Debriefing for Subjects of Experiment

Robert T. Arrigo: Programming
Gordon Redding: Author
Additional Credits:
This module was supported by National Science Foundation Grants #9981217 and #0127561.

Explanation of the Experiment

You have finished the experiment. Thank you for your participation!

You may have recognized that the lines together with the angles form the famous Muller-Lyer illusion. The illusion is that a line with angles less than 90(deg) (i.e., arrow junctions) looks shorter than the same line with angles greater than 90(deg) (i.e., fork junctions). One explanation of this illusion is that the two-dimensional line junctions are interpreted as linear perspective drawings of three-dimensional corners. These drawings activate brain mechanisms that compute the inverse perspective to recover the size of the virtual corners that produced the drawings. Lines with arrow junctions depict convex corners in front of the picture plane of the drawing, while lines with fork junctions depict concave corners behind the picture plane. To produce the same size line in the drawing, the virtual corners must be a different sizes; the convex corner must be smaller than the concave corner. Hence the illusion: The line with arrow junctions looks smaller than the line with fork junctions.

The present experiment tests this linear-perspective hypothesis by rotating the virtual corners about their vertical picture plane axes to produce drawings with asymmetrical angles. To maintain the same size lines in the drawings the corners closer to the picture plane: The rotated convex corner in front of the picture plane (depicted by asymmetrical arrow junctions) must be increased in size, while the size of the roatated concave corner behind the picture plane (depicted by asymmetrical fork junctions) must be decreased in size. Hence, the prediction is that the illusion will decrease for both the drawings of with arrow junctions and fork junctions.

A control stimulus is used to provide a measure of each part of the illusion. The control stimulus is a line the same size as the lines with arrow and fork junctions, but with T junctions. These T junctions do not activate linear perspective rules and the control stimulus provides a measure of perception of picture plane size. The lines with arrow junctions should be reported as having a smaller size than the control ine, while the lines with fork junctions should be reported as having a larger size than the control line. The various lines without junctions are intended to provide practice at the task of matching the length of lines.

A large amount of data must be analyzed before we know the results. In the meantime, you may address your questions to Gordon M. Redding my email at gredding@ilstu.edu

Again, we want to thank you very much for your participation in this experiment.

Please click the Exit button to leave the experiment.

Copyright: 2000