| ISLE | Chapter 2
| Experiment: Method of Limits: Dot Detection
In the method of limits, stimuli are presented in a graduated scale, and participants must judge whether they detected the stimulus or not. The researcher hopes to pick a value that is readily detected and a value that is never detected and then several values between. These values are then presented in order, going from the most intense to the least intense, a descending staircase, and from least intense to most intense, an ascending staircase.
In this experiment, we will start with a descending staircase. Since the most intense stimulus is readily detectable, you should have no trouble responding with a yes you detected it. The next stimulus to be presented will be the next most intense. If you detect that stimulus, you will again respond with a yes. Then the next most intense stimulus will be presented. This series will be continued until you respond with a no you did not detect the stimulus. This response is called a crossover point as you have crossed from detection over to nondetection.
Then a descending staircase will begin with the least intense stimulus. You should not be able to detect in and respond with a no you did not detect it. Then the next more intense stimulus in the series will be presented. This series will continue until you respond that you did detect the stimulus. This response is also a crossover response as you have crossed from nondetection over to detection.
These two types of staircases, descending and ascending, will continue until you have completed all that the experiment requires. Your threshold is the average of these crossover points.
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 steps there are in the staircase (series).
Number of Staircases: how many staircases (series) will be presented (always an even number).
Minimum Value of Relative Dot Luminance: what is the least intense dot to be displayed.
Maximum Value of Relative Dot Luminance: what is the most intense dot to 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. A dot will be presented above this mark. After it has been presented, buttons will appear asking if you have seen the dot. Click or press Yes (or press the 'z' key) if you have and No (or press the '/' key) if you have not seen the dot. When the experiment has been completed, you will be instructed that you can view your results.
Your data will be presented on this tab where the trials will be on the x axis and the intensity value of the stimulus will be displayed on the y axis. To see your threshold, which is the average of the crossovers, click the button that says Show Threshold. To display the trial by trial data, click the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.