1. The barn owl (Tyto alba)
amazing performance on sound localization tasks

2. azimuth 3D Space of sound localization Distance
Elevation (vertical plan)
Distance
azimuth
(horizontal plan)

3. 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

4. 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

5. 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)

6. 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

7. 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

8. azimuth 3D Space of sound localization Distance
Elevation (vertical plan)
Distance
azimuth
(horizontal plan)

9. 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

10. 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)

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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!!

18. 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
Why does frequency matter for sound location?

19. 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

20. 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

21. 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)

22. 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

23. 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

24. 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

25. 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

26. 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”)

27. guitar- many harmonics
Timbre: differences in number & relative strength of harmonics
guitar- many harmonics
400
800
1200
1600
2000
2400
Frequency (Hz)
flute - few harmonics (only one actually)
400
800
1200
1600
2000
2400
Attack (onset) and decay (offset) also affect timbre
-low harmonics build up faster, high harmonics decay slower

28. 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"

29. How do we perceive objects in our world? Summary of Gestalt rules
Good fit, closure, simplicity
Similarity
Good continuity
Proximity
Common fate
Familiarity (meaningfulness)
Common region
Connectedness
Synchrony
Classical Gestalt “laws”
Modern Gestalt “laws”

30. Read the relevant pages on the “Gestalt Rules” of visual perception from Chapter 4 (see following slide)

31. Auditory Scene Analysis
Gestalt Psychology & auditory grouping
Similarity:
Location
Timbre
Pitch
Temporal Proximity:
Onset
Offset
Synchrony
Good Continuation
(melody)
Familiarity (experience)
pitch
time

32. 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.

33. Timing cues for Gestalt “laws”
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)

34. Frequency components that change together are grouped together
Frequency components that change together are grouped together. Those that do not change are grouped separately (law of synchrony, proximity &/or common fate)

35. 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)

36. The tendency to perceive “good continuation” results
In perceptual “filling in”, or closure