These sounds at different locations create an auditory space , which exists all
around, wherever there is sound. This locating of sound sources in auditory space
is called auditory localization .
Comparing location information for vision and hearing. Vision: The bird and the
cat, which are located at different places, are imaged on different places on the
retina. Hearing: The frequencies in the sounds from the bird and cat are spread
out over the cochlea, with no regard to the animals’ locations.
This means that two tones with the same frequency that originate in different
locations will activate the same hair cells and nerve fibers in the cochlea. The
auditory system must therefore use other information to determine location. The
information it uses involves location cues that are created by the way sound
interacts with the listener’s head and ears.
There are two kinds of location cues: binaural cues, which depend on both ears,
and monaural cues, which depend on just one ear. Researchers studying these
cues have determined how well people can locate the position of a sound in three
dimensions: the azimuth , which extends from left to right; elevation , which
extends up and down; and the distance of the sound source from the listener. In
this chapter, we will focus on the azimuth and elevation.
Binaural Cues for Sound Localization
Binaural cues use information reaching both ears to determine the azimuth (left
right position) of sounds. The two binaural cues are interaural time difference and
interaural level difference. Both are based on a comparison of the sound signals
reaching the left and right ear
Interaural Time Difference
The interaural time difference (ITD) is the difference between when a sound
reaches the left ear and when it reaches the right ear
If the source is located directly in front of the listener, at A, the distance to each
ear is the same; the sound reaches the left and right ears simultaneously, so the
ITD is zero. However, if a source is located off to the side, at B, the sound reaches
the right ear before it reaches the left ear.
The magnitude of the ITD can be used as a cue to determine a sound’s location
ITD is an effective cue for localizing lowfrequency sounds
Interaural Level Difference
The other binaural cue, interaural level difference (ILD) , is based on the
difference in the sound pressure level (or just “level”) of the sound reaching the
two ears. A difference in level between the two ears occurs because the head is a
barrier that creates an acoustic shadow , reducing the intensity of sounds that
reach the far ear. This reduction of intensity at the far ear occurs for high
frequency sounds, but not for lowfrequency sounds
Example: Because the ripples are small compared to the boat, they bounce off the
side of the boat and go no further. Now imagine the same ripples approaching the
cattails in Figure 12.5d. Because the distance between the ripples is large compared to the stems of the cattails, the ripples are hardly disturbed and continue
on their way.
Notice that at high frequencies, there is a large difference between the ILD for
sounds located at 10 degrees (green curve) and 90 degrees (blue curve). At lower
frequencies, however, there is a smaller difference between the ILDs for sounds
coming from these two locations until, at very low frequencies, the ILD is a very
poor indicator of a sound’s location.
The Cone of Confusion
Because the time and level differences can be the same at a number of different
elevations, they cannot reliably indicate the elevation of the sound source. Similar
ambiguous information is provided when the sound source is off to the side.
These places of ambiguity are illustrated by the cone of confusion
All points on this cone have the same ILD and ITD
In other words, there are many locations in space where two sounds could result
in the same ILD and ITD.
Monaural Cue for Localization
Monaural cue —a cue that depends on information from only one ear.
The primary monaural cue for localization is called a spectral cue , because the
information for localization is contained in differences in the distribution (or
spectrum) of frequencies that reach each ear from different locations.
Differences in the way the sounds bounce around within the pinna create different
patterns of frequencies for the two locations ( King et al., 2001). The importance
of the pinna for determining elevation has been demonstrated by showing that
smoothing out the nooks and crannies of the pinnae with molding compound
makes it difficult to locate sounds along the elevation coordinate
They determined how localization changes when the mold is worn for several
weeks, and then what happens when the mold is removed.
After measuring initial performance, Hofman fitted his listeners with molds that
altered the shape of the pinnae and therefore changed the spectral cue.
Localization performance is poor for the elevation coordinate immediately after
the mold is inserted, but locations can still be judged at locations along the
Apparently, the person had learned, over a period of weeks, to associate new
spectral cues to different directions in space.
