Hearing - the Sense of Audition

Generalities.

     The sense of hearing gives us the ability to perceive and evaluate the meaning of sounds. Do not get confused with the term audition which is more like the quantification and qualification of that perception. Sounds are mechanical vibrations that propagates through the matter (sounds do not propagate in the interstellar void). Human hearing allows for the perception of vibrations that have frequencies between 16 Hz (low pitched sound) and 16,000 Hz (16 kHz; high pitched sound). Note that is in the best cases, in people with very good hearing. For the low and high pitched sounds, a higher intensity of the vibrations may be required to perceive them. But, when they are to intense, sounds may be perceived with pain and even cause damages to our auditory system. The frequencies that are best perceived by the human ear are those around 1 to 3 kHz. Curiously, this corresponds and even surpasses the range of high pitched for talking or signing. This maybe why alarms have high pitched tone, so that even from a distance we will heard very well. On the other hand, when we talk to each other, at lower pitch, we do not need to yell.

     To be perceived and interpreted, the sound vibrations need to reach our brain. Even though those vibrations could be transmitted through our skull, this mode of transmission is absolutely not efficient. It is our auditory system which is specialized for the optimal transmission of those sound vibrations. The auditory system consists of the outer ear, the middle ear, the inner ear and the central pathways leading the auditory information all the way to our brain cortex where the auditive information could be interpreted and remembered.

     At the level of the outer ear, the sound vibrations penetrate the ear via the auricle (the pinna), where they can be slightly amplified as they funnel to the ear canal. Then, they travel along that ear canal, up to the eardrum (the tympanic membrane). Like a drum skin this membrane starts vibrating. On the other side of the eardrum, in the middle ear, the sound vibrations are transmitted by the ossicles. These are three little bones, the malleus (the hammer), the incus and the stapes. The last of these bones, the stapes, sits on a small membrane of the cochlea, the oval window. The cochlea, an organ that has a spiral shape, belong to the inner ear. This is where the sound vibrations will make small hair cells to vibrate and transduce those mechanical vibrations into electrical signals that will then travel to the brain, first via the auditory nerve, then through different nuclei of the brainstem, the pons, the mesencephalus and up to the cerebral cortex where most of the interpretation and memorisation will occur.

The outer (external) ear.

     The outer ear constists in the auricle (the pinna), which is the part of the ear that is externally visible, and the auditory canal (better named the external acoustic meatus). For details about the naming of the different parts of the auricle, consult my page about the external view of the head and the neck. And, at the end of the external acoustic meatus there is the tympanic membranne, often called the eardrum.

     The auricle is mostly made of cartilage, a smooth connective tissue resembling a bone but that is not calcified. Inside the auricle there are some little muscles that, in the animal would permit movements of the auricle, but not very important in human (maybe some human can move their auricle, but that would mostly be an exception). In human, it is mainly the time difference between the perception between the two ears that allows for the localisation of sounds. Finally, the auricle also help a little for the perception of sounds of middle-pitch frequency by amplifying them by 5 to 20 decibels.

The middle ear.

     Like we saw in the previous section, the tympanic membranne is located at the end of the acoustic meatus. This membranne vibrates, like a drum skin, when the sound waves reach it. On the other side of the tympanic membranne there are three little bones, the ossicles. The first one is the maleus, often named the hammer, which is glued to the back of the tympanic membranne and relay those vibrations to the other ossicles. The second and the third ossicles are respectively the incus and the stapes. Those three ossicles are linked by small ligaments which permit the transmission of the vibrations and, at the same time, a certain attenuation of the sounds (vibrations) that may be too loud. The last ossicle, the stapes, sits on the oval window of the cochlea of the inner ear to relay the sound vibrations. One important thing to note about the middle ear is that it is connected to the nasal part of the pharynx (the nasopharynx) by a small conduit named the auditory tube (also called the Eustachian tube). This tube allows for pressure equilibrium between the outer ear and the middle ear.

The inner ear.

     The inner ear is located within the skull. There are two parts to the inner ear: the semicircular canals involved in equilibrium and the cochlea (ressembling a snail shell) that is responsible for the sounds perception. In this page, I am only discussing the transmission of the sound informations.

     The cochlea has a spiral shape having a little more than two and a half turns. The mobiolus, the canal inside the spiral, have three cavities separated by thin membrannes. These channels are the vestibular duct (scala vestibuli), the tympanic duct (scala tympani) and the cochlear duct (scala media).

