Handbook of Psychology, Volume 4: Experimental Psychology

Auditory Anatomy and Physiology 129

tuning curve represents the tonal level required for a thresh-
old number of neural discharges as a function of the fre-
quency of the tone. The sharp V-shape tuning curves indicate
that auditory nerve fibers are highly tuned to the frequency of
stimulation; thus, they reflect the biomechanical traveling
wave action of the cochlear partition.
The discharges of auditory nerve fibers are also synchro-
nized to the vibratory pattern of acoustic stimulation. Fig-
ure 5.8 shows histograms for auditory nerve fibers indicating
that the timing pattern of acoustic stimulation is preserved in
these fibers up to about 5000 Hz (Geisler, 1998; Pickles,
1988). The auditory nerve discharges during only one phase of
the sound, and the probability of discharge is proportional to
the instantaneous amplitude of the sound. Thus, the temporal
pattern of neural discharges in auditory nerve fibers depicts the

temporal structure of sound’s pressure wave form for those

frequency components that are lower in frequency than ap-

proximately 5000 Hz.

The number of discharges and number of discharging

fibers increases in proportion to stimulus level. However, the

discharge rate of individual auditory nerve fibers varies over

only about a 40–50 dB range. Thus, although increased dis-

charge rate does provide information about a sound’s overall

amplitude, a simple relationship between discharge rate and

sound amplitude cannot account for the range of sound inten-

sity that most animals are capable of processing.

Thus, auditory nerve fibers are highly frequency-selective,

discharge in synchrony with the acoustic stimulus (at least up

to 5000 Hz), and change their discharge rates in proportion

to sound level. The discharge rate of any particular auditory

Figure 5.8 Time-locked post-stimulus-time (PST) histograms to the sum of two pure tones. In the top row, tone 2 is 20 dB more intense than

tone 1. In the middle row there is a 15 dB difference between the two tones, and in the bottom row the difference is 10 dB. For all cases the time

domain waveform of the summed sinusoids is superimposed on top of the PST histogram. The nerve discharges only during one phase of the wave-

form (the positive-going sections of the waveform). The PST histogram displays the ability of the nerve to discharge in synchrony with the period

of the input stimulus envelope, at least for low-frequency stimulation. Source: From Yost (2000), based on a figure from Hind, Anderson,

Brugge, and Rose (1967), with permission.