Localization remained excellent immediately after removal of the ear molds
The Physiology of Auditory Localization
The Auditory Pathway and Cortex
The auditory nerve carries the signals generated by the inner hair cells away from
the cochlea and toward the auditory receiving area in the cortex.
Auditory nerve fibers from the cochlea synapse in a sequence of subcortical
structures —structures below the cerebral cortex. This sequence begins with the
cochlear nucleus and continues to the superior olivary nucleus in the brain
stem, the inferior colliculus in the midbrain, and the medial geniculate nucleus
in the thalamus. From the medial geniculate nucleus, fibers continue to the primary auditory
cortex (or auditory receiving area, A1 ), in the temporal lobe of the cortex.
Acronym SONIC MG (a very fast sports car), which represents the three
structures between the cochlear nucleus and the auditory cortex, as follows: SON
= superior olivary nucleus; IC = inferior colliculus; MG = medial geniculate
Processing in the superior olivary nucleus is important for binaural localization
because it is here that signals from the left and right ears first meet
Auditory signals arrive at the primary auditory receiving area (A1) in the temporal
lobe and then travel to other cortical auditory areas:
o the core area , which includes the primary auditory cortex (A1) and some
o the belt area , which surrounds the core, and
o the parabelt area, receives signals from belt area
The Jeffress Neural Coincidence Model
The Jeffress model of auditory localization proposes that neurons are wired so
they each receive signals from the two ears
How the circuit proposed by Jeffress operates. Axons transmit signals from the
left ear (blue) and the right ear (red) to neurons, indicated by circles.
o Sound in front. Signals start in left and right channels simultaneously.
o Signals meet at neuron 5, causing it to fire.
o Sound to the right. Signal starts in the right channel first.
o Signals meet at neuron 3, causing it to fire. (Coming from the right, it gets
a head start)
This neuron and the others in this circuit are called coincidence detectors ,
because they only fire when both signals coincide by arriving at the neuron
simultaneously. The firing of neuron 5 indicates that ITD = 0.
This has been called a “place code” because ITD is indicated by the place (which
neuron) where the activity occurs.
One way to describe the properties of ITD neurons is to measure ITD tuning
curves , which plot the neuron’s firing rate against the ITD.
Graph: IDT vs. Firing Rate
Broad ITD Tuning Curves in Mammals
The “range” indicator below each curve indicates that the gerbil curve is much
broader than the owl curve. The gerbil curve is, in fact, broader than the range of
ITDs that typically occur in the environment
Responses recorded from a neuron in the left auditory cortex of the monkey to
sounds originating at different places around the head. The firing of a single
cortical neuron to a sound presented at different locations around the monkey’s
head is shown by the records at each location. his neuron responds to sounds
coming from a number of locations on the right.
According to this idea, there are broadly tuned neurons in the right hemisphere
that respond when sound is coming from the left and broadly tuned neurons in the
left hemisphere that respond when sound is coming from the right. To summarize research on the neural mechanism of binaural localization, we can
conclude that it is based on sharply tuned neurons for birds and broadly tuned
neurons for mammals. The code for birds is a place code because the ITD is
indicated by firing of neurons at a specific place. The code for mammals is a
distributed code because the ITD is determined by the firing of many broadly
tuned neurons working together
Localization in Area A1 and the Auditory Belt Area
Found that destroying A1 decreased, but did not totally eliminate, the ferrets’
ability to localize sounds.
Showed that deactivating A1 in cats by cooling the cortex results in poor
These studies also showed that destroying or deactivating areas outside A1
Gregg Recanzone (2000) compared the spatial tuning of neurons in A1 and
neurons in the posterior area of the belt. He found that neurons in A1 respond
when a sound is moved within a specific area of space and don’t respond outside
that area. When he then recorded from neurons in the posterior belt area, he found
that these neurons respond to sound within an even smaller area of space,
indicating that spatial tuning is better in the posterior belt area. Thus, neurons in
the belt area provide more precise information than A1 neurons about the location
of sound sources.
