Remember the Ebbinghaus illusion (shown again below)? How do you think a psychologist might use this illusion to learn about mental processes or behavior? Read on to see an actual example from a psychologist at Colorado State University.

Ebbinghaus in the Real World

Imagine that you are in a golf competition in which you are putting against someone with the same experience and skill that you have. There is one problem: Your opponent gets to putt into a hole that is 10% larger than the hole you have to use. You’d probably think that the competition was unfairly biased against you.

Figure 1. Do you suspect that the perceived size of a golf hole will affect putting performance?

Now imagine a somewhat different situation. You and your opponent are about equal in ability and the holes you are using are the same size, but the hole that your opponent is using looks 10% larger than the one you are using. Would your opponent have an unfair advantage now?

If you read the earlier section on the Ebbinghaus effect, you have an idea how psychologists could exploit your perceptual system (and your opponent’s) to test this very question. Psychologist Jessica Witt and her colleagues Sally Linkenauger and Dennis Proffitt recruited research participants with no unusual golf experience to participate in a putting task. They competed against themselves rather than against another person.

The experimenters made the task challenging by using a hole with a 2-inch diameter, which is about half the diameter of the hole you will find on a golf course. An overhead projector mounted on the ceiling of their lab allowed them to project Ebbinghaus’s circles around the putting hole. Some participants saw the putting hole surrounded by circles that were smaller than the hole in the center; the other half saw surrounding black circles that were larger.

Participants putted from about 11½ feet away. They took 10 putts in one condition, and then 10 in the other condition. Half of the participants putted with the large surrounding circles first and half saw the small surrounding circles first. This procedure is called counterbalancing. If there is any advantage (e.g., getting better over time with practice) or disadvantage (e.g., getting tired of putting), counterbalancing assures that both conditions are equally exposed to the positive or negative effects of which task goes first or second. Failure to take account of this type of problem means that you may have a confounding variable—practice or fatigue—that influences performance. A confounding variable is something that could influence performance, but is not part of the study. We try to control (that is, neutralize) potentially confounding variables so they cannot be the cause of performance differences. So, for instance, if everyone did the large surrounding circles condition first and then the small surrounding circles, then differences in performance could be due to order of conditions (leading to practice or fatigue effects) rather than the size of the surrounding circles. By counterbalancing, we don’t get rid of the effects of practice or fatigue for any particular person, but—across all the participants—practice or fatigue should affect both conditions (both types of Ebbinghaus circles) equally.

The experimenters wanted to know two things. First, did they actually produce the Ebbinghaus illusion? Remember: there is no guarantee that people see or think the way your theory says they should. So just before the participant started putting in a particular condition, they drew a circle using a computerized drawing tool, attempting to match the exact size of the putting hole. This is better than simply asking, “do you see the illusion?” The drawing task attempts to directly measure what they perceive.

Second, the experimenters wanted to see if the perceived size of the hole influenced putting accuracy. They recorded the success or failure of each putt. Each participant could get a score of 0 to 10 successful putts in each condition.

Methods Summary

Recap the steps you’ve read about thus far:

1. The participant practices putting to get used to the task.
2. The participant completes the first condition (large surrounding circles for half of the participants and small surrounding circles for the other half).
   • The participant draws a circle corresponding to their estimation of the actual size of the putting hole. This allows the experimenters to determine if the Ebbinghaus effect actually occurred.
   • The participant putts 10 times in this condition.
3. Participant completes the second condition (whichever condition they have not yet done).
   • The participant draws a circle corresponding to their estimation of the actual size of the putting hole.
   • The participant putts 10 times in this condition.

Now see if you can guess the results of this study.

This is not the only experiment that has used a sports context to study the effects of illusions. Other experiments have shown that people hit softballs better when the balls are perceived as larger. People score higher in darts when the board appears larger. Athletes kick field goals and return tennis balls more successfully when the goal posts or tennis balls appear larger. In all of these studies, the balls or boards or goal posts were not actually larger, but they were perceived as larger because the experimenters created illusions. Skilled athletes often report that targets appear larger or time slows down when they are “in the zone”, as if practice and skill create their own perceptual illusions that increase confidence and make difficult challenges feel easier.

A Final Note: Science Doesn’t Always Produce Simple Results

Professor Witt’s study had interesting results; however, they weren’t quite as simple as we have made them seem. The researchers actually had two different hole sizes: 2 inches and 4 inches. The Ebbinghaus circles were adjusted to be relatively larger or smaller than the putting hole.

The Ebbinghaus illusion worked for the smaller (2 inch) putting holes, but not for the larger (4 inch) putting holes. In other words, when people drew the circles as they perceived them (the “manipulation check” dependent variable), they drew different sized circles for the 2 inch holes (the Ebbinghaus illusion), but the same size circles for the 4 inch holes (no Ebbinghaus illusion).

For the larger (4 inch) putting holes, putting accuracy was the same for the two different conditions. This didn’t bother the experimenters, because—as we have already noted—the participants did not experience the Ebbinghaus illusion with the larger holes. If the holes were perceived as the same, then self-confidence should not have been affected and, in turn, putting should not have been better in one condition than the other.

In the research paper, the experimenters suggest a few technical reasons that the larger hole might not have produced the Ebbinghaus illusion, but they admit that they have no definitive explanation. That’s okay. Science often yields messy results—and these can be the basis for new experiments and sometimes for really interesting discoveries. The world is not as simple as our theories try to make it seem. Happily, in science, as in many aspects of life, you learn more from your failures than your successes, so good scientists don’t try to hide from results they don’t expect.