| ISLE | Chapter 2
| Experiment: Magnitude Estimation: Dot Brightness
Magnitude Estimation is a psychophysical method in which participants judge and assign numerical estimates to the perceived strength of a stimulus. This technique was developed by S. S. Stevens in the 1950s (e.g., Stevens, 1956). Magnitude estimation usually works in the following way. A stimulus is presented, and the participant simply gives a number that indicates how intense she perceive that stimulus. In a common variation, an experimenter presents a standard stimulus, which is often called the modulus, and assigns it a particular value, say 20. Then the participant must judge subsequent stimuli and give them a numerical value comparing it to the standard stimulus or modulus. So if the participant thinks the new stimulus is twice as intense as the standard, it should be assigned a 40. If the next tone is just a bit less intense than the standard, it will be assigned a 15. If the tone is heard to be much less intense than the standard, it might receive a 5 on this hypothetical scale.
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 stimulus to see how this might affect your ability to detect the dot. The settings include the following:
Dot Diameter: the diameter of the dot in pixels.
Dot Position: the distance the dot is above the fixation mark (in number of diameters of fixation mark).
Background Level: brightness of the background against which the dot is to be detected (in the basic intensity values of the screen). The dot values are differences from this level.
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 Levels of Relative Dot Luminance: how many different levels of intensity of your stimulus.
Number of Repetitions: how many times each stimulus will be presented.
Use Modulus: controls if a comparison stimulus (the modulus) be presented. Enter the
value for this standard stimulus. A value of 50 is the default.
Minimum Value of Relative Dot Luminance: what is the least intense dot that can be displayed.
Maximum Value of Relative Dot Luminance: what is the most intense dot that can be displayed.
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.
Keep your eyes fixated on the red plus sign in the middle of the screen. If you use the modulus,
it will be a dot flashed above the fixation mark. The value of the modulus will be shown below
the fixation mark. Then the stimulus will be presented. This stimulus, a dot, will be flashed above this mark.
After the stimulus dot has been removed, a box will appear for you to type your judgment.
If
you have a keyboard, you can use the numbers to type your estimate of the stimulus. Back space
will remove the last
number typed and the delete key will clear the stimulus. Press the enter key when you are done. If you do not have a keyboard or wish to use your mouse,
you can use the number buttons after the Done button to enter the numbers. The B button is the back space and the
C button will act as the delete button. You can press the done button to go to the next trial.
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 different levels of intensity of the stimulus. On the y-axis will be the average magitude estimation of that intensity. To display the data showing the average magnitude estimation for each stimulus level, click or tap the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.