Week 13: Neurobiology of Hearing Part 2

General principles of sensory coding Reception Transduction Coding Parallel processing Our sensory systems deceive us constantly Perception is an active process

Reception

Transduction

We now have tonotopic frequency encoding - and - We now have the encoding of intensity We now have the encoding of frequency by tonotopic organization. We now have tonotopic frequency encoding - and - The beginnings of intensity coding

Localizing sound We have two main cues available to localize sound: Interaural Intensity Differences IID Interaural timing differences ITD We discussed last class that Intensity differences don’t work well for low frequency sounds And the challenge of timing differences is that sound is fast and the head is narrow so timing differences less than one millisecond!

Sound frequency and wavelength

High frequency sounds create a “sound shadow” High frequency sounds create a “sound shadow”. Low frequency sounds do not! What is the cutoff point? Head width! Think about it: across all the sizes of animals – what is the frequency of their communication signals, and how big are their heads? But : What is left when lambda is greater than head diameter (lower frequencies)? Timing difference! What is difference in time of arrival if sound travels at 350 m/s and the head is 25 cm wide? 350/0.25 = 0.000714 seconds?

Finally: we are in the brain! So how does the brain decode location?

So how does the brain decode location?

Computing the IID (intensity difference)

High frequency sounds create a “sound shadow” High frequency sounds create a “sound shadow”. Low frequency sounds do not! What is the cutoff point? Head width! Think about it: across all the sizes of animals – what is the frequency of their communication signals, and how big are their heads? But : What is left when lambda is greater than head diameter (lower frequencies)? Timing difference! What is difference in time of arrival if sound travels at 350 m/s and the head is 25 cm wide? 350/0.25 = 0.000714 seconds?

Computing the ITD (timing difference)

Always better as an animation.

What about elevation? What about sounds directly in front or directly behind? Head-related Transfer Function (HRTF)

What about sounds directly in front or directly behind? Head-related Transfer Function (HRTF)

Higher order processing So how does the brain decode location?

Our sensory systems deceive us constantly Perception is an active process Sound reverse Overcoming noise Tritone paradox McGurk Effect (start 1:21) deutsch.ucsd.edu/psychology/ Top-down processing vs. bottom-up processing: How previous experience changes what we perceive Sound reverse: Matlab program - shows that we filter out echoes Overcoming noise: Point out that our auditory system “fills in the gaps” even when the noise overrides what we are listening to. We will see something very similar in the case of vision.’ (Then point out that there is no tone during the noise) Tritone paradox:

Restoring Lost Hearing: Cochlear implants We have two main cues available to localize sound: Interaural Intensity Differences IID Interaural timing differences ITD We discussed last class that Intensity differences don’t work well for low frequency sounds And the challenge of timing differences is that sound is fast and the head is narrow so timing differences less than one millisecond!

Hearing with cochlear implants https://www.youtube.com/watch?v=SpKKYBkJ9Hw