 |
It has been suggested that Fechnerian psychophysics be merged into this article or section. () |
Psychophysics is a subdiscipline of psychology dealing with the relationship between physical stimuli and their subjective correlates, or percepts. Psychophysics has been described variously as “the scientific study of the relation between stimulus and sensation”[1] or, more completely, as “the analysis of perceptual processes by studying the effect on a subject’s experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions.”[2] It is a general-purpose theory that can be applied to any sensory system. The techniques of "classical" or Fechnerian psychophysics are still widely used, although the theoretical background is heavily influenced by Signal detection theory.[3]
Contents |
History
Many of the classical techniques and theory of psychophysics were formulated in 1860 when Gustav Theodor Fechner published Elemente der Psychophysik.[4]. He coined the term "psychophysics", described research relating physical stimuli with how they are perceived, and set out the philosophical foundations of the field. Fechner wanted to develop a theory that could relate matter to the mind, by describing the relationship between the world and the way it is perceived. He was influenced by the work of German physiologist Ernst Heinrich Weber [5][6] Fechner's work formed the basis of psychology as a science. Wilhelm Wundt, the founder of the first laboratory for psychological research, built upon Fechner's work.
One author[7] has argued that the medieval scientist Alhazen should be considered the "founder of psychophysics." Although al-Haytham made many subjective reports regarding vision, there is no evidence that he used quantitative psychophysical techniques and such claims have been rebuffed.[8]
Psychophysicists usually employ experimental stimuli that can be objectively measured, such as pure tones varying in intensity, or lights varying in luminance. All the senses have been studied: vision, hearing, touch (including skin and enteric perception), taste, smell, and the sense of time.
Regardless of the sensory domain, there are three main topics in the psychophysical classification scheme: absolute thresholds, discrimination thresholds, and scaling.
The most common use of psychophysics is in producing scales of human experience of various aspects of physical stimuli. Take for an example the physical stimulus of frequency of sound. Frequency of a sound is measured in hertz, cycles per second. But human experience of the frequencies of sound is not the same as the frequencies. For one thing, there is a frequency below which no sounds can be heard, no matter how intense they are (around 20 Hz depending on the individual) and there is a frequency above which no sounds can be heard, no matter how intense they are (around 20,000 Hz, again depending on the individual). For another, doubling the frequency of a sound (e.g., from 100 Hz to 200 Hz) does not lead to a doubling of experience. The perceptual experience of the frequency of sound is called pitch, and it is measured by psychophysicists in mels.
More analytical approaches allow the use of psychophysical methods to study neurophysiological properties and sensory processing mechanisms. This is of particular importance in human research, where other (more invasive) methods are not used due to ethical reasons.
Areas of investigation include sensory thresholds, methods of measurement of sensitivity, and signal detection theory.
Thresholds
A threshold (or limen), is the point of intensity at which the participant can just detect the presence of, or difference in, a stimulus. Stimuli with intensities below the threshold are considered not detectable, however stimuli at values close to threshold will often be detectable some proportion of the time. Due to this, a threshold is considered to be the point at which a stimulus, or change in a stimulus, is detected some proportion p of the time. There are two kinds of thresholds: absolute[9] and difference.[6]
An absolute threshold is the level of intensity of a stimulus at which the subject is able to detect the presence of the stimulus some proportion of the time (a p level of 50% is often used). An example of an absolute threshold is the number of hairs on the back of one's hand that must be touched before it can be felt - a participant may be unable to feel a single hair being touched, but may be able to feel two or three as this exceeds the threshold.
A difference threshold is the magnitude of the difference between two stimuli of differing intensities that the participant is able to detect some proportion of the time (again, 50% is often used). To test this threshold, several different 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, however study of difference thresholds still occurs, for example in pitch discrimination tasks.[10][5]
Discrimination
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.
Experimentation
In psychophysics, experiments seek to determine whether the subject can detect a stimulus, identify it, differentiate between it and another stimulus, and describe the magnitude or nature of this difference.[5][6]
Classic Methods of experimentation
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.[11]
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.[11]
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 procedure 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.[11]
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.[11]
References
- ^ Gescheider G (1997). Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates, ix.
- ^ Bruce V, Green P R, Georgeson M A (1996). Visual perception, 3rd, Psychology Press.
- ^ Gescheider G (1997). "Chapter 5: The Theory of Signal Detection", Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates.
- ^ Gustav Theodor Fechner (1860). Elemente der Psychophysik (Elements of Psychophysics).
- ^ a b c Snodgrass JG. 1975. Psychophysics. In: Experimental Sensory Psychology. B Scharf. (Ed.) pp. 17-67.
- ^ a b c Gescheider G (1997). "Chapter 1: Psychophysical Measurement of Thresholds: Differential Sensitivity", Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates.
- ^ Omar Khaleefa (1999). "Who Is the Founder of Psychophysics and Experimental Psychology?". American Journal of Islamic Social Sciences 16 (2).
- ^ Aaen-Stockdale, C.R. (2008). "Ibn al-Haytham and psychophysics". Perception 37 (4): 636–638. doi:.
- ^ Gescheider G (1997). "Chapter 2: Psychophysical Measurement of Thresholds: Absolute Sensitivity", Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates.
- ^ Gescheider G (1997). "Chapter 4: Classical Psychophysical Theory", Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates.
- ^ a b c d Gescheider G (1997). "Chapter 3: The Classical Psychophysical Methods", Psychophysics: the fundamentals, 3rd, Lawrence Erlbaum Associates.
|
|||||||||||||||||||||||
Wikipedia content modification information:
- This page was last modified on 9 August 2008, at 19:57.
Wikipedia Authorship and Review
Wikipedia content provided here is not reviewed directly by PediaView.com. Wikipedia content is authored by an open community of volunteers and is not produced by or in any way affiliated with PediaView.com.
Wikipedia Usage Guidelines
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article on "Psychophysics".
The URL for this specific entry is:
All Wikipedia text is available under the terms of the GNU Free Documentation License. (See Copyrights for details). Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc.