auditory cortex

Question 1

what are the three divisions of the cochlear nucleus?

Answer 1

anteroventral cochlear nucleus
posteroventral cochlear nucleus
dorsal cochlear nucleus

Question 2

describe the tonotopic organisation of the cochlear nucleus

Answer 2

each part of the cochlear nucleus has a tonotopic organisation such that it gets innervated by a range of frequencies going from the apex to the base of the cochlear

Question 3

what are the two models required to code sound localization?

Answer 3

-interaural time differences (the difference in time regarding which ear the sound reaches first)

-interaural intensity differences
(the difference in how loud the sound is, ie, if the sound came from the right side, then the sound would be louder near the right ear)

Question 4

what part of the brain involves integrating sound localisation coming from both ears?

Answer 4

superior olivary nucleus in the brainstem (pons)

Question 5

what part of the superior olivary nucleus involves processing interaural time differences?

And what part of the cochlear nucleus does it receive input from?

Answer 5

medial superior olive (MSO)

receives inputs from the anteroventral cochlear nucleus from both the right and left ears.

Question 6

What part of the superior olivary nucleus involves processing interaural intensity differences?

And what part of the cochlear nucleus does it receive input from?

Answer 6

lateral superior olive (LSO)

LSO receives excitatory inputs from the ipsilateral side the anteroventral cochlear nucleus and inhibitory inputs from the contralateral side of the anteroventral cochlear nucleus (indirectly, via MNTB)

Question 7

describe the process of interaural intensity difference coding in the LSO

Answer 7

if the sound is coming from the left ear, this will generate excitatory inputs from the ipsilateral anteroventral cochlear nucleus to the LSO and inhibitory inputs indirectly by MNTB interneurons from the contralateral anteroventral cochlear nucleus to the LSO. so there will be greater excitation from the left ear compared to inhibition from right ear so net excitation to higher centres. in addition, in the right ear there is greater inhibition from left ear then excitation from right ear so net inhibition to higher centres

Question 8

describe the process of interaural time difference coding in the MSO

Answer 8

The MSO is comprised of (post-synaptic) neurons ranging from A to E, and here would be branches of pre-synaptic neurons coming from both the left and right ears that innervate each of these post-synaptic neurons of the MSO. However, they are innervated at different times but compensated by a longer and shorter pathway, so that they code together. But if the left and right ears both receive the sound simultaneously, they will innervate neuron C at the same time, allowing for the perception of hearing sound from both ears at the same time.

If the sound comes from the left ear first, the auditory nerve afferents on the left side will be innervated first and will take the route of the longer pathway to reach branch 5 for the innervation of (post-synaptic) neuron E to compensate for the delay of sound reaching the right ear, which will take the shorter route to innervate branch one on the right side, innervating the same post-synaptic neuron E. this allows us to estimate the interaural time difference such that the sound source came from the left ear.

Question 9

What does the detection of interaural time differences require in order to be efficient?

Answer 9

the precise spike timing
(phase locking of the AN afferents)

Question 10

AN afferents can phase lock between what frequency ranges?

Answer 10

between low to mid-frequency

Question 11

which cells have shown strong phase locking, where in the cochlear nucleus are they located and where do its innervations project to

Answer 11

bushy cells, located in the anteroventral cochlear nucleus, projecting to the MSO

Question 12

what is ipsilateral and contralateral

Answer 12

ipsilateral means same side
whereas contralateral means opposite side

Question 13

what are the different parts of auditory cortex?

Answer 13

core, belt, parabelt

Question 14

What are the two parallel auditory processing streams, including the different parts involved and the regions of the brain they interact with depending on which stream?

Answer 14

venral stream: core, belt, parabelt, temporal and prefrontal areas
dorsal stream: core, belt, parabelt, parietal and prefrontal areas

Question 15

which streams have ‘what’ and ‘where’ functions

Answer 15

ventral is the what
dorsal is the where

Question 16

what area does the auditory cortex occupy in the brain?

Answer 16

lateral and dorsal surface of the superior temporal gyrus

Question 17

where is the primary auditory cortex located?

Answer 17

in heschl’s gyrus, buried in the sylvian fissure

Question 18

Discuss the evidence for and against the Marmoset Twitter call, with the initial assumption that sound may have meaning.

Answer 18

for: the marmoset monkeys responded only to the natural twitter call non-manipulated, but when they manipulated it to speeding it ip or slowing it down, and even reversing it, they did not respond, raising assumption that sound may have a deeper meaning.

against: when they used the same marmoset twitter call on ferrets, they found that they responded equally to reversed and natural sound upon training, suggesting that the change in response was not significant when the results were compressed over a longer time period, ie every 300 milliseconds, but when they distributed the result over a shorter time window, ie every 5 milliseconds, there was a distinguishable response

suggests that it was not a semantic response to the calls but rather response to the acoustic features of the twitter calls (pitch, amplitude, and frequency)

Question 19

describe the evidence supporting the existence of two separate ‘what’ and ‘where’ processing streams in monkeys

Answer 19

They placed electrodes in the anterolateral and caudal lateral areas of the auditory cortex.

neurons within the AL did not show location sensitivity but showed strong call selectivity suggesting that it may involve the ventral ‘what’ streams

whereas nerone in the CL did not show strong call sensitivity but strong spatial location sensitivity, suggesting it may be involved in the dorsal ‘where’ stream