# Psychophysics

The psychophysics is one of the history of science is the oldest psychological research. It refers to the legitimate interaction between subjective psychological (mental) experience and quantifiable, so objective physical stimuli as the triggering processes. Psychophysics was founded in 1860 as a branch of experimental psychology by Gustav Theodor Fechner and is based on the preliminary work of Ernst Heinrich Weber . It is sometimes viewed as a sub-area of sensory physiology , perceptual psychology or psychophysiology . Sometimes also with aesthetic questions of taste .

(1) External psychophysics
(2) Internal psychophysics
(3) Physiology

The distinction between external and internal psychophysics goes back to Fechner . While external psychophysics measures the connection between irritation of the sensory organs and experience, internal psychophysics deals with the relationships between neuronal processes and experience. In most cases, “psychophysics” is primarily understood to be the investigation of stimulus-experience contexts, while the area of ​​internal psychophysics is now assigned to cognitive neuroscience . These assignments result from the fact that Fechner was only able to postulate internal psychophysics in theory, but was unable to research it due to a lack of neuroscientific methods .

## history

The discovery of a constant relationship between the length of the strings of a lyre and the basic chords of music, attributed to Pythagoras (approx. 570–510 BC), is based on the legend of Pythagoras in the smithy . Nevertheless, the term Pythagorean tuning has been retained for the tuning based on the fifth as the most harmonious interval , cf. Circle of fifths . The tone intervals designated as harmonic have the following vibration ratios: octave 1: 2, fifth 2: 3, fourth 3: 4 and major third 4: 5. With these remarks, the bow is already curious about aesthetic issues such as B. those of the golden section , on which Fechner has also written empirical studies.

One of the earliest approaches to psychophysical scaling comes from Claudius Ptolemy (approx. 100–175 AD) around 150 AD. He suggested a useful measure for the size of the stars based on the apparent brightness of the stars.

## Sub-areas

In addition to the distinction between internal and external psychophysics, one can differentiate the work areas with regard to the sensory modalities. Many psychophysical works are specialized in one sense - for example on visual or auditory perception (see also psychoacoustics ), the sense of smell or touch  . In addition, one can differentiate between four different question areas:

1. On the one hand, the perception threshold (absolute threshold) can be examined. It is researched how strong the stimulation of a given sense organ must be for a reaction to take place. Due to adaptation effects , the perception threshold can vary considerably in different contexts .
2. Furthermore, the stimulus differentiation can be examined. How different do two stimuli have to be so that they are perceived as different in a given context ( difference threshold )? Using the uncertainty interval , physically different stimuli are defined that trigger the same sensation. When it comes to color perception , one also speaks of metamerism .
3. Another topic is stimulus recognition . One can ask, for example, when a blue triangle is recognized as a blue triangle. As a rule, the presence of a stimulus is registered (stimulus detection) before identification is possible.
4. A final area of ​​investigation is the scaling , which not only examines whether a person has recognized a stimulus, but whether they can estimate how strong the stimulus is. The scaling capabilities can vary significantly in different situations.

## Psychophysical Laws

Ernst Heinrich Weber (1795–1878)

Three classic laws have been formulated in psychophysics. The Weber's Law describes the observation that the difference threshold is in an almost fixed ratio to the stimulus intensity: The stronger the stimulus, the greater must be the stimulus difference to notice this difference. The formal description is:

${\ displaystyle {\ frac {\ Delta R} {R}} = const. = k}$

It stands for the stimulus difference and for the stimulus. The quotient is also known as Weber's fraction . An example: With k = 1/10 , with a stimulus of 10 units , you need a stimulus difference of one unit to notice it. With a weight of 20 g you would therefore need 2 g to register a difference. With a weight of 20 kg, on the other hand, there would be 2 kg fluctuation. ${\ displaystyle \ Delta R}$${\ displaystyle R}$

Fechner's law (also known as the Fechner scale ) describes the relationship between stimulus and experience intensity:

${\ displaystyle E = k \ cdot \ log R + f}$

${\ displaystyle E}$represents in the formula the sensitivity level, the stimulus level. The sensitivity corresponds to the logarithm of the stimulus intensity, multiplied by a constant , to which a further (small) constant is added. In other words, Fechner's law says that the intensity of sensation (approximately and in a suitable range) increases with the logarithm of the intensity of the stimulus. A doubling of the stimulus strength does not result in a doubling of the sensitivity, but only an increase of . Fechner derived his scale from Weber's law, using the postulate that the difference threshold (which varies depending on the stimulus intensity) corresponds to a constant increase in sensation. The sensitivity is then the integral of Weber's fraction. Like all sensory / perception psychological laws, Fechner's law only applies within a certain area of ​​application. Fechner scales are used, for example, for volume measurement (as sound pressure level in dB ) and brightness measurement . ${\ displaystyle R}$${\ displaystyle k \ cdot 30 \%}$

Exemplary course of an experiment with an adaptive threshold determination method (here the so-called transformed staircase method). Up to the first reversal point (which is not counted here) a larger step size and the simple-up / down-staircase method are used so that you can quickly move into the area of ​​the threshold.

In the middle of the 20th century, Stanley S. Stevens (1957) found a way to directly measure sensitivity levels. To this end, he introduced various methods of so-called magnitude estimation. The sensitivity levels found using these methods can be well described over a wide range by a power function ( Stevens power function ):

${\ displaystyle G = k \ cdot R ^ {m}}$

The exponent represents the modality-specific power, which is less than one for most senses (with the exception of pain and the sensation of length) and the subjectively estimated size of an attribute. According to Stevens, the average estimate of the size of an attribute applies roughly as a power function of the intensity of the stimulus. Stevens' law is often shown in a double-logarithmic representation; in it the connection is a straight line, the slope of which is given by the exponent . ${\ displaystyle m}$${\ displaystyle G}$${\ displaystyle m}$

In most of the representations, Steven's law is presented as the more correct compared to Fechner's law. However, this common textbook opinion is also contradicted.

## Science and philosophy-historical significance

Psychophysics developed very early in relation to other cognitive science disciplines. Weber and Fechner's central works date from the mid-19th century. Her systematic and empirical research into stimulus-experience relationships has had a great influence on many scientists and philosophers. Examples are Hermann von Helmholtz and Ernst Mach , Max Weber and Wilhelm Wirth . Sigmund Freud continued the principle of psychic energy introduced by Fechner ( psychodynamics ). Fechner's theory of sensation touches on the body-soul problem which is central to the philosophy of spirit . So psychophysics was one of the science historically important pioneers of a scientific study of consciousness processes .

At the same time, Fechner's work also offers a critical-skeptical perspective on the natural sciences. As a critic of materialism, Fechner was not of the opinion that mental events can be reduced to physical events . Nevertheless, he tried to establish connections between these events with psychophysics. Fechner's methodological approach can also be found today in part among neuroscientists who are looking for neural correlates of consciousness without wanting to make a metaphysical statement about the nature of consciousness.

## literature

• Gustav Theodor Fechner: Elements of psychophysics . 1860.
• Walter H. Ehrenstein : Psychophysics . In: Lexicon of Neuroscience. Volume 3, Spektrum Verlag, Heidelberg / Berlin 2001.
• SS Stevens: Psychophysics . New York 1975, OCLC 164635229 .
• Max Weber: On the psychophysics of industrial work. In: Collected essays on sociology and social policy. Tübingen 1988, ISBN 3-16-845371-4 , pp. 61-255.
• Manuel Kühner, Heiner Bubb, Klaus Bengler, Jörg Wild: Adaptive procedures in psychophysics - efficient determination of absolute and difference thresholds. In: Ergonomics up-to-date. No. 13, 2012. ( www.lfe.mw.tum.de ( Memento from September 4, 2014 in the Internet Archive ))