Download Directional Perception in the Human Auditory System

J. Undergrad. Sci. 3: 135-140 (Fall 1996)
Organismic & Evolutionary Biology
Directional Perception in the Human Auditory System
SUSAN J. SHAW
Human perception of auditory direction was tested in
the horizontal and mid-sagittal planes. When the auricular funneling and baffling effects are bypassed by extensions of the ear canals, the accuracy of perception
worsens significantly. Providing artificial baffling posterior to the extensions of the ear canals improves perception. The accuracy of perception also worsens when
the auricular funneling is eliminated by a modeling material filling the conchae while leaving artificial canals
for sound entrance. These results indicate that both the
auricular funneling and baffling contribute significantly
to the anterior versus posterior differentiation. Similarly,
the accuracy of superior versus inferior perception is
worsened by the modeling material filling the conchae.
Prior demonstrations improve the accuracy of perception when the artificial canals created by the modeling
material are non-horizontal. However, prior demonstrations do not significantly improve the accuracy of perception when the artificial canals are horizontal. This
suggests that the natural slope of the ear canals provides a learnable cue for distinguishing superior versus inferior sounds. The findings can explain why the
hearing aids worsen accuracy of directional perception
and why the human ear canals are not horizontal.
Introduction
Sound localization is generally believed to be primarily
a binaural phenomenon. The interaural disparities in time
and intensity are the two most important mechanisms by
which animal and human ears perceive locations of sounds
in space.1-4 It has been shown that neurons in various parts
of the auditory pathway are very sensitive to differences in
arrival time and intensity between the two ears.5-8 The sound
localization acuity varies with azimuthal direction, but does
not vary significantly with stimulus duration.9
However, the interaural disparities in time and intensity
do not provide all of the cues for sound localization. For
example, obviously other cues will be needed to localize a
sound that emanates from the mid-sagittal plane because
the sound will arrive at both ears at practically the same
time with practically the same intensity.10 It has been shown
that changes in spectral content of wide-band auditory stimuli
can serve as a relative cue for auditory distance.11
The role of the external ears, including the auricles and
the ear canals, has received relatively little attention. It has
been shown that distortions of the auricular shapes can
adversely affect the auditory directional perception.10, 12 The
cues provided by the auricles and the ear canals for directional perception will be explored in this study.
It is known that the auricles have funneling and baffling effects on sounds and that the auricles help in distinguishing between sounds coming from the front and the
back.3 It is not very clear yet whether funneling or baffling
or both contribute to this differentiation of anterior versus
posterior sounds. This is explored in the first part of this
study.
The barn owl is one of the most widely studied animals
for sound localization. It has been shown that the barn owl
has the best ability to accurately localize sounds out of all
the species that have been studied. The barn owl’s left ear
is higher than the eye level and points downward while its
right ear is lower than the eye level but points upward. This
unique anatomy gives barn owls exceptional ability to localize sounds in the vertical plane.13, 14 In humans the lateral
two thirds of the external ear canals slant upward, while the
medial one third of the canals slants downward. Whether or
not this carries any physiological meaning in human auditory function has attracted very little attention. The second
part of this study further explores the roles of the auricles
and the upward slant of the lateral two thirds of the ear canals in directional perception in the mid-sagittal plane.
Methods
To present sounds from different directions at the same
distance, a circular frame was built. The frame consists of a
circular plastic tube supported by wood and metal strips,
forming a circle with a diameter of 172 cm. It was calibrated
for 360 degrees.
Twelve identical 400±100 Hz direct current mini-buzzers were utilized. The buzzers were from the Radio Shack
Company, Fort Worth, Texas. They were checked with a
Realistic Sound Level Meter, which was also from the Radio
Shack Company, to ensure that the buzzers produced the
same sound intensities. The sound levels produced by the
twelve 400 Hz buzzers were all within 72±2 dB at 86 cm, the
testing distance in this study.
In the first part of this study, twelve buzzers were
mounted on the circular frame, positioned in the horizontal
plane. The buzzers were evenly spaced 30° apart. Each
buzzer was connected through a wire and a switch to a sixvolt battery. In the second part of this project, three buzzers
were mounted on the circular frame, which was positioned
in the mid-sagittal plane. The buzzers were 45° apart and
each buzzer was similarly connected through a wire and a
switch to a six-volt battery.
