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Hall of Illusions


Applet design: Al Seckel  Programing: Akos Feher  Barberpole©1997 IllusionWorks, L.L.C.

 Barber Pole

     You can get some interesting illusions when you move contours or figures behind apertures. In this demonstration, you can perform experiments on one of the most famous of all aperture problems - the barber pole illusion. You might be quite surprised at what you perceive!


The effect of different shaped occluders

     In the first configuration you will see a horizontal aperture and a series of diagonal lines moving in a left/right direction. Press the Show button to see the true movement of the lines behind the occluder. The lines are moving in an left/right direction. No surprise.

     Hide the lines again, and then switch to a different shaped occluder. For example, the vertical rectangle. What happens to your perception of the direction of the movement of the lines? Do the lines now appear to be moving in either an upwards or downwards spiral? Every point on the diagonal line is moving in a horizontal line, yet you perceive all points as moving in a vertical direction! You might recognize this illusion from a barber shop -- hence the name. A series of slanted lines moving in a horizontal direction will give the impression of an upwards or downwards spiral movement.  Check the true movement of the lines by revealing the motion and then hide the motion again.

     Try different shapes, such as the cross and the circle. In what direction do the moving lines appear to move when occluded by these different shapes?  What affect does the shape of the occluder have on your perception of the direction of movement of the lines?

The effect of the direction of the moving lines.

     In the previous examples, you were occluding lines with different shapes, while the lines underneath were moving in a left/right direction.  Each shape (the cross and the circle were equivalent) had different perceived directions of motion.

     What happens when you change the direction of the lines to an up/down direction?  Is there any difference in your perception of the direction of movement? In other words, when the lines are moving left/right behind a vertical aperture the direction of movement perceived is along the vertical direction. Is there a difference in your perception of the movement when the lines are actually moving up/down behind the vertical occluder?

     You will find that there is no perceived difference of movement, whether the lines are moving in an left/right direction or a up/down direction. It makes no difference what shape you use. Most people find this quite surprising.

What's Going On?

     A moving bar, contour, or figure moving behind an aperture provides ambiguous information about the direction of movement. In the case of contours or figures seen through an aperture, your visual system regards the surface as occluding stripes that move behind the occluder. This follows from the numerous T-junctions (where the ends of the line meet with the inside of the aperture), which indicate that the surface was behind and not bounded by the aperture. This depth cue, however, is in conflict with binocular disparity, which indicates that the diagonal stripes are on the same depth plane.

     The shape of the aperture usually determines the perceived direction of motion. Thus, a vertically elongated aperture makes vertical motion dominant whereas a horizontally elongated aperture makes horizontal motion dominant. In the case of a circular or cross aperture, the perceived direction of movement is usually orthogonal to the orientation of the stripes. The perceived direction of movement relates to the termination of the line's end points within the inside border of the occluder. The vertical aperture, for instance, has longer edges at the vertical orientation, creating a larger number of terminators unambiguously moving vertically. Functionally, this mechanism ensures that we perceive the moving pattern as a rigid surface moving in one direction.

     Adelson and Movshon showed how the perception of the movement of the lines behind an aperture can be caused by a variety of possible movements of the contour.

(Graphic to come)

     In the figure above there is a circular aperture behind which is a vertical red line. The dashed black arrows represent the different possibilities (directions) the line could be moving behind the aperture to give the same perceived direction of movement. For example, the red line can be moving directly to the right as indicated by the solid arrow or by other directions of movement indicated by the dashed arrows.

     Adelson and Movshon reasoned that the length of the dashed arrow to the constraint line (blue) would indicate the velocity of movement that would be necessary to create the movement corresponding to the solid arrow. For example, if the line is moving upward at an angle, it has to be moving faster than if it were moving straight across to the right, if both movements are to be perceived as the same movement across the aperture.

     Shin Shimojo, Gerald Silverman, and Ken Nakayama showed that by manipulating binocular disparity (making the stripes appear in the front or back) they could change the perceived direction of motion, even though they left the status of the terminators unchanged.

     This  indicates that local measurements of motion (as accomplished by orientation-selective, motion sensitive neurons) by themselves are insufficient to specify true motion direction. This indicates that we cannot understand the perception of motion solely in terms of low-level motion signals. Even for the coding of motion direction, the visual system needs information about the layout of surfaces in three-dimensional space.


History Of The Striped Barber Pole

     In the Middle Ages, hair was not the only thing that barbers cut. They also performed surgery, tooth extractions, and bloodletting.

     The barber pole as a symbol of the profession is a legacy of bloodletting. The barber surgeon's necessities for that curious custom were a staff for the patient to grasp (so the veins on the arm would stand out sharply), a basin to hold leeches and catch blood, and a copious supply of linen bandages. After the operation was completed, the bandages would be hung on the staff and sometimes placed outside as advertisement. Twirled by the wind, they would form a red & white spiral pattern that was later adopted for painted poles. The earliest poles were surmounted by a leech basin, which in time was transformed into a ball. One Interpretation of the colors of the barber pole was that Red represented the blood, Blue the veins, and White the bandages.

Entire web site©1997 IllusionWorks, L.L.C.