PSYC 212 Lecture Notes - Lecture 14: Musical Note, Real Face, Natural Sounds

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20 Jul 2016
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Pitch, Location, and music perception: 22:13
Telling where sounds are coming from:
Intensity of the sound- Louder, perceived as being closer to you- tell you
your approx.. distance from a source alone.
Inter-aural time difference: Sounds are waves travelling in air, as they strike
one ear, if they are coming from one side they will strike one ear before the
other- that time diff. might be a cue for the brain to tell where the sound is
coming from.
-If an object is closer to one side of your head than the other, the effect on
your auditory perception:
The first comment was that it was the intensity of the sound, but the
intensity diff. between two ears is one of the strongest cues for sound
location.
Whispering in someones ear- Whisper in one ear, the sound is faint enough
to not get through to the other side of the persons head, so the other person
next to them cannot hear it.
Any sound coming from one side, will be stronger heard on one ear than the
other.
Those two cues- inter-aural time diff.. and inter-aural intensity diff.- Those
are great for telling sounds coming from the right side to the left and maybe
a little bit behind as well.
Sound coming from above or below- How do we tell:
The angle at which it hits the pinna affects elevation judgements.
We need a coordinate system to first describe where sound is coming from.
-The plane horizontal- the azimuth plane, the vertical plane is called the
elevation plane- trying to find where sounds are in an infinite sphere around
our head- challenge of sound location, where is it coming out in this
spherical space around us.
Describing any point in this spherical space by describing the azimuth, and
then the elevation.
Figure 6.21:
The intensity diff. of the sound between two ears, can vary also as a function
of not just the position but also as a function of the frequency of the sound.
The way the sounds are dampened by the head are affected by the
frequency of the sound you hear.
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Case of hearing a sound from the front- the difference of the intensities are
heard on the two ears is always the same, regardless of frequency, but as
you move
-The intensity of the waves are identical in the two years- in front.
The way sound is effected by the shape of your head is depended on the
frequency of the sound.
Figure 6.20:
High frequency sounds will get blocked by your head, if you hear a high
frequency tone coming from one way, it will get blocked, not get through to
the other ear very much- strong inter-aural intensity diff.
Low frequency sounds- can bend around your head as well, aren’t as useful
for sound location, or can be more ambiguous- the inter-aural intensity diff.
can be more ambiguous.
When we whisper, we whisper in a high-pitch voice, we cling all the deep
sounds out so we will not be heard.
Q: Hear music from neighbour –low bass music- high frequency sounds will
get filtered by the construction material around you, low frequency sounds
tend to resonate through or with the material so you typically hear it, part of
the reason why if any of you have these surround sound systems, they can
have the bass generator, and somehow that is not confusing to your ear that
all the base is coming all from the front. The high frequency sounds will be
very localized, so you need multiple speakers to generate them from diff.
angles, so you can simulate the inter-aural intensity diff.
Anti-lautering devices- sound generator produces high frequency sounds
that teenagers can hear and not adults- cause of hearing loss.
Interaural time diff.- another cue telling us where sound is coming from on
the azimuth.
The time that it takes the sound to reach the two years will differ, and here
you are plotting the interaural time diff. as a function fo the azimuth angle.
The diff. in miliseconds, that it would take for sound to come from 0
degrees, vs. 180 and90 being at the center.
As you move to the very side, the interaural time diff. is the heighest- that
time diff., the time of incidence- the sound pressure wave is lets say staring
at 180, it will hit one ear before- that tiny time diff. between the incidence of
that pressure from my snap on one ear vs the other- signal that brain uses
to know that the sound must have came from one sound.
Interaural Time Diff. Calculation by Neurons:
Simple schematic description of how it might be able to do it- Neurons- each
one is a cochlear neuron from the left ear and the other right side is from
the right ear- Each one of these nerves has multiple synapses onto these
candidate inter-aural intensity detectors.
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-Each one of these little dots is meant to be sensitive to one inter-aural
intensity diff.
How would it do that.-In the left ear- lets say you snap your finger on the
right side, that nerve will become activated, the action potential starts
travelling down, realeses something at each dot, meanwhile the sound has
travelled to the other ear and started causing an action potential on the left
side, by the time the action potential travels down, the other will have hit
another nerve- and then suddenly that nerve will have coincident release of
both nerves releasing neurotransmitters on the same time- twice as much
neurotransmitter release at the exact same time, it will fire.
Two events happening from that same side- action potential coming in,
releasing while the other ear is being stimulated and the action potential is
coming down and just as it comes down it meets the activity from the other
ear in one neuron/nerve(dot), and that one neuron happens to have twice as
much neurotransmitter release, and as a result, it will fire.
If nothing happens in the other ear at all, numbed the entire organ of corti in
the other ear, nothing coming from the other side, so you will not have an
inter-aural intensity diff., no neural signal coming in.
The idea here is to create a lagged system, where signals coming in from
one side can meet signals coming I nfrom the other side in a manner that
spatially seperates them- so you can have neurons that each have sensitivity
to one inter-aural time diff.
EX. Lets say the sound is coming from straight in the front, the snap of the
finger is heard by the two ears at the same time, the two signals are coming
in and will meet right half way on the central neuron- the time it takes for
one of them to meet the other neural signal is going to be about the same,
and so your brain has learned that when that neuron in the middle gets
activated- no intensity diff. between the two.
Brain would have learned that when most right neuron gets activated, sound
was coming in from one side.- basically, the neurons activity itself signifies
what the inter-aural time diff. is.
This is a cochlear neuron(blue squares)- basilar membrane has vibrated,
organ of corti has transduced that by bending of cilia of hair cells, some
cochlear neuron will now start firing to signal that they have the sound
coming in, that neuron is firing, meaning it is sending action potentials down
its axon, and the result is neurotransmitter release onto these other
neurons.
The purpose of this is to specifically have a neural mechanism that calculates
the inter-aural time difference.
At each point, the idea here is that at each location, as the action potential
coming from this nerve is propagating through these branches, they are
releasing neurotransmitters but those are not enough to force these neurons
above threshold. They do not depolarize the neurons enough to cause them
to generate action potentials- need the neurotransmitter from the other side.
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