During the tests, the subjects were blindfolded so that
they could not see which switch was being turned on. Twenty
voluntary subjects of 18 to 50 years, mean 36 years, 10
males and 10 females, were tested. No subject had a past
history of hearing problems.
The buzzers were sounded in random order, with each
stimulus lasting two seconds. For horizontal plane testing,
after each stimulus the subject was instructed to point with
his/her arm in the perceived direction of the sound. For midsagittal plane testing, the subject was instructed to give a
verbal selection of one out of the three buzzer directions as
being closest to the perceived sound direction. If unsure
about the perceived sound direction, the subject was requested to give a best guess. This study was performed in a
soundproof, echo-free room.
For tests in the horizontal plane, the circular frame was
set up at the level of each subject’s ears, with the head in
SUSAN J. SHAW is a first-year student at Northwestern University. She completed the research described in this article under the supervision of Sharon Writer, Marilyn Hettick, Norm
Fjeldsted, and David Shaw, M.D., M.P.H. Ms. Shaw was honored as a Westinghouse Science Talent Search finalist for this work, and she received first place in the 1995 California State
Science Fair. She aspires to a career in academic medicine.
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Org. & Evol. Biol.
Figure 1. Plan view of buzzer directions in the
horizontal plane. Subject’s head is at the center
of the circular frame.
Figure 2. Plan view of buzzer directions
in the mid-sagittal plane. Subject’s head
is at the center of the circular frame.
Figure 3. Schematic drawing of ear canal
extensions and artificial baffling sheets.
the center. The directions in this plane were designated as
shown in Figure 1. For tests in the mid-sagittal plane, the
subject’s head was again located at the center of the circle.
The subject’s ear level was at the same distance from the
roof and from the floor of the room. The directions in this
plane were designated as shown in Figure 2. No head movement was allowed during the tests.
Horizontal Plane: Effect of Auricular Funneling and
Baffling. A series of related experiments were performed to
explore the role of the auricles in human auditory directional
perception. The auricles are known to have funneling and
baffling effects on sounds. These experiments were designed
to explore whether the funneling effect, the baffling effect, a
combination of the two, or neither of the two are related to
the human’s ability to distinguish anterior versus posterior
sounds.
In Experiment I, an extension structure that projected laterally beyond the auricle was put in the opening of each external ear canal. The purpose of this design was to let the sound
enter the ear canal through the extension structure, thereby
bypassing the auricular funneling and baffling effects. Various
types of structures had been tried until a modified plastic stethoscope was judged the most satisfactory. The stethoscope was
modified from UniScope, manufactured by Bio-Dynamics, Inc.,
Indianapolis, Indiana. The short ear pieces were replaced with
long 4.2 cm ear pieces. The ear pieces were long enough to
extend laterally beyond the auricles of every subject. Each ear
piece had an internal diameter of 4.5 mm. The inner canal of
the U-shaped tube, which connected the two ear pieces, was
occluded. The elasticity of the U-shaped tube helps to press
the ear pieces tightly against the ear canal openings. A 4.4 mm
hole was drilled through the lateral wall of each ear piece to
allow sound waves to enter. The design of the ear canal extensions is to mimic the sound entrance of the behind-the-ear (BTE)
hearing aids.
The design of Experiment II included the ear canal extensions plus artificial baffling structures. The artificial baffling structures were made of flat plastic sheets, 6.5 cm x
2.5 cm. The size of the sheets was comparable to the size
of human auricles, which have average lengths of 6-6.5 cm
and average lateral protrusions of 2-2.5 cm. The plastic
sheets were attached to the ear pieces of the modified stethoscope. To simulate the baffling effect of the auricles, the
plastic sheets were kept vertical in the coronal plane and
attached lateral to and posterior to the entrances of the ear
pieces, in a fashion similar to the spatial relationship between the auricles and the opening of the ear canals. (See
Figure 3). In contrast to the concave shape of the auricles,
the plastic sheets were flat so that the baffling effect is preserved while the funneling effect is mostly eliminated.
