The method of constant stimuli means that threshold is determined by presenting the observer with a set of stimuli of which
some are above the threshold and of which some are below the
threshold but that the set of stimuli are presented in a
random order. This differs from the method of limits where stimuli are presented in fixed orders.
In the method of constant stimuli, you always present all stimuli.
This technique prevents the observer from being able to predict or anticipate what the next stimulus will be.
However, the method of constant stimuli can be quite time-consuming.
In the method of constant stimuli, the stimulus that is detected 50% of the time and not detected 50% of the time
is considered to be the threshold (see the figure, Figure 2.4 from the book).
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 type of stimulus and 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:
Type of Method of Constant Stimuli: Traditional or Forced-Choice. In Traditional,
asked if the stimulus is present. In Forced-Choice, you will be asked if the stimulus is in one of two locations or one of two
time periods. For more on Forced-Choice see here.
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.
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.
Traditional Method Instructions: After the dot 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.
Forced-Choice Method Instructions: In addition to the dot being above the fixation mark, it will be either to the right or to the left of the fixation mark. After the dot has been presented, buttons will appear asking if the dot is to the left or right of the fixation mark. Click the proper button, or press Z for left or the / key for right. If you do not know which side the dot appeared you must guess.
You 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 proportion of trials where you reported detecting the stimulus for each intensity. To see your threshold, which is determined by a simple linear interpolation method, click the button that says Show Threshold. The value will be shown below the graph and highlighted on the graph. To display the data showing the proportion of trials you detect for each stimulus intensity, click the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.