binaural hearing and sound localisation Flashcards
what happens in the inferior colliculus
where horizontal and vertical signals converge
MGN
the part of the thalamus specialised to hearing
where do teh spiral ganglion cells project to?
the dorsal cochlear nucleus
the posterior ventral cochlear nucleus
the anterior ventral cochlear nucleus
what cells type does the dorsal cochlear nucleus mainly contain?
fusiform
what cells does the posterior ventral cochlear nucleus mainly contain?
octopus
pathway of the dorsal acoustic stria
projects from the DCN
1) sends excitatory input to the contralateral central nucleus of the inferior colliculus
2) projects to the contralateral NLL
the NLL sends inhibitory signals to the central nucleus of the inferior colliculus
pathway of the intermediate acoustic stria
projects from the PVCN
1) sends excitatory input to the central nucleus of the inferior colliculus
2) projects to the NLL
the NLL sends inhibitory signals to the central nucleus of the inferior colliculus
what is the main cell type in the PVCN?
octopus
what are the main cell types in AVCN?
spherical bushy and globular bushy
what is the tonotopic organisation of hair cells?
highest frequency at the base and lowest frequency at the apex
where is the tonotopic organisation of the basilar membrane (hair cells) conserved?
in the spiral ganglion cells that are innervated by the inner hair cells of the cochlear, at the cochlear nucleus and at all levels up to the auditiry cortex
what frequencies does phase locking occur at?
20Hz- 4000 Hz
what frequencies does tonotopic firing occur at?
4000Hz - 20,000 Hz
what is phase locking?
the frequency of the sound wave is low enough that the ganglion cells can fire action potentials in time locked onto the frequency of the soundwave
what is the maximal firing rate of a neuron/ what is the maximal frequency for phase locking?
1000Hz
what is the volley theory?
phase locking that occurs at low frequencies that are larger than 1000 Hz
neurons cannot respond to every wave
groups of spiral ganglion neurons respond to the same part of a wave but they dont respond to every wave
combination of the neurons code for frequency of the sound wave
coded for by the amount of neighbouring spiral ganglion cells that get activated
what happens to spiral ganglion cells activation in the volley theory as frequency increases?
the basilar membrane will vibrate the sides more, so more neighbouring spiral ganglion cells will become activated
frequency range for the volley theory
1000Hz- 4000Hz
when wavelength is smaller than barrier width
shadow generated behind the barrier
shadow
area where sound is quieter behind a barrier
when wavelength is larger than barrier width
wave diffracts around the barrier
travels as if the barrier isn’t there
what size barrier does the human head generate?
0.2m
at what frequencies is the wavelength of sound larger than 0.2m? (diffracts)
20Hz to 1500Hz
what is the delay of sound from the left to right ear when sound when wavelength >0.2m
0.6ms
why can we rely on interneural timing differences when wavelength is <0.2m
the wavelength will reach the left ear and the right ear before the next wavelength appears
at what frequencies is a shadow generated behind the head?
1500Hz- 20,000 Hz
wavelength <0.2m
what two mechanisms determine sound direction
interneural level difference (ILD)
interneural timing difference
what frequency can ILD be used for?
greater than 1500Hz
what frequency can ITD be used for
less than 17000 Hz
where are there neurons sensitive to different interneural timing delays
Medial superior olive
what time delay can the neurons in teh MSO discriminate between?
0.11 msec
giving a precision of 2*
interneural timing delay neural pathway
1) Sound from the left side initiates activity in the left anterior ventral cochlear nucleus.
2) projects to the ipsilateral and contralateral medial superior olive
3) Very soon, the sound reaches the right ear, initiating activity in the right AVCN
.
Meanwhile the first impulse has travelled farther along its axon
3) Both impulses reach a olivary neuron at the same time, and summation of synaptic potentials generates an action potential
- Timing difference from left and right ears leads to summation of signals at the neuron selective for specific time difference
why is summation of signals in the medial superior olive necessary?
either signal alone creates an EPSP, but not large enough to reach threshold for an action potential
pathway of ITD after the MSO
MSO projects to CNIC
projects to MGN
projects to auditory cortex
ILD: AVCN projects to
ipsilateral LSO (glu, excitatory)
contralateral MNTB (glu, excitatory)
ILD: MNTB projects t0
LSO on the ipsilateral side (Gly, inhibitory)
MNTB stands for
medial nucleus of trapezoid body
what is the role of the LSO neuron
calculate the difference between the inhibitory and excitatory signals sent by each ear
if the volume is the same on either side of the ear?
equal signals will reach the auditory cortex
if the sound reaching the left ear is louder than that reaching the right ear?
the signal will be greater in the right auditory cortex
ILD: where does the LSO project to?
ipsilateral CNIC (inhibitory, gly)
contralateral CNIC (excitatory, glu)
what is sound localisation in the vertical plane determined by?
differences in direct and reflected sounds due to teh pinna
what can lead to a lost ability to detect direction of sound in the vertical plane?
sticking a tube in the ear or taping the pinna
neural pathway of vertical sound processing
1) Processed by the dorsal cochlear nucleus
-Has different layers of processing differences in the direct sound and the reflected sound, so it can work out what vertical orientation the sound came from
2) Send the information contralaterally to the central nucleus of the inferior colliculus, where it integrates with the horizontal signal
3) projects to the MGN and then the auditory cortex
features of sound that aid determination of direction in the vertical plane
- Most sounds aren’t pure tone, have different varying frequencies
- At different vertical planes, sounds have different spectral pattern
