lecture 14 - adrian rees Flashcards
auditory cortex in monkey:
in the core region there is
the primary auditory cortex (A1)
the rostral area (R)
the rostral temporal area (RT)
in each area in the core region there is a tonotopic representation of frequency.
in A1 the frequency runs..
high to low
in R its
low to high
in RT its
high to low
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
low to high frequency
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
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)
two strategies for encoding frequency
- place code - maintaining spatial representation of frequency established on the basilar membrane in the cochlea (tonotopic organisation)
- temporal code - firing of nerve fibres and neurons synchronised to the sound waveform (phase locking)
action potentials synchronise with the…
peaks of the sounds waveform
the AP represent the sound wave form in pattern of firing. the smallest interval between APs is…
the time between the peaks which is equal to the period of the sound
1/period =
frequency
when we record from the bushy cells in the cochlear nucleus the APs are much more
tightly clustered around the peak of the sound waveform
this is because each globular bushy cells gets input from…
several auditory nerve fibres
combined input causes a more precise response (greater chance that the cells will capture every period of the sound)
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
> 3000-5000Hz
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…
the range over which firing changes with level
how big is this range
about 40 dB
human hearing extends over 120 dB
how do we hear this range if the dynamic range of the fibres is 40dB
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)
they need a higher level of sound before they respond and they have no spontaneous rate. they are also linear (not sigmoidal)
the low SR fibres synapse with the IHC on the…
modiolar side
(high SR on the pillar side)
in the blest areas (surround the core areas) there is segregation of ‘what’ and ‘where’ information
‘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
posterior auditory field for ‘where;
anterior auditory field for ‘what’
