|
(Continued from page 119)
were made bright enough and electrical amplifiers were stable enough to allow neurophysiologists to devote their time to experiments rather than to equipment problems.
In 1927, Forbes, Miller, and O'Connor presented a series of rapidly repeated clicks to a cat's ear and were able to record the volley of nerve impulses. Wever and Bray performed a series of experiments that led to the development of the cochlear microphonic 2 years later by Saul and Davis. Wever and his associate listened to words spoken into a cat's ear that had been recorded and fed into a telephone receiver. To their amazement, the words were intelligible!
By 1935, Derbyshire and Davis had established the hair cells of the organ of Corti as the generator of the cochlear microphonic. Their work was published as a definitive study of the action potentials in the auditory nerve. Four years later a microintracellular electrode was developed by Hodgkin and Huxley to record the membrane potential and the action potential of the giant axon of the squid. The technique of response averaging to the ear and introduced electrocochleography (E Coch G). Because of the need to place an electrode directly on the basal coil of the cochlea, this procedure never gained wide acceptance in the United States. Sohmer and Feinmesser were able to use ''far field" electrodes to perform E Coch G by placing the active electrode in the ear canal rather than through the tympanic membrane and on to the cochlea. By 1971, Jewett and Williston had established a definitive description of BSAER, and in 1974 Hecox and Galambos expanded this definition to the audiometry of infants and adults
(Vertex) Potentials: On the basis of electrode placement, the first distinction among evoked potentials is anatomical, ''at the vertex'' or "in the ear.'' In the first subdivision, one or two reference electrodes are placed on the ear lobe or mastoid while the active electrode is attached to the top of the head or vertex. Vertex potentials measured in this fashion are classified by latency as fast, middle, slow, or late. This method of recording evoked potentials is called ''far field" technique.
The second subdivision is referred to as E Coch G, and the reference electrodes are placed on the ear lobe and the active electrode is placed within the middle ear or external auditory canal. If classified by latency (as the V potentials), these evoked potentials would be referred to as the first potentials.
Continuing Response: The second distinction among evoked potentials is physiological and is based upon whether it is a continuing (alternating current AC) or sustained (direct current DC) potential. The continuing potentials have been assigned specific names and abbreviations.
Cochlear Microphonic (CM). A continuing AC response generated chiefly by the external hair cells in the organ of Corti.
Summating Potential (SP). A continuing DC response generated in both internal and external hair systems, depending upon whether it is negative or positive.
(Continued on page 121)
|
|
