1. 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.

4. ABR binaural interaction component

5. MLR binaural interaction component

6. Binaural responses detectable in most newborns

7. Newborn binaural responses suggest limitations on binaural processing

8. Conclusion Binaural evoked potentials have not been well described in human infants

9. Morphological and physiological evidence of binaural development in nonhumans What limits binaural processing during development?

10. Lateral superior olive: IID circuit

11. Medial superior olive: ITD circuit

12. Responses of LSO neurons to IID

13. Responses of MSO neurons to ITD

14. Normalized spike rate?

15. Immature neurons don’t respond much

16. Immature LSO provides less information about IID

17. Range of IIDs eliciting a response increases with age.

18. Immature phase locking will lead to poor ITD processing

19. 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.

20. Forming a map of auditory space ITD 30 µs IID 4 dB Spectral shape Intensity -5 dB 20 degrees visual angle in azimuth 5 degrees visual angle in elevation.6 meters away ITD -10 µs IID -2 dB Spectral shape Intensity -6 dB -10 degrees visual angle in azimuth - 5 degrees visual angle in elevation.6 meters away

21. The auditory system is laid out by frequency and calculates auditory space Auditory scene Intensity X Frequency X Time representation in the ear Calculated spatial representation in the brain 20, -10,.6 22, -7,.6 -10, -20,.6 20, -20,.4 buzz hum click ring Neural computation of auditory space

22. The visual system is laid out spatially View Spatial representation on retina Retinotopic representation in the brain 20, -10,.6 22, -7,.6 -10, -20,.6 20, -20,.4

23. Visual and auditory spatial representations are superimposed Scene Spatial representation on retina Multimodal spatial representation in the brain 20, -10,.6 22, -7,.6 -10, -20,.6 20, -20,.4 buzz hum ring click Intensity X Frequency X Time representation in the ear Spatial representation in auditory pathway

24. Normal development of SC response in guinea pigs Azimuthal plane Neurons respond to sounds in these locations

25. Effects of visual and auditory experience on spatial maps

26. Effects of abnormal auditory experience on spatial maps

27. Effects of dark rearing on spatial maps

28. Brief normal exposure is sufficient for normal spatial maps

29. Spectral as well as interaural cues are important

30. Abnormal experience can produce unusual neural responses. Normal experience Disparate experience

31. Where does experience have its effects? Scene Spatial representation on retina Multimodal spatial representation in the brain 20, -10,.6 22, -7,.6 -10, -20,.6 20, -20,.4 buzz hum ring click Intensity X Frequency X Time representation in the ear Spatial representation in auditory pathway

32. 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 isn’t the only sense that can define space.

33. 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.