Ewald Hering

Early years

Hering was born in Altgersdorf in Saxony, Germany. He grew up in a probably poor family, son of a Lutheran pastor. Hering attended gymnasium in Zittau and entered the university of Leipzig in 1853. There he studied philosophy, zoology and medicine. He completed an M.D. degree in 1860.

It is somewhat unclear how Hering trained to do research. At the time Johannes Müller was perhaps the most famous physiologist in Germany. Hering seems to have applied for studying under his direction but was rejected (ref needed), which might have contributed to his animosity towards von Helmholtz, Müller's protégé. However, in Leipzig E. H. Weber and G. T. Fechner were conducting groundbreaking studies founding what would become the field of psychophysics. Although there is no evidence that Hering ever studied under their direction, in his later years he will proudly acknowledge himself a "student of Fechner".

After graduating, he practiced as a physician in Leipzig. With little time to do research and even scarcer financial resources, he turned to binocular vision and the problem of the horopter. There, he surprised the scientific community when he published, as a completely unknown scientist, his own mathematical derivation of the horopter independently from Hermann von Helmholtz, who was by then considered one of the best German scientist and mathematician. Hering went as far as ridiculing Helmholtz's (unimportant) mathematical errors in his derivation of the horopter.

University posts

Hering was subsequently appointed professor of physiology at the military academy of Vienna until 1870. With better resources he conducted important studies in physiology, in particular on the cardiac and respiratory systems. In 1870, he succeeded Purkinje at the university of Prague where he remained for the next 25 years. There he became involved in fierce arguments between nationalistic Czechs who wanted the university taught in the language of the land, and a minority of German professors. Eventually a separate German university was created in 1882 and Hering became its first rector.

In his late years, Hering returned to Germany where he became professor at the university of Leipzig in 1895, aged 61. He retired in 1915 and died of tuberculosis 3 years later. He was an atheist.

Binocular vision

Hering studied binocular vision. He derived, almost simultaneously with Helmholtz, the theoretical shape of the horopter. Despite identical results, Hering's derivation was far more modern, using recently developed projective geometry, and elegant. Helmholtz himself qualified Hering's approach of "very elegant, comprehensive and complete". Subsequently, Hering empirically estimated the shape horopter. He noticed, alongside with Helmholtz and Hillebrand, that the empirical horopter does not perfectly match the theoretical horopter, a phenomenon now named the Hering–Hillebrand deviation.

He is also famous for the Hering's law of visual direction which describes the perceived egocentric direction of an object in function of the direction of this object in the two retinaes of the two eyes. Unbeknownst to Hering, and other visual scientists of the time, a similar law had been proposed by Alhazen in the 10th century and by Wells in the 18th century, although both their laws were different and not completely correct.

Eye movements

Hering then studied eye movements. He developed the Hering's law of equal innervation to describe the conjugacy of eye movements in animals. According to this law eye movements are always equal in intensity in the two eyes but not in direction. Eye movements can therefore be either conjugate (in the same direction such as saccades or smooth pursuit) or disjunctive (such as vergence eye movements). Hering's law of equal innervation is best described by Müller's stimulus where the fixation point changes position in 1 eye but not the other eye. Simplicity conducts that only the misaligned eye should move to refoveate. Hering's law predicts that because the eyes must always move by equal amounts, both eyes should move in the new binocular direction of the target (see Hering's law of visual direction above), then move in opposite direction to adjust vergence to that of the target. In other words, the eye in which the target did not move will move away and then back at the target. This prediction was experimentally confirmed by Yarbus in his seminal work on eye movements. However it is now known that strong deviations from Hering's law exist.

Color theory

Hering disagreed with the leading theory developed primarily by Thomas Young, James Clerk Maxwell and Hermann von Helmholtz. Young proposed that color vision is based on three primary colors: red, green, and blue. Maxwell demonstrated that any color can be matched by a mixture of 3 primary colors. This was interpreted by Helmholtz as proof that humans perceive colors through 3 types of receptors, while white and black would reflect the amount of light.

Hering instead believed that the visual system worked based on a system of color opponency, though he had little empirical evidence to support this claim. In this model, colors are perceived through receptors sensitive to three couples of opponent colors: red-green, yellow-blue and white-black. Later, Erwin Schrödinger demonstrated that the first and the second model are mathematically equivalent.

We now know that if the human eye possesses 3 types of receptors as proposed by Young, Maxwell, and Helmholtz, they then combine in 3 opponent channels as proposed by Hering. In this way, both the Hering and Helmholtz theories are correct.

Physiology

Hering made significant contributions to the field of physiology as well as psychology. In particular he demonstrated with his student Breuer the Hering–Breuer reflex, or that artificially inflating the lungs triggers an automatic signal triggering expiration. Then deflating the lungs in turns triggers a new signal inducing respiration. I.e. inspirations and expirations are an endless reflex loop triggering each other. He also showed the Traube-Hering reflex, or that inflating the lungs triggers an acceleration of the heart.

Other research

In 1861, Hering described an optical illusion which now bears his name – the Hering illusion. When two straight and parallel lines are presented in front of radial background (similar to the spokes of a bicycle), the lines appear as if they were bowed outwards. The Orbison illusion is one of its variants, while the Wundt illusion produces a similar, but inverted effect.

Hering first suggested the idea of organic memory in an 1870 lecture for the Imperial Academy of Science in Vienna. Hering took influence from the idea of inheritance of acquired characteristics and suggested that memories could be passed on through generations by germ cells.

The Hering–Helmholtz controversy

Hering spent most his life arguing violently with Helmholtz. The controversy was not only scientific but also philosophical; Hering was a nativist, Helmholtz an empiricist. Helmholtz also came from a higher social class and was always considered a prodigy, while Hering had to go through a very hard time in his early career. Hering and Helmholtz disagreed on almost everything and the controversy lasted long after the end of both of their lives. Hering however was by far the most aggressive of the two, and was always prompt to point out any mistake that Helmholtz might have made, sometimes even going so far as to insult him ("It is likely that the great Helmholtz in his dozing state..."). Helmholtz's faction (though probably not Helmholtz himself) spread rumors which accorded to Hering the need for help to do his mathematical work and that he was clinically insane ("He has been, as I have heard, mentally ill").

Publications