The barn owl

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• The barn owl
• (Tyto alba)

amazing performance on sound localization tasks
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3D Space of sound localization
Elevation (vertical plan)
Distance

• azimuth

(horizontal plan)
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Cues about Azimuth Interaural Time Difference
(ITD)
Binaural Cues when both ears are needed to
process information
i.e. ITD 60 microseconds
20 cm
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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

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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)
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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
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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
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3D Space of sound localization
Elevation (vertical plan)
Distance

• azimuth

(horizontal plan)
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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

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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)
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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)

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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

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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

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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 lt1-msec apart (left v. right)
  dont quite sound centered (locates more in
  direction of 1st sound)
• But, we perceive this VERY short Interaural Time
  Difference

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Precedence Effect direct vs. indirect sound

• Two Sounds arriving gt1-to-5-msec apart (left v.
  right)
• 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

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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 wouldnt be able to
  localize

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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!!

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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?

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• 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

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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

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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)

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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

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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

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• 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

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• 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)
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• Timbre differences in number relative strength
  of harmonics

Attack (onset) and decay (offset) also affect
timbre
-low harmonics build up faster, high harmonics
decay slower
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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"
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How do we perceive objects in our world? Summary
of Gestalt rules

1. Good fit, closure, simplicity
2. Similarity
3. Good continuity
4. Proximity
5. Common fate
6. Familiarity (meaningfulness)
7. Common region
8. Connectedness
9. Synchrony

Classical Gestalt laws
Modern Gestalt laws
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Read the relevant pages on the Gestalt Rules of
visual perception from Chapter 4 (see following
slide)
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Auditory Scene Analysis
Gestalt Psychology auditory grouping
pitch

• Similarity
• Location
• Timbre
• Pitch
• Temporal Proximity
• Onset
• Offset
• Good Continuation
• Familiarity (experience)

time
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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 proximity.
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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 common fate)
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Frequency components that change together are
grouped together. Those that do not change are
grouped separately (law of synchrony, proximity
/or common fate)
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Stream segregation spectral cues Gestalt laws
Spectral separation Frequencies that are
similar (i.e., octaves, chords) are grouped
together (law of similarity, familiarity) Harmoni
city Frequencies that are harmonically related
may be grouped together (law of
familiarity) Spectral profile Frequency
components whose relative amplitudes remain
constant across the sound may be grouped
together. (law of good continuation)