1. From sound to hearing
Nisheeth
26th February 2019

2. Sound localization

3. Localization Accuracy vs. Frequency
Signature of a dual-mechanism process?

4. Localization Accuracy vs
Localization Accuracy vs. Frequency: Low Freq – Interaural Time Difference High Freq – Interaural Intensity Difference ΔI ΔT

5. Sound Shadowing (Interaural Intensity Difference –IID)
High-frequency sound waves are “blocked” by the human head and cast a “shadow” at the far ear
(Strong IID cue)
Low-frequency sound waves wrap easily around the head and cast little or no sound shadow (Weak IID Cue) ΔI

6. Characteristic parameter: head size
Heads are 0.3 meters around
Sound frequencies higher than ~3000 Hz wrap around the head

7. IID = f(Location, Frequency)
Straight
Ahead
Straight
Behind
Right Ear
(Perpendicular)

8. ITD versus Location ΔT Straight Ahead Straight Behind Right Ear
(Perpendicular)

9. Delay Line Theory (How to Build a Cell tuned to delta-T Signals)
Delta-T = 200 microsec

10. “Active” Localization (Continuous Sound Sources)

11. Straight Ahead vs. Straight Behind
Relatively good localization performance despite same IID and ITD levels (i.e., zeros) Differential sound distortion (“coloration”) introduced by interaction with pinna Modifying shape of pinna causes immediate reduction in localization accuracy (Hoffman, et al., 1998) Listening through the ears of another yields “ahead” vs. “behind” confusion (chance performance)

12. Building blocks of the world of sound
Auditory cognition

13. Auditory cognitive concepts
Speech-related
Phonemes, visemes and their relationship
Music-related
Rhythm perception and motor entrainment

14. Phoneme-viseme mapping
Visemes are mouth configurations associated with sound
Fundamental to learning to speak
And also to hear (McGurk & McDonald, 1976)

15. Several possible clusterings

16. Rhythm Strongly dependent on periodicity
Motor periodicity: repetition of motor actions
Heartbeat
Circadian rhythm
Chewing
Walking
Auditory periodicity is beat detection
Inference of a periodic pulse from a complex sound stimulus
Entrainment
Synchronizing motor actions to inferred beats

17. The musical hierarchy

18. Automated beat extraction
Pop music has concentrated energy at the beat frequency
Easy to extract algorithmically
Classical music and jazz have syncopated rhythms
Silent events matter significantly in beat inference
Very poorly understood
Homework: try picking out a jazz beat before the bass kicks in, or a Hindustani piece’s rhythm during alaaps where the tabla is quiet

19. The anatomy of entrainment
Rhythm perception involves brain regions also associated with motor output and production
Cerebellum
Basal ganglia
SMA
Can explain simple tapping
Not more complex aspects of musicality
Still terra incognita

20. Animal music cognition
Simple motor entrainment is shared by humans with at least parrots and pinnipeds
But (apparently) not with other mammal species, e.g. dogs, cats
Maturational feature in humans > 2 years
Massive social influence is also an important research area
(Kotz, Ravignani & Fitch, 2018)