Psychophysics
Introduction
Psychophysics studies the relationship between physical properties of the world and our psychological perception of them.
A very basic psychophysical experiment, described in the textbook, would involve playing tones of various volumes and asking experiment participants to say whether or not they hear each tone. Based on their responses, we could calculate each participant’s hearing threshold—the volume level at which they can just barely hear the tone.
In this activity, we’ll conduct a psychophysical experiment using the 100-pixel-long lines shown at left. The central line in this illustration is rotated 30° clockwise from vertical. We’ll use this as our reference angle. The leftmost line is rotated 29° from vertical—one degree away from the reference angle. The rightmost line is rotated 33° from vertical—three degrees away from the reference.
The psychophysical question we will be asking is how well your visual system can discriminate the angular differences between the reference line and the subtly rotated test lines (either –1° or +3° from the test line).
Go ahead and start the experiment now by clicking the “Start or Restart Experiment” link on the left.
After you have done a few trials, read about bias in psychophysics experiments and how signal detection theory accounts for it.
Instructions
Click the “Start or Restart Experiment” link on the left to get to the experimental part of the activity. Then click the START button to begin a trial. After a brief delay, you’ll see three diagonal lines. The middle line may or may not be exactly the same angle as the two outer lines. Click the appropriate link below the START button to register your guess.
Start or Restart Experiment
On each trial of our psychophysics experiment, you will see three lines. The outer two lines will always be rotated 30° clockwise from vertical (remember, this is our reference angle). The inner line may be identical to the outer lines, or it may be rotated away from these reference lines by 1° or 3°.
Examine the stimuli as long as you like, then click one of the two links to say whether you think the middle line is the same angle as or a different angle than the outer lines. If you’re not sure, take a guess.
Click the START button to start a new trial.
You can reset your results if you wish.
Experiment
Examine the stimuli as long as you like, then click one of the two links to say whether you think the middle line is the same angle as or a different angle than the outer lines. If you’re not sure, take a guess.
Results
The middle line was misaligned by 1° relative to the outer two lines.
You responded that the middle line was misaligned relative to the outer lines.
Therefore, you were correct. In signal detection theory terms, the result of this trial was a hit.
The middle line was misaligned by 1° relative to the outer two lines.
You responded that the three lines were the same.
Therefore, you were incorrect. In signal detection theory terms, the result of this trial was a miss.
The middle line was exactly the same as the outer two lines.
You responded that the three lines were the same.
Therefore, you were correct. In signal detection theory terms, the result of this trial was a correct rejection.
The middle line was exactly the same as the outer two lines.
You responded that the middle line was misaligned relative to the outer lines.
Therefore, you were incorrect. In signal detection theory terms, the result of this trial was a false alarm.
Overall results so far:
You have run 0 trials.
| Relative Misalignment | ||
|---|---|---|
| 0° | 1° | 3° |
| 0/0 | 0/0 | 0/0 |
| Correct rejection rate: N/A | Hit rate for 1°: N/A | Hit rate for 3°: N/A |
Bias and Signal Detection Theory
As we noted in the introduction, a basic psychophysical experiment might involve playing tones of various volumes and asking participants to report whether or not they hear each tone.
If we were to apply this “classical” psychophysical method to the stimuli in this activity, the middle line would always be misaligned relative to the outer lines. We would then vary the degree of misalignment back and forth until we found the angle at which you were just able to detect that the middle line was misaligned.
One problem with this kind of method is that participants may be (and indeed, usually are) biased to respond in one way or another. For example, if an observer knows he has poor vision, he may have little confidence in his ability to do the task, so he may always respond that he thinks the lines are perfectly aligned, even when he does sometimes suspect that the lines look misaligned. On the other hand, a hot-shot observer may claim to always see the misalignment, even when the lines are so similar that there’s no way her visual system could have detected it.
A good way to address the bias issue is to modify the experimental procedure slightly as we’ve done here, so that on half of the trials the lines really are perfectly aligned. In this modified procedure, the task is no longer to say whether or not you can see the misalignment, but whether or not the lines really are misaligned (this is a subtle, but important, distinction).
If the lines are different enough, an observer should always be able to accurately say whether or not the middle one is misaligned, so she will be accurate 100% of the time. But if the misalignment is near the threshold of vision for this task, the observer will have to guess most of the time, so her accuracy level would be close to 50%. Furthermore, the participant who always claims to see a misalignment would end up with the same overall score (50% correct) as the participant who always claims the lines are identical (think about this for a moment and you should see why).
This modification turns the experiment into a signal detection experiment. The signal is a misaligned center line, and the experimental question is how much orientation difference is needed so that the participant can reliably detect the signal. The outcome of each trial is classified according to the following rules:
- If the center line really is misaligned and you correctly say it is misaligned, that is a hit.
- If the center line is really identical to the outer lines, but you incorrectly say it is misaligned, that is a false alarm.
- If the center line is misaligned but you incorrectly say the three lines are identical, that is a miss.
- Finally, if the center line is really identical to the outer lines and you correctly say they are identical, that is a correct rejection.
Signal detection theorists have developed statistical tools for using hit, false alarm, miss, and correct rejection rates to distinguish an observer’s bias from their sensitivity—the true measure of their perceptual ability to do the task at hand. The calculation of these statistics is beyond the scope of this textbook but the basic concept should be fairly easy to understand.
If you do enough trials of our experiment you should find that your hit rate for 3°-misaligned central lines is greater than your hit rate for 1°-misaligned central lines, since your visual system is almost certainly more sensitive to larger misalignments than to smaller misalignments. View your results so far and see if this is the case.