Audition
Fall, 2002

The auditory system detects sound waves. Sound waves are produced by the compression and rarefication of air (Figure 11.1).
 

In order to understand the auditory system, we need to understand the three components of sound (Figure 11.2):

1) Pitch is determined by sound wave frequency. That is, the number of waves passing a particular point each second. Humans can hear sounds with a frequency as low as 20 Hz and as high as 20,000 Hz.

2) Intensity, or loudness, is determined by the force of the waves. This is measured by wave amplitude.

3) Timbre is the quality of the sound (i.e., the purity of the wave). Most sounds are composed of multiple overlapping frequencies. The mechanism used to code timbre is similar to that used to code pitch.

  The function of the ear is to convert this mechanical energy into neural activity.
That is, vibrations in the air must be converted into action potentials.

The parts of the ear are (Figure 11.3):

The pinna funnels sound waves into the auditory canal.
The tympanic membrane or eardrum converts air vibrations to mechanical energy.
The ossicles are 3 little bones inside head that amplify the force of the sound waves.
The oval window is a membrane that creates waves inside the cochlea.
The cochlea is the location of the sensory receptors for audition. The cochlea is filled with fluid because fluid does not compress. Thus, the vibrations from the ossicles are accurately transmitted to the cochlea. Movement of the oval window creates waves inside the cochlea. These waves cause the basilar membrane to move (Figures 11.7 & 11.8). The sensory receptors for audition sit on the basilar membrane. These cells are called hair cells because they have stereocilia projecting off the top. The stereocilia are attached to the overlying rigid tectorial membrane. Movement of the basilar membrane causes the stereocilia to bend (Figure 11.12).
  Movement of the stereocilia in one direction opens K+ channels (Figures 11.13 & 11.14).
Movement of the stereocilia in the opposite direction causes K+ channels to close. The depolarizations and hyperpolarizations produced by movement of the stereocilia regulates transmitter release. Pitch is coded by the location of hair cells on the basilar membrane (Figure 11.9). Because the membrane is thin and tight at the basal end and thick and flexible at the apex, sound waves of different frequencies maximally displace the membrane at different locations. For example, a high pitch sound will produce a greater vibration at the basal end than the apex, whereas a low pitch sound will produce a greater vibration at the apex. This is the same principle at work with string instruments (e.g., piano). Timbre is caused by overlapping waves activating different parts of the membrane simultaneously.
 

Loudness is coded by creating a bigger wave which causes a greater displacement of the basilar membrane. This does two things to hair cells besides potentially damaging them. A bigger wave causes:

a) more hair cells to be activated (population code).
b) a greater displacement of the stereocilia which causes more transmitter to be released (frequency code).   There are two types of hair cells on the basilar membrane. Each appears to have a different function:
a) Inner hair cells appear to by the primary sensory neurons for audition. Although there is only a single layer of these (approximately 3500 neurons), they are heavily innervated by the auditory nerve (Figures 11.10 & 11.15).
b) Outer hair cells appear to dampen and amplify sounds by expanding and shrinking. Although there are 3 rows of these neurons (approximately 15,000 neurons), only 5% of the auditory nerve innervates these neurons.
The primary neural pathway for the perception of sound travels through many structures (Figure 11.17): 1. Hair cell
2. Auditory nerve (8th cranial nerve)
3. Cochlear nucleus (medulla)
4. Superior olive nucleus
5. Inferior colliculus
6. Medial geniculate nucleus (MGN) in the thalamus
7. Auditory cortex (superior temporal gyrus). Hearing problems can be caused by damage to: a) Neurons (Hair cell to cortex)
b) Membranes & bones (called conduction deficits). Vibrations of the skull can still be heard. Localization of sounds

The auditory system uses several mechanisms to localize sounds. For example:

Low frequency sounds (20-2000 Hz) are coded by phase comparisons between the two ears (called interaural time delay).
High frequency sounds (2000-20,000 Hz) are coded by intensity comparisons between the two ears (called interaural intensity difference).

Neurons in the superior olive nucleus compare inputs from the two ears. A line of neurons in the superior olive nucleus receives input from both ears. A sound from the right will reach the right ear before the left ear. Thus, the action potential generated in the right ear will get a head start and travel a greater distance before synapsing on a neuron at the same time as the action potential coming from the left ear. Depending on what the time difference is between the ears, a different neuron will receive both inputs at the same time (see Figure 11.25).
 

 Return to Psych 372 Syllabus