The afferent fibers in the auditory division of the eighth cranial nerve end in dorsal and ventral cochlear nuclei (Figure 1). From there, auditory impulses pass by various routes to the inferior colliculi, the centers for auditory reflexes, and via the medial geniculate body in the thalamus to the auditory cortex located on the superior temporal gyrus of the temporal lobe. Information from both ears converges on each superior olive, and beyond this, most of the neurons respond to inputs from both sides. In humans, low tones are represented anterolaterally and high tones posteromedially in the auditory cortex.
Fig1. Simplified diagram of main auditory (left) and vestibular (right) pathways superimposed on a dorsal view of the brainstem. Cerebellum and cerebral cortex have been removed. For the auditory pathway, eighth cranial nerve afferent fibers form the cochlea end in dorsal and ventral cochlear nuclei. From there, most fibers cross the midline and terminate in the contralateral inferior colliculus. From there, fibers project to the medial geniculate body in the thalamus and then to the auditory cortex located on the superior temporal gyrus of the temporal lobe. For the vestibular pathway, the vestibular nerve terminates in the ipsilateral vestibular nucleus. Most fibers from the semicircular canals terminate in the superior and medial divisions of the vestibular nucleus and project to nuclei controlling eye movement. Most fibers from the utricle and saccule terminate in the lateral division, which then projects to the spinal cord. They also terminate on neurons that project to the cerebellum and the reticular formation. The vestibular nuclei also project to the thalamus and from there to the primary somatosensory cortex. The ascending connections to cranial nerve nuclei are concerned with eye movements.
The responses of individual second-order neurons in the cochlear nuclei to sound stimuli are like those of the individual auditory nerve fibers. The frequency at which sounds of the lowest intensity evoke a response varies from unit to unit; with increased sound intensities, the band of frequencies to which a response occurs becomes wider. The major difference between the responses of the first- and second-order neurons is the presence of a sharper “cutoff” on the low-frequency side in the medullary neurons. This greater specificity of the second-order neurons is probably due to an inhibitory process in the brainstem. In the primary auditory cortex, most neurons respond to inputs from both ears, but strips of cells are stimulated by input from the contralateral ear and inhibited by input from the ipsilateral ear.
The increasing availability of positron emission tomography (PET) scanning and functional magnetic resonance imaging (fMRI) has greatly improved the level of knowledge about auditory association areas in humans. The auditory pathways in the cortex resemble the visual pathways in that increasingly complex processing of auditory information takes place along them. An interesting observation is that although the auditory areas look very much the same on the two sides of the brain, there is marked hemispheric specialization. For example, Wernicke area is concerned with the processing of auditory signals related to speech. During language processing, this area is much more active on the left side than on the right side. Wernicke area on the right side is more concerned with melody, pitch, and sound intensity. The auditory pathways are also very plastic, and, like the visual and somatosensory pathways, they are modified by experience. Examples of auditory plasticity in humans include the observation that in individuals who become deaf before language skills are fully developed, sign language activates auditory association areas.
Conversely, individuals who become blind early in life are demonstrably better at localizing sound than individuals with normal eyesight.
Musicians provide additional examples of cortical plasticity. In these individuals, the size of the auditory areas activated by musical tones is increased. In addition, violinists have altered somatosensory representation of the areas to which the fingers they use in playing their instruments project. Musicians also have larger cerebellums than nonmusicians, presumably because of learned precise finger movements.
A portion of the posterior superior temporal gyrus known as the planum temporale is located between Heschl gyrus (transverse temporal gyrus) and the sylvian fissure (Figure 2). It has an almost triangular shape that is larger in the left than in the right cerebral hemisphere in about three quarters of the population. It coincides in part with Wernicke area in the left hemisphere and likely is involved in language related auditory processing. A curious observation is that the planum temporale is larger than normal on the left side in musicians and others who have perfect pitch. Also curiously, this size asymmetry appears to be absent in individuals with schizophrenia.
Fig2. Asymmetries in the size of the left and right planum temporale as shown in a brain sectioned horizontally along the plane of the sylvian fissure. The triangular shape of the planum temporale is larger in the left than in the right cerebral hemisphere in about three-quarters of the population. This region coincides in part with Wernicke area in the left hemisphere and is involved in language-related auditory processing. Plane of section shown in the insert at the bottom. (Reproduced with permission from Kandel ER, Schwartz JH, Jessel TM [editors]: Principles of Neural Science, 3rd ed. New York, NY: McGraw-Hill; 1991.)
