Chapter 10 The Auditory Brain and Perceiving Auditory Scenes

Question 1

Ascending Pathways

Answer 1

From the Ear to the Brain
*Type I auditory nerve fibers carry signals from inner hair cells in cochlea to the ipsilateral cochlear nucleus in brain stem

Question 2

Cochlear nucleus

Answer 2

a structure in the brain stem (one on each side of the brain); it receives signals via Type I auditory nerve fibers from inner hair cells in the ipsilateral ear

• Ipsilateral cochlear nucleus -> contralateral inferior colliculus (via nerve tract, lateral lemniscus) -> contralateral medial geniculate body (MGB) -> contralateral auditory cortex
• Ipsilateral cochlear nucleus travel directly or indirectly (via synapse in the contralateral trapezoid body) -> contralateral olivary complex -> contralateral inferior colliculus -> contralateral MGB and auditory cortex
• Second pathways: ipsilateral cochlear nucleus -> ipsilateral superior olivary complex -> ipsilateral inferior colliculus

Question 3

Inferior colliculus

Answer 3

a structure in the midbrain (one on each side of the brain); a stop on the ascending auditory pathway.

Question 4

Medial geniculate body (MGB)

Answer 4

a structure in the thalamus (one on each side of the brain); the next stop on the ascending auditory pathway after the inferior collicullus

Question 5

Superior olivary complex

Answer 5

a structure in the brain stem (one on each side of the brain); a stop on the ascending auditory pathway receiving signals from both cochlear nuclei.

Question 6

Preferred frequency

Answer 6

inner hair cells produce a large burst of action potentials in Type I auditory nerve fibers and sustained slower firing rate above baseline rate

• Others: initial strong response but quickly return to baseline rate
• Some others: gradually increasing their firing rate to a moderate level without any initial strong response

Question 7

Descending pathways

Answer 7

From brain to ear
*Neural signals from superior olivary complex back to outer hair cells -> modulating outer hair cells’ motile response
*Help protect the ear from damage by activating the acoustic reflex and to be involved in attention by blocking task-irrelevant ascending auditory signals while passing task-relevant ones
o Similar in visual system: feedback from cortex modulates activity in lateral geniculate nucleus to block task-irrelevant visual signals
*Top-down signals from cortex affect subcortical structures

Question 8

Auditory cortex

Answer 8

Part of the cerebral cortex, tucked into the lateral sulcus (Sylvian fissure) on top of the temporal lobe; consists of the auditory core region, belt and parabelt

Question 9

Primary auditory cortex (A1)

Answer 9

• Auditory core region, Rostral core, rostrotemporal core
• In A1 and rostrotemporal core: neurons with high characteristic frequencies are located at the posterior (back) end; neurons with low characteristic frequencies are located at the anterior (front) end
• In rostral core: the arrangement is opposite; posterior neurons have low characteristic frequencies; anterior neurons have high characteristic frequencies
• Orientation tuning of neurons can be broad or narrow in visual cortex; the frequency tuning of neurons in auditory cortex can also be
• The researchers speculated that neurons with broad tuning widths might be involved in integrating component frequencies of complex sounds, as part of the process of discriminating and recognizing sound sources.
• This discrimination and recognition process appears to be carried forward in the belt and parabelt, which are thought to be analogous to areas beyond V2 in the visual pathways.
• Unlike neurons in the auditory core region, neurons in the belt and parabelt don’t respond strongly to pure tones but instead appear to be tuned to more complex stimuli containing multiple frequencies, which are, the type of stimuli we’re most likely to encounter in everyday life.

Question 10

Belt

Answer 10

along with the parabelt, a region of cortex wrapped around and receiving signals from auditory core region

Question 11

Parabelt

Answer 11

along with the belt, a region of cortex wrapped around and receiving signals from the auditory core region

Question 12

Tonotopic map

Answer 12

arrangement of neurons within auditory brain regions such that the characteristic frequencies of the neurons gradually shift from lower at one end of the region to higher at the other end; echoes the arrangement of characteristic frequencies along basilar membrane; according to frequency

Question 13

“What” pathway

Answer 13

the identity of sound sources, extends from core regions into the belt and parabelt and then into anterior parts of the temporal cortex

Question 14

“Where” pathway

Answer 14

the location of sound sources, extends from the core regions into posterior parts of the auditory cortex and eventually into the posterior parietal cortex

Question 15

Localizing sounds

Answer 15

• In audition, there is no corresponding explicit representation of location- the cochlea is organized tonotopically, with position in the cochlea representing frequency, not spatial location
• To represent the location of sound sources, the auditory system has instead evolved an exquisitely sensitive method based on comparing aspects of the sound arriving at the two ears
• > In vision, stereoscopic depth perception uses an analogous comparison of information in the two retinal images
• A polar coordinate system based on two mutually perpendicular planes centered on the head is used to specify the locations of sound sources in 3-D space.

