12. Auditory System: Structure and Function
Part 2 of 2

Lincoln Gray, Ph.D.
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Tonotopic Organization

Physical characteristics of the basilar membrane cause different frequencies to reach maximum amplitudes at different positions. Much as on a piano, high frequencies are at one end and low frequencies at the other. High frequencies are transduced at the base of the cochlea whereas low frequencies are transduced at the apex. Figure 12.7 illustrates the way in which the cochlea acts as a frequency analyzer. The cochlea codes the pitch of a sound by the place of maximal vibration. Note the position of the traveling wave at different frequencies. (Beware! It may initially seem backwards that low frequencies are not associated with the base.) Select different frequencies by turning the dial. If audio on your computer is enabled, you will hear the sound you selected. Hearing loss at high frequencies is common. The average loss of hearing in American males is about a cycle per second per day (starting at about age 20, so a 50-year old would likely have difficulty hearing over 10 kHz). If you can't hear the high frequencies, it may be due to the speakers on your computer, but it is always worth thinking about hearing preservation.

As you listen to these sounds, note that the high frequencies seem strangely similar. Think about cochlear-implant patients. These patients have lost hair-cell function. Their auditory nerve is stimulated by a series of implanted electrodes. The implant can only be placed in the base of the cochlea, because it is surgically impossible to thread the fine wires more than about 2/3 of a turn. Thus, cochlear implant patients probably experience something like high frequency sounds.

Figure 12.8 shows the range of frequencies and intensities of sound to which the human auditory system responds. Our absolute threshold, the minimum level of sound that we can detect, is strongly dependent on frequency. At the level of pain, sound levels are about six orders of magnitude above the minimal audible threshold. Sound pressure level (SPL) is measured in decibels (dB). Decibels are a logarithmic scale, with each 6 dB increase indicating a doubling of intensity. The perceived loudness of a sound is related to its intensity. Sound frequencies are measured in Hertz (Hz), or cycles per second. Normally, we hear sounds as low as 20 Hz and as high as 20,000 Hz. The frequency of a sound is associated with its pitch. Hearing is best at about 3-4 kHz. Hearing sensitivity decreases at higher and lower frequencies, but more so at higher than lower frequencies. High-frequency hearing is typically lost as we age.

The neural code in the central auditory system is complex. Tonotopic organization is maintained throughout the auditory system. Tonotopic organization means that cells responsive to different frequencies are found in different places at each level of the central auditory system, and that there is a standard (logarithmic) relationship between this position and frequency. Each cell has a characteristic frequency (CF). The CF is the frequency to which the cell is maximally responsive. A cell will usually respond to other frequencies, but only at greater intensities. The neural tuning curve is a plot of the amplitude of sounds at various frequencies necessary to elicit a response from a central auditory neuron. The tuning curves for several different neurons are superimposed on the audibility curves in Figure 12.8. The depicted neurons have CFs that vary from low to high frequencies (and are shown with red to blue colors, respectively). If we recorded from all auditory neurons, we would basically fill the area within the audibility curves. When sounds are soft they will stimulate only those few neurons with that CF, and thus neural activity will be confined to one set of fibers or cells at one particular place. As sounds get louder they stimulate other neurons, and the area of activity will increase.

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