Hearing

Question 1

What is sound

Answer 1

Pressure waves in the air

It is a sequence of pressure waves which propagate through this elastic net.

Question 2

What are the properties of air

Answer 2

Air is ‘elastic;’ if you try to compress it, it will push back.
Air has both elasticity and mass; so we can imagine air as being made up of little ‘lumps of air’ where each lump is connected to the next lump by an elastic spring.

Question 3

How is the spectrum of the sound determined

Answer 3

Any complex sound wave can be produced by adding together sinusoidal sound waves (pure tones)
The amplitudes and frequencies of these components determine the spectrum of the sound.

Question 4

What is the head-related transfer function (HRTF)

Answer 4

When sound is transformed from the external ears (pinnae) from free-field sound to the sound at the ear drum.
HRTF adds localisation information into the signal (this is why ears are shaped as they are).

Question 5

What happens in the middle ear

Answer 5

It contains a set of intricate, interconnected bones: stapes, incus and malleus.
These amplify the motion of the ear drum into pressure waves transmitted via the oval window into the fluid-filled cochlea (in the inner ear)

Question 6

What happens in the inner ear (cochlea)

Answer 6

Basilar membrane inside the cochlea vibrates according to the frequency structure of the sound.
Hair cells attached to the basilar membrane then transduce this mechanical signal into action potentials in auditory nerve.

Question 7

What is the tonotopic map in the cochlea

Answer 7

It is a map of sound frequency that is created by the basilar membrane through its vibrations.
This is achieved through its structure: wide and stiff at the base, narrow & loose at apex.
The apex vibrates for a low frequency while the brave vibrates for a high frequency.
The cochlea separates out the different frequency components of a complex sound.
Thus, different auditory nerve fibres represent different frequencies = place code of frequency

Question 8

What is phase locking

Answer 8

When auditory nerve fibres produce spikes that are phase-locked (synchronised) to the vibration of the basilar membrane.

Question 9

What are the two ways that frequency components of sound are represented

Answer 9

Place of activity - tonotopic mapping or place coding
Timing of activity - temporal coding in which sounds with higher frequencies produce higher rates of synchronised firing.

Question 10

What is the main task of subcortical processing

Answer 10

Sound source localisation utilising cross-ear differences in:
Sound wave amplitude
Sound wave arrival time

Question 11

What are the properties of the visual nerve

Answer 11

The retina is 2-dimensional
Provides shape information
Provides colour information
Task for visual system to separate out visual objects relatively easy

Question 12

What are the properties of the auditory nerve

Answer 12

Basilar membrane provides inofrmation on one dimension only:
Frequency content of the incoming sound wave
This sound wave is the the sum of all signals from around
Task for auditory system to separate out auditory objects is huge.
Subcortical pathway performs (at least) source localisation as part of the solution.

Question 13

How is the auditory cortex organised

Answer 13

Core (C): primary fields (HG) receiving input from thalamus
Belt (B): secondary fields surrounding core
Parabelt (PB): secondary fields next to belt (STG)
Hierarchal organisation: activity progresses from C to B to PB
Parallel organisation: activity propagates along multiple C-B-PB paths

Question 14

What happens in the cortical visual areas

Answer 14

Processing is distributed and specialised: each area represents specific type of information; orientation, colour, motion, etc.
What and where processing is achieved through two processing streams.

Question 15

What do the auditory cortical fields do

Answer 15

Core fields respond to pure tones and complex sounds.
Belt and parabelt fields respond to complex sounds and noise, and less vigorously to pure tones.
No tonotopic organisation in parabelt.
However, there is little evidence of the kind of specialisation seen in visual cortex.

Question 16

Which features do cells in the auditory cortex show selectivity for

Answer 16

Intensity
Bandwidth
Sound source location

Question 17

What are the ‘what’ and ‘where’ processing streams

Answer 17

Anterior ‘what’ stream: specialisation in sound source identity (e.g. semantic meaning)
Posterior ‘where’ stream: specialisation in sound source location.
But this is a debated issue; what and where info is mixed

Question 18

What are the properties of spectrotemporal receptive fields

Answer 18

Sharp and wide tuning to frequency
Tuning to sounds with complicated special structure
Tuning to sounds whose spectral structure changes

Question 19

How is the auditory cortex modulated

Answer 19

By top-down effects which can be seen in discrimination tasks:
- Attention modulates STRFs (Fritz et al., 2003)
- Working memory shows up as sustained activity in
auditory cortex (Huang et al., 2016)
- • Perceptual learning changes mappings in auditory cortex
(Polley et al., 2006)

Question 20

Describe perceptual learning in the auditory cortex

Answer 20

Polley et al (2006) - Rats trained to discriminate stimuli either according to pitch or loudness
Pitch discrimination training leads to expansion of frequency representation
Loudness discrimination training leads to expanded loudness representations
Thus, representations are modulated by top-down effects of learning

Question 21

Describe surface recordings from cortex

Answer 21

Mesgarani & Chang (2012, Nature) presented sentences to human epileptic patients.
Surface recordings from auditory cortex
Method: stimulus spectrogram reconstruction from neural population responses + using machine learning classifier to decode stimulus from neural
response.
In the mixed condition, auditory cortex is responding as if the sentence being attended to is being presented alone.

