Perception 3: Low level hearing

Question 1

What is the difference between our perception of vision vs sound?

Answer 1

Map of vision, but no map of sound - different sounds converge into one

Question 2

What are sound waves?

Answer 2

Variations in air pressure
(sound causes vibration in air particles, change in air pressure is picked up by ear)
This is where auditory perception begins

Question 3

The physical properties of a sound wave determine the perceptual properties
What perceptual properties do amplitude and frequency determine?

Answer 3

Amplitude -> loudness
Frequency -> pitch

However, relationship between frequency and pitch is more complicated than that for amplitude and loudness
More frequent oscillations = higher pitch

Question 4

What does Fourier’s theorem state?

Answer 4

Any complex sound waveform can be created using a finite number of sinusoids

Question 5

The lowest frequency present is called the fundamental frequency, or f0 (can also be f sub1)
What are harmonics?

Answer 5

Integer multiples of f0
Sounds vibrate at fundamental frequency but also at harmonic frequencies
Harmonics are found in natural sounds

Question 6

What are the three stages of how the human brain interprets sound waves?

Answer 6

1. Delivering the sound stimulus to the receptor
2. Converting the physical stimulus into an electrical signals
3. Inferring perceptual qualities (e.g. loudness, pitch) from electrical signals

Question 7

What do the outer and middle ear structures do?

Answer 7

Translate air vibrations into liquid vibrations of the inner ear

Sound hits ear drum - ossicles vibrate (malleus, incus, stapes)
Vibrate onto the oval window and cochlea

Question 8

What does the cochlear partition contain? What does this do?

Answer 8

The basilar membrane - this reacts to liquid vibrations in the scala vestibuli and scala tympani
Vibrates up and down in response to liquid vibration

Question 9

What does the movement of the basilar membrane cause?

Answer 9

Causes cilia (hair cells) to move laterally

Question 10

What does the corti do?

Answer 10

Translate vibration info into a neural impulse

Question 11

What direction does the tectorial membrane vibrate? What does this result in in combination with the basilar membrane vibrations?

Answer 11

Left to right
Since basilar membrane vibrates up and down, this causes friction on tips of hair cells - causing lateral movement
Cilia tips move left to right

Question 12

What happens when hair cells bend in one direction?

Answer 12

Links are stretched, these are connected to ion flow gates, so when they are pulled, these gates open - positively charged potassium ions flow inside of the hair cell - inside of hair cell has positive charge relative to outside which causes depolarisation - releases neurotransmitter
When they bend in other direction - hyperpolarisation

The vibrational stimulus causes oscillations of excitation and inhibition in the hair cells

Question 13

The basilar membrane responds like a travelling wave. What does this mean about the physical properties of it?

Answer 13

Physical properties of the membrane mean that different places along it respond preferentially to different frequencies

Base is stiff and narrow => high frequencies
Apex is flexible and wide => low frequencies

Sound travels from base to apex like a travelling wave
Can map hz onto basilar membrane

Question 14

What happens to frequency tuning curves as frequency (hz) increases (higher pitch)? What consequences does this have for perception?

Answer 14

The tuning curves get increasingly wide
So wide that higher frequencies cannot be distinguished from one another as easily as low frequencies

Higher frequencies are more perceptually similar to one another

Question 15

Is pitch derived from place encoding? (area of basilar membrane that is vibrating the most)

Answer 15

Not quite.

Pitch of a complex sound corresponds to its f0
BUT Removing f0 does not change its perceived pitch
Sound sounds the same without f0 - so perception of pitch cannot just be explained by part of basilar membrane vibrating

Therefore, brain must engage in some kind of pattern recognition - we hear the difference between different frequencies

Question 16

Is pitch derived from time encoding - what is the temporal theory of pitch?

Answer 16

In addition to frequency specificity, auditory nerve fibers are phase-locked with respect to the stimulus
- action potential in nerve fiber only responds to increase in sound pressure, not decrease - each nerve fiber responds to peaks

This is because action potentials only occur when an increase in sound pressure occurs (i.e. a peak in the waveform)

• Time intervals between action potentials are integer multiples of the period (p) of the waveform
• The population of responses across many fibers convey the sound’s frequency

Question 17

Why is phase locking in the temporal theory of pitch a favourable explanation?

Answer 17

Phase locking is only reliable up to 5 kHz, AND human pitch perception breaks down above 5 kHz

Ability of action potentials to rise and fall at 5k hz is lost - neurons cannot fire at that frequency - shows this is down to time

Question 18

Summarise place and time theory of pitch perception:

Answer 18

1. Place theory
   There is a frequency to place conversion in the cochlea;
   The perceived pitch is simply related to the place of maximum response on the basilar membrane
2. Time theory
   The time pattern of neural impulses reflects the frequency of the stimulus (phase locking)
   The perceived pitch is related to the time intervals between nerve spikes.

