higher auditory processing
1. Sound Localization – comparing sounds from both ears
Localizing sound means determining where a sound is coming from in space. The brain uses three main cues to
achieve this:
1. Interaural Time Difference (ITD)
2. Interaural Level Difference (ILD)
3. Head-Related Transfer Function (HRTF)
These cues are based on comparisons between what each ear hears, and they are processed in different parts of
the auditory system.
Sound is first processed in the superior olive nuclei and needs input from both ears and start comparing
the sounds
2. Interaural Time Difference (ITD)
• Definition: ITD is the difference in arrival time of a sound at each ear.
• Works with: Low-frequency sounds (below ~1500 Hz).
• Why?: Low-frequency waves have long wavelengths, making it easier for the brain to compare when
the same wavefront reaches both ears.
How It Works
• When a sound comes from one side, it reaches the closer ear first before the further ear.
• The superior olivary nucleus in the brainstem compares the timing of the sound reaching each ear.
• This process is similar to binocular disparity in vision, which compares images from both eyes.
ITD Circuit (Neural Mechanism)
• The olivary nuclei contain neurons that act as motion detectors for sound.
, • Each neuron responds to a specific time difference.
• If sound from the left arrives earlier at the left ear, the neuron will fire accordingly.
• This is similar to the Reichardt motion detector, which detects motion in vision by using time delays.
3. Interaural Level Difference (ILD)
• Definition: ILD is the difference in sound intensity (loudness) between the two ears.
• Works with: High-frequency sounds (above ~1500 Hz).
• Why?: High frequencies don’t bend around objects (including the head) as much, so they create a
sound shadow.
How It Works
• The ear closer to the sound source receives a louder sound.
• The opposite ear receives a softer sound due to the shadow effect caused by the head.
• The medial geniculate nucleus (MGN) in the thalamus processes this difference.
• This cue only works for high frequencies because low frequencies wrap around the head, making
loudness similar in both ears.
4. Head-Related Transfer Function (HRTF)
• Definition: The way sound is modified by the shape of the head, ears (pinna), and body before reaching
the eardrum.
• Works with: High-frequency sounds (above ~4000 Hz).
• Why?: High-frequency sounds reflect off surfaces and create unique spectral changes.
How It Works
• Sound interacts with the pinna, torso, and head, altering its frequency spectrum.
• This spectrum is different depending on the sound’s elevation (height) and whether it comes from
front or back.
• The brain learns to associate these frequency changes with specific directions.
• We don’t know exactly where in the brain this matching process happens, but we know it occurs.
5. Problems in Sound Localization
Front-Back Ambiguity
• ITD and ILD tell us whether a sound is coming from left or right but not if it is in front or behind.
• The sound source can be at different positions but still have identical ITD and ILD values.
,Elevation Ambiguity
• ITD and ILD are not very useful for determining if a sound is above or below the listener.
• Elevation changes only slightly affect ITD and ILD unless they are extreme (e.g., sound coming from
directly above or below).
• This creates a "cone of confusion", where multiple locations share the same ITD and ILD.
How HRTF Solves These Problems
• The brain uses HRTF to differentiate between sounds in front vs. behind and above vs. below.
• Each individual has a unique HRTF based on their ear shape and body structure.
• The brain learns and adapts to this over time.
6. Adaptation to HRTF (Hofman et al., 1998)
• Scientists tested sound localization by altering the shape of the pinna.
• When a new ear shape was introduced, subjects lost the ability to determine elevation.
• Over 23 days, subjects adapted and regained their ability to localize elevation.
• This suggests that HRTF learning is plastic (changeable) and can adjust over time.
Summary
• ITD helps localize low-frequency sounds by comparing arrival times at each ear.
• ILD helps localize high-frequency sounds by comparing loudness differences.
• HRTF helps distinguish front vs. back and elevation by analyzing frequency changes caused by the
body.
• The brain processes these cues in different regions and is capable of adaptation.
1. What are the three main cues the brain uses to localize sound?
The brain primarily relies on three cues to determine the direction of a sound: Interaural Time Difference
(ITD), Interaural Level Difference (ILD), and Head-Related Transfer Function (HRTF). ITD helps localize
low-frequency sounds by comparing the arrival times of a sound wave at each ear. ILD is useful for high-
frequency sounds, as it detects differences in loudness between the two ears due to the head blocking some
of the sound. HRTF, the most complex cue, helps differentiate whether a sound comes from the front, back,
above, or below by analyzing how the ear, head, and torso modify sound frequencies before they reach the
eardrum.
2. What type of sounds does ITD work best with, and why?
ITD is most effective for low-frequency sounds, typically below 1500 Hz. This is because lower frequencies
have longer wavelengths, allowing the brain to detect the time difference between when the same
wavefront reaches both ears. For high-frequency sounds, the wavelengths are shorter, making it difficult to
distinguish whether a sound wave arriving at one ear is part of the same wave cycle as the one reaching the
other ear.
