lecture 15 + 16 - adrian rees Flashcards
two distinct planes of sound localisation
azimuth - horizontal plane
elevation - vertical plane and front/back
need two ears to compare interaural (between the ears) differences in…
time and intensity
two interaural cues used to localise sound
- interaural time difference (ITD)
- interaural level difference (ILD)
ITD has two components to it
- onset time of the sound
- on going phase differences between the waveforms reaching the two ears
ITD is generated by path difference
additional distance sound must travel to reach ear furthest from the source
2 types of ITD
difference in onset time (short sounds)
differences in interaural phase (for longer sounds)
phase difference between the two ears due to
path difference which results in a time difference
interaural time difference is maximal at…
90 degrees
as the frequency of the sound increase the wavelength of the sound decreases so the peaks and troughs get closer together. we then have multiple peaks and troughs over this path difference. so we can end up with the sound being in the same phase in both ears even though the sound source is off to the side so the que is not very informative about where the sound is located. so when sounds above 1500Hz this phase difference que becomes of less use
frequencies above 1500Hz
we use ILD as when sound waves are directed at the head there is a…
shadow created
difference in level of the sound between the two ears
why does this not work for low frequencies
when the wavelength of the sound is long and is greater than the diameter of the head, the sound waves bend around the head (diffract) and join up on the other side (no shadow)
when theres multiple frequencies in complex sounds we use both ques simultaneously (ITD and ILD)
the spherical and globular bushy cells (SBC and GBC) from the cochlear nucleus send projections into…
the superior olivary complex (SOC)
the SOC contains multiple nuclei such as
the medial superior olive (MSO)
the lateral superior olive (LSO)
medial nucleus of trapezoid body (MnTB)
lateral nucleus of trapezoid body (LnTB)
LSO is responsible for the
level differences
MSO is responsible for the
time differences
the MSO receives input from the SBCs and GBCs on from both left and right ears
MSO recieves direct input from the…
SBCs
MSO also recieves inhibitory inputs from the…
GBCs
but the GBCs in themselves are excitatory. the drive neurons in the MNTB and LNTB and the neurons in the MNTB and LNTB are..
inhibitory (glycinergic neurons)
firing of the MSO neurones sends connections down stream in the auditory pathway to the inferior colliculus
LSO receives inputs from the SBC on the same side and from GBCs from the opposite side via the…
MNTB
so you get excitation from the same side and inhibition from the other side
LSO (level differences)
can compare the balance of excitation from one ear with the inhibition from the other ear.
if the sound is on the ipsilateral side (90 degrees) then the excitatory input will be strong.
the sound on the contralateral side will be lower, so this side inhibitory and also the strength of the inhibition will be lower than the strength of the excitation
if we move the sound to the front…
the sound level in the two ears should be the same so the excitation and inhibition should be the same
if the sound is moved round to the contralateral sound the inhibition will be stronger than the excitation
comparing the outputs from the two LSO we get a signal telling us where the sound is located in space
how does the brain detect ITD
compares phase differences between waveforms at the ears
done by phase locking - firing of neuron indicates the peak of the waveform
if the sound arrives later in the left ear than the right ear the peaks of the wave form will be different.
phase locking now causes two streams of action potentials with the APs from the left ear being slightly delayed
model proposed by Jeffress (1948)
1. in the MSO there is coincidence detection neurons
only fire when an impulse arrives from both the left and right side at the same time
if there is an interaural delay there is delay lines which
cancel interaural delay
impulses from leading ear travel further to reach the coincidence cell than impulses from the ear
use the delay cancellation process as a measure of wear the sound is in space
convert interaural time differences into spatial representation
delay lines
if the sound is off to the left
impulse travels further up the network of coincidence cells than the impulse from the other side travels down it
they meet in coincidence further up the ladder
converted the time difference into a spatial recognition
problems with jessress hypothesis
when you record from these neurones you find that they are delay sensitive (respond best when theres a delay)
in most cases they respond best when the contralateral ear is the lead ear
in many of them , the peaks of these functions lie outside the range that this animal can generate interaural delays
the range of interaural delays is dependent on the size of the head
bigger the head the wider the interaural range of delays
most of the peaks are on the edge of the range of delays
when you block the glycinergic inhibition the ITD response function peaks at
0 delay
this group suggests that inhibition makes the delay sensitive
MSO has dendrites
the inputs are excitatory from SBC and theyre going to the dendrites
inhibition from the GBCs via MNTB neurons makes contact with the cell body of the MSO
inhibition gets to the MSO neurone really quickly as it goes directly to the cell body and takes longer for the excitation as it goes through the dendrites
also the synapse in the MNTB is a huge and fast synapse (called the calyx of held)
argue that the inhibition effects the excitatory input coming in from the contralateral side.
in order to get the cell to fire at the same time you need to activate this input first
but franken found that blocking the inhibition depended on how long the strychnine was applied
interaural time and intensity differences can be same for more than one location (immediately in front and behind the head)
identified by
sound reflect from the pinna diferently if theyre infront/ behind
pyramidal neurons in the DCN are selective for
position of spectral notches created by pinna interactions