Experiment III required placing a modeling material in the
conchae portion of the auricles, and leaving artificial canals for
sound entrance. The eraser end of a pencil, with a transverse
diameter of 7 mm, was gently inserted at an upward angle of
15° into the external ear opening during the procedure. There
were two purposes of using the pencil. One was to prevent the
modeling material from entering the ear canal, and the other
was to create an artificial ear canal. A square head protractor
from the General Tools Manufacturing Company in New York
was used to measure the angles. Each concha was filled with
the modeling material until the surface was flush. The pencil
was then removed, leaving an artificial canal 7 mm in diameter
slanting 15° upward, which is similar to the natural slope of the
lateral part of the external ear canals. After trying several modeling materials, Play-Doh, from Playskool, Inc., Pawtucket,
Rhode Island, was found to be the most satisfactory. The purpose of this design was to eliminate the funneling function of
the conchae while leaving the auricular baffling effect mostly
unaffected.
Experiments I, II and III were compared with the control, in which no material was attached to the outer ears or
put in the conchae.
While the subject was still blindfolded, the buzzers were
randomly rearranged before each new experiment. Before
each horizontal plane experiment, the subjects received a
standard verbal explanation of the task but received no demonstration of the buzzers. This was to ensure that the subjects’ directional perception under various experimental setups was not influenced by prior experience.
Vertical Plane: Effect of Ear Canal Slants. The auditory
differentiation of superior versus inferior sounds in the midsagittal plane was tested. The subjects were asked to distinguish among three different directions of sounds: Superior 45°, Level 0°, and Inferior 45°, as shown in Figure 2.
In the control, the subjects did not have any modeling
material put into the conchae. In Experiments IV through
VII, the conchae of the subjects were filled with a modeling
material as before, except that the artificial canals slanted
in different directions. The directions of the artificial canals
created by the modeling material were 15° upward for
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Figure 4. Directional perception in the horizontal plane: perceived sound directions vs. actual sound directions. In 4A, the control, the natural auricular
funneling and baffling were not inhibited. The directions that subjects thought the sounds came from were very close to the actual ones. 4B is a scattergram of
Experiment I, which utilized ear canal extensions to bypass both auricular funneling and baffling. This resulted in front-back reciprocal confusion. 4C shows the
additional effect of artificial posterior baffling in Experiment II, created by attaching plastic sheets behind the ear extensions. Confusion partially decreased in the
posterior half-field (210°-330°). In 4D, Experiment III, the conchae was filled with a modeling material, thus eliminating natural auricular funneling. This resulted in
significant anterior versus posterior reciprocal confusion, though more so in the anterior half-field (30°-150°).
Experiment IV, 45° upward for Experiment V, 45° downward
for Experiment VI, and 0° (horizontal) for Experiment VII.
For each experiment, each buzzer was tested ten times in
random sequence. The buzzers were randomly replaced
after each experiment.
Each experiment in the mid-sagittal plane was initially performed with prior verbal explanation but no prior demonstra-
tion with the buzzers. Then each experiment was repeated after three demonstrations of buzzing from each of the three
sound directions. The buzzers used for demonstration were
replaced before testing. The purpose of the demonstrations
was to explore whether there was any cue learnable by the
subjects to consciously adjust their directional perception under different slanting angles of the artificial canals.
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Results
Horizontal Plane: Effects of Auricular Funneling and
Baffling. The results of the control and Experiments I, II
and III are shown in Figure 4. The scattergrams show each
subject’s perception of sound direction for a particular orientation of a buzzer.
Auricular Funneling and Baffling Effects Unmodified; Control
Experiment. The scattergram in Figure 4A shows the individual
results of the twenty subjects for sounds emanating from the
horizontal plane. Most of the perceived directions are very close
to the correct orientations of the buzzers, as anticipated.
