Poggendorf Illusion |
Introduction
If it was your job to chalk the baselines of a playing field for a softball game, you might sight down one of the baselines to make sure that it was straight. If the baseline looked straight to you, you might be confident that it was straight. And, usually, our vision is veridical; that is, what we see corresponds to the actual physical world. Now, however, we will investigate a phenomenon that is non-veridical. In cases like the one described below, the arrangement of lines on a surface has a perceptual appearance that does not correspond to its physical measurement. Such a phenomenon is called an illusion.
Illusions act like windows into perceptual processing. The mismatch between perception and physical measurement allows the experimenter to investigate how perception changes as aspects of the illusory stimulus are changed. Illusions are thought by some to give special insight into how perceptual processing usually works. Suppose that your perception is your visual system's best guess as to what the physical world really is. Then the illusion may unveil what kind of inference the brain makes and what properties of the stimulus made the brain guess the way it did. This theoretical perspective on perception and on illusions is called a mediational approach, and assumes that some kind of cognitive processing (here, unconscious reasoning or an unconscious inference) must mediate between the physical stimulus and the perception of that stimulus.
In this experiment, you will study the Poggendorf illusion. A vertical rectangle is intersected by a straight line at an angle through its side. If you are unable to see the part of the line that is inside the rectangle, the line parts that you can see no longer look like they belong to a straight line. The purpose of this study is to measure your perceptual error for the Poggendorf figure while varying some of its aspects.
Procedure
Open Psyk.trek on your computer. When the introductory screen appears, click the arrows to the left or right of the central box to get to Simulations. Click on Simulations and choose Simulation 3, The Poggendorf Illusion. Continue with the experiment as instructed by the program. Using the mouse, you will be able to vertically drag the line part on the right side of the illusion. When the line appears to match collinearly with the line part on the left side of the illusion, click the box, "Done".
Record your error at the end of each trial. A number will appear on the screen that measures how many vertical points your estimate was from the true position that an actual straight line would have occupied.
Once the ten trials are finished, your data will be graphed to test for three different effects. Sketch the main features of each graph on your data sheet.
Results and Discussion
Given the following information, graph your data:
A. Compare the effects of angle by drawing a bar graph of these data. Based on your graph, what can you conclude about the effect of angle on the magnitude of your perceptual error?
B. The first three trials used the same conditions. Compare your results on these three trials by drawing another bar graph. Did your performance change in any systematic way over those three trials? Explain.
C. Trials 1, 6, and 9 used stimuli at the same 45o angle. However, the width of the rectangle varied. Knowing that the box width was 60 cm for Trial 1, 100 cm for Trial 6, and 140 cm for Trial 9, graph the effects of rectangle width on accuracy. Based on your graph, what can you conclude about the effects of the width of the rectangle on the magnitude of your perceptual error?
What was your average perceptual error?
What was the range and standard deviation of your data?
What factors influenced your performance on each trial?
Terms and Concepts
References
Weiten, W. (1998). Psyk.trek: A multimedia introduction to psychology (CD-ROM). Pacific Grove, CA: Brooks/Cole Publishing Co.