| ISLE | Chapter 2
| Experiment: Method of Constant Stimuli: Frequency Discrimination
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 motion of the stimulus to see how this might affect your ability to detect 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 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 greatest 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. The first is the standard tone, and the second is the target tone. Click or press Yes (or press the Z key) if you hear that the second tone sounds higher in pitch and No (or press the / key) if you do not hear that the second tone has a higher pitch. 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 frequency difference 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 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.