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Chapter 10

PSYB51 Chapter 10


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier
Chapter
10

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Chapter 10: Hearing in the Environment
Sound Localization
-Even though sound travels very fast, the pressure wave will not arrive at both ears at
the same time
-The intensity of sound is greater at the ear closer to the source
Interaural Time Difference
-Interaural time difference the difference in time between a sound arriving at one
ear vs the other
We tell where a sound is coming from by determining which ear receives the
sound first
-Azimuth: the angle of a sound source on the horizontal plane relative to a pnt in the
center of the head between the ears. It is measured in degrees, with 0 degrees
being straight ahead. The angle increases clockwise toward the right, with 180
degrees being directly behind
Figure 10.4 page 251
Red circles indicate positions from which a sound will reach the right ear before
the left ear; blue circles show positions from which a sound will reach the left
ear first. The size and brightness of each circle represent the magnitude of the
ITD
-ITDs are largest about 640 microseconds when sound come directly from the left or
directly from the right
A sound coming from directly in front of or directly behind ITD = 0
A sound source located at 60 degrees produce an ITD of 480 microseconds; at
an angle of -20 degrees an ITD of -200 microseconds
Physiology of ITDs
-Binaural input enters almost every stage of the auditory nervous system after the
auditory nerve.
As information moves upward through the system, with every additional synapse
the timing between the two ears is likely to become less precise
-The medial superior olives are the first places in the auditory system where inputs
from both ears converge
There are neurons here whose firing rates increase in response to very brief
time difference
-The interpretation of ITDs is critically dependent on the size of the head
Interaural Level Difference
-Interarual level difference the difference in level b/w a sound arriving at one ear vs
the other
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-Sounds are more intense at the ear closer to the sound source b/c the head partially
blocks the sound pressure wave from reaching the opposite ear
-*Sounds are more intense at the ear that is closer to the sound source, and less
intense at the ear farther away from the source
-*The ILD is larges at 90 and -90 degrees, and is nonexistent at 0 degrees and 180
degrees
-Between these two extremens, the ILD generally correlates with the angle of the
sound source, but because of the irregular shape of the head, the correlation is not
quite as great as it is with ITDs
-Difference between ILD and ITD: the head blocks high frequency sounds much more
effectively than it does low frequency sounds
Long wavelengths bend around the heard
ILDs are greatest for high frequency tones, and provides the best info about
sound source location
-Physiology of ILDs
-Neurons that are sensitive to intensity diff found in the lateral superior olives (LSO): a
relay station in the brain stem where inputs from both ears contribute to the detection
of the interaural level difference
-Excitatory connections come from the isperlateral ear; inhibitory connections from
the contralateral ear via the medial nucleus of the trapezoid body
-What makes neurons in the LSOs so sensitive to differentiate in intensity is the
competition between excitatory and inhibitory inputs
-Cones of Confusion
-An ITD of -480 microseconds arises from an angle of -60 or an angle of -120
-ILDs for these two angles are also identical
-Cone of confusion
-A region of positions in space where all sounds produce the same time and level
differences (ITD and ILD)
-Wallach study: head position is fixed and is surrounded by a vertically oriented,
cylindrical screen covered with vertical stripes.
-The cylinder starts to rotate and you experience an induced motion effect
-A sound is activated that is located directly in front of you
-You heard the sound as either coming from directly above or directly below you the
only two pnts from which the ITD and ILD of a sound remain constant
-Experiments like the one above establish that cones of confusion are real perceptual
phenomenon
-In the real world, our heads are not held fixed and as we move our heads, the ITD
and ILD of a sound source shift, and only one spatial location will be consistent
-Pinna and Head Cues
-The pinnae serve to funnel sound energy into the ear canal
-They funnel certain sound frequencies more efficiently
-The intensity of each frequency varies slightly according to the direction of the sound
-Tiny microphones can be inserted inside your auditory canals
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