Question 22

What are some of the subcomponents of speech processing.

Answer 22

• Basic auditory processing of complex sound
• Distinguishing speech from non-speech
• Identifying speech sounds (e.g. vowels)
• Semantics at word, sentence, and discourse level
• Recognising prosodic aspects of speech
• Recognising individual voices, accents, & style of speech
• Recognising emotions
• Merging auditory and visual information

Question 23

What is the traditional model of speech processing

Answer 23

Speech production occurs in Broca’s area in inferior frontal gyrus (IFG)
Speech comprehension occurs in Wernicke’s area in
posterior superior
temporal gyrus (pSTG)
But this no longer holds

Question 24

Speech intelligibility

Answer 24

Scott et al. (2000) did a PET
study:
They contrasted responses to intelligible vs. non -intelligible speech stimuli
Larger responses to intelligible speech in left anterior STG
(so not Wernicke’s area)

Question 25

Does context support speech perception

Answer 25

Yes.
Obleser et al (2007), did an fMRI study looking at processing of degraded speech (~speech in noise)
It has been found that semantic predictability enhances intelligibility.
This enhancement in intelligibility is associated with a left-lateralized widly distributed system becoming active.
This system is associated with semantic processing.

Question 26

What is the problem of studying comprehension

Answer 26

Contrasts are needed to study the processing of intelligible speech.
Unintelligible stimuli tend to differ from intelligible stimuli in terms of acoustic complexity or
structure.
We can’t be sure that differences in brain activation reflect intelligibility only.

Question 27

What is intelligibility in increased activations

Answer 27

Hakonen et al. (2017), did an fMRI study and found:
Intelligibility leads to increased activity in Anterior Cingulate Cortex (ACC), Frontal Pole (FP),
and right Frontal Operculum.
These areas associated with retrieval mode of episodic memory.
Episodic memory needed in comprehension of speech in noise.

Question 28

What is intelligibility in decreased activations

Answer 28

Intelligible speech leads to decreased activation in auditory cortex.
This might be due to predictive coding:
Top down signals dampen predictable bottom-up signals in sensory cortex.

Question 29

What is visual binding

Answer 29

The process of integration where all information is put together to generate integrated percept (because the visual cortex has many retinotopic maps, each representing a specific feature; colour, orientation, motion, form).

Question 30

How is auditory processing serial

Answer 30

Because the temporal ordering of sound events is crucial for speech to make sense.

Question 31

What are the challenges of visual perception

Answer 31

The information arriving in the cortex comes in short discontinuous bursts of up to 200ms, but there is no info coming in between the bursts.

Question 32

What are the challenges of auditory perception

Answer 32

Information arrives in a continuous stream,
Information from different sound sources is mixed, but sound sources are perceived as unmixed.
In this kind of temporal binding, the time order of incoming information is essential.

Question 33

How do you test for memory in sensory systems

Answer 33

Tested by changing past stimulation: if there is an effect, this is called context sensitivity.

Question 34

What is adaptation/ forward masking

Answer 34

Repeating the same stimulus within hundreds of milliseconds leads to response attenuation in primary auditory cortex

Question 35

What is forward facilitation

Answer 35

Playing one stimulus and then another can lead to increased responses to the probe.

Question 36

What are event-related responses

Answer 36

Brain responses time-locked to stimulus presentation and averaged over many stimulus presentations.
They reflect sensory processing.
They can be measured in EEG: event-related potential (ERP)
Or they can be measured in MEG: event-related field (ERF)

Question 37

What is the oddball paradigm

Answer 37

When a ‘deviant’ is randomly thrown into a standard sequence to test whether attention is being paid.

Question 38

What stimulations can the standard and deviant differ along

Answer 38

• Frequency
• Intensity
• Sound duration
• Sound source location
• Speech sound identity
• Tone sequences
And more

Question 39

What are some psychophysical effects of deviants

Answer 39

Infrequent stimuli cause surprise. 
Orienting response/orienting reflex:
• Skin conductance
response
• Heart rate changes
• Eye movements
• Halting of ongoing activity
• Involuntary reorienting of
attention

Question 40

What is the Mismatch Negativity (MMN)

Answer 40

The difference in ERP that the deviant produces compared to the standard.
It is elicited by any perceptible difference between the standard and deviant.

Question 41

What does MMN reflect

Answer 41

Adaptation to standards, response recovery to deviants. Clear index of context sensitivity, memory, and temporal binding.
Clear index of change detection, which might subserve the orienting response (OR).
The auditory cortex keeps track of stimulus probabilities.
Linked to auditory sensory memory.
If we understood how MMN is generated, we might learn something about temporal binding more generally.