| ISLE | Chapter 2
| Experiment: Forced-Choice Method: Dot Threshold
The forced-choice methods were developed as a means to deal with problems associated with traditional psychophysical methods.
In the traditional methods, participants can guess or even alter their response levels to appear more or less sensitive
than they are, and the experimenter would not know. So, in forced-choice methods, the structure of the trial changes.
In the most
basic forced-choice experiment, the stimulus can be in one of two places or in one of two intervals.
Then, instead of asking you if you see or hear the stimulus, you are asked where or when the stimulus occurred.
If you do not know when or where the stimulus is, you have to guess. That is the forced in forced-choice.
In this experiment, the forced-choice is paired with a method of constant stimuli experiment. The way the stimuli
are chosen follows the method of constant stimuli while the trials are run, according to forced-choice. This change
leads to a bit of problem. In method of constant stimuli, the threshold is determined as the point where you
detect the stimulus 50% of the time. Well, if you are guessing and there are only two options, when you cannot really
detect the stimulus, you will be correct 50% of the time. To adjust for this issue, the threshold is determined as the
stimulus intensity you correctly detect 75% of the time in a forced-choice experiment.
To review method of constant stimuli, look here.
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:
Type of Method of Constant Stimuli: Traditional or Forced-Choice. In Traditional,
you be
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. Forced-choice is default in this version of the experiment.
Number of Levels of Relative Dot Luminance (display values): 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 (display values): what is the least intense dot to be displayed.
Maximum Value of Relative Dot Luminance (display values): 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.
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.
Traditional Method Instructions: After the dot has been presented, buttons will
appear, asking if you have see 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. 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 correctly reported detecting the stimulus postion (for forced-choice) for each intensity. To see your threshold, which is the 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 each stimulus intensity click the Show Data button.
Change the settings below to alter the stimulus parameters in this experiment.