| ISLE | Chapter 2
| Experiment: Forced-Choice Method: Frequency Discrimination
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 frequency differences. The settings include the following:
Standard Frequency: the frequency of the standard tone or how fast the sound cycles, in Hertz (Hz) or cycles per second.
Tone Duration: how long the tones in the experiment will play, in seconds.
Gain: the relative intensity of the sound (1.0 is full intensity and 0 is that the sound is off).
Time between the tones: the blank interval between when the first and second tones, in seconds.
Play: allows you to play the current version of the standard tone.
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 Frequency Difference: how many steps there are in the staircase (series).
Number of Repetitions: how many staircases (series) will be presented (always an even number).
Minimum Value of Frequency Difference: what is the least frequency difference between the standard (lower frequency tone) and the target (higher frequency tone).
Maximum Value of Frequency Difference: what is the greates frequency difference between the standard (lower frequency tone) and the target (higher frequency tone).
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. Two tones will be played. One of the tones will have a higher frequency and may sound to have a higher pitch. Click or press First (or press the Z key) if you hear that the first tone sounds higher in pitch and Second (or press the / key) if you hear that the second tone has a higher pitch. If you cannot tell the difference, make your best guess. 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.