Okay, now assume that a sound a occurs a on the right side.
This sound will reach the right ear fills.
And the left ear a little bit later, 300 microseconds later, let's say.
The spikes that are locked to the peaks of the sine wave will occur 300
microseconds earlier here, than here. However, this axon is longer.
And if its length is such that the travel time from this cochlear nucleus to MSO is
exactly 300 microseconds longer, than the travel time from this cochlear nucleus to
the MSO. The spikes that originated from this, it
will reach the MSO at exactly the same time as the spikes that originated from
that ear. In consequence, the spikes reaching the
MSO neurons from both sides will arrive at the same time and this MSO neurons
have a special property, their coincidence detectors.
The fire mostly when spikes occur simultaneously on both sides.
So, the MSO will see the spikes at the same time, and the reason that it will
see the spikes at the same time is because of this internal delay caused by
the axon length and it will fire. On the other hand, for this specific
neuron if the sums come exactly from the middle, the spikes will occur