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Development of
Sound Localization
2
How do the neural mechanisms
subserving sound localization
develop?
Overview of the development of
sound localization
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Gross localization responses are observed soon
after the cochlea begins to function and in
newborn humans.
The precision of sound localization improves
between birth and 5 years of age.
Localization under complex listening conditions
takes longer to develop.
Experience appears necessary for the formation
of auditory spatial maps.
Overview of this lecture
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Electrophysiological evidence of development of
binaural hearing mechanisms in humans.
Morphological and physiological evidence of
development of binaural hearing mechanisms in
nonhumans.
Limitations imposed by immature peripheral
coding.
Development of spatial maps and role of
experience.
ABR binaural interaction component
MLR binaural interaction component
Binaural responses detectable in
most newborns
Newborn binaural responses suggest
limitations on binaural processing
Conclusion
Binaural evoked potentials have not
been well described in human
infants
Morphological and
physiological evidence of
binaural development in
nonhumans
What limits binaural processing
during development?
Lateral superior olive: IID circuit
Medial superior olive: ITD circuit
Responses of LSO neurons to IID
Responses of MSO neurons to ITD
Normalized spike rate?
Immature neurons don’t respond
much
Immature LSO provides less
information about IID
Range of IIDs eliciting a response
increases with age.
Immature phase locking will lead
to poor ITD processing
Conclusions re: interaural cue calculation
in the immature auditory system
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The circuits used in calculating interaural
differences are in place when the cochlea
starts to function.
The immature responses of neurons that
provide input to the superior olive limit
interaural cue calculation.
The neurons of the superior olive may also
be immature, independent of their inputs.
Forming a map of auditory space
ITD 30
-10µs
µs
IID 4
-2dB
dB
Spectral shape
Intensity -6
-5 dB
-10
20degrees
degreesvisual
visualangle
angleininazimuth
azimuth
- 55 degrees
degrees visual
visual angle
angle in
in elevation
elevation
.6.6meters
metersaway
away
The auditory system is laid out by
frequency and calculates auditory space
Auditory scene
Intensity X
Frequency X Time
representation in
the ear
22, -7, .6
20, -10, .6
buzz
hum
-10, -20, .6
ring
20, -20, .4
click
Neural
computation of
auditory space
Calculated
spatial
representation in
the brain
The visual system is laid out
spatially
View
Spatial
representation on
retina
22, -7, .6
Retinotopic
representation in
the brain
20, -10, .6
-10, -20, .6
20, -20, .4
Visual and auditory spatial representations
are superimposed
Spatial representation in auditory pathway
Intensity X
Frequency X
Time
representation in
the ear
22, -7, .6
20, -10, .6
buzz
Scene
Multimodal
spatial
representation in
the brain
-10, -20, .6
hum
ring
click
Spatial
representation on
retina
20, -20, .4
Normal development of SC
response in guinea pigs
Azimuthal plane
Neurons respond to
sounds in these
locations
Effects of visual and auditory
experience on spatial maps
Effects of abnormal auditory
experience on spatial maps
Effects of dark rearing on spatial
maps
Brief normal exposure is sufficient
for normal spatial maps
Spectral as well as interaural cues
are important
Abnormal experience can produce
unusual neural responses.
Normal
experience
Disparate
experience
Where does experience have its effects?
Spatial representation in auditory pathway
Intensity X
Frequency X
Time
representation in
the ear
22, -7, .6
20, -10, .6
buzz
Scene
Multimodal
spatial
representation in
the brain
-10, -20, .6
hum
ring
click
Spatial
representation on
retina
20, -20, .4
Implications: Blind people and
sound localization
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Blind people (and visually deprived guinea
pigs) have same discrimination-type sound
localization abilities as sighted people.
Interestingly, they are able to localize
sound sources by pointing as well as
sighted people.
Conclusion: Vision isn’t the only sense that
can define space.
Conclusions
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Not surprisingly, binaural evoked responses can
be evoked from newborn infants, although the
morphology of some responses change with age
Whether binaural interaction or improvements in
monaural coding is responsible for changes in
response is not clear.
Normal multimodal experience is required for the
formation of auditory maps of space.