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Library of Congress Cataloging-in-Publication Data
Goldstein, E. Bruce, [date J
Sensation and perception I E. Bruce Goldstein.-6th ed.
p. em.
Includes bibliographical references and index.
ISBN 0-534-63991-7
l. Senses and sensation. 2. Perception. I. Title.
QP431 .G64 2002
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Frequency (kHz)
located near the base of the cochlea and is most sensitive at about 18,000 Hz (Russell & Sellick, 1977).
The frequency to which the hair cell is most sensitive
is called the characteristic frequency of the cell.
Figure 10.32 shows that the tuning curves for auditory
nerve fibers are similar to hair-cell tuning curves.
It is clear from the physiological evidence that
specific frequencies are signaled by basilar membrane vibration at specific places along the cochlea,
and this determines which nerve fibers along the
cochlea will fire. The idea that frequencies are represented by the firing of fibers located at specific places
along the cochlea has been confirmed by the results
of psychophysical experiments.
Psychophysical Masking and Place Coding
One confirmation of place theory has been provided
by the results of experiments using a psychophysical
technique called auditory masking. The basic principle behind masking is that one tone, if intense
enough, can mask or decrease our perception of
another tone that is occurring at the same time.
The procedure for a masking experiment is
shown in Figure 10.33. First, the threshold for hearing is determined across a range of frequencies, by
determining the lowest intensity at each frequency
that can just be heard (Figure l0.33a). Then, a masking stimulus is presented at a particular place along
the frequency scale, and, while the masking stimulus
Figure 10.32
Frequency tuning curves of cat
auditory nerve fibers. The characteristic frequency of each fiber is
indicated by the arrows along the
frequency axis. (From Palmer,
1987.)
is sounding, the thresholds for all frequencies are
redetermined (Figure 10.33b).
We will describe an experiment by J. P. Egan and
H. W. Hake (1950) in which they used a masking
noise that contained frequencies ranging from 365 to
455 Hz. When Egan and Hake measured the thresholds in the presence of this masking noise, they
observed the result shown in Figure 10.34, which
indicates how the masking noise increased the original threshold at each frequency. Notice that frequencies near the masking frequencies are affected the
most. Also, notice that this curve is not symmetrical.
(a)
Measure
threshold
Frequency
(b)
Remeasure
threshold with
the masking
tone present
Masking tone
Figure 10.33
The procedure for a masking experiment. (a) Threshold is
detem1ined across a range of frequencies. Each arrow indicates a frequency where the threshold is measured. (b) The
threshold is redetennined at each frequency (small arrows)
in the presence of a masking stimulus (large arrow).
Base
Apex
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Figure 10.34
Results of Egan and Hake's (1950) mashing
experiment. The threshold increases the most
near the f-equencies of the mash 6 noise,
and the masking effect sfJreads more to high
f·equencies. (Listen to WebTutor, Masking
Q)
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10
100
200
300
500 700 1,000
2,000 3,000
5,000
10,000
High and Low Frequencies.)
Frequency of test tone (Hz)
That is, the masking effect spreads more to high frequencies than to low frequencies.
We can relate this asymmetrical effect of the
masking tone to the vibration of the basilar membrane, by looking at Figure 10.35, which reproduces
the vibration patterns caused by 200- and 800-Hz test
tones and a 400-Hz masking tone. We can see how a
400-Hz masking tone would affect the 200- and 800Hz tones by noting how their vibration patterns overlap. Notice that the pattern for the 400-Hz tone,
which is shaded, almost totally overlaps the pattern
for the higher-frequency 800-Hz tone, but that it does
not overlap the peak vibration of the lower-frequency
;Mask
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Figure 10.35
Vibration patterns caused by 200- and 800-H:z test tones
and the 400-Hz mask, taken from basilar membrane vibration patterns in Figure 10.29. The pattern for a 400-Hz tone
is used for the masking pattern (shaded). Since the mask
actually contains a band of frequencies, the actual pattern
would be wider than is shown here. It would, however, still
be asymmetrical and would overlap the 800-Hz vibration
more than the 200-Hz vibration.
200-Hz tone. We would therefore expect the masking
tone to have a large effect on the 800-Hz tone but a
smaller effect on the 200-Hz tone, and this is exactly
what happens. Compare the heights of the curve in
Figure 10.34 at 200 and 800 Hz. Thus, Bekesy's
description of the way the basilar membrane vibrates
predicts the masking function in Figure 10.34.
The Basilar Membrane
as a Frequency Analyzer
We've seen that there is a great deal of evidence to
support the idea that a particular frequency causes
maximum activity at a specific place along the basilar
membrane. Auditory researchers have carried this
idea of frequency being represented along the basilar
membrane a step farther by proposing that the cochlea operates as if it consists of a bank of filters, each of
which processes a limited band of frequencies.
One way to visualize this bank of filters is to look
back at the neural tuning curves in Figure 10.32.
Each of these curves represents the band of frequencies that causes auditory nerve fibers located at different places along the basilar membrane to fire. For
example, Curve A is for a fiber near the apex of the
basilar membrane of a cat that responds to frequencies
between about 300 and 900 Hz. Curve B is for a fiber
near the base of the basilar membrane that responds
to frequencies between about 20,000 and 30,000 Hz.