Hearing

How Information about Sound is Carried in the Auditory System

The basic structure of the early stages of the auditory system is illustrated in Figure 1. Sounds are transmitted through the outer ear (the pinna and ear canal, or meatus) and middle ear (which includes three very small bones, called the malleus, incus, and stapes, collectively known as the ossicles) into the inner ear, which includes the cochlea. Within the spiral-shaped cochlea, there is a kind of ribbon, called the basilar membrane, which runs from the tip of the spiral (the apex) to the outer end of the spiral (the base). The ribbon is surrounded by fluids. When a sound enters the ear, the basilar membrane moves up and down. Each place on the basilar membrane is tuned to respond best to a limited range of frequencies. Low frequencies produce their biggest response toward the apex of the cochlea, and high frequencies produce their biggest response toward the base.

Figure 1 Schematic illustration of the structure of the peripheral auditory system, showing the outer, middle, and inner ear

Lying on top of the basilar membrane are specialized sensory cells called hair cells. These run in rows along the length of the basilar membrane. One type of hair cell, called the inner hair cell, responds to the movement of the basilar membrane by generating an electrical signal that in turn leads to the release of a neurotransmitter that triggers activity in the neurons of the auditory nerve. Each neuron derives its response from the vibration at a specific place on the basilar membrane.

[Page 384]Information about the characteristics of sounds is carried in the auditory nerve in three basic ways:

• 1.
  By the rate of firing of individual neurons, which will be referred to as the “amount” of neural activity. The more vibration there is at a given place, the greater is the amount of activity in neurons connected to that place. It is commonly believed that the subjective loudness of a sound is related to the amount of neural activity evoked by that sound, although this idea has been disputed.
• 2.
  By the distribution of activity across neurons. Each neuron is tuned so that it responds most strongly to a specific frequency, called the characteristic frequency (CF); the tuning reflects the tuning of the place on the basilar membrane that drives the neuron. The distribution of the amount of neural activity as a function of CF is called the excitation pattern. The excitation pattern conveys “place” information since the CF at the peak of the excitation pattern is related to the place on the basilar membrane that is excited most.
• 3.
  By the detailed time pattern of the neural impulses and especially the time intervals between successive nerve impulses. This form of information is known as “temporal” information. Neural impulses tend to be evoked at times corresponding to a specific phase of the waveform on the basilar membrane (for example, at the peaks of the waveform), an effect called phase locking. As a result, for a periodic sound, the time intervals between successive nerve impulses are approximately integer multiples of the period of the sound. For example, if the sound has a frequency of 500 hertz (Hz), the period is 2 milliseconds (ms), and the time intervals between successive nerve impulses would be close to 2, 4, 6, 8, 10, … ms. Phase locking breaks down at high frequencies (above about 36 kHz [kilohertz] in most mammals), but the upper limit in humans is not definitely known. Studies of pitch perception suggest that phase locking is very weak for frequencies above about 5 kHz.

In addition, information about sounds is conveyed by the differences between the two ears in all the above. In particular, differences in intensity at the two ears (primarily conveyed by differences in neural firing rate) and differences in the time of arrival of sounds at the two ears (conveyed mainly by subtle differences in the exact timing of nerve spikes) play a strong role in determining the perceived location of sounds.

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