The auditory system comprises both the periphery, which deals with transduction of acoustic signals into a neural signal, and the auditory pathway, which is responsible for the processing of this neural signal (Pulkki & Karjalainen, 2015). The periphery consists of three individual segments: the outer ear, middle ear and inner ear (Plack, 2014) (see Figure 3.1). Each segment contributes towards the conversion of compressions and rarefactions in the surrounding medium into the neural representation that governs perception of sounds in space. This section provides a brief overview of how sound is processed as it travels through the auditory system.
The outer ear
As sound arrives at the ear, the first structure that it meets is the pinna, which is the external part of the ear. The pinna’s complex structure makes slight modifications to the spectral characteristics of sounds depending on the direction from which the sound reaches the ear (Plack, 2014). These spectral modifications are decoded within the auditory pathway to determine the location of sound sources (Plack, 2014).
From the pinna, sounds enter the ear canal (external auditory meatus), which is a short tube which has an opening in the pinna at one end and is terminated by the eardrum (tympanic membrane) at the other end (Plack, 2014).
Figure 3.1: Annotated diagram of the human ear indicating the outer, middle, and inner ear sections. Adapted from (Plack, 2014, p. 54)
The middle ear
The pressure variations in the ear canal cause the eardrum to move, converting the sound waves into mechanical vibrations (Plack, 2014). The eardrum is then connected to a series of small bones: incus, malleus, and stapes, collectively referred to as the ossicles (Plack, 2014) (shown in Figure 3.1). The incus is attached to the eardrum and conducts vibrations on to the malleus which passes them on to the stapes. The stapes connects the middle ear to the inner ear and is attached to the oval window at the base of the cochlea. The middle ear serves as an impedance mechanism to improve the transfer of energy from the air to the inner ear (Pulkki & Karjalainen, 2015).
The inner ear
The inner ear comprises the cochlea and the semicircular canals (Pulkki & Karjalainen, 2015). As the latter is not involved in hearing (Pulkki & Karjalainen, 2015), this section focusses on the function of the cochlea.
The cochlea is a spiral structure, formed from a coiled fluid-filled tube (Plack, 2014). The tube itself is constructed of three channels, the scala vestibuli, the scala tympani and the scala media (Plack, 2014) (see Figure 3.2). The oval window is at the base of the scala vestibuli.
Figure 3.2: Annotated cross-section of the cochlea. Adapted from (Plack, 2014, p. 56)
The scala vestibuli is separated from the scala media by Reissner’s membrane and the scala media and the scala tympani are separated by the basilar membrane (Plack, 2014). The scala vestibula and the scala tympani contain the same fluid and are joined by a hole at the top of the spiral (Plack, 2014).
At the base of the scala tympani there is a membrane, similar to the oval window, called the round window (Plack, 2014). Vibrations of the oval window cause compression and rarefaction in the fluid within the cochlea, which causes the Reissner’s membrane and the basilar membrane to vibrate (Plack, 2014). As the basilar membrane’s thickness changes from the base to the apex of the cochlea, becoming wider and looser as it approaches the apex, different frequencies of vibration lead to different locations of the basilar membrane resonating (Plack, 2014). This means that sounds are broken into their composite frequencies that are represented at different locations along the basilar membrane. For this reason, it is common to think of the basilar membrane as a bank of band-pass filters.
The organ of Corti sits on top of the basilar membrane and comprises sets of inner and outer hair cells) which have stereocilia that extend above the organ of Corti (Pulkki & Karjalainen, 2015). With the outer hair cells, the stereocilia embed into the tectorial membrane above, while the stereocila of the inner hair cells do not (Plack, 2014). When the basilar membrane vibrates, so does the tectorial membrane. This movement causes the stereocilia to bend, which causes the hair cell to release a neurotransmitter (Plack, 2014). This is then received by receptors of neurons of the auditory nerve and causes electrical spikes (action potentials), which form the neural signal within the auditory pathway (Plack, 2014).
As neurons attach to hair cells at specific points on the basilar membrane, each neuron is associated with a specific frequency depending on the hair cell’s location. In practice, this
means that when a region of the basilar membrane is excited by its characteristic frequency, there is an increase in the rate of action potentials in the neurons from the hair cells at that location. The rate of the action potentials is dependent on the amplitude of the basilar membranes movement, increasing with the amount of displacement until it reaches the limits at which action potentials can be generated (Plack, 2014). In addition to conveying information about the frequency that is excited, this neural code also provides information regarding the phase vibration (Plack, 2014).
Auditory Pathway
The auditory nerve runs from the cochlea to brain where the signal is decoded to inform our perception of sound. Different areas of the brain are thought to be responsible for extracting different information from the neural representation produced by the cochlea. The exact function of the different nuclei through which the signals pass, are still the matter of research (Plack, 2014). As the neuropsychology is not the primary concern of this thesis, this section does not go into the details of the each of the stages involved in this process but provides a broad overview.
Up to this point, the description has considered only the activity in a single ear. At this point it is, however, important to consider neural codes arriving from both cochlea. For each stage in the processing of the neural code, there is a pair of specialised nuclei—one on each side of the brainstem (Plack, 2014). As the neural signals are passed between specialised nuclei, important information pertaining to the location of the sound and its characteristics are extracted (Plack, 2014). After this, the resulting signals are passed to the auditory cortex (Plack, 2014). The auditory cortex is thought to perform the higher level processing of sounds and communicates with other parts of the brain to integrate information from the various modalities and to analyse semantics of auditory information (Plack, 2014).
This discussion has considered only afferent (periphery to cortex) signals. There are, however, also efferent connections (originating in the brain), which are thought to control the movement of the basilar membrane under high sound pressure levels, and alter the processing of sounds in the brainstem (Plack, 2014).
Figure 3.3: Diagram showing the median, transverse and frontal planes. Adapted from (Blauert, 1997, p. 14). Locations are referred to in (azimuth, elevation) format.