6 : 4/ Neural Mechanisms of Hearing 1 15 



lobe of the cerebral cortex is buried deep in one of the folds of the 

 cortex and is hard to study. In other mammals, the cerebral projection 

 is on or nearer the exposed portions of the cortex. In these latter 

 animals, there are always two and, in some animals, three areas where 

 responses appear (in the unconscious animal) when the ear is stimulated. 

 Each of these areas is connected to both ears. Within each cerebral 

 projection area, specific smaller areas correspond to specific spots along 

 the basilar membrane. 



A detailed examination of the acoustic pathway shows that several 

 neurons are involved. The first is located in the spiral ganglion within 

 the inner ear. The nerve fibers leaving this ganglion join those from the 

 vestibular portion of the ear to form the eighth cranial nerve. Within 

 the brain, the vestibular and auditory fibers separate. Those from the 

 cochlea go to one of two nuclei in the lower brain stem known as the 

 dorsal and ventral cochlear nuclei. Some fibers leaving these have synapses 

 with other neurons associated with reflex actions and balance. Others 

 go to synapses in another nucleus in -the lower brain stem called the 

 superior olivary complex. Some fibers synapse in the superior olivary 

 complex on the same side, others on the opposite side of the brain, and 

 still others pass through without interruption, joining fibers from the 

 superior olivary complexes and passing up the brain stem. In the 

 nuclei of the lateral lemniscus farther along the brain stem, some of 

 the auditory fibers end, and others pass through uninterrupted. 



In the midbrain level, some of the auditory fibers end at synapses 

 in the inferior colliculus. From here, some fibers cross over to synapses 

 in the opposite inferior colliculus. All of the fibers of the auditory 

 tract have synapses in another nucleus of the midbrain, the medial 

 geniculate body. Fibers of these neurons finally reach the auditory 

 areas of the cerebral cortex. 



The groups of nerve fibers in the brain stem "fire" in such a fashion 

 as to reconstitute the original sound wave, or at least almost do so. 

 Where this synchronization starts is not known. Wever has proposed 

 that it occurs in the cochlea, that in some fashion the nerve fibers fire 

 in volleys to reproduce the over-all form of the incident pressure wave. 

 The manner in which this could occur is shown in Figure 8, for 15 nerve 

 fibers. Observe that none fires too often, but that there is a certain 

 over-all synchrony. This effect need not occur in the cochlea. It 

 could just as well originate at the first or even second synapse. This 

 semisynchronous action is called the volley theory. It states, in its 

 simplest form, that below some frequency, say 100 cps, the number of 

 nerve fibers excited varies with the instantaneous pressure. From 300 to 

 3,000 cps, the volley-type effect reproduces the form of the incident 

 sound wave, whereas above 3,000 cps it cannot follow, but reproduces 