Question 16

Azimuth

Answer 16

In the horizontal plane, azimuth refers to the side-to-side dimension, the angle left or right of the median plane

Question 17

Elevation

Answer 17

In median plane, elevation refers to the up-down dimension, the angle above or below the horizontal plane

Question 18

Distance

Answer 18

the distance from the center of the head in any direction.

Question 19

Minimum audible angle

Answer 19

minimum angular separation between a reference sound source and a different sound source emitting a tone of the same frequency that yields 75% correct judgments about the relative horizontal positions of the two sources
*The smaller the minimum audible angle, the more accurately the listener can perceive the azimuth of the sound source.

Question 20

Acoustic shadow

Answer 20

area on the other side of the head from a sound source in which the loudness of the sound is reduced because the sound waves are partially blocked by the head; it has a much greater effect on high-frequency sounds than on low-frequency sounds.

Question 21

Interaural level difference (ILD)/ Interaural intensity difference (IID)

Answer 21

the difference in the sound level of the same sound at the two ears.

• 0 degree azimuth- directly in front of the listener- exhibit zero ILD
• ILD increase steadily for sound sources from 0 degree to 90 degree azimuth
• ILD decreases steadily back to zero at 180 degree azimuth- directly behind the listener
• ILD typically increases with frequency at any given azimuth
• ILD is a good cue for perceiving the azimuth of pure tones at high frequencies but not as good at low frequencies

Question 22

Interaural time difference (ITD)

Answer 22

the difference in arrival time of the same sound at the two ears
*Most people have an ITD threshold microseconds or less, which means that the 292 microseconds ITD of a sound at an azimuth of 45 degree is more than sufficient.

Question 23

Cone of confusion

Answer 23

a hypothetical cone-shaped surface in auditory space; when two equally distant sound sources are located on a cone of confusion, their locations are confusable because they have highly similar ILD and ITD.

• Sounds from all such sources would have highly similar ILD and ITD
• Head movement

Question 24

Perceiving Elevation

Answer 24

The pinna- that funnels sound waves into the auditory canal

• Provide information used to judge elevation
• They reflect off the bumps and ridges and reverberate (echo) slightly, which amplifies some frequencies and attenuates others, changing the shape of the frequency spectrum.
• The exact nature of the modification depends to some extent on the azimuth of the sound source- which largely accounts for the ILD asymmetries
• But depends even more particularly on its elevation

Question 25

Spectral shape cue

Answer 25

pinna-induced modification in a sound’s frequency spectrum; it provides information about the elevation of the sound source.

• Each person’s pinnae are unique
• Works best for broadband sounds
• Contain a wide range of frequencies
• People are indeed quite poor at judging the elevation of pure tones

Question 26

Perceiving Distance

Answer 26

• ILD, ITD, and the spectral shape cue- provide little information about distance.
• If the listener knows the sound level of the source, then the perceived loudness can be used to judge at least whether the source is relatively near or far.
• Inverse square law- the amplitude of a sound decreases in proportion to the square of the distance to the source.
• > The reduction in level is greater for high frequencies than for low frequencies, which results in a progressive “blurring” of a sound as the source grows more distant
• > This is analogous to using atmospheric perspective as a depth cue in vision.
• Echoes in situations where there are many hard surfaces to reflect sound waves
• > You can perceive the relative proportion of each type of sound energy, direct versus reflected.
• Loudness and frequency are provided by the movement of sound sources toward or away from the listener.

Question 27

Doppler effect

Answer 27

frequency of a sound emitted by a moving sound source is higher in front of the sound source than behind it; the frequency rapidly decreases as the sound source passes the listener.

