Studies of Acquired and Inherited Deafness in Animals
Authors: Lurie M. H. 
Published: September 2005

These studies of deafness in animals were chiefly of the end organ for the reception of sound (organ of Corti). The functions of the organ of Corti were studied by means of the Wever and Bray phenomena, then correlated by histological examination. Animals studied were chiefly cats, dogs, guinea pigs and mice. In all these animals the organ of Corti is the same as in the human. The difference between human and animal hearing is the interpretation of sound detected by the organ of Corti. The interpretation of sound stimulating the organ of Corti is done by the brain. The function of the organ of Corti is to analyze the sound stimulation and transpose this stimulus into nerve impulses. The nerve from the organ of Corti (cochlea nerve) transmits these nerve impulses to the brain and the brain interprets these impulses.

The chief causes of deafness are: (1) disturbances of the transmission apparatus (the middle ear apparatus), usually called conduction or middle ear deafness; (2) disturbances of the end organ for reception of sound, usually called perception or nerve deafness. Deafness resulting from intercranial lesions cannot as a rule be differentiated from perception deafness. Conduction deafness in animals shows a similar pathology to that found in the human. Changes occur in the middle ear cavity, resulting usually from infection, preventing sound from reaching the organ of Corti in a normal manner. To overcome this increased resistance of the middle ear to transmission of sound, the intensity of sound must be increased.

It must be remembered that the organ of Corti does not care how sound reaches it, for if it is a normal organ of Corti, it will respond in a normal manner; if abnormal, in an abnormal manner. Bone conduction is just another means of sound reaching the organ of Corti.

The studies of the organ of Corti in animals deafened by disease, toxic poisoning and exposure to loud sounds for long periods of time, have given definite clues to the cause of perception deafness in the human. The lesions found in these animals are of a similar character, showing that the end result is the same in the organ of Corti. This is a degeneration of the external hair cells in the organ of Corti, and in more marked cases, a loss also of the internal hair cells, followed by a degeneration of the nerve cells. The loss of these special sensory hair cells in animals, when studied by the Wever and Bray method, has shown changes of response to sound similar to those found in the human being suffering from perception deafness.

Studies of inherited deafness have shown that deafness of this type may vary from complete degeneration of the organ of Corti to only partial degeneration. The evidence at present from studies of the waltzing guinea pig shows that the organ of Corti develops to normal before birth and then degenerates after birth. This degeneration is similar to that found in animals deafened by toxic poisoning, by disease and exposure to loud sounds.

From these studies the following conclusions with regard to perception deafness are drawn. (1) Degeneration of the external hair cells of the organ of Corti causes partial loss of hearing. (2) Internal and external hair cells when degenerated cause a complete loss of hearing for the region involved. This may result in (a) loss of part of the range; (b) an island of deafness. (3) External hair cells are for the detection of sounds of minimum intensity and are not concerned primarily with pitch. (4) Internal hair cells are more for the detection of pitch and do not begin to function until the intensity of sound has been increased to about 20 db above the threshold of the external hair cells.

These conclusions are amplified in the main article and the difficulties of developing hearing aids for the perception type of deafness are also discussed.