lecture 14 - adrian rees

Question 1

auditory cortex in monkey:
in the core region there is

Answer 1

the primary auditory cortex (A1)
the rostral area (R)
the rostral temporal area (RT)

Question 2

in each area in the core region there is a tonotopic representation of frequency.
in A1 the frequency runs..

Answer 2

high to low

Question 3

in R its

Answer 3

low to high

Question 4

in RT its

Answer 4

high to low

Question 5

fMRI shows that in humans A1 region runs high to low and then is reverses when it gets to human R region as it goes

Answer 5

low to high frequency

Question 6

RFs in ferret auditory cortex show plasticity:
selectivity of neurons for different frequencies can change depending on what the task is that the animal is undertaking.
animal has been trained to listen out for a tone (400Hz), when the tone is played the animal gets a reward.
this causes

Answer 6

the receptive field to change and the area that corresponds to region where 400Hz would stimulate this cell has become enhanced (increasingly sensitive to the 400Hz tone)

Question 7

two strategies for encoding frequency

Answer 7

1. place code - maintaining spatial representation of frequency established on the basilar membrane in the cochlea (tonotopic organisation)
2. temporal code - firing of nerve fibres and neurons synchronised to the sound waveform (phase locking)

Question 8

action potentials synchronise with the…

Answer 8

peaks of the sounds waveform

Question 9

the AP represent the sound wave form in pattern of firing. the smallest interval between APs is…

Answer 9

the time between the peaks which is equal to the period of the sound

Question 10

1/period =

Answer 10

frequency

Question 11

when we record from the bushy cells in the cochlear nucleus the APs are much more

Answer 11

tightly clustered around the peak of the sound waveform

Question 12

this is because each globular bushy cells gets input from…

Answer 12

several auditory nerve fibres
combined input causes a more precise response (greater chance that the cells will capture every period of the sound)

Question 13

we have phase locking because the receptor potential of the hair cell is fluctuating with the waveform of the sound at least for low frequencies.
the probability of an AP is greatest when the receptor potential is most depolarised. at high frequencies the oscillations are very small so rather than getting APs which synchronise with the peaks of the waveform we get APs that occur when the sound is on but they are not synchronised
what frequency does this happen

Answer 13

> 3000-5000Hz

Question 14

spontaneous activity when no sound
as you increase the level of the sound (dB) we find the threshold where the firing exceeds the spontaneous background activity
as we increase further we get linear increase in firing
then we reach saturation when firing does no increase despite increasing sound level
the dynamic range is…

Answer 14

the range over which firing changes with level

Question 15

how big is this range

Answer 15

about 40 dB

Question 16

human hearing extends over 120 dB
how do we hear this range if the dynamic range of the fibres is 40dB

Answer 16

we have low spontaneous rate nerve fibres which respond to higher sound levels (the fibres we were talking about prior are high spontaneous rate, so they fire quite a lot even in silence)

Question 17

they need a higher level of sound before they respond and they have no spontaneous rate. they are also linear (not sigmoidal)

Question 18

the low SR fibres synapse with the IHC on the…

Answer 18

modiolar side
(high SR on the pillar side)

Question 19

in the blest areas (surround the core areas) there is segregation of ‘what’ and ‘where’ information

Answer 19

‘where’ information from V1 passes through the parietal lobe to get to the pre frontal cortex
‘what’ information from V1 passes through the temporal lobe to get to the pre frontal cortex

Question 20

posterior auditory field for ‘where;
anterior auditory field for ‘what’