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