Download Interaural Time Difference

The barn owl
(Tyto alba)
amazing performance on sound localization tasks
3D Space of sound localization
Elevation (vertical plan)
Distance
azimuth
(horizontal plan)
Cues about Azimuth: Interaural Time Difference (ITD)
eagle at 10 degrees azimuth
Binaural Cues: when
both ears are needed to
process information
x
y
x - y = .02m
speed of sound = 331 m/s
x - y = 60 microseconds
i.e. ITD = 60 microseconds
20 cm
Physiology of Interaural Time
Difference
– Superior Olivary Complex: (medial superior
olives): First place where input converges from
two ears
– ITD detectors form connections from inputs
coming from two ears during first few months
of life
• Newborns can do it (poorly)
• 2-month olds progressively better
• Indicative of lower brain (non-cortical) structures
Processing information on the azimuth:
Interaural Level Difference (ILD)
Binaural Cues: calculations are done comparing ears on pressure
changes (i.e., differential activity at each level of the ear)
Interaural Level Difference (ILD) is also influenced
by the size of the “shadow” cast from the head
Shadow differentially influences
high frequency waves (knocks
them out) compared to large/low
frequency waves
Acoustic
Shadow
How the size of your head influences Interaural Level Differences?
Low frequency waves
are longer/wider & “less
frequent”
Whereas, high frequency
waves occupy a smaller
space
Takes a smaller “obstacle”
to block out smaller waves
3D Space of sound localization
Elevation (vertical plan)
Distance
azimuth
(horizontal plan)
Information about the vertical plane: Elevation
Spectral Cues
Sound reflects off the head
and differing locations of
the pinna
These reflections differ as a
function of whether the
sound is coming from
higher or lower locations
Differing “reflections” cause variations in amplitude (loudness) at
differing frequencies (changing cues in the spectrum) that reveal
important information about the elevation of sound sources
Directional transfer function (DTF)
Information about Distance
• Differing effect of short distance (i.e., within
arms length) v. longer distances
• Short distances are dramatically influenced by
interaural level differences (ILD)
Information for distance as we get farther away
• Sound level – changes in distance change amplitude
(loudness/SPL/dB)
– Mostly useful for familiar stimuli
• Frequency changes – loss of high frequencies through the
atmosphere over longer distances
• Movement parallax – exactly the same as in vision – nearer
objects seem to move faster than farther objects in “sound
space”
• Reflection – a source of multiple sound inputs – the greater
the distance the greater opportunity for reflected
information
Sound localization in a complex environment:
The influence of reflected sound (echoes)
• Echoes (reflected sound or “reverberation”)
occur to some extent in all natural situations
• Echoes depend on characteristics of the
room or other space
• Echoes contribute to sound quality
• Echoes provide cues about the space in
which they occur
• Echoes could potentially complicate sound
localization
Precedence Effect: direct vs. indirect sound
Simultaneous sounds that are symmetrically located to
either side:
• Source of sound perceived as “centered” (called Fusion)
• Sounds arriving <1-msec apart (left v. right) don’t quite
sound centered (locates more in direction of 1st sound)
 But, we perceive this VERY short Interaural Time Difference
Precedence Effect: direct vs. indirect sound
Two Sounds (left v. right ears) arriving >1-to-5-msec
apart :
•
•
location perceived as direct from 1st sound
called Precedence Effect
Two sounds arriving to the listener more than 5-msecs
apart:
• hear two different sounds
• called Echo Threshold
Precedence Effect
Auditory system “deadens” sounds that are arriving
under 5-msec apart
Prevents hearing echoes in most day-to-day settings
The effect of deadened echoes, being able to localize
sound
Too many echoes and you wouldn’t be able to localize
Physiology of sound localization
• One synapse from cochlea to cochlear nucleus (CN)
• One synapse from CN to Olivary Complex (lateral superior
olive)
• Location information processing is done very fast!!
Physiological basis for localization
• Jenkins & Merzenich (1984): lesions of very
specific frequency channels in the auditory cortex
(cats)
– Inability to localize sound
• Stroke patients with damage to frequency channels
in the auditory cortex
– Inability to localize sound
• Why does frequency matter for sound location?