Moving Beyond the Temporal Lobe: Auditory Where (and What) Pathways
Two auditory pathways extend from the temporal lobe to the frontal lobe. These
pathways, like the what and where pathways
The what pathway, which starts in the front (anterior) part of the core and belt
and extends to the prefrontal cortex. The what pathway is responsible for
identifying sounds. The where pathway, which starts in the rear (posterior) part of
the core and belt and extends to the prefrontal cortex. This is the pathway
associated with locating sounds.
Thus, the posterior belt is associated with spatial tuning, and the anterior belt is
associated with identifying different types of sounds. This difference between
posterior and anterior areas of the belt represents the difference between where
and what auditory pathways.
temporarily deactivating a cat’s anterior auditory areas by cooling the cortex
disrupts the cat’s ability to tell the difference between two patterns of sounds, but
does not affect the cat’s ability to localize sounds. Conversely, deactivating the
cat’s posterior auditory areas disrupts the cat’s ability to localize sounds, without
affecting the cat’s ability to tell the difference between different patterns of sounds
Lesion and cooling studies indicate that A1 is important for localization.
However, additional research indicates that processing information about location
also occurs in the belt area and then continues farther in the where processing
stream, which extends from the temporal lobe to the prefrontal area in the frontal
lobe. Hearing Inside Rooms
If you are listening to someone playing a guitar on an outdoor stage, some of the
sound you hear reaches your ears after being reflected from the ground or objects
like trees, but most of the sound travels directly from the sound source to your
ears (Figure 12.20a). If, however, you are listening to the same guitar in an
auditorium, then a large proportion of the sound bounces off the auditorium’s
walls, ceiling, and floor before reaching your ears
The sound reaching your ears directly, along path 1, is called direct sound ; the
sound reaching your ears later, along paths like 2, 3, and 4, is called indirect
Perceiving Two Sounds That Read the Ears at Different Times
The speaker on the left is the lead speaker, and the one on the right is the lag
speaker. If a sound is presented in the lead speaker followed by a long delay
(tenths of a second), and then a sound is presented in the lag speaker, listeners
typically hear two separate sounds—one from the left (lead) followed by one from
the right (lag). But when the delay between the lead and lag sounds is much
shorter, something different happens. Even though the sound is coming from both
speakers, listeners hear the sound as coming only from the lead speaker. This
situation, in which the sound appears to originate from the lead speaker, is called
the precedence effect because we perceive the sound as coming from the source
that reaches our ears first
The precedence effect governs most of our indoor listening experience
The precedence effect means that we generally perceive sound as coming from its
source, rather than from many different directions at once.
Architectural acoustics , the study of how sounds are reflected in rooms, is
largely concerned with how indirect sound changes the quality of the sounds we
hear in rooms. The major factors affecting indirect sound are the size of the room
and the amount of sound absorbed by the walls, ceiling, and floor.
If most of the sound is absorbed, then there are few
sound reflections and little indirect sound. If most of the
sound is reflected, there are many sound reflections and a
large amount of indirect sound. Another factor affecting
indirect sound is the shape of the room. This determines how
sound hits surfaces and the directions in which it is reflected.
he amount and duration of indirect sound produced by a room is expressed as
reverberation time —the time it takes for the sound to decrease to 1/1000th of its
original pressure (or a decrease in level by 60 dB).
If the reverberation time of a room is too long, sounds become muddled because
the reflected sounds persist for too long. In extreme cases, such as cathedrals with
stone walls, these delays are perceived as echoes, and it may be difficult to
accurately localize the sound source. If the reverberation time is too short, music
sounds “dead,” and it becomes more difficult to produce highintensity sounds. Acoustics in Concert Halls
Intimacy time: The time between when sound arrives directly from the stage and
when the first reflection arrives. This is related to reverberation but involves just
comparing the tim