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Sensation and Perception

Chapter 11: Auditory Brain and

Perceiving Auditory Scenes

Lecture Outline

• Ascending and descending auditory pathways – Cochlear nucleus – Superior olivary complex – Inferior colliculus – Medial geniculate body /

nucleus

• Auditory cortex – Core, belt, parabelt – Tuning curves – “What” and “where” pathways

• Sound localization – Azimuth

• ILD & ITD • Front-back confusions • Cone of confusion

– Elevation • Spectral shape cue

– Distance • Inverse square law, echoes,

Doppler effect

– Ventriloquism effect

• Auditory scene analysis – Grouping by harmonic

coherence – Grouping by synchrony or

asynchrony – Grouping by frequency

similarity – Grouping by temporal proximity

• Perceptual completion of occluded sounds

• Perceptual organization

Ascending Pathway

Descending Pathway

• Information also flows from the cortex to the cochlea

– Modulate the motile response of the outer hair cells

– Attention – block task-irrelevant ascending auditory signals

– Activate the acoustic reflex

Auditory Cortex

• Top of temporal lobe

• Core – A1

• Pure tones

• Belt – Complex sounds

• Parabelt – Complex sounds

• Tonotopic organization

Auditory Tuning Curves

“What” and “Where” Pathways

Sound Localization

Perceiving Azimuth

• Interaural Level Differerence (ILD; IID; IAD)

• Most sounds are closer to one ear than the other

• Inverse square law

• Acoustic shadow

ILD

• Greatest for sounds directly to the left of the left ear or to the right of the right ear.

• Smallest for sounds anywhere on the median plane

• Works best for higher frequencies

• Processed by the lateral superior olive

ITD

• Interaural Time Difference (ITD)

• Greatest (~600 μs) when sound directly to left of left ear or to the right of the right ear

• 0 for sound anywhere on the median plane; front-back confusions • Minimum audible angle (MAA) – 1° to 10 °

Neural Basis of ITD

Cone of Confusion

Elevation Localization

• Spectral shape cue

Elevation Localization

Distance Localization

• For sounds of familiar loudness, inverse square law

• Inverse square law is greater for higher frequencies than lower frequencies; results in “blurring”

– Blurring can be used to determine distance

Distance (m) p (μP) 20 log10(p/p0)

1 900,000 / 12 = 900,000 93 dB

2 900,000 / 22 = 225,000 81 dB

3 900,000 / 32 = 100,000 74 dB

5 900,000 / 52 = 36,000 65 dB

10 900,000 / 102 = 9,000 53 dB

20 900,000 / 202 = 2,250 41 dB

Distance Localization

• With echoes, compare direct sound intensity to reflected sound intensity

– Direct > reflected  sound is near

– Direct < reflected  sound is far

• Changes in intensity

• Doppler effect – when a sound source is moving, frequency is higher in front of the source than behind the source

Ventriloquism Effect

• Sounds tend to be localized on the basis of visual cues when visual and auditory cues conflict

– Visual and auditory events must occurs close together in time

– Visual and auditory events must be plausibly linked

– Visual and auditory events must be close in space

Auditory Scene Analysis

• Auditory stream

• Auditory scene

Grouping by Harmonic Coherence

• In (a) the frequencies are all harmonics (integer multiples of fundamental)

– Grouped as a single

auditory stream

• In (b) the harmonics are grouped as one auditory stream and the non-harmonic is perceived as a separate auditory stream

Grouping by Harmonic Coherence

• In (a), two separate sound sources produce harmonics of a single sound

– Perceived as a single stream

• In (b), two separate sound sources produce harmonics, but not of a single sound

– Perceived as two auditory streams

Grouping By Synchrony or Asynchrony

Grouping by Frequency Similarity

Grouping by Temporal Proximity

Perceptual Completion of Occluded Sounds

https://www.youtube.com/watch?v=ZyvyGMkzNQc

Perceptual Organization

• Effect of past experience

• A melody is played with notes alternating between octaves

• Listeners find it difficult to identify the song

• But after they hear the normal melody, they can then hear it in modified version using melody schema