In reality both place- and time-based coding contribute to pitch perception.

Question 19

In hearing what are sounds from all locations combined into?

Answer 19

A single waveform that the ear detects - superimposed together (no spatial mapping in the cochlear)

Question 20

On what three dimensions can sound be localised?

Answer 20

1) Azimuth - left/right
2) Elevation - up/down
3) Distance - how far away

Each requires different types of acoustic cue (some monoaural, some binaural)

Question 21

What is the interaural level difference for detecting azimuth (left or right localisation) of sounds?

Answer 21

• Sounds reaching the contralateral ear (opposite ear to where the sound came from) are attenuated - head has shadowing effect
• Better for higher frequencies

Question 22

What is the interaural timing difference for detecting azimuth (left or right localisation) of sounds?

Answer 22

• Sounds detected in the contralateral ear arrive later than at the ipsilateral ear due to further travel distance
• Better for lower frequencies

Question 23

Since the interaural level difference and the interaural timing difference are binaural cues, what do they require?

Answer 23

Central processing in the CNS

Question 24

Wightman and Kistler (1992)
Artificially manipulated interaural timing differences (ITD) of broadband sounds.
Other cues (e.g. ILD) held constant)

i.e. they simply digitally delayed the arrival of one sound at one ear
Ppts had to locate where sound was coming from

What did results show?

Answer 24

ITD manipulated to be 0 = listeners think sound is at 0 degrees
Then far left and far right clusters for -45 and +45

Subjects become more accurate when low frequencies are removed

This suggests the following dual mechanism:
1) ITDs are dominant when low frequencies are present
2) ILDs are dominant when high frequencies are present

Question 25

Are ITDs or ILDs more dominant with high or low frequencies?

Answer 25

1) ITDs are dominant when low frequencies are present 2) ILDs are dominant when high frequencies are present

Question 26

What do the pinna and head affect?

Answer 26

how sound waves are absorbed, reflected, and diffracted. - These interactions create frequency-specific distortions (spectral cues) that vary depending on the elevation of the sound source. Sound arriving at ear canal is different from sound arriving at pinna Sound coming from above pinna is perceived differently as pinna changes this - sound waves hit different parts of the pinna Learn over time through sensorimotor reinforcement that sound that sounds a certain way is likely to be coming from above or below

Question 27

Hofman et al (1998) gave people “new ears” (molds inside of pinna) and showed what about elevation perception?

Answer 27

a) That elevation perception becomes significantly impaired (left/right was not impaired) b) That elevation perception can be re-learned over a period of days Had to make saccades towards where they thought sound was coming from This demonstrates neural plasticity in the auditory system

Question 28

The brain combines several contextual cues to infer distance of a sound source. How do sound level and sound spectrum alter with distance?

Answer 28

Sound level decreases with distance Sound spectrum - high frequencies travel less distance

Question 29

What is the direct-to-reverberant energy ratio?

Answer 29

Reverberation increases with distance Direct decreases with distance So ratio decreases with distance

Question 30

What does the main central auditory pathway involve?

Answer 30

- Cochlear nucleus - Superior olivary nucleus - Inferior colliculus (sudden sounds) - Medial geniculate nucleus (contains more advanced maps of space) - Primary auditory cortex

Question 31

What happens at the superior olivary nucleus?

Answer 31

The first site to combine left and right … and is therefore the first site that can support azimuth localisation

Question 32

How are interaural timing differences encoded by a neural circuit?

Answer 32

Algorithmic models of interaural timing difference (ITD) coding typically consist of coincidence detectors Individual neurons 1-9 preferentially respond to a particular position If signals from both ears reach neuron 5 - the sound entered each ear at the same time, therefore sound came from the center If signals reach neuron 3 - the sound entered the right ear earlier than the left ear as this has had more time to travel along its axon (assuming left side starts at 1)

Question 33

In what animal did Carr and Konishi first observe the coincidence detector neurons for ITD?

Answer 33

Owls - the laminar nucelus (owl equivalent of the superior olivary nucleus)

Question 34

What does the inferior colliculus do?

Answer 34

It integrates binaural and spectral information to form a spatial map (more in multisensory lecture). Ordered representation of frequencies in the sound Inferior colliculus also encodes spectral (frequency) in a tonotopic map Schreiner and Langner (1997) studied in the cat Deeper neurons represent higher sound frequencies - tonotopic organisation

Question 35

What does the human primary auditory cortex (A1 - located on Heschl's gyrus) consist of?

Answer 35

Tonotopically organised maps

Question 36

Is there a spatial map in A1?

Answer 36

No - just tonotopic auditory frequency mapping

Question 37

High resolution (7T) fMRI supports the existence of mirrored tonotopic maps in A1 - what does this mean?

Answer 37

There are at least two different maps - middle has low frequency representations, ends have high frequency representations The function of these separate maps is unclear… … but could be linked to different cortical streams