The Fechner Weber Principle

┬ę1997 Dennis Leri

WEBER-FECHNER PRINCIPLE: An approximate psychological law relating the degree of response or sensation of a sense organ and the intensity of the stimulus. The law asserts that equal increments of sensation are associated with equal increments of the logarithm of the stimulus, or that the just noticeable difference in any sensation results from a change in the stimulus which bears a constant ratio to the value of the stimulus.

In the bright midday sun you light a candle. Does anyone notice it getting brighter? Will you identify my voice if I call you on your cellular phone at a rock concert? You're carrying the downside of a refrigerator up a flight of stairs and someone puts a hammer on the fridge, do you sense the difference? Mostly, the Fechner Weber Principle or Law holds that you won't notice a difference. Moshe Feldenkrais invoked the Fechner Weber Law in discussing the necessity of reducing effort while learning. The Fechner Weber principle marked the beginning of the science of psychophysiology and yet all its implications have not been played out in that field.

Ernst Heinrich Weber (1795-1878) was the German anatomist and physiologist who first introduced the concept of the just-noticeable difference, that is, the smallest difference perceivable between two similar stimuli. Weber was a professor at the University of Leipzig from 1818 until 1871. He is known chiefly for his work on sensory response to weight, temperature, and pressure; he described a number of his experiments in this area in De Tactu (1834;"Concerning Touch"). Weber determined that there was a threshold of sensation that must be passed before an increase in the intensity of any stimulus could be detected; the amount of increase necessary to create sensation was the just-noticeable difference. He further observed that the difference was a ratio of the total intensity of sensation, rather than an absolute figure; thus, a greater weight must be added to a 100-pound load than to a 10-pound load for a man carrying the load to notice the change. Similar observations were made on other senses, including sight and hearing. Weber also described a terminal threshold for all senses, the maximum stimulus beyond which no further sensation could be registered.

Weber's findings were elaborated in Der Tastsinn und das Gemeingef├╝hl (1851; "The Sense of Touch and the Common Sensibility"), which was considered to be "the foundation stone of experimental psychology." Weber's empirical observations were expressed mathematically by Gustav Theodor Fechner, who called his formulation Weber's law.

Gustav Theodor Fechner (1801-1887) was a German physicist and philosopher and a key figure in the founding of psychophysics, the science concerned with quantitative relations between sensations and the stimuli producing them. At the age of 16 he enrolled in medicine at the University of Leipzig where he studied anatomy under Weber. No sooner had he received his medical degree, however, than his interest began to shift toward physics and mathematics.

Fechner's psychological interests began to manifest themselves toward the end of the 1830's in papers on the perception of complementary and subjective colors. In 1840, the year in which an article on subjective afterimages appeared, Fechner suffered a nervous collapse. Exacerbated by a painful injury to the eyes sustained while gazing at the sun during his research, Fechner's ailment manifested itself in temporary blindness and prostration. He resigned his position at Leipzig and went into a lengthy period of virtual seclusion during which his interests turned increasingly toward metaphysics. In 1848, the year of his return to the University as Professor of Philosophy, he completed Nanna, a metaphysical treatise that contains his first explicit, philosophical treatment of the problem of the relationship of mind to body.

In Nanna, and in the more important Zend-Avesta (1851), Fechner sketched out a dual-aspect, monistic, pan-psychical mind/body view. In a famous metaphor Fechner likened the universe, which is at one and the same time both active consciousness and inert matter, to a curve that can be regarded from one point of view as convex and from another as concave yet still retains its essential integrity. In line with this approach to mind/body, Fechner laid out a future program for psychophysics -- to demonstrate the unity of mind and body empirically by relating increase in bodily energy to corresponding increase in mental intensity.

Between 1851 and 1860, Fechner worked out the rationale for measuring sensation indirectly in terms of the unit of just noticeable difference between two sensations, developed his three basic psychophysical methods (just noticeable differences, right and wrong cases, and average error) and carried out the classical experiments on tactual and visual distance, visual brightness, and lifted weights that formed a large part of the first of the two volumes of the Elemente der Psychophysik. Fechner's aim in the Elemente was to establish an exact science of the functional relationship between physical and mental phenomena. Distinguishing between inner (the relation between sensation and nerve excitation) and outer (the relation between sensation and physical stimulation) psychophysics, Fechner formulated his famous principle that the intensity of a sensation increases as the log of the stimulus (S = k log R) to characterize outer psychophysical relations. In doing so, he believed that he had arrived at a way of demonstrating a fundamental philosophical truth: mind and matter are simply different ways of conceiving of one and the same reality.

