2.3 The Physiology of Directional Hearing
2.3.1 The Peripheral Auditory System
The function of the peripheral auditory system is to convert sound pressure in the acoustical domain to neural firing rates at the auditory nerve, which can be further processed by the central nervous system. A simplified schematic representation of the auditory periphery is shown in Figure 2.2. Although this system is generally described in much more detail, here we focus only on elements which are explicitly related to the computational models employed in this thesis.
Figure 2.2: Simplified schematic representation of the peripheral auditory path- way, adapted from (Chittka and Brockmann, 2005). Figure modified and used under the creative-commons license.
Waves arriving at the pinna travel down theauditory canal, which is an irregu- larly shaped tube whose average diameter is about 0.65cm horizontally and 0.9cm vertically, and whose length is approximately 2.5−3.5cm. The auditory canal is sealed on its inner side by a conically-shaped tissue called the tympanic mem- brane. This structure can be viewed as an open-closed tube, whose fundamental
resonant frequency lies within the 2-5kHz range. In the middle ear, the mechani- cal vibration of the tympanic membrane induces motion in three inter-connected
ossicles, namely the malleus, incus and stapes4. The stapes then pushes on an ovally-shaped membrane covering the oval window, which is the intersection be- tween the middle ear and the cochlea. As the cochlea is filled with endolymph and perilymph, which are fluids whose acoustical properties resemble those of slightly salted water, the middle ear acts as an impedance matching mechanism, allowing for an effective transmission of acoustic power.
A cross section of cochlea is shown in Figure 2.3. It is an incompressible- fluid-filled spiral structure residing within the temporal bone, along which lies the basilar membrane and Reissner’s membrane who divide it into three sub- spaces, namely the scala tympani, scala media and scala vestibuli.
scala vestibuli scala media scala tympani tectorial membrane Reissner's membrane organ of corti OHC basilar membrane
nerve fibres IHC
Figure 2.3: Cross-section of the cochlea. IHC and OHC stand for inner hair cells
and outer hair cells respectively. Figure modified and used under the creative- commons license.
When the stapes pushes and pulls on the oval window, the fluid in the cochlea induces motion on the basilar membrane which vibrates at different places ac- cording to the spectral content of the signal. This mechanism in essence performs a frequency analysis of the signal, which entails that information is further trans- mitted in separate frequency channels. From a logical point of view, the process
can be seen as if the signal is passed through a network of auditory filters each having its own critical bandwidth. In this work, auditory filters are either re- ferred to by their centre frequencies or by an index number, ˙k, whose relation to its centre frequency fc is given by (Glasberg and Moore, 1990)
fc= 228.833
e0.11 ˙k
−1 (2.24)
Attached to the basilar membrane is the organ of corti whose function is to convert the mechanical vibrations of the membrane to electrical signals that can be further transmitted to the brainstem. The organ of corti features sensory hair cells, which deflect from their resting position when the basilar membrane vi- brates and causes the organ on corti to move with it respectively. This deflection is due to the existence of the tectorial membrane which pushes against the tips of the hair cells when the organ of corti is moved5. When hair cells are deflected, their tip-links stretch to allow influx of fluid from the inner part of the organ of corti. Because this fluid is saturated with potassium ions which have a pos- itive electric charge, each corresponding hair cell membrane develops a voltage potential across itself. Thus, the mechanical fluctuations of the basilar mem- brane are represented as periodic changes in voltage potentials of corresponding hair cells. This behaviour resembles a capacitor in an electric circuit. In a sim- ilar manner, charging and discharging of the cell is governed by a certain time constant, which causes the system to respond differently to different stimulating frequencies. At low frequencies the system is able to faithfully reproduce the fine structure of the waveform, as fluctuations are sufficiently slow and allow for a complete depolarisation-repolarisation process to occur. At high frequencies, the system is not able to keep up with stimulus, which results in continuous depo- larisation of the cell’s membrane potential. This manifests itself as DC-offset in system’s output signal.
Unlike traditional neurons, hair cells do not fire an action potential as a re- 5In fact, this is thought to happen only with outer hair cells, whereas motion of theinner
hair cells is thought to take place because of motion of fluid trapped between the organ of corti and the tectorial membrane.
action to depolarisation. Instead, they have synaptic connections with neurons of the spiral ganglion, which form the axons of the auditory nerve. This connec- tion is accomplished by a form of glutamatergic neurotransmission, in which the rate of the cell’s glutamate release is dependent on the deflection of the hair cell. The greater the deflection, the more depolarised the membrane voltage and the greater the release of glutamate. Since the firing rate of spiral ganglion nerves6 is dependent on the rate of glutamate release, then deflection of hair cells (and therefore the instantaneous amplitude of the stimulating signal) is encoded as time-varying neural firing rates.