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Is musical practice increasing the size of auditory cortex, or do
people with large auditory cortex become musicians?
Human psychoacoustical performance
Frequency differencesFrequency differencesPressure ratio between softest and loudest sounds…
Pressure ratio between softest and loudest sounds…
Hair motion at absolute threshold…Hair motion at absolute threshold…
Perceptual learning
• Partially non-specific– Playing tetris improves frequency
discrimination
• Partially due to passive exposure
• But also to some extent requires active task performance
Animal models of auditory plasticity
• Classical conditioning– Fear conditioning: associating a sound with a
foot shock
• Environmental enrichment and relatives– Manipulating the environment can have both
beneficial and disruptive effects on the auditory system
• Spatial hearing
timeam
plitu
de
Interaural Time Differences (ITDs)
Interaural Level Differences (ILDs)
Binaural Cues for Localising Sounds in Space
Interaural Time Difference (ITD) Cues
ITD
ITDs are powerful cues to sound source direction, but they are ambiguous (“cones of confusion”)ITDs are powerful cues to sound source direction, but they are ambiguous (“cones of confusion”)
Disambiguating the cone of confusion
• Sounds on the median plane (azimuth 0, different elevations) have zero binaural disparities
• This is a special case of the cone of confusion
• Nevertheless, humans and other animals can determine the elevation of a sound source
Binaural Cues in the Barn Owl
Barn owls have highly asymmetric outer ears, with one ear pointing up, the other down. Consequently, at high frequencies, barn owl ILDs vary with elevation, rather than with azimuth (D). Consequently ITD and ILD cues together form a grid specifying azimuth and elevation respectively.
Processing of Interaural Time Differences
Interaural time difference
MS
O n
euro
n re
spon
s e
Sound on the ipsilateral side
Contra- lateral side
Medial superior olive
To the Inferior Colliculus
Preservation of Time Cues in AVCN
• Auditory Nerve Fibers connect to spherical and globular bushy cells in the antero-ventral cochlear nucleus (AVCN) via large, fast and secure synapses known as “endbulbs of Held”.
• Phase locking in bushy cells is even more precise than in the afferent nerve fibers.
• Bushy cells project to the superior olivary complex.
• Auditory Nerve Fibers connect to spherical and globular bushy cells in the antero-ventral cochlear nucleus (AVCN) via large, fast and secure synapses known as “endbulbs of Held”.
• Phase locking in bushy cells is even more precise than in the afferent nerve fibers.
• Bushy cells project to the superior olivary complex.
sphericalbushy
cell
sphericalbushy
cell
endbulbof Held
endbulbof Held
VIII nervefiber
VIII nervefiber
The coincidence detection model of Jeffress The coincidence detection model of Jeffress (1948) is the widely accepted model for low-(1948) is the widely accepted model for low-
frequency sound frequency sound localisationlocalisation
Window
SemicircularCanals
Left Ear Right EarCochlear Nucleus
MSO
0 s Time Delay
0 s
0 s Time Delay
Auditory Nerve ActivityLarge calyx synaptic ending
Cochlear Nucleus
Window
SemicircularCanals
Left Ear Right EarCochlear Nucleus
MSO
300 s Time Delay
300 s
Arrives at left ear 300 s later than at the right
Coincident spikes
Auditory Nerve ActivityLarge calyx synaptic ending
Cochlear Nucleus
Window
SemicircularCanals
Left Ear Right EarCochlear Nucleus
MSO
300 s Time Delay 0 s Time Delay
0 s300 s
0 s Time DelayArrives at left ear 300 s later than at the right
Coincident spikes
Auditory Nerve ActivityLarge calyx synaptic ending
Cochlear Nucleus
Processing of Interaural Level Differences
Interaural intensity difference
LS
O n
eur o
n r e
s po n
se
Sound on the ipsilateral side
Contralateralside
C > II > CLateral superior olive
To the Inferior Colliculus
The Calyx of Held
• MNTB relay neurons receive their input via very large calyx of Held synapses.
• These secure synapses would not be needed if the MNTB only fed into “ILD pathway” in the LSO.
• MNTB also provides precisely timed inhibition to MSO.
Caird and Klinke 1983Frequency (kHz)
0.125 0.12532 32
Sou
nd le
vel (
dB S
PL)
20 20
100100
Ipsilateral Contralateral
The Superior Olivary Nuclei – a Summary
• Most neurons in the MSO respond best to sounds that occur earlier in the contralateral ear.
• Most neurons in the LSO respond best to sounds that are louder in the ipsilateral ear.
• Space representation is crossed, and therefore LSO projects mostly contralaterally and MSO ipsilaterally.
MNTBMNTB
MSOMSO
LSOLSO
CNCN CNCN
Midline
Inhibitory ConnectionExcitatory Connection
ICIC ICIC
Orientation responses to auditory and visual stimuli are congruent…
Auditory orientation response
Visual orientation response