• > Whether the source is approaching or receding
• > When an approaching source reaches its closest point

Question 28

Echolocation

Answer 28

sound localization based on emitting sounds and then processing the echoes to determine the nature and location of the object that produced the echoes

• Provides information about azimuth, elevation, and distance, as well as information about the size and shape of the target and about the physical characteristics of the target’s surface.
• Human can use echolocation quite accurately to judge their distance from walls or other objects.

Question 29

Ventriloquism effect

Answer 29

when the visual system and the auditory system give you conflicting information about the location of a sound source, your perception tends to be dominated by the visual information, and you hear the sound as coming from the visually determined location.

• > Especially powerful when the visual information matches your previous experience
• > The visual and auditory events must be reasonably close together in time.
• > The two events must be plausibly related.
• > The two events must be plausibly close together in space.

Question 30

Medial superior olive (MSO)

Answer 30

part of the superior olivary complex in the brain stem

• > Contain neurons that function as a mechanism for detecting specific ITDs and thus representing the azimuth of sound sources.
• > Both the left and right MSO receive signals from both the left and right cochlear nuclei.

Question 31

Coincidence detectors

Answer 31

they fire only if the signals from the two cochlear nuclei arrive at the same time.

• > Population coding
• Neurons in the auditory cortex are tuned for different ILDs, and the responses of a population of differently tuned neurons provide a neural code for sound localization using ILDs.

Question 32

Auditory scene

Answer 32

all the sound entering your ears during the current interval of time

Question 33

Auditory scene analysis

Answer 33

the process of extracting and grouping together the frequencies emitted by specific sound sources from among the complex mixture of frequencies emitted by multiple sound sources within the auditory scene.

Question 34

Auditory stream

Answer 34

an assortment of frequencies occurring over time that all “go together” because they were emitted by the same sound source or by related sources.
*Analogous to perceptual organization in vision, where the visual system organizes the visual scene into a set of distinct objects.

Question 35

Overlap in frequency (audition); overlap in space (vision)

Answer 35

• Vision: group the regions that belong together into a single visual object; separating out the regions that belong to other, overlapping visual objects
• Auditory: group the frequencies that go together as frequencies belonging to a single auditory stream; separating out the frequencies that belong to other, simultaneous auditory streams, despite the occurrence of all these frequencies in the same stretch of time.

Question 36

Two possibilities how to be interpreted when all the sounds come from one location

Answer 36

• All the frequency components were produced by a single sound source, that the mark the beginning of a single auditory stream
• Multiple independent sound sources all happened, by chance, to begin producing sounds at the same moment in time.
• The former: use the simultaneous onset of all the frequency components as a basis for concluding that they are part of a single auditory stream.
• > • Analogous to the principles used by the visual system to group visual features in order to make a best guess about the objects making up the visual scene

Question 37

Harmonic coherence

Answer 37

frequencies that are harmonics of the same fundamental frequency tend to be grouped together as part of the same auditory stream

Question 38

Synchrony

Answer 38

frequencies that begin, end, or change at the same time also tend to be grouped together

Question 39

Auditory system entertain two possibilities

Answer 39

• All the harmonics are coming from a single source or that many independent sources are emitting frequencies that, completely by chance, are related to each other in precisely this way.
• The former is much more likely and is the interpretation that the auditory system strongly tends to prefer.

Question 40

Sequential grouping

Answer 40

• Auditory system uses to group frequencies that occur at different times- sequentially rather than simultaneously
• Similarity in the frequencies of sequential tones
• Temporal proximity (how close together in time they occur)

Question 41

Grouping by Frequency Similarity

Answer 41

*The auditory system uses the frequency similarity of sequential tones to group those tones into a single auditory stream or into multiple separate auditory streams (ABAABA)

Question 42

Grouping by Temporal Proximity

Answer 42

• Tones that are close together or far apart in time.

Question 43

Perceptual Completion of Occluded Sounds

Answer 43

• The listener’s auditory system automatically assumes that the glides are making an uninterrupted sound that rises smoothly to a high frequency, declines smoothly to a low frequency, rises smoothly again, and so on- just as when you seem actually to perceive your friend’s words when the bus goes by.
• This auditory phenomenon is analogous to the visual phenomenon of perceptual interpolation where the observer perceives partly occluded edges or surfaces as continuing behind an occluder.