The Law of Audiometric Assessment

The one course that challenges most audiology students no matter where in the world they study is Psychoacoustics.  Psychoacoustics and the Just Noticeable Differences (JNDs) and Just Not Noticeable Differences (JNNDs) have mystified audiologists, otolaryngologists, and hearing researchers for over 150 years.  Where did JNDs come from?  Who started these concepts?  These are the questions for this week’s Hearing International.

Just so happens that on January 26, 2013 audiologists, otolaryngologists, and hearing researchers will remember the 135th anniversary of the death of Ernst Heinrich Weber, inventor of the JND, the JNND and other psychophysical measurements.  Additionally, January 18 marks the 40th anniversary of the death of one of the 20th Century’s most celebrated Experimental Psychologists, (the late) Stanley Smith (S.S.) Stevens of Harvard University. Dr. Stevens’ research and that of his colleagues at the Harvard Psycho-Acoustic Laboratory was instrumental in establishing the basis for today’s audiological testing; a couple of centuries earlier, Dr. Weber is said to have begun the field of Experimental Psychology with his research into the senses and particularly into hearing. Boring (1950) states that by coining the phrase, just noticeable difference (JND) to refer to the smallest perceptible difference between two sensations, Weber amassed 19th century data in support of the general principle that a JND in the intensity of a sensation is a function of the change in the magnitude of a stimulus by a constant factor of its original magnitude. Although it has since been shown that there are significant limitations in the generality of this relationship–not only across other sensory systems but even within touch itself–it would be hard to overstate the importance of Weber’s discovery for the emerging science of experimental psychology.  In articulating these threshold discrimination measurements relationship Gustav Fechner later termed “Weber’s Law,” as providing an existence of proof for the possibility of establishing quantitative relationships between variations in physical and mental events. Fechner’s Law is actually a method of explaining Weber’s Law.

While S.S. Stevens modified Weber/Fechner Law into Stevens Power Law in 1957, it was Weber who began the actual science and the study of psychacoustic phenomenon.  Weber made important discoveries about the sense of touch and invented the idea of the just-noticeable difference between two similar physical stimuli. A product of his invention of JNDs was the founding of the field of psychophysics, the branch of psychology that studies the relations between physical stimuli and mental states.  Since audiology has its beginnings in the field Experimental Psychology, it is fitting to honor these anniversaries by taking a short look at Weber and his discoveries.

Ernst Heinrich Weber was born June 24, 1795, in Wittenberg, Germany, as the 3rd of 13 children. His father, Michael Weber, was a professor of theology at the University of Wittenberg.   Michael Weber was no doubt a Lutheran as the University of Wittenburg is where Martin Luther began his movement in the 1500s.  In fact the town is actually called Lutherstadt Wittenberg after Luther.  Childhood was a busy time for young Ernst as there were lots of children in the house, academic conversations, and much interest in physics as well.  Childhood is important to all of us but was very influential to the founder of the field of psychophysics.  While young Ernst and brothers Eduard and Wilhelm were growing up, their house was also occupied by Ernst Florens Friedrich Chladni, later to be known as the the “Father of Acoustics.” A physicist at the University of Leipzig, Chladni’s important works include research on vibrating plates and the calculation of the speed of sound for various gases.  The boys interaction with Chladni excited their interest in physics as a basis of all natural sciences and obviously the home environment was an influence for Ernst as well as his brothers.  Ernst, Eduard and Wilhelm worked together on their research at the University of Leipzig.  While Ernst was known for his psychphysical research, Eduard went on to contributing to knowledge to circulation of blood and proved the inhibitory power of the Vagus Nerve, while brother Wilhelm contributed to knowledge of acoustics and sound wave motion.  Who knows, without the influence of Chladni there may never have been JNDs and JNNDs.

Weber learned Latin in secondary school, and began to study medicine in 1811 at the University of Wittenberg. He received his doctor of medicine degree in 1815, specializing in comparative anatomy, and became a lecturer at the University of Leipzig in 1817; he was promoted to professor of anatomy the following year. At Leipzig, Weber became assistant at the medical clinic run by J. C. Clarus, qualified as docent in 1817 with a work on the comparative anatomy of the nervus sympathicus, and the following year became extraordinary professor of comparative anatomy. In 1821 he was nominated to the chair of human anatomy, which in 1840 was joined with physiology. In 1865 he gave up physiology and supported the appointment of Carl Ludwig, who established an independent physiological insitute that attracted many foreign students. In 1871 Weber retired from the chair of anatomy at the University of Leipzig.  Weber made his name studying touch, pain, sight, hearing, taste, and smell. He was one of the first psychologists to really experiment. Unlike many of his contemporaries, he did not just sit at a desk and speculate about human mental states and perceptions. Instead, he, with the assistance of his brothers Eduard and Wilhelm, actually tested human subjects to discover how they reacted to physical stimuli. He published the results of many of his experiments about touch in De Tactu in 1834.

So, Ernst Weber and brothers Eduard and Wilhelm developed and expanded the notion of JNDs and JNNDs, which today audiologists use as the basis of audiometric threshold measurement.  The contributions of Weber, his brothers and the refinement of his theories by Fechner, Stevens and others greatly contributed to the development of a field where  JNDs and JNNDs in hearing are assessed everyday.

A Hearing International salute to those that set the basis for our profession some 150 years ago and to those who refined his basic experiements into a real assessment technique.