Poggendorff Illusion Description

The Poggendorff illusion is a misalignment effect produced by the interaction of diagonal line elements with horizontal and vertical edges. Although the line segments in each picture are collinear, the right segment looks too high to be continuous. The rectangles are perceived as upright blocks while the line seems to recede into the distance. The right segment is seen as farther away and therefore higher in the visual plane.

The strength of the illusion above is reduced by showing the rectangle lying in the same plane as the diagonal line or by aligning the diagonal with imaginary lines converging at a vanishing point.

This variant above demonstrates the effect for the curved edge of a circle which does not depend on depth perception. The black bar partitions the circle into two arcs; the smaller left arc seems to draw in on itself because the eye underestimates the angle of the covered segment.


Zöllner variant. If perceived in three dimensions, the thicker lines with horizontal crossings seem to lie flat in a plane. The vertical crossings on the other thick lines seem to prop them above the plane. This makes the near ends of these thick lines higher while the far ends of all of the thick lines recede to the same vanishing point. Therefore, the angles of the lines seem to be different. The thicker lines can be perceived as straight if the thinner crossbars are grouped together (as a decorative pattern, perhaps). However, this view seems to increase the offset of the two parts of each of the thin crossbars.

This kind of illusion can also lead to distortions in the reading of graphs; although the vertical distance between the two lines above is always the same, the slanted sections look closer in value than the straighter sections.


An Interactive Poggendorf Illusion Demonstration

     Can you connect the two white line segments?  Use your left mouse button to independently move the two tilted white lines up and down along either side of the gray rectangle.  When you think that the two lines are positioned such that they lie along the same line, press the "Connect Me" button.  If you have positioned the two lines so that they are connected, a green line will be drawn between the two white lines.  If you did not, a red line will be drawn.

Hold or click these buttons to change the width of the gray rectangle
Hold or click these buttons to change the angle of the two white line segments
This button will connect the two line segments
If the applet is blank, press this button

NOW EXPERIMENT WITH THE ILLUSION!

  • Click on the gray, occluding box to make it disappear.  Now try to connect the two white lines.  Is it easier without the occluding box?  Click on the box again to make it re-appear.
  • Press the "Increase Box" or "Decrease Box" button to change the width of the box.  Now try to connect the two white lines.  As the distance increases or decreases between the two lines, does the task become easier or harder?  Remove the gray box after you've changed its size and try again!
  • Press the "Increase Angle" or "Decrease Angle" button to change the tilt of the two white lines.  Now try to connect the two white lines.  How does the difficulty of the task change with the angle of tilt? What happens if you make the lines horizontal?

     As you can see, there are many visual factors that affect the strength of this illusion.  However, by using such casual observation, we cannot mathematically measure how much an effect these factors have; moreover, it is difficult to understand precisely why they have an effect.  Experimental psychological techniques are needed to address these questions.  We therefore now invite you to participate in a short 5-10 minute experiment.  


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