So then, I'm going to concentrate again on the calculation of the interaural time
differences, on the presentation of the interaural time differences and the
reason for that is because this is the best studied part of the circuit.
It's not the only part but the best studied part of the circuit.
So we have the inferior colliculus and in inferior colliculus some representation
is still separate from different frequencies.
So remember that in the cochlear we already separated sounds into its
frequency component and this frequency components are processed separately as
you go up. Along the stations of the auditory mid
brain so that for example, neurons that are sensitive to eight kilohertz lie in a
different place than neurons that are sensitive to six kilohertz and neurons
that are sensitive to four kilohertz. For each one of these frequencies, you
have a bunch of neuron which is sensitive to a difference interaural time
difference. So if you go to the eighth kilohertz
place, you will find neurons that are sensitive to fire most when the
interaural time difference is zero. So they are sensitive to some sounds
right in the middle, and you will have neurons that are sensitive most to
interaural time differences of a few tens of microseconds to to one side or to the
other. And so you have a range of neuron with,
range of interaural time difference activity.
Same thing as six kilohertz, you have a bunch of neurons, all of them are
sensitive to six kilohertz but if you present the sound with different ITD's,
you will see that some of the neurons fire when the ITD is zero, other, when
the ITD is 20 or 40 or 50 or 100 micro seconds.
The same thing for the four kilohertz neurons.
Now, when you represent space, the frequency is immaterial as sounds can
come from the right and be at eight kilohertz or four kilohertz or six
kilohertz, all three of them together. So, if you want to represent space, you
want to somehow integrate over, get rid of the frequency dependence.
And this is exactly what happens in the next type of processing in the barn owl.
The neurons in the inferior colliculus project to a to the next station on the
pathway, which is called ICX. So, that's an external nucleolus of the
inferior collicullus, and this projection has a very special structures.
All neurons that are sensitive for ITD of zero microseconds, from four kilohertz at
six kilohertz to eight kilohertz all converge together to the same location in
the ICX. All neurons which are sensitive to ITD of
50 micro seconds, independent of the frequency for six or eight kilohertz, all
converge to a different place in the ICX and so on and so forth.
So, in the ICX, you get an axis of interaural time difference.
Which is now independent of frequency, independent of which sound you've
presented, what are the properties of the sound that you've presented.
The neurons in the ICX will be sensitive only to the interaural time difference of
these sounds. So, the ICX is the first place where you
get a. Real, truthful roughly stimulus
independent space map, auditory space map for neurons are sensitive to space and
not to other features of the sound. So this is the ICX.
Next station is the optic tectum. Okay, as I told you before the optic
tectum has now a, a lot of different inputs including visual inputs and the
visual inputs are ordered in the optic tectum, so visual inputs come from
straight ahead would go to one point those that go slightly sideways to a
different point and those that goes to far sideways.
Will go to a third point in the optic tectum.
And then what is right now magic but we will see in a moment how it comes about
is the fact that each location in the ICX which is determined by its IT interval
time difference in sensitivity. Project to the optic tectum to a location
which corresponds, to the, correct azimuth of the visual field.
So, neurons whose best ITD is zero microseconds would project to neurons
whose visual field is straight ahead. And neurons with ITD sensitivity center
is 50 microseconds. 50 microseconds in the barn owl is about
20 degrees to the side. So they will project the location in the
optic tectum where the visual fields are 20 degrees to the side, and so on and so
forth. So we have a projection of the space map.
which is defined in term of interaural time differences of the level of the ICX.
Into a space map which is both visual and auditory and the two maps match.