|Figure 1. J. Richard Ewald, 1855-1921, a portrait by his daughter. From (Jongkees 1966)|
J.R. Ewald (2/14/1855-7/22/1921) was a German physiologist to whom we owe important insights concerning vestibular function. He was born in Berlin, the son of painter Arnold Ewald. In 1880, he started to work in the Strassburg physiological laboratory of Friedrich Leopold Goltz (1834-1902). Ewald married Bertina, the daughter of the physiologist Moritz Shiff. In 1900, he was appointed to the chair of physiology at Strassburg, taking Goltz’s place.
|Table 1: Ewald’s three laws:|
|1. A stimulation of the semicircular canal causes a movement of the eyes in the plane of the stimulated canal|
|2. In the horizontal semicircular canals, an ampullopetal endolymph movement cases a greater stimulation than an ampullofugal one.|
|3. In the vertical semicircular canals, the reverse is true.|
Ewald is best known for his description of Ewald’s 3 laws. (Ewald 1882) These were obtained from research on pigeons, using a “pneumatic hammer”. The hammer was a plunger placed into the semicircular canals of awake pigeons. Ewald would push or pull on the canals using his “hammer”, and observe the eyes. The eyes jump (causing “nystagmus”) with particular vectors depending both on the particular one of the three semicircular canals that was being stimulated by the hammer, and the direction of the fluid flow in the canal. Heavy criticism was leveled at Ewald, and in fact, an entire issue of a journal was devoted to the topic "On the case for repeal of Ewald's second law" (e.g. Hallpike, 1961) .
Ewald’s first law, that the eyes move in the plane of the stimulated canal, was not novel, as Marie-Jean-Pierre Flourens (1794-1867) had established that cutting the nerve to a semicircular canal of pigeons caused eye movements in its own plane.
Ewald’s second law was found to be of much greater interest, as it had profound clinical implications. The directional asymmetry implies that in persons who have lost inner ear function on one side, the remaining side should produce higher velocity nystagmus when the head is being rotated towards the remaining intact ear (ampullopetal), but less when the head is being rotated towards the side of vestibular loss. Ewald’s second and third laws is the basis for the head-impulse test currently used as a bedside method of determining whether or not vestibular function is lost in one horizontal semicircular canal, such as is often the case after a bout of vestibular neuritis(Cremer, Halmagyi et al. 1998).
Ewald's second law is also thought to be the cause of the Head Shaking Nystagmus.
In the recording above, a nystagmus is seen following vigorous head-shaking. This is thought to be due to a combination of Ewalds 2nd law and the velocity storage mechanism (Hain et al, 1987).
The most common mechanism suggested for these laws is that firing in the vestibular nerve can be excited, nearly without limit, but can be inhibited only to zero.
If this is indeed the case, then the asymmetry between excitation and inhibition should be most profound for strong inputs, and very little seen for weak inputs. In other words, one would expect equivalent responses to warm and cold caloric irrigations, but different responses for strong "impulses" of rapid head movement.