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).
The user typically starts the experiment by clicking the “Start or Restart Experiment” link on the left. Read below for a description of the activity.
Description of activity
On each trial of our psychophysics experiment, the user sees 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°.
The user then clicks one of the two links to say whether they think the middle line is the same angle as or a different angle than the outer lines. There is a results section that keeps track of how often the user is correct or incorrect for the three conditions: same angle, 1° rotated, and 3° rotated. The next section describes the activity in terms of signal detection theory.
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:
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If the center line really is misaligned and you correctly say it is misaligned, that is a hit.
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If the center line is really identical to the outer lines, but you incorrectly say it is misaligned, that is a false alarm.
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If the center line is misaligned but you incorrectly say the three lines are identical, that is a miss.
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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 a user does enough trials of the experiment they typically find that their hit rate for 3°-misaligned central lines is greater than their hit rate for 1°-misaligned central lines, since their visual system is almost certainly more sensitive to larger misalignments than to smaller misalignments.