Auricular Funneling and Baffling Effects Bypassed (mimicking
BTE hearing aids); Experiment I. The ear canal extensions,
which allowed sounds to enter the extensions at locations lateral to the auricles, were designed to eliminate both funneling
and baffling effects of the auricles. The scattergram in Figure
4B shows that there is very little confusion of right versus left,
although there is tremendous confusion of anterior versus posterior. However, the confusion is not random. Subjects tend to
be confused between reciprocal directions in the anterior and
the posterior half fields. For example, for sounds coming from
the 120° (right anterior 30°) direction, 9 out of 20 (45%) responses were within ±30° of the correct 120° direction; while 6
out of 20 (30%) of the responses were within ±30° of the reciprocal 240° (right posterior 30°) direction. The orderly anteriorposterior confusion suggests that the interaural time disparity
is still working to enable the subjects to narrow the responses
down to either around the correct directions or around the reciprocal directions. If the sound sources of 180° (due right direction) and 360° (due left direction) are excluded, the rest can
be divided into anterior and posterior fields. The results show
that, for sounds from the anterior field, 57% are perceived as
coming from the anterior field; while 43% are perceived as coming from the posterior field. For sounds from the posterior field,
61% are perceived as from the posterior field, while 39% are
perceived as from the anterior field.
Auricular Funneling and Baffling Effects Bypassed, Artificial
Baffling Created; Experiment II. For the anterior field, the
results are similar to those of Experiment I, as portrayed in
Figure 4C. For sounds from the anterior field, 60% are perceived as coming from the anterior field, while 40% are perceived as coming from the posterior field. The confusion is
decreased for sounds from the posterior field, where 71%
are perceived as from the posterior field, while 29% are
perceived as from the anterior field.
Auricular Funneling Effect Bypassed (mimicking ITE hearing
aids); Experiment III. For sounds emanating from the anterior
field, the results are similar to Experiment II, with 56% being
perceived as from the anterior field and 44% being perceived
as from the posterior field, as portrayed in Figure 4D. The accuracy is better for sounds emanating from the posterior field,
for which 73% are perceived as coming from the posterior field,
while 27% are perceived as coming from the anterior field.
The results of the control and Experiments I, II and III
are compared in Figure 5, with the data of statistical analysis by χ2 test shown below. In comparing Experiment I with
the control, the χ2 is 242 for the anterior field and 130 for the
posterior field, with P < 0.005 for both. This indicates that
the auricles have significant effect on anterior versus posterior directional perception for 400 Hz sounds in the horizontal
Figure 5. Directional perception in the horizontal plane: percentage of
perceived sound directions in the correct half-field. A is the control; B
(Experiment I) is with ear canal extensions; C ( Experiment II) is with artificial
baffling sheets added to the ear canal extensions; D (Experiment III) is with
modeling material filling the conchae. Comparison between A and B: Anterior
field, χ2 = 242 (P < 0.005); Posterior field, χ2 = 130 (P < 0.005). Comparison
between B and C: Anterior field, χ2 = 0.37 (P > 0.25); Posterior field, χ2 = 4.2 (P
< 0.05). Comparison between A and D: Anterior field, χ2 = 256 (P < 0.005);
Posterior field, χ2 = 49 (P < 0.005).
plane. This suggests that bypassing the auricular funneling
and baffling by the ear canal extensions worsens accuracy
of directional perception.
Comparison between Experiments I and II reveals that
there is no significant difference in the anterior field (χ2 =
0.37, P > 0.25). However, there is significant difference in
the posterior field (χ2 = 4.2, P < 0.05). Thus the artificial
baffling posterior to sound entrance can improve accuracy
of directional perception in the posterior field but does not
significantly affect the anterior field.
Comparison between Experiment III and the control reveals significant differences for both the anterior and posterior fields (χ2 = 256, P < 0.005, and χ2 = 49, P < 0.005,
respectively). This suggests that the funneling effect of the
conchae does significantly affect the anterior versus the
posterior auditory differentiation. The anterior field is more
affected than the posterior field. This shows that a material
filling the conchae, in a manner similar to ITE hearing aids,
can significantly worsen the accuracy of directional perception in the horizontal plane. Even though the auricular baffling and funneling effects are neither directly measured nor
completely separated, the results suggest that they both
contribute to anterior versus posterior auditory differentiation.
