Auditory Neuroscience: How We Hear
Auditory neuroscience is the study of how the brain processes sound, allowing us to hear and make sense of it. This involves a complex series of steps, starting with the ear and culminating in the auditory cortex of the brain.
The Anatomy of Hearing
To understand how we hear, it's essential to examine the different parts of the ear and their functions.
Outer Ear
The outer ear helps guide sound waves into your external auditory canal, or ear canal.
Middle Ear
In the middle ear, sound waves cause the eardrum to vibrate, which in turn vibrates three tiny bones called ossicles. These ossicles consist of 3 bones: the malleus, incus, and stapes, which amplify the sound.
Inner Ear
The amplified sound waves then enter the inner ear, specifically the cochlea. Within the cochlea are sensory cells known as “hair cells”. These hair cells convert the vibrations into electrical impulses.
From Ear to Brain
Once the sound is converted into electrical impulses, the signal travels through the auditory nerve to the brain and eventually arrives at the auditory cortex.
Frequency and the Auditory Cortex
Humans can hear sounds in the range of 20 to 20,000 Hz (Hertz). The frequency of a soundwave is how high or low the tone of a sound is.
The brain processes sound into different frequencies. Different areas of the auditory cortex are organized by frequency, as shown in the picture below.
Neural Communication and Plasticity
Neurons are the fundamental units of the brain. While parts of the body outside the brain are usually referred to as “nerves,” they work the same way. Neurons consist of a cell body, dendrites, and an axon, along with various organelles. Dendrites receive signals from other neurons. The point where two neurons connect to each other is called a synapse.
Synapses allow different neurons in the brain to work together. The brain is capable of changing how they connect two neurons; this change is called plasticity. As you learn new things, new connections are made, and some existing connections change to become stronger or weaker, allowing you to perform tasks with greater precision as you learn.
For example, when sitting at a piano and a specific note is sent to the brain, the brain must learn to play that note. It needs to learn which key to play and also which keys not to play on either side of it. This means that neurons that respond to the target frequency must learn to suppress their response to the target frequency.
Review of the Hearing Process
Let’s review how your brain hears. The process starts with the ear collecting sound waves and converting sound into an electrical impulse. This electrical signal is then transmitted to the auditory cortex of your brain.
Synaptic Function and Hearing Loss
The Rutherford lab studies the smallest parts of the ear called synapses. Synapses are the sites of communication between two neurons or between a neuron and a sensory receptor cell.
According to Mark Rutherford, PhD, assistant professor in the Department of Otolaryngology, “All information about your acoustic environment is carried to the brain by auditory nerve fibers." Too much glutamate causes over-excitation and can be toxic to nerve cells. This process, called excitotoxicity, can lead to neurodegeneration and hearing loss.
The lab is currently studying the composition of glutamate receptors to assess vulnerability. The first study is looking at sex differences in susceptibility to hearing loss, with collaborator Maria Rubio, PhD, at University of Pittsburgh. The second study is looking at synaptic pathology in aging, particularly Alzheimer’s patients, in collaboration with John Cirrito and Carla Yuede in the Department of Neurology at Washington University.
Central Auditory Pathways
The auditory system transforms sound waves into distinct patterns of neural activity, which are then integrated with information from other sensory systems to guide behavior, including orienting movements to acoustical stimuli and intraspecies communication. The first stage of this transformation occurs at the external and middle ears, which collect sound waves and amplify their pressure, so that the sound energy in the air can be successfully transmitted to the fluid-filled cochlea of the inner ear.
In the inner ear, a series of biomechanical processes occur that break up the signal into simpler, sinusoidal components, with the result that the frequency, amplitude, and phase of the original signal are all faithfully transduced by the sensory hair cells and encoded by the electrical activity of the auditory nerve fibers. One product of this process of acoustical decomposition is the systematic representation of sound frequency along the length of the cochlea, referred to as tonotopy, which is an important feature preserved throughout the central auditory pathways.
The earliest stage of central processing occurs at the cochlear nucleus, where the peripheral auditory information diverges into a number of parallel central pathways. Accordingly, the output of the cochlear nucleus has several targets. One of these is the superior olivary complex, the first place that information from the two ears interacts and the site of the initial processing of the cues that allow us to localize sound in space. The cochlear nucleus also projects to the inferior colliculus of the midbrain, a major integrative center and the first place where auditory information can interact with the motor system. This pathway ultimately reaches the primary auditory cortex for conscious perception.
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 MGB does not act as a simple relay centre: it has reciprocal connections with the auditory cortex and mediates refinement of the incoming information. 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.
| Structure | Function |
|---|---|
| Outer Ear | Collects and guides sound waves |
| Middle Ear | Amplifies sound waves |
| Cochlea | Converts vibrations into electrical impulses |
| Auditory Nerve | Transmits electrical signals to the brain |
| Auditory Cortex | Processes and interprets sound |