, 3. Where in the brain is ITD processed?
The processing of ITD occurs in the superior olivary nucleus, a structure located in the brainstem. This
nucleus receives input from both ears and compares the time at which the same sound arrives at each ear.
By analyzing these tiny differences, the brain can determine the horizontal location of the sound source.
4. How does ITD help in sound localization?
When a sound originates from one side of the head, it reaches the closer ear first, followed by the opposite
ear with a slight delay. The superior olivary nucleus in the brainstem detects this delay and determines the
direction of the sound based on how much earlier it arrived at one ear. The brain uses specialized neurons
that are tuned to fire only when sound signals from both ears arrive simultaneously. This process is similar to
how binocular vision works in depth perception, where the brain compares images from both eyes to gauge
distance.
5. What type of sounds does ILD work best with, and why?
ILD is most effective for high-frequency sounds, generally above 1500 Hz. High frequencies have shorter
wavelengths, which do not easily bend around objects, including the head. This creates a sound shadow,
making the sound quieter in the further ear. The brain detects this difference in loudness and uses it to infer
the location of the sound. In contrast, low-frequency sounds tend to wrap around the head, making them
roughly equal in intensity at both ears, which is why ILD is not useful for localizing them.
6. Where in the brain is ILD processed?
The processing of ILD takes place in the medial geniculate nucleus (MGN) of the thalamus. This region
compares the loudness levels between both ears and helps determine the horizontal direction of high-
frequency sounds. Unlike ITD, which is processed in the brainstem, ILD requires higher-level processing in
the auditory pathway.
7. Why doesn’t ILD work for low-frequency sounds?
Low-frequency sounds have longer wavelengths, which allows them to diffract (bend) around objects,
including the head. As a result, they reach both ears with very little difference in intensity, making ILD an
ineffective localization cue for low frequencies. The brain, therefore, relies more on ITD for low-frequency
sound localization.
8. What is the “cone of confusion” in sound localization?
The cone of confusion refers to a set of locations in space where sounds produce similar ITD and ILD
values, making it difficult to determine the exact origin of the sound. Sounds coming from the front and back
or from above and below can have the same ITD and ILD, leading to ambiguity. Despite this, humans are
generally good at distinguishing these locations, thanks to HRTF, which provides additional spectral
information to resolve the confusion.
9. How does the brain resolve the cone of confusion?
1. Sound Localization – comparing sounds from both ears
Localizing sound means determining where a sound is coming from in space. The brain uses three main cues to
achieve this:
1. Interaural Time Difference (ITD)
2. Interaural Level Difference (ILD)
3. Head-Related Transfer Function (HRTF)
These cues are based on comparisons between what each ear hears, and they are processed in different parts of
the auditory system.
Sound is first processed in the superior olive nuclei and needs input from both ears and start comparing
the sounds
2. Interaural Time Difference (ITD)
• Definition: ITD is the difference in arrival time of a sound at each ear.
• Works with: Low-frequency sounds (below ~1500 Hz).
• Why?: Low-frequency waves have long wavelengths, making it easier for the brain to compare when
the same wavefront reaches both ears.
How It Works
• When a sound comes from one side, it reaches the closer ear first before the further ear.
• The superior olivary nucleus in the brainstem compares the timing of the sound reaching each ear.
• This process is similar to binocular disparity in vision, which compares images from both eyes.
ITD Circuit (Neural Mechanism)
• The olivary nuclei contain neurons that act as motion detectors for sound.
, • Each neuron responds to a specific time difference.
• If sound from the left arrives earlier at the left ear, the neuron will fire accordingly.
• This is similar to the Reichardt motion detector, which detects motion in vision by using time delays.
3. Interaural Level Difference (ILD)
• Definition: ILD is the difference in sound intensity (loudness) between the two ears.
• Works with: High-frequency sounds (above ~1500 Hz).
• Why?: High frequencies don’t bend around objects (including the head) as much, so they create a
sound shadow.
How It Works
• The ear closer to the sound source receives a louder sound.
• The opposite ear receives a softer sound due to the shadow effect caused by the head.
• The medial geniculate nucleus (MGN) in the thalamus processes this difference.
• This cue only works for high frequencies because low frequencies wrap around the head, making
loudness similar in both ears.
4. Head-Related Transfer Function (HRTF)
• Definition: The way sound is modified by the shape of the head, ears (pinna), and body before reaching
the eardrum.
• Works with: High-frequency sounds (above ~4000 Hz).
• Why?: High-frequency sounds reflect off surfaces and create unique spectral changes.
How It Works
• Sound interacts with the pinna, torso, and head, altering its frequency spectrum.
• This spectrum is different depending on the sound’s elevation (height) and whether it comes from
front or back.
• The brain learns to associate these frequency changes with specific directions.
• We don’t know exactly where in the brain this matching process happens, but we know it occurs.
5. Problems in Sound Localization
Front-Back Ambiguity
• ITD and ILD tell us whether a sound is coming from left or right but not if it is in front or behind.
• The sound source can be at different positions but still have identical ITD and ILD values.
,Elevation Ambiguity
• ITD and ILD are not very useful for determining if a sound is above or below the listener.
• Elevation changes only slightly affect ITD and ILD unless they are extreme (e.g., sound coming from
directly above or below).
• This creates a "cone of confusion", where multiple locations share the same ITD and ILD.
How HRTF Solves These Problems
• The brain uses HRTF to differentiate between sounds in front vs. behind and above vs. below.
• Each individual has a unique HRTF based on their ear shape and body structure.
• The brain learns and adapts to this over time.
6. Adaptation to HRTF (Hofman et al., 1998)
• Scientists tested sound localization by altering the shape of the pinna.
• When a new ear shape was introduced, subjects lost the ability to determine elevation.
• Over 23 days, subjects adapted and regained their ability to localize elevation.
• This suggests that HRTF learning is plastic (changeable) and can adjust over time.
Summary
• ITD helps localize low-frequency sounds by comparing arrival times at each ear.
• ILD helps localize high-frequency sounds by comparing loudness differences.
• HRTF helps distinguish front vs. back and elevation by analyzing frequency changes caused by the
body.
• The brain processes these cues in different regions and is capable of adaptation.
1. What are the three main cues the brain uses to localize sound?
The brain primarily relies on three cues to determine the direction of a sound: Interaural Time Difference
(ITD), Interaural Level Difference (ILD), and Head-Related Transfer Function (HRTF). ITD helps localize
low-frequency sounds by comparing the arrival times of a sound wave at each ear. ILD is useful for high-
frequency sounds, as it detects differences in loudness between the two ears due to the head blocking some
of the sound. HRTF, the most complex cue, helps differentiate whether a sound comes from the front, back,
above, or below by analyzing how the ear, head, and torso modify sound frequencies before they reach the
eardrum.
2. What type of sounds does ITD work best with, and why?
ITD is most effective for low-frequency sounds, typically below 1500 Hz. This is because lower frequencies
have longer wavelengths, allowing the brain to detect the time difference between when the same
wavefront reaches both ears. For high-frequency sounds, the wavelengths are shorter, making it difficult to
distinguish whether a sound wave arriving at one ear is part of the same wave cycle as the one reaching the
other ear.
, 3. Where in the brain is ITD processed?
The processing of ITD occurs in the superior olivary nucleus, a structure located in the brainstem. This
nucleus receives input from both ears and compares the time at which the same sound arrives at each ear.
By analyzing these tiny differences, the brain can determine the horizontal location of the sound source.
4. How does ITD help in sound localization?
When a sound originates from one side of the head, it reaches the closer ear first, followed by the opposite
ear with a slight delay. The superior olivary nucleus in the brainstem detects this delay and determines the
direction of the sound based on how much earlier it arrived at one ear. The brain uses specialized neurons
that are tuned to fire only when sound signals from both ears arrive simultaneously. This process is similar to
how binocular vision works in depth perception, where the brain compares images from both eyes to gauge
distance.
5. What type of sounds does ILD work best with, and why?
ILD is most effective for high-frequency sounds, generally above 1500 Hz. High frequencies have shorter
wavelengths, which do not easily bend around objects, including the head. This creates a sound shadow,
making the sound quieter in the further ear. The brain detects this difference in loudness and uses it to infer
the location of the sound. In contrast, low-frequency sounds tend to wrap around the head, making them
roughly equal in intensity at both ears, which is why ILD is not useful for localizing them.
6. Where in the brain is ILD processed?
The processing of ILD takes place in the medial geniculate nucleus (MGN) of the thalamus. This region
compares the loudness levels between both ears and helps determine the horizontal direction of high-
frequency sounds. Unlike ITD, which is processed in the brainstem, ILD requires higher-level processing in
the auditory pathway.
7. Why doesn’t ILD work for low-frequency sounds?
Low-frequency sounds have longer wavelengths, which allows them to diffract (bend) around objects,
including the head. As a result, they reach both ears with very little difference in intensity, making ILD an
ineffective localization cue for low frequencies. The brain, therefore, relies more on ITD for low-frequency
sound localization.
8. What is the “cone of confusion” in sound localization?
The cone of confusion refers to a set of locations in space where sounds produce similar ITD and ILD
values, making it difficult to determine the exact origin of the sound. Sounds coming from the front and back
or from above and below can have the same ITD and ILD, leading to ambiguity. Despite this, humans are
generally good at distinguishing these locations, thanks to HRTF, which provides additional spectral
information to resolve the confusion.
9. How does the brain resolve the cone of confusion?