Vertical Plane: Effect of Ear Canal Slants. The subjects’ ability
to differentiate sounds emanating from three different elevations, Superior 45°, Level 0° and Inferior 45°, in the mid-sagittal plane were studied in Experiments IV to VII. Figure 6 and
Table 1 show the results of the control and the four experiments with various slanting angles of the artificial canals and
compares the data before and after demonstrations. The first
column of the table shows the direction of the three buzzers,
while the second row lists the perceived sound directions. The
data of statistical analysis by χ2 test is also shown in Figure 6.
Conchae and Ear Canal Slopes Unmodified; Control
Experiment. Without demonstration, for the Superior 45°
sounds, 85% are correctly perceived as coming from Superior 45° direction; while 12.5% are perceived as coming from
Level 0° direction and 2.5% are perceived as coming from
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Figure 6. Directional perception in the mid-sagittal plane: percentage of
correct distinctions among superior, level, and inferior sound directions.
A is the control, with no modeling material in the conchae. Demonstrations did
not significantly improve distinction: χ2 = 1.79 (P > 0.25). In B (Experiment IV)
conchae were filled with modeling material to make artificial canals sloping
15° upward. After demonstrations, percentage of correct distinctions improved
significantly: χ2 = 10.6 (P < 0.05). C (Experiment V ) is with 45° upward artificial
canals. Demonstrations also increased correctness in this experiment
significantly: χ2 = 31.7 (P < 0.005). D (Experiment VI), with 45° canals downward,
also showed improvement after demonstration: χ2 = 10.9 (P < 0.05). In E
(Experiment VII) artificial canals were level at 0°, which showed no significant
improvement after demonstrations: χ2 = 2.81 (P > 0.25). In comparing A and
B, the differences made by the artificial canals were significant for both before
(χ2 = 418, P < 0.005) and after demonstrations (χ2 = 350, P < 0.005).
Inferior 45° direction (Table 1A). For the Level 0° sounds,
86% of the responses are correct. For sounds coming from
the Inferior 45° buzzer, 88% of the responses are correct.
Thus, for the control, overall 86.3% of the responses are
correct when the testing was done without prior demonstration. The overall correct percentage improves only slightly
to 87.8% after 3 demonstrations. The difference before and
after demonstrations is insignificant, as evidenced by the
small χ2 (1.79) and the P being greater than 0.25. This suggests that the subjects have already learned enough from
daily experience and the three demonstrations do not significantly improve the results.
Conchae Filled for 15° Upward Artificial Canals; Experiment
IV. In this experiment, the slope of the artificial canals, created by the modeling material filling the conchae, is similar
to the slanting direction of the lateral portion of the human
ear canals. Without demonstration, overall only 59.2% of
the responses are correct (Table 1B). The overall correct
percentage improves to 65.8% after 3 demonstrations. Compared with the control, the χ2 is 418 before demonstrations
and 350 after demonstrations, with P < 0.005 for both. This
shows that modeling material occupying the conchae, as
ITE hearing aids do, does significantly worsen accuracy of
directional perception in the mid-sagittal plane. Comparison before and after demonstrations within Experiment IV
shows that the χ2 = 10.6 and P < 0.05. This suggests that a
cue is available for the subjects to learn to improve the results after demonstrations.
Conchae Filled for 45° Upward Artificial Canals; Experiment V.
Without demonstration, 47.3% of the responses are correct.
After three demonstrations, the figure rises to 57.3% (Table
1C). Comparing before and after demonstrations, χ2 = 31.7,
Table 1. Directional perception in the mid-sagittal plane: percentage of
the perceived sound directions for each of the three buzzer directions. A
= control; B = Experiment IV (artificial canals 15° upward); C = Experiment V
(artificial canals 45° upward); D = Experiment VI (artificial canals 45° downward);
E = Experiment VII (artificial canals horizontal). * = correct directional distinction.
which is evidence of significant improvement after demonstrations (P < 0.005). Thus, similar to Experiment IV, there is a cue
for the subjects to learn from the demonstrations.
