Week 9 : The auditory brain, scene analysis & sound localization

Question 1

MAIN Ascending auditory pathway

Answer 1

• left off with the bundle of type 1 nerve fibres that exit the cochlea as the auditory nerve
• 1st stop = cochlear nucleus in the brain stem… trapezoid body attached
• 2nd stop = superior olivary complex on opposite side of the brain
• 3rd stop = inferior colliculus of the midbrain
• 4th stop = medial geniculate nucleus in the thalamus… projects to cortex but also gets even more info back from cortex
• 5th stop = primary auditory cortex (temporal lobe)

Question 2

More stops…

Answer 2

• there are many more stops along this pathway between the sensory organ and the cortex then there were in visual pathways
• there is more cross talk between the 2 hemispheres
• the dominant pathway sees info arriving at the right cochlea project to the left auditory cortex… but this is not super confined

Question 3

Efferent fibers…

Answer 3

• fibres carry info from the brain toward more peripheral structures
• 3 rows of outer hair cells serve as an amplifier within the cochlea & boost small sound signals
• the signals that control this amplification arrive via nerve fibres from the brain stem

Question 4

Afferent fibers

Answer 4

• fibres that carry info toward the brain are called afferents
• there are a few afferent projections from outer hair cells, which provide feedback to the brain about their function
• the majority of the input to the brain from cochlea comes from the inner hair cells that transduce sound

Question 5

8th cranial nerve

Answer 5

• the auditory nerve leaves from the basilar membrane in the cochlea
• the vestibular nerve also leaves from the the 3 ring shaped canals
• they come to gather to form the 8th cranial nerve
• the nerve passes through a narrow passage in the skull, before the 2 divisions separate again

Question 6

the colocation of the auditory & vestibular nerve + the root of passage thru the skull give rise to 2 things…

Answer 6

1. this narrow pathway is why head injury can result in hearing loss as the ascending nerve may shear against the bone
2. why balance disorders often result from the growth of small tumours on the auditory nerve (acoustic neuroma)

Question 7

Auditory nerve fiber tuning

Answer 7

• each of the ganglion cells makes contact with with only 1 hair cell
• so… nerve fibre maintains the frequency based organization
• at very high sound levels, fibres respond over a broad frequency range but have a fine tuning for a specific low level sound (tuning curve)

Question 8

Tonotopic organization

Answer 8

• frequency based organization
• maintained at every stage of the ascending auditory pathway
• not each location the brain focuses on the same things but the frequency-tuned gradient is present at every point in the path
• this preserves the auditory brains ability to discriminate sounds based on pitch

Question 9

A1…

Answer 9

• primary auditory cortex lies along the top of the temporal lobe
• buried deep in lateral sculls, where temporal lobe meets parietal & front lobes

Question 10

auditory core region

Answer 10

• the auditory core is in the centre of the auditory cortex (A1)
• there is a lot less known about this than V1
• the core has a frequency based organization
• around the core is the belt region & a tertiary parabelt region (complex sounds)… no tonotopic organization here

Question 11

dorsal “where” pathway

Answer 11

Heads toward parietal lobe where it integrates with info from the visual stream to track object location and movement, as well as plan how to interact with objects in the environment

Question 12

ventral ‘what’ pathway

Answer 12

• Travels along ventrolateral temporal lobe towards frontal brain areas
• Integrates with regions involved in attention, memory and emotion to discern the nature of a sound source (identify sound)

Question 13

Broadman areas 44 & 45

Answer 13

• Collectively known as Broca’s area
• Associated with the processing of speech semantics (particular meaning of words or phrases)

Question 14

Auditory scene analysis

Answer 14

• refers to our ability to distinguish the different sounds in the ambient environment
• each source of complex sound waves produces unique sound profiles… but they are combined together in the air so that sound arriving at ear is a single highly complex waveform
• our auditory system separates features out from the complex waveform & identify each object in the environment, shift attention between them & asses how to interact w/ them

Question 15

Bregman’s view of auditory scene analysis…

Answer 15

• auditory system uses heuristic rules to determine which frequencies go with which other frequencies and which are associated with which objects
• centers on the ability to group different patterns of sound together

Question 16

3 basic types of rules…

Answer 16

1. timing (temporal segregation)… sounds produces at the same time may be grouped together
2. space (spacial segregation)… sounds coming from the same place in space are grouped together
3. frequency (spectral segregation)… sounds of the same frequency/harmonic pattern are grouped together

Question 17

timing (temporal segregation)

