The Pathway of Sound Perception: How We Hear
Our auditory system converts pressure waves into meaningful sounds. This translates into our ability to hear the sounds of nature, to appreciate the beauty of music, and to communicate with one another through spoken language. This section will provide an overview of the basic anatomy and function of the auditory system.
The ear can be separated into multiple sections.
The outer ear includes the pinna, which is the visible part of the ear that protrudes from our heads, the auditory canal, and the tympanic membrane, or eardrum. The middle ear contains three tiny bones known as the ossicles, which are named the malleus (or hammer), incus (or anvil), and the stapes (or stirrup). The inner ear contains the semi-circular canals, which are involved in balance and movement (the vestibular sense), and the cochlea.
The Journey of Sound Waves
Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate. This vibration results in movement of the three ossicles. As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells, which are auditory receptor cells of the inner ear embedded in the basilar membrane. The basilar membrane is a thin strip of tissue within the cochlea.
Sitting on the basilar membrane is the organ of Corti, which runs the entire length of the basilar membrane from the base (by the oval window) to the apex (the “tip” of the spiral). The organ of Corti includes three rows of outer hair cells and one row of inner hair cells. The hair cells sense the vibrations by way of their tiny hairs, or stereocillia. The activation of hair cells is a mechanical process: the stimulation of the hair cell ultimately leads to activation of the cell. As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain.
Auditory Pathways in the Brain
A sensory pathway that carries peripheral sensations to the brain is referred to as an ascending pathway, or ascending tract. The sensory pathway of audition ascends through three brainstem nuclei. Audition begins by traveling along the vestibulocochlear nerve, which synapses with neurons in the cochlear nuclei of the superior medulla. The cochlear nuclei receives information from the cochlea.
The spiral ganglion houses the cell bodies of the first order neurons (ganglion refers to a collection of cell bodies outside the central nervous system). These neurones receive information from hair cells in the Organ of Corti and travel within the osseous spiral lamina.
The vestibular nerve joins the cochlear nerve entering the internal acoustic meatus, and from this point onward they are collectively called vestibulocochlear nerve. The nerve enters the cranium through the internal acoustic meatus and travels a short distance (around 1 cm) to enter the brainstem at the cerebellopontine angle. From the dorsal cochlear nucleus, most fibres cross the midline and ascend in the contralateral lateral lemniscus.
From the ventral cochlear nucleus, some fibres also ascend in the lateral lemniscus bilaterally. However, most fibres from the ventral cochlear nucleus decussate to the contralateral superior olivary nuclei in a region known as the trapezoid body. Although the ventral cochlear nuclei neurons decussate at the trapezoid body, some fibres synapse at the ipsilateral superior olivary nucleus. The superior olivary nucleus is located just next to the trapezoid body.
In summary, in both the dorsal and ventral nuclei, some fibres decussate while others do not. For that reason, information from both ears travels bilaterally in each lateral lemniscus. This is important because supranuclear lesions (i.e. above the level of the cochlear nucleus) will not lead to serious hearing impairment.
The next part of the ascending process of auditory information in the brainstem is the superior olivary nucleus. The superior olivary nucleus takes the information from the cochlear nucleus and begins the process of interpreting and combing information. After the superior olivary nucleus, auditory processing continues on to a nucleus called the inferior colliculus (IC). The inferior colliculus (IC) is a midbrain structure that integrates the vast majority of ascending auditory information and projects via the thalamus to the auditory cortex.
The IC is also a point of convergence for corticofugal (information from the cerebral cortex) input and input originating outside the auditory pathway. The medial geniculate nucleus of the thalamus then receives the auditory information from the three brainstem nuclei. The MGB does not act as a simple relay centre: it has reciprocal connections with the auditory cortex and mediates refinement of the incoming information. Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing. This pathway ultimately reaches the primary auditory cortex for conscious perception.
The primary auditory cortex (A1) is located in the superior temporal gyrus, right under the lateral fissure. These are pathways that do not lead to primary auditory cortex.
Sound Waves and Perception
As mentioned above, the vibration of the tympanic membrane is what triggers the sequence of events that lead to our perception of sound. Sound waves travel into our ears at various speeds and amplitudes. Like light waves, the physical properties of sound waves are associated with various aspects of our perception of sound. The frequency of a sound wave is associated with our perception of that sound’s pitch. High-frequency sound waves are perceived as high-pitched sounds, while low-frequency sound waves are perceived as low-pitched sounds.
As was the case with the visible spectrum, other species show differences in their audible ranges. For instance, chickens have a very limited audible range, from 125 to 2000 Hz. Mice have an audible range from 1000 to 91000 Hz, and the beluga whale’s audible range is from 1000 to 123000 Hz.
The loudness of a given sound is closely associated with the amplitude of the sound wave. Higher amplitudes are associated with louder sounds. Loudness is measured in terms of decibels (dB), a logarithmic unit of sound intensity.
Although wave amplitude is generally associated with loudness, there is some interaction between frequency and amplitude in our perception of loudness within the audible range. For example, a 10 Hz sound wave is inaudible no matter the amplitude of the wave. Of course, different musical instruments can play the same musical note at the same level of loudness, yet they still sound quite different. This is known as the timbre of a sound.
| Sound | Decibels (dB) | Potential Hearing Damage |
|---|---|---|
| Whisper (5 feet away) | 20-30 | No |
| Normal Conversation | 60 | No |
| Heavy Traffic | 70-80 | Possible with prolonged exposure |
| Food Processor | 80-90 | Yes |
| Subway Train (20 feet away) | 90-100 | Yes |
| Rock Concert | 110-120 | Yes |
| Jackhammer | 120-130 | Yes |
A typical conversation would correlate with 60 dB; a rock concert might check in at 120 dB. A whisper 5 feet away or rustling leaves are at the low end of our hearing range; sounds like a window air conditioner, a normal conversation, and even heavy traffic or a vacuum cleaner are within a tolerable range.
However, there is the potential for hearing damage from about 80 dB to 130 dB: These are sounds of a food processor, power lawnmower, heavy truck (25 feet away), subway train (20 feet away), live rock music, and a jackhammer. About one-third of all hearing loss is due to noise exposure, and the louder the sound, the shorter the exposure needed to cause hearing damage (Le, Straatman, Lea, & Westerberg, 2017). Listening to music through earbuds at maximum volume (around 100-105 decibels) can cause noise-induced hearing loss after 15 minutes of exposure. Although listening to music at maximum volume may not seem to cause damage, it increases the risk of age-related hearing loss (Kujawa & Liberman, 2006).