P6 - Hearing

Question 1

What are the 3 main sound localisation cues?

Answer 1

• ITDs (interaural time differences)
• ILDs (interaural level differences)
• spectral cues

Question 2

What are ITDs?

Answer 2

Interaural Time Differences
–> a sound on one side of the head will arrive at the nearer ear first, producing a timing difference between the two ears (because of differences in path length between the sound source and each ear)

Question 3

What are ILDs?

Answer 3

Interaural Level Differences
–> a sound on one side of the head will be louder in the nearer ear, producing a difference in sound level between the two ears (because the head blocks sound by casting an acoustic shadow)

Question 4

What are spectral cues?

Answer 4

Spectral cues
–> when sound hits the pinna, it bounces around inside it in a way that amplifies some frequencies and attenuates others. This changes the spectrum of the sound (i.e. the amount of energy at different frequencies), but does so in a way that depends on the direction of the sound

Question 5

How is horizontal sound localisation achieved?

Answer 5

By using ITDs and ILDs, the brain can tell whether a sound is on the left or right

Question 6

How is vertical sound localisation achieved?

Answer 6

By using spectral cues (i.e. comparing the amount of energy at different frequencies), the brain can tell the elevation of a sound as well as whether it is in front or behind

Question 7

What sound localisation cues does the brain rely on at low frequencies (<2000Hz)?

Answer 7

Low frequencies:

• -> brain relies on ITDs
• -> ILDs and spectral cues are small

Question 8

Why are ILDs small at low frequencies?

Answer 8

ILDs are small because low frequencies are not blocked by the head very well due to diffraction

Question 9

What sound localisation cues does the brain rely on at high frequencies (>2000Hz)?

Answer 9

High frequencies:

• -> brain relies on ILDs and spectral cues
• -> brain not very sensitive to high frequency ITDs

Question 10

Why is the brain less sensitive to high frequency ITDs

Answer 10

Phase-locking in the auditory nerve provides crucial information about the precise timing of sounds.

But at high frequencies, phase-locking fails (because auditory nerve fibres can’t keep up with the extremely rapid peaks and troughs in the sound, although the auditory nerve may still keep track of the envelope)

Question 11

How is a psychometric function for ITDs created?

Answer 11

For each ITD you tested, plot the percentage of times that the subject heard the sound on the right. Then, fit an S-shaped curve to your data.

Question 12

Psychometric function for ITDs:

What does the 50% point represent?

Answer 12

The ITD that the subject hears on the right 50% of the time. This is the ITD that causes
participants to guess randomly (i.e. 50% of the time they guess right and 50% of the time they guess left)

Question 13

Psychometric function for ITDs:

What does the 75% point represent?

Answer 13

The ITD that the subject hears on the right 75% of the time. This is deemed to be a
reliable rightward response because it is too consistent to reflect random guessing.

Question 14

What is an ITD threshold and how do you calculate it?

Answer 14

How much you have to change the ITD to go from random guessing (50% Point) to a consistent
rightward response (75% Point)
--> smallest change in ITD that can be reliably detected

75% point - 50% point

Question 15

What is the spatial threshold?

Answer 15

smallest change in location that can be reliably detected

Question 16

What is the maximum ITD and how do you calculate it?

Answer 16

Maximum time taken for sound to travel from one ear to the other (which will be experienced when a sound is very close to one ear)

πr/c

With πr = distance between ears (r is radius of head)
c = speed of sound in air = 343m/s