Development of Sound Localization 2

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Development of Sound Localization 2

• How do the neural mechanisms subserving sound
  localization develop?

2
Overview of the development of sound localization

• 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.

3
Overview of this lecture

• 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.

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ABR binaural interaction component
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MLR binaural interaction component
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Binaural responses detectable in most newborns
7
Newborn binaural responses suggest limitations on
binaural processing
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Conclusion

• Binaural evoked potentials have not been well
  described in human infants

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Morphological and physiological evidence of
binaural development in nonhumans

• What limits binaural processing during
  development?

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Lateral superior olive IID circuit
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Medial superior olive ITD circuit
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Responses of LSO neurons to IID
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Responses of MSO neurons to ITD
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Normalized spike rate?
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Immature neurons dont respond much
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Immature LSO provides less information about IID
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Range of IIDs eliciting a response increases with
age.
18
Immature phase locking will lead to poor ITD
processing
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Conclusions re interaural cue calculation in the
immature auditory system

• 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.

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Forming a map of auditory space
-10 degrees visual angle in azimuth - 5 degrees
visual angle in elevation .6 meters away
20 degrees visual angle in azimuth 5 degrees
visual angle in elevation .6 meters away
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The auditory system is laid out by frequency and
calculates auditory space
Calculated spatial representation in the brain
Intensity X Frequency X Time representation in
the ear
Auditory scene
22, -7, .6
20, -10, .6
buzz
-10, -20, .6
hum
ring
click
20, -20, .4
Neural computation of auditory space
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The visual system is laid out spatially
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Visual and auditory spatial representations are
superimposed
Spatial representation in auditory pathway
Multimodal spatial representation in the brain
Intensity X Frequency X Time representation in
the ear
22, -7, .6
20, -10, .6
buzz
-10, -20, .6
hum
Scene
ring
20, -20, .4
click
Spatial representation on retina
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Normal development of SC response in guinea pigs
Azimuthal plane
Neurons respond to sounds in these locations
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Effects of visual and auditory experience on
spatial maps
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Effects of abnormal auditory experience on
spatial maps
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Effects of dark rearing on spatial maps
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Brief normal exposure is sufficient for normal
spatial maps
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Spectral as well as interaural cues are important
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Abnormal experience can produce unusual neural
responses.
Normal experience
Disparate experience
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Where does experience have its effects?
Spatial representation in auditory pathway
Multimodal spatial representation in the brain
Intensity X Frequency X Time representation in
the ear
22, -7, .6
20, -10, .6
buzz
-10, -20, .6
hum
Scene
ring
20, -20, .4
click
Spatial representation on retina
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Implications Blind people and sound localization

• 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 isnt the only sense that can
  define space.

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Conclusions

• 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.