| ISLE | Chapter 6
| Experiment: Simultaneous Color Contrast: PSE
Simultaneous color contrast occurs when our perception of one color is affected by a color that surrounds it. The effect occurs when a color is surrounded by its opponent color and not via other colors or achromatic backgrounds. A green circle will seem greener if the green circle is surrounded by a red background, and a red circle will seem redder if it is surrounded by a green background. Similarly, a blue circle will seem bluer if it is surrounded by a yellow background, and a yellow circle will appear more yellow if it is surrounded by a blue background.
In the figure belown the two central circles are identical. But do they look that way? If you are not sure, cover up the surrounding circles and then look at the two central circles.
To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.
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.
Standard (Left) Center: adjust the three sliders, one for each primary on your monitor, to create the color in your standard stimulus, positioned on the left. The color of your setting is shown on the right of the red primary slider.
Standard (Left) Surround: adjust the three sliders, one for each primary on your monitor, to create the color for the circle that surrounds your standard stimulus. The color of your setting is shown on the right of the red primary slider.
Comparison (Right) Surround: adjust the three sliders, one for each primary on your monitor, to create the color for the circle that surrounds your comparison stimulus, positioned on the right. The color of your setting is shown on the right of the red primary slider.
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 Center: 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 a space button or the 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 be instructed that you can view your results.
Your data will be presented on this tab. On the x-axis will be 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.