Conchae Filled for 45° Downward Artificial Canals; Experiment
VI. Without prior demonstration, only 39.5% of the responses
are correct (Table 1D). This is quite poor because it is close
to random guessing for which about 33% would be correct.
Repeating the tests after 3 demonstrations improves the
overall correct percentage to 46%. Comparison between
before and after demonstrations shows that χ2 = 10.9 and P
< 0.05. Again, there is a cue from which the subjects can
learn.
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Conchae Filled for 0° Artificial Canals; Experiment VII. The
data show that 43% of the responses are correct before
demonstrations, while 45.5% of the responses are correct
after three demonstrations (Table 1E). In comparing these
two, χ2 = 2.81 and P > 0.25. Surprisingly, demonstrations do
not significantly improve the results. This indicates that there
is insufficient cue for the subjects to learn from the demonstrations when the artificial canals are horizontal.
The above results in the mid-sagittal plane testing can be
easily compared in Figure 6. These results suggest that horizontal ear canals would be the worst anatomic structure for
distinguishing sounds from different elevations in the mid-sagittal plane. When the ear canals are slanted either upward or
downward, there seems to be a cue for the subjects to learn to
improve the directional perception in the mid-sagittal plane.
Thus, both the conchae and the upward slanting of the human
ear canals contribute to the directional perception for sounds
from different elevations in the mid-sagittal plane.
Org. & Evol. Biol.
the mid-sagittal plane. Several subjects participated in extra testing. In this testing, the subjects received additional
demonstrations by being told the correct direction of the
sound after every incorrect answer. The percentages of correct answers further improve in those experiments with nonhorizontal artificial canals. However, for the experiment that
includes horizontal artificial canals, the percentage of correct answers does not improve significantly by extra demonstrations. This further shows that horizontal artificial canals provide little learnable cue for distinction between superior and inferior sounds. This finding suggests an explanation of why the human ear canals are not horizontal.
Acknowledgments
I would like to thank Marilyn Hettick, Sharon Writer and
Norm Fjeldsted for their advice and provision of equipment.
Also thanks to David Shaw, M.D., M.P.H., for assistance in
data analysis.
Discussion
References
People wearing hearing aids often have poor auditory directional perception. It is often believed that the underlying
cause is hearing loss accompanied by hearing distortion. Can
hearing aids help in the directional perception? Or do hearing
aids actually worsen directional perception? The ear canal extensions simulate the behind-the-ear (BTE) hearing aids that
receive sounds at locations outside the auricles. The results
show that the ear canal extensions, which bypass auricular
funneling and baffling, significantly worsen accuracy of directional perception. The most popular aids, in-the-ear (ITE), occupy the conchae. When a modeling material was used to fill
the conchae, as these ITE aids would, the results show significant worsening of directional perception. Similar results are
seen when the conchae are filled, leaving 15° upward artificial
canals. This indicates that this type of hearing aids, through
their occupancy of the conchae, deprive the wearers of an important cue in directional perception.
It is practically impossible to completely separate the
auricular funneling and baffling effects. This project is believed to be the first one attempting to explore the effects of
the auricular funneling and baffling separately. The results
suggest that each function does contribute individually to
the anterior versus posterior auditory differentiation.
Accuracy of directional perception in the mid-sagittal
plane worsens significantly after the conchae are filled with
a modeling material, leaving artificial canals at about the
normal angle. This indicates that the conchae also contribute to the auditory distinction between superior and inferior
sounds in the mid-sagittal plane. This is similar to the findings by Gardner and Gardner11 and by Oldfield and Parker12
that distortions of the shapes of the pinnae result in considerable degradation in sound localization. Since the opening
of the ear canals are not located at the center of the conchae, the funneling for superior sounds should be somewhat different from that of inferior sounds. It is postulated
that the eccentric locations of the ear canal openings in the
conchae and the asymmetric shapes of the conchae probably provide cues for auditory directional distinction.
The results in the mid-sagittal plane testing also show
that the slopes of the ear canals provide a cue for distinction between superior and inferior sounds. The nature of
the cue has yet to be determined.
The number of demonstrations influences the results
in the testing for effect of ear canal slants with sounds from
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