Psychophysical Methods Laboratory

1. Classical Methods of Psychophysics

The classical methods, as illustrated by G. T. Fechner, include the Method of Limits, the Method of Constant Stimuli, and the Method of Adjustment. Before you begin experiencing the three methods, be sure that you know the terminology involved. On your worksheet are a number of terms. For each term, write a definition in the accompanying box. One useful resource is the Precision and Accuracy web site (Ono, York University). (The dictionary is on the right side of the screen.) The definitions actually differ slightly, depending on whether you're conducting a study of absolute or difference thresholds and also depending on the specific method used. Nonetheless, provide a simple/basic definition for each term.

Method of Limits

First, to experience the Method of Limits, go through the demonstration provided by Professors Tan and Pizlo (Purdue). Choose the truncated Method of Limits using the Müller-Lyer Illusion. Click on the Start Experiment button. In the next window, choose the Short Session. (You may have to enlarge the window to get the buttons to show.) You'll go through 20 trials. On each trial, you'll adjust the length of the left line (Test Line) to get it to match the length of the line on the right (Reference Line). When you are finished, you'll see your data. Record the Point of Subjective Equality (PSE) and the Difference Limen (DL), which is the same as the jnd, on your worksheet.

Method of Adjustment

Next, within that same site, choose the Method of Adjustment for the Müller-Lyer Illusion. Click on the Start Experiment button. Again, choose the Short Session, which may require that you resize the window. For the 20 trials, adjust the Test Line, using the buttons to make it shorter or longer. When you're confident that you've matched the length of the Reference Line, click on the Next Trial button. When you are finished, you'll see your results. Record the Mean and Standard Deviation on your worksheet.

Method of Constant Stimuli

For this demonstration, we'll make use of Ono's Precision and Accuracy page, where you'll be judging the length of horizontal lines. First, click on Method of Constant Stimuli: Overview and Objectives. After reading that information, click on the Operating Instructions. Take the 5 practice trials to be sure that you're comfortable with the pace of responding. When you're all set, choose Tutorial and Quiz and then Proceed with Trials. Answer the quiz questions on your lab worksheet.

2. Signal Detection Experiment

In order to give you a sense of what it's like to participate in a signal detection experiment, you should use Safari (on a Mac) as your browser. Go to Indiana's Signal Detection Experiment and follow the instructions in your worksheet.

3. Magnitude Estimation Judgments

The magnitude estimation procedure is actually fairly simple (from the participant's perspective), even though participants often report that they are not performing well on the task. As a participant, all that you need to do is to assign a number that seems to best fit a particular stimulus. For this exercise, we're going to use squares of various sizes. The squares will appear on the screen in front of you. Your task is to judge the size of each square. The numbers that you use can be quite arbitrary. What's important is that your numbers "capture" the relationships among the stimuli. Thus, if you've given some square a 1.0 and another square appears to be half of thesize of that square, you'd have to give it a 0.5. If a square appears to be 10 times larger than a square that you'd rated a 2, then you'd assign it a 20. Needless to say, you'd never use 0, because a stimulus will always be present.

After each square appears you should record your judgment of the size of the stimulus on your worksheet. [You may need to scroll down just a bit to see the link to the next trial. Ideally, you wouldn't be able to look back at previous responses (or stimuli), so don't peek!]

Click here to begin presenting the stimuli.

4. Category Scale Judgments

Another means of learning about a person's perception of a stimulus is to use a category scale. Researchers have used scales with varying numbers of categories. For our purposes, we'll use a 7-point category scale. You'll see each of the stimuli again and you need to judge the size of each square using the 7-point category scales found in your worksheet. Simply place an X within the box that seems to best represent the size of the square that you're seeing (from Very Small to Very Large).

Click here to begin presenting the stimuli.

You're almost finished! Click here to graph your data.

5. Graphing the Magnitude Estimation Data

Here are the relative areas of the six squares:

The values for the squares are already entered on the x-axis. Set up the y-axis so that it will "capture" the values that you used to judge the areas of the squares. Then plot each of your responses as a function of the area of the square that you were judging. Connect each of the points on your graph and describe the nature of the relationship between the judged areas and actual areas.

Note that you could also graph the category scale data if you were so inclined. (You would treat the first scale point as a 1, the second scale point as a 2, etc.)