| ISLE | Chapter 2
| Experiment: Method of Adjustment: Point of Subjective Equality (PSE)
The method of adjustment means just that. The observer controls the level of the stimulus and adjusts it to be at the perceptual threshold. The participant does so by increasing or decreasing the level of the stimulus until it feels like it is just at the detectable level. This is an intuitive measure for most participants because it mirrors many normal activities like adjusting the volume control on a radio or a dimmer switch on a light.
The method of adjustment is very useful for matching one stimulus to another to determine the point of subjective equality or, more commonly, the point of subjective equality (PSE), which is a different dependent measure than a threshold. The PSE is the settings of two stimuli in which the observer experiences them as identical. In vision research, the experimenter could present a stimulus of particular brightness. The participant would have to adjust another stimulus to be equally bright to the first one. One particular use of the PSE is to measure the strength of an illusion. If the point of subjective equality is systematically different than the point of objective equality, then you have a measure of an illusion.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
On this tab, you can adjust the various parameters of the motion of the stimulus to see how this might affect the aftereffect. The settings include the following:
Stimulus Size: the size or diameter of each of the
large outer circles. The value is the percentage of the half of the screen side
that the circle is on.
Size of Inner Circle: the size of the smaller circle that is centered in the larger circle, as a percentage of the size of the larger circle.
Position: the position of the circles from the center. It is a
percentage of the distance that the circle can be moved. If the
circles are too big and too close, they might overlap,
and if they are too big and too far out, the circle might not be entirely on the screen.
Background Level: the intensity of the area around the circles. The units are the units used by the display to create intensity.
Reset: at the top of the settings page is a Reset button. Pressing this button restores the method settings to their default values.
On this tab, you can adjust how the method will work. The settings include the following:
Number of Trials in the Experiment: how many times you will make your adjustment.
Range of Variation of Comparison Circle: over what range will
the starting value of the stimulus
change. The range is a percentage of the total range of values the stimulus can have.
Reset At the top of the settings page is a Reset button. Pressing this button restores the stimulus settings to their default values.
On the Experiment tab, press the space bar or the Start button
on the screen to start the experiment.
You do not need to keep your eyes fixated on the central fixation mark, but it might be helpful.
Two stimuli will be presented one to each side of this fixation mark.
These stimuli are each composed of two circles: a large outer circle and
a smaller inner circle. The circle on the left will not change during the experiment and by convention is called the standard. The circle on the right is the stimulus you will change, at least the color of the central circle. By convention, this stimulus is called the comparison.
At the same time that the stimuli appear, a slider will appear below the stimulus area. Use
the slider to increase
or decrease the blueness of the comparison central circle on the right.
Your task is to adjust the color of the inner circle of the comparison
on the right so that it looks as similar as possible in color to the color of the central circle of the standard on the left. Use the slider at the bottom of the screen.
When you have accomplished this goal, click the Match button to the right of the slider to proceed to the next trial.
You can also use the arrow keys to adjust the slider. Up and right arrow keys will increase the intensity of the dot and
down and left arrow keys will decrease the intensity of the dot. The enter key will act like the Match button.
When you have finished all of your trials, you will instructed that you can view your results.
You data will presented on this tab. On the x-axis the trials. On the y-axis will be the value of your dependent measure that you set when you pressed the Match button on that trial. To see your PSE, click the button that says Show PSE. To display the data for all of the trials, click the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.