     If you unfold the cochlea, like for the figure on the left, you will notice that it is large in the proximal area (close to the middle ear) and thin at its distal end (the helicotrema) where the scala vestibuli communicates with the scala tympani. As I told you before, the stapes sits on the oval window at the entrance of the scala vestibuli. The sound waves propagate along that duct and come back via the scala tympani where they dissipate at the level of the round window. The cochlea is filled with liquid and sound waves propagate easily and, depending on the wave length the vibrations will not get to the end of the cochlea but rather dissipate through the scala media containing the hair cells that will vibrate in reaction to the sound waves. The vibrating hair cells (of the organ of Corti) will then transduce this mechanical movement into an electrical signal that will travel via the cochlear nerve to reach different brain structure where the sounds will be interpreted.

     Because the cochlea is large at its entrance and very thin at the end, the sounds of various wavelenghts do not travel all the way. The high pitched sounds (high frequency, short wavelength) are perceived at the entrance of the cochlea whereas the low pitched ones (low frequency, long wavelength) are perceived its tip.

Position in the cochlea where the sound vibrations are transduced.

     Here we have few anatomical pictures about the inner ear.

The inner ear.

Section of the inner ear.

Cut accross the bony cochlea.

The central pathways.

     Electrical activity generated by the hair cells travel via the cochlear nerve fibers to different brain nuclei where auditory informations could be proccessed and interpreted. Note that the cochlear nerve fibers (carrying auditory information) will merge with vestibular nerve fibers (carrying equilibrium information) to form the vestibulocochlear nerve (the eighth cranial nerve (VIII)). Roughly, each nervous fiber carry the information of one hair cell of the organ of Corti. Hence, each fiber will carry the auditory information about one sound wave, its frequency and its amplitude. But of course, things are usually more complicated than that. Sounds could be quite complex and even for a sound of a single (pure) frequency wave, hundreds of hair cells could be stimulated at once. In addition, there are some afferent fibers that will bring inward information from the brain. These inward impulses can modulate hair cell activities to render them more or less sensitive.

     All this to say that even at the level of the cochlea there is already some kind of information processing. For example, increasing our level of attention and focussing will favor the amplification of certain sounds of weak intensity, or help discriminating a specific voice in a loud crowd. On the oppsite, sounds that are too loud could be attenuated in order to protect our hearing.

     The auditory nerve fibers emerging from the cochlea have their first relay at the level of the cochlear nucleus, located at the junction of the medulla and the pons. From there, this sensory information split into four branches. Two of those nervous bundles travel on the same side of the ear that perceived the sound (ipsilateral) and two cross (decussate) on the other side of the brain (contalateral). And, its the opposite for the other ear, allowing for the stereo perception. Two mechanisms help to localize those sounds : the time difference between their perception by one ear versus the perception of the same sound by the other ear, and the difference in the sound amplitude (loudness) of a sound which is attenuated by the head before it reaches the opposite ear.

     One bundle, the dorsal acoustic stria, climbs ipsilaterally in the brain and, at the level of the pons, decussate to the contralateral side to reach the nucleus of the lateral lemniscus. At this level, some branches cross back ipsilaterally while other remain contralateral and eventually reach the inferior Colliculus on both side of the midbrain. The more medial bundles pass by both the ipsilateral and contralateral superior olivary nuclei in the medulla, and climb up to the inferior colliculus of the midbrain. Then, only the contralateral fibers will travel up to the medial geniculate nucleus where the auditory information may blend with other sensory information. Finally, auditory information will then reach the primary auditory cortex. Hence, the sounds heard by the left ear will be mainly interpreted in the right hemisphere of the brain, and vice-versa for the right ear.

     All the way, these different pathways are organized in a tonotopic fashon, meaning the these structures have a defined organization in relation to the sounds frequencies. The processing of the sounds is organized gradually from the lower pitched (frequency) to the higher ones. At the level of the primary auditory cortex, the lower frequencies are processed in the anterior aspect, whereas the higher ones are gradually processed posteriorly. Two part of the auditory cortex could be recognized : the primary and the secondary (association) cortices. The primary cortex is where the sounds are recognized and memorized. The associative auditory cortex, larger in surface, is where the the sounds could be associated with other sensory modalities in order to add some context to those sounds. At this level, things could be very complex as we need to consider many cultural factors like the language, the distinction and isolation of a particular voice in a crowd of people, as well as auditory hallucinations or illusions. Finally, there are relationships between the associative auditory cortex and other areas involved in heard and spoken languages. In addition, we know the language functions are mostly processed in the left hemisphere (side) of the brain, especially in right-handled people, or in the right hemisphere in many lefties, though for some people both hemispheres could be involved.

Links.

Britanica encyclopedia site about hearing :https://www.britannica.com/science/ear