Lecture 21 Flashcards

Question

Interaural level difference (ILD):

Answer 1

it's the diff in the sound pressure (amplitude) of a stimulus hitting one ear vs. another as a sound stimulus is coming in you get a reduction in the intensity as the sound stimulus goes through your head difference in sound pressure level reaching the two ears • Reduction in intensity occurs for high frequency sounds for the far ear. – The head casts an acoustic shadow, blocking out (reflecting) and absorbing some of the high-frequency pressure wave. • This effect doesn’t occur for low frequency sounds. Works best above 800 Hz. the sound pressure waves interfered with by the head

Answer 2

``` blocking out (reflecting) and absorbing some of the high-frequency pressure wave. ```

Answer 3

...farther to the side. – This is shown in psychophysics experiments where they measure the frequencies hitting the ears.

Answer 4

they are not effective for detecting differences in elevation. • An additional cue is necessary for reliable elevation judgments (monaural cues).

Answer 5

...azimuth plane.

Answer 6

– Consider points A and B. The level and time differences of signals reaching each ear is zero. – There is a similar ambiguity at points C and D. – These points are said to lie on a cone of confusion. – Multiple (infinite!) cones of confusion are possible. all the points are the same distance and can have the same shadowing effect: not getting any time or level differences

Answer 7

is a spectral filter when sound comes in it bounces off them and the frequencies are affected

Answer 8

unique to each head how the shape of the pinna and the head uniquely affect the intensities of frequencies – This is a spectral cue since the information for location comes from the spectrum of frequencies.

Answer 9

``` Measurements have been performed by placing small microphones in ears and comparing the intensities of frequencies with those at the sound source. ``` "what it's like to sound like you"

Answer 10

manipulated the pinna – Listeners’ accuracy was measured while locating sounds differing in elevation (baseline). – They were then fitted with an ear mold that changed the shape of their pinna. – If the shape of the pinna is important for elevation judgments, subjects should show a large decrease in performance (compared to baseline)… – Right after the molds were inserted, performance was poor for elevation but was unaffected for azimuth (spectral cues not important for left and right). – After 19 days (they kept the molds in this whole time), performance for elevation was close to original performance. – Once the molds were removed, performance remained high (didn't go back to some baseline where they had to re-learn). – This suggests that there might be two different sets of neurons—one for each set of spectral cues (new and old) = a sparse code OR a more broadly distributed population code that could adjust and encode for both. – Nice example of experience-dependent plasticity.

Answer 11

it's a low frequency but where you're sitting in the room changes whether it's an ITD or HRTF (based on the altitude one is sitting in the room)

Answer 12

– It appears that ITD and ILD azimuth cues are most effective to the side, but we have difficulty detecting those on the cone of confusion. – Remember that perception is an active process!! No single observation is used to localize a sound. We orient to make ITD and ILD zero in front of us (till you're staring right at the object). – Spectral cues (HRTF) are always used: very good when straight on cause that's what we have the most experience with. – We also use expectations (top-down information) to help localize.

Answer 13

are mainly for orienting. | In real applications, we move our heads and take many ‘auditory samples’ with all localization cues.

Answer 14

Two basic approaches have been proposed (using ITD as a model) 1. Narrowly tuned ITD neurons 2. Broadly tuned ITD neurons

Answer 15

neurons responsive to small changes respond to small differences in timing • Respond to specific time differences only. One neuron gives a location. This is a form of specificity coding.

Answer 16

lots of neurons responding to large changes in timing differences • Respond to broad range of time differences. Location is calculated from a range of neural responses. This is a form of distributed coding.

Answer 17

how the system is sensitive to small changes – These neurons receive signals from both ears. – Coincidence detectors fire only when signals arrive from both ears simultaneously (sensitive to the different when the sound hits one ear and the other). – If signal reach both ears at the same time, then ITD is zero. – If a signal comes in on the right earlier, it will travel father along these cells and meet the left signal at a different location .