Answer 17

• When the time between occurrences of the same note becomes super short, we are more likely to perceive two different sound sources, each producing a single frequency repeatedly… composers use this to create illusion of 2 sound streams from one instrument
• but… if the 2 sounds at the same frequency separation are played slower, we perceive a single sound source which alternates
• sounds linked/correlated in time

Question 18

Space (spatial segregation)

Answer 18

• Compares loudness, timing and frequency distribution of sounds reaching our left and right ears
• can distinguish where in space sounds are coming from & determine whether they are together/seperate

Question 19

frequency (spectral segregation)

Answer 19

• as the difference between an alternating frequency range increases we are more likely to separate them as two different sound sources
• harmonics have highly specialized spacing, so frequency components that occur at multiples of some fundamental frequency are highly likely to be perceptually grouped together … especially if the components have the same onset/offset times
• the equal spacing between these harmonics is important… if we were to take one away or change its frequency we would perceive 2 different sound objects

Question 20

auditory development

Answer 20

• auditory system develops early
• functional at 25th week of pregnancy
• 2 day old infants can recognize moms voice
• all hair cells & auditory nerve continue to change & develop thru the first few years of life
• Infants have equivalent thresholds across the frequency range as do young adults

Question 21

sounds around us…

Answer 21

• sound travels faster in water than air (so harder to localize it in water)
• to localize an object we need to know If it is… left/right of us (azimuth), above/below us (elevation) & in front/behind us (distance)
• median plane = above/below (elevation)
• horizontal plane = left or right of head (azimuth)

Question 22

2 kinds of cues to sound localization…

Answer 22

1. interaural time difference cues (one ear before other)
2. interaural level difference cues (clear, unobstructed path to one ear only w/ acoustic shadow to other)

Question 23

Interaural time difference

Answer 23

• time interval between when a sound enters one ear & when it enters the other ear
• auditory system an detect a millisecond difference in timing from ears & use this info
• this gives us the location of the object along the azimuth
• 90 degrees = 640 microseconds difference (max) / 20 degrees = 200 microseconds
• For a region in the brain to compute the interaural time difference, it must have access to information from both ears… superior olives

Question 24

interaural level difference

Answer 24

• difference in loudness and frequency distribution between the 2 ears
• Our ears can detect loudness differences between the left and right ears
• The head casts an acoustic shadow which changes the loudness/frequency distribution to each ear
• Much more prominent for high-frequency sounds than for low-
• computed in the superior olive

Question 25

interaural differences in the superior olive

Answer 25

• the connectivity between the two ears in the brain allow us the ability to interpret these tiny timing differences
• it gets info from both ears
• cells here are tuned to incredibly small differences in the signals that arise from the 2 cochlear nuclei

Question 26

Jeffress model

Answer 26

• proposed to explain how interaural time difference (ITD) sensitive cells might function in the superior olive
• Input from the left and right cochlear nuclei each target a series of ITD sensitive cells in the superior olive that have characteristic arrangement
• The idea is that signal arriving from the left and right ear will only cause neural activity to be generated within the ITD sensitive cell simultaneously activated by both inputs (arriving at same time)
• Inputs from the two ears are delayed by various amounts relative to one another by the relative length of the axons & we can tell

Question 27

Cone of confusion

Answer 27

• region of positions in space in which sounds create the same interaural time & interaural level differences
• we can’t tell where its coming from
• easily fixed cause head can be moved

Question 28

Human pinna (elevation perception)

Answer 28

• shape of the pinna matters
• The patterns of ridges and folds differentially shape sound signals that arrive from different elevations
• so, the pinna gives an auditory cue to judge whether sounds come from above or below
• The sound from each elevation differs significantly as a function of source elevation
• If you modify the pinna (hoops) can alter a listeners ability to accurately localize sound elevation
• Thankfully, over time we remap these cues and the ability can be restored (20 days about)

Question 29

detecting distance (3)

Answer 29

1. internal knowledge of loudness of familiar sounds
2. frequency (high frequency sounds show greater decrease in loudness as a function of distance than low-) e.g. distant vs close thunder)
3. proportion of direct sound to reflected sound (close by will have larger rations of direct sound than reflected)

Question 30

biosonar (bats)

Answer 30

• bats can track fully mosquitos (hear sounds over 100,000hz)
• biosonar… echolocation… using amount of sound reflected & the production & perception to get useful info abt an objects size & pattern of movement
• this depends on the bats ability to produce & perceive ultra-high frequency sounds
• begin vocalizing in search mode, during which their calls are long & steady in frequency
• once they detect prey, vocalization gets shorter & more frequency which gives them higher number of auditory glimpses until prey is captured
• info on distance, size & speed of target
• Daniel pitch, famous echolocator