Now what if you superimpose the two? Do you see them sliding past each other at right angles? The answer is no; you see the plaid moving horizontally (indicated by arrow in g). Perception researchers Edward H. Adelson of the Massachusetts Institute of Technology and J. Anthony Movshon of New York University have done some clever experiments to show that, contrary to naive intuition, this effect does not happen simply by averaging the vectors of the two stripes. It happens because of a principle they dubbed “intersection of constraints.” Each grating’s motion is compatible with a family of vectors, and the region of overlap—where the two families overlap—is taken as the “true” direction of motion. Intriguingly, motion-sensitive cells in areas of the brain (including one called MT) at work early in the visual hierarchy of motion processing respond to the direction of each grating separately (“component motion”), whereas cells at a higher level respond to the overall direction of the plaid (“plaid motion”). It is as though the cells were integrating the output of the component sensitive cells by deploying the intersection-of-constraints algorithm.
There is an alternative model to the intersection of constraints. Notice in g that even though the motion of the component stripes is ambiguous the intersections between the lines are moving unambiguously horizontally. These crossover points might “capture” and drag along the gratings horizontally (analogous to the role of the sharp tips in the vertical aperture or barber pole).
At present, no compelling reason exists to choose one model over the other; the former (intersection of constraints) is more mathematically elegant and might appeal to a cosmologist, whereas the latter (a messy “shortcut”) might appeal to a biologist.
The original barber pole pattern is supposed to depict blood and bandages, harking back to an era when barbers were also surgeons. Little did they realize that the illusion could provide such razor-sharp insights into human motion perception.
This article was originally published with the title Sliding Stripes.