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Types of threshold
Several different sensory thresholds have been defined;
- Absolute threshold: the lowest level at which a stimulus can be detected.
- Recognition threshold: the level at which a stimulus can not only be detected but also recognised.
- Differential threshold the level at which an increase in a detected stimulus can be perceived.
- Terminal threshold: the level beyond which a stimulus is no longer detected.
A threshold (or limen), is the point of intensity at which the participant can just detect the presence of a stimulus. Stimuli with intensities below the threshold are not detectable. However, a subject does not detect the same stimulus every time; thus thresholds are considered as an average of trials rather than an absolute limit (Snodgrass, 1975). Because of this, in experimentation the proportion of trials at a given stimulus which the subject reports detecting the stimulus is known as p.
There are two kinds of thresholds: absolute and difference. An absolute threshold is the level in some property of a stimulus at which the subject is able to detect the presence of the stimulus some proportion of the time (p; 50% is often used). An example of an absolute threshold is inability to feel the brush of a single hair on the back of the hand, though touching the same place with several hairs may be detectable - that is, it may exceed the threshold.
A difference threshold is the magnitude of the difference between two stimuli of differing intensities that the subject is able to detect some proportion of the time (again, 50% is often used). To test this threshold, several difference methods are used. The subject may be asked to adjust one stimulus until it is perceived as the same as the other, may be asked to describe the magnitude of the difference between two stimuli, or may be asked to detect a stimulus against a background.
Absolute and difference thresholds are sometimes considered similar because there is always background noise interfering with our ability to detect stimuli (Snodgrass, 1975), however study of difference thresholds still occurs, for example in pitch discrimination tasks.
Psychophysical experiments have traditionally used three methods for testing subjects' perception in stimulus detection and difference detection experiments: the method of limits, the method of constant stimuli, and the method of adjustment (Snodgrass, 1975).
Method of limits
Wilhelm Wundt invented the method of limits. The subject reports whether he or she detects the stimulus. In ascending method of limits, some property of the stimulus starts out at a level so low that the stimulus could not be detected, then this level is gradually increased until the participant reports that they are aware of it. For example, if the experiment is testing the minimum amplitude of sound that can be detected, the sound begins too quietly to be perceived, and is made gradually louder. In the descending method of limits, this is reversed. In each case, the threshold is considered to be the level of the stimulus property at which the stimuli is just detected.
In experiments, the ascending and descending methods are used alternately and the thresholds are averaged. A possible disadvantage of these methods is that the subject may become accustomed to reporting that they perceive a stimulus and may continue reporting the same way even beyond the threshold (the error of habituation). Conversely, the subject may also anticipate that the stimulus is about to become detectable or undetectable and may make a premature judgment (the error of expectation).
To avoid these potential pitfalls, Georg von Bekesy introduced the staircase method in 1960 in his study of auditory perception. In this method, the sound starts out audible and gets quieter after each of the subject's responses, until the subject does not report hearing it. At that point, the sound is made louder at each step, until the subject reports hearing it, at which point it is made quieter in steps again. This way the experimenter is able to "zero in" on the threshold.
Method of constant stimuli
Instead of being presented in ascending or descending order, in the method of constant stimuli the levels of a certain property of the stimulus are not related from one trial to the next, but presented randomly. This prevents the subject from being able to predict the level of the next stimulus, and therefore reduces errors of habituation and expectation. The subject again reports whether he or she is able to detect the stimulus.
Method of adjustment
Also called the method of average error, the method of adjustment asks the subject to control the level of the stimulus, instructs them to alter it until it is just barely detectable against the background noise, or is the same as the level of another stimulus.
In discrimination experiments, the experimenter seeks to determine at what point the difference between two stimuli, such as two weights or two sounds, is detectable. The subject is presented with one stimulus, for example a weight, and is asked to say whether another weight is heavier or lighter (in some experiments, the subject may also say the two weights are the same). At the point of subjective equality (PSE), the subject perceives the two weights to be the same. The just noticeable difference (JND), or difference limen (DL), is the difference in stimuli that the subject notices some proportion p of the time (50% is usually used for p).
The methods of limits, constant stimuli and adjustment can be used in difference detection by asking the subject to detect a difference between stimuli rather than detect a single stimulus.
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