6 : 3/ Neural Mechanisms of Hearing 1 1 1 



maintained at ground potential. It greatly reduces the pick-up of 

 unwanted signals by the central conductor. In a similar manner, the 

 fluid in the vestibular and tympanic ducts is at body potential and shields 

 the central conductor. 



With sufficiently sensitive equipment, it can be shown that many 

 shielded cables have a d-c potential between the inner conductor and 

 the shield. A similar d-c potential exists across the basilar membrane. 

 It can also be shown that any shielded cable acts as a microphone con- 

 verting alternating pressures into electrical signals. Many people 

 believe the cochlear potentials are of a similar nature, that is, unwanted 

 electrical signals resulting from mechanical vibrations. These are called 

 microphonics when they occur in an electronic circuit. By analogy, the 

 cochlear potentials are referred to as microphonics. 



Whether the cochlear potential plays any role other than that of a 

 microphonic is not known. The cochlear potential is absent in some 

 deaf cats which lack hair cells ; it may be associated with the hair cells 

 in some fashion. Most investigators feel that the hair cells are inti- 

 mately associated with initiating the nerve potential. Similar hair cells 

 are found at the nerve endings in the inner ear associated with balance 

 and acceleration. The exact manner in which the electrical impulses 

 in the nerve fibers are initiated is not known. (Nor, for that matter, is 

 it known for most sensory nerve endings.) 



The description to this point includes most of the outstanding features 

 of the known actions of the cochlear portion of the inner ear. It appears 

 necessary to assign to the nervous system both the acuity of tonal dis- 

 crimination and also the reconstitution of the individual nerve impulses, 

 to have the integrated form of the original pressure wave. 



3. Arm Analogs and Neural Sharpening 



The exact mechanism by which the nervous system carries out an 

 extremely sharp frequency analysis is not known. However, it is a 

 familiar fact that the nervous system does sharpen many types of 

 stimuli. Thus, when a bright spot is focused on the retina, the sensi- 

 tivity of the eye to surrounding areas is decreased. In bright light, 

 this has the advantage of eliminating the effect of stray light. Simi- 

 larly, if two compass points are pressed against the skin of the forearm 

 at distances greater than about 2.5 cm apart, two sensations are received. 

 At around 2.5 cm the two sensations weaken each other, whereas, at 

 still closer distances, the two sensations add to each other. In the 

 latter case, the person feels the stimulus midway between the two actual 

 compass points. This is illustrated in Figure 5. 



