| ISLE | Chapter 2
| Experiment: Method of Adjustment: Dot Threshold
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. In a light threshold study, the method of adjustment would ask the observer to adjust the light source to be the dimmest light that person can just detect. The advantage of this technique is that it can quickly yield a threshold for each participant, but a disadvantage is that it leads to great variance from one participant to the next and between successive trials for each participant.
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 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 Trials in the Experiment: how many times you will make your adjustment.
Range of Variation of Relative Dot Luminance: 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.
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 Space button on
the screen to start the experiment.
Keep your eyes fixated on the red plus sign in the middle of the screen. A dot will be presented
above this mark.
At the same time that the dot appears, a slider will appear below the stimulus area. Use the slider
to increase
or decrease the intensity of the dot.
Your task is to adjust the dot so that you can just see it.
When you have accomplished this goal, click or tap the Done 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 Done button.
When you have finished all of your trials, you will instructed that you can view your results.
Your data will be presented on this tab. On the x-axis is 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 threshold, click the button that says Show Threshold. 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.