While the philosophical message of the Elemente was largely ignored, its methodological and empirical contributions were not. Fechner may have set out to counter materialist metaphysics; but he was a well-trained, systematic experimentalist and a competent mathematician and the impact of his work on scientists was scientific rather than metaphysical. He combined methodological innovation in measurement with careful experimentation. Mental events could, Fechner showed, not only be measured, but measured in terms of their relationship to physical events. In achieving this milestone, Fechner demonstrated the potential for quantitative, experimental exploration of the phenomenology of sensory experience and established psychophysics as one of the core methods of the newly emerging scientific psychology. Later research has shown, however, that Fechner's equation is applicable within the mid range of stimulus intensity and then holds only approximately true.

He later delved into experimental aesthetics and sought to determine by actual measurements which shapes and dimensions are most aesthetically pleasing. He was also a proponent of panpsychism (from Greek pan, 'all'; psyche, 'soul'), a philosophical theory asserting that a plurality of separate and distinct psychic beings or minds constitute reality. Panpsychism is distinguished from hylozoism (all matter is living) and pantheism (everything is God). For Gottfried Wilhelm Leibniz, the 17th-century German philosopher and a typical panpsychist, the world is composed of atoms of energy that are psychic. These monads have different levels of consciousness: in inorganic reality they are sleeping, in animals they are dreaming, in human beings they are waking; God is the fully conscious monad.

In 19th-century Germany, Arthur Schopenhauer asserted that the inner nature of all things is will -- a panpsychistic thesis. And Gustav Theodor Fechner, the founder of experimental psychology and an ardent defender of panpsychism, contended that even trees are sentient and conscious. In the United States, Josiah Royce, an absolute idealist, not only followed Fechner in affirming that heavenly bodies have souls but also adopted a unique theory that each species of animal is a single conscious individual -- incorporating into itself the individual souls of each of its members.

So, now we are able to place the Fechner Weber Principle in its proper historical context. While the metaphysical implications of the principle were important to Fechner, its impact on his contemporaries was decidedly methodological. We can appreciate it as the first attempt to scientifically coalesce or imbricate the material and the mental. Specifying just noticeable differences in any sensation, that is, heavier, brighter, louder to be the result of a change in a stimulus bearing a constant ratio to the value of the stimulus the Fechner Weber Principle relates quantities to qualities. The Feldenkrais Method raises the question, when contemplating the Fechner Weber principle, just how is it that we can lower the background stimulation to enable us to detect just noticeable differences at lower thresholds. While learning with reduced effort is its own reward, somehow the different strata of our experience are reconfigured via a Feldenkrais lesson. In reconfiguring previous configurations we are face to face, so to speak, with the most intimate dynamical machinations of habit.

We know from our reading of Piaget that as we act so we sense, or even that action is cognition. Our experience, being grounded in the sensory motor substrate, is plastic and amenable to great variation. Sensory motoric operations are grounded in evolutionary processes. The habits of the species, the so called phylogenetic learnings and learning processes, make it possible to sequence and stratify our actions so that we can maintain sentience and participate in acculturation. The habits of a culture as enacted by each of us are the so called ontogenetic learnings. All the contingencies of life -- diet, locale, ancestors, etc. -- impact our personal history. Our personal history is encoded in the temporalizations and spatializations signified by what we attend to and what we can attend to. We live in the textures of the upsurge of phenomenal existence. The generic possibilities of bright or dark, hot or cold, wet or dry, smooth or rough, sudden or slow and so on are instantiated in the unexpected reflection of the sun in a window, the coolness of the morning fog, the dryness of the flour diminishing as it changes to dough and so on. Sensing differences is a function of the intensity of a stimulus relative to the intensity of the ongoing level of stimulation. Interpreting those differences makes them meaningful. No differences, no meaning.

By design, a Feldenkrais lesson evokes the archaic phylogenetic dynamics of organic learning. Those species specific processes are, in our personal history, often poorly integrated and socialized. Lessons resocialize them. By intelligently reshuffling the phylogenetic and the ontogenetic we can do more with less effort. We reset the change point at which we can detect just noticeable differences. New distinctions can be drawn because of newly differentiated sensory motoric operations. Our attention is drawn to different differences. Thresholds below which we perceive nothing and above which perceive something are shifted.

If philosophers forever ponder the question "Why is there something rather than nothing?" thinkers and researchers from Weber to Fechner to Moshe Feldenkrais have begun to ask and answer the question "How there is something rather than nothing?" In-habiting the world means living in it. A habitat is a house. The Fechner Weber Principle is a habit our species uses to live in this world. The various set points of background stimulation to emergent percept are established by us as learned habits. As Feldenkrais practitioners we can, through the means at our disposal, use our species specific set of habits to reorganize our socially acquired habits. Learning is habit forming.