Cognition Laboratory Experiments

John H. Krantz, Hanover College,

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Mental Imagery



Statistical Concepts

Hanover College
Psychology Department

Instructions for the Poggendorf Illusion Experiment


When you click on the link below, you will be presented with the experiment setup screen.  On this screen will be the variables you can set to define your condition.  Here is a list of the variables and their settings

Variable Settings
Number of Trials This variable controls the number of trials per condition.  The number of conditions is controlled by the next two variables.  You can pick from 5 to 200.
Number of Bar Widths One of the variables you can adjust is the width of the bar in the Poggendorf illusion.  This variable allows you to determine how many different bar widths you wish to test.  The value for bar width is proportional to the width of the screen from 0 (no bar) to 0.5 (half of the image).  Below this slide, sliders will appear to allow you set the bar width for each condition.
Number of Angles The other variable is the angle of the line that crosses the central bar.  An angle of 0 means a horizontal line and angles can go up to 60 deg from horizontal in a clockwise rotation.
  You may have up to 10 conditions in this experiment.

After you have finished making your settings, press the Done button at the bottom of the screen.  The Poggendorf Illusion experiment screen will then be presented.  Press the button at the top of the page or the space bar to begin the experiment.  After a brief delay the Poggendorf stimulus will appear.  Your task is to adjust the line on the right side of the screen so that it lines up with the line on the left side of the screen.  In other words, you want this line to appear as one line.  You can do this three ways:

  1. press the up and down buttons to the right of the Poggendorf stimulus.
  2. press the up and down arrow keys on your keyboard.
  3. click and/or drag your mouse over the right edge of the bar moving and/or dragging the right hand line along with your mouse pointer.

The conditions are selected randomly by the computer and the initial position of the right hand bar also is determined randomly..

At the end of the experiment, your results will be presented. You will get the mean and standard deviation of your average positions and the average time it took you to make your adjustments for each condition.  You can also get your trial by trial results.  Closing your results window(s) will take you back to the setup window so you can run another experiment.

Click here to start the experiment.