Sound localization is
influenced by multiple
factors:
• Location cues for each of
the 3 planes (Horizontal,
vertical, distance)
• Interaural level
differences (ILD) is
particularly sensitive to
Frequency
Physiological basis for localization in the
monkey auditory cortex
• Cells respond differentially to specific interaural time
delays
– Interaural time difference “detectors:” cells that respond “best”
to specific time delays between the two ears
• Cells have been identified in the right (not left)
hemisphere that respond “best” when there is movement
between either the source or the perceiver
• “Panoramic” Neurons
– Fires to stimuli in all locations surrounding the perceiver, but…
– …neural firing rate varies (increase v. decrease) as a function of
location in space
Neurons in the Inferior Colliculus are tuned to multiple
sound parameters:
•Frequency
•Intensity
•Duration
•Direction and rate of change of frequency modulation (FM)
•Rate of change in amplitude modulation (AM)
•The interval between two sounds
•Other more complex sound patterns
(not all are tuned to every parameter)
Integration in the Inferior Colliculus
The inferior Colliculus received convergent
information from multiple lower brainstem pathways.
This convergence performs a number of functions:
• Integration of information about binaural intensity
and time differences
• Integration of information contained in different
spectral (frequency) ranges
• Integration of information occurring at different times
Auditory Scene Analysis
• What happens in natural situations?
– Acoustic environment can be a busy place
– Multiple sound sources
– How does auditory system sort out these sources?
– Source segregation and segmentation, or auditory
scene analysis
Considering sound quality: timbre
• What have we considered in terms of sound?
–
–
–
–
Fundamental frequency, pitch (hi/low)
Amplitude, intensity, loudness (hi/low)
Duration (long/short)
Location (horizontal, vertical, distance)
• What else? Sound quality (complexity)  timbre
• Timbre: Psychological sensation by which a
listener can judge that two sounds that have
the same loudness and pitch, but are
dissimilar;
– Conveyed by harmonics and other high
frequencies
– Perception of timbre depends on context in
which sound is heard
– Provides information about auditory scene
Auditory Scene characteristics: Attack and Decay of sound
The parts of a sound during which amplitude: (i) increases
(onset or “attack”), or (ii) decreases (offset or “decay”)
Timbre: differences in number & relative strength of harmonics
guitar- many harmonics
400
800 1200 1600 2000 2400
Frequency (Hz)
400
800 1200 1600 2000 2400
flute - few harmonics
(only one actually)
Attack (onset) and decay (offset) also affect timbre
-low harmonics build up faster, high harmonics decay slower
Gestalt Psychology
In response to Wilhelm Wundt (1879) who proposed that
“perception” was a function of “sensation”
Gestalt psychologists were struck by the many ways in which our
perceptions transcend the simple sensations from which they are
built… and the importance of the “organization of perception”
"The whole is different/greater than the sum of the parts"
How do we perceive objects in our
world? Summary of Gestalt rules
1.
2.
3.
4.
5.
6.
Good fit, closure, simplicity
Similarity
Good continuity
Proximity
Common fate
Familiarity (meaningfulness)
7.
8.
9.
Common region
Connectedness
Synchrony
Classical Gestalt
“laws”
Modern Gestalt
“laws”
Read the relevant pages on the
“Gestalt Rules” of visual
perception from Chapter 4
(see following slide)
Auditory Scene Analysis
Gestalt Psychology &
auditory grouping
pitch
Similarity:
Location
Timbre
Pitch
Temporal Proximity:
Onset
Offset
Synchrony
Good Continuation
(melody)
Familiarity (experience)
time
Factors that contribute to auditory stream segregation:
Binaural cues
Spatial separation: Frequencies coming from different points in
Space produce different Interaural Level and Timing Differences
Frequency components with the same ILD/ITD values will be
grouped together.
This would be an example of the Gestalt law of (spatial) proximity.
Stream segregation:
Timing cues for Gestalt “laws”
Temporal separation: Frequency components that occur close
together are grouped (law of proximity (near in time)).
Temporal onsets and offsets: Frequencies that have the same
onset and offset time belong together. (law of synchrony)
Temporal modulations: Frequency components that change
together belong together. (i.e., law of good continuation and
the law of common fate)
Frequency components that change together are grouped together.
Those that do not change are grouped separately (law of synchrony,
proximity &/or common fate)
Stream segregation: spectral cues & Gestalt “laws”
Spectral separation: Frequencies that are “similar” (i.e., octaves,
chords) are grouped together (law of similarity, familiarity)
Harmonicity: Frequencies that are harmonically related may be
grouped together (law of synchrony, law of familiarity)
Spectral profile: Frequency components whose relative amplitudes
(e.g., soft or loud) remain constant across the sound may be
grouped together. (law of good continuation)
The tendency to perceive “good continuation” results
In perceptual “filling in”, or closure