as vertical because of the closer vertical proximity of the moving black rectangles. They estimated the extent of the completed surfaces by finding the horizontal separation required to make the perception bistable, as in the simple Ramachandran-Anstis bistable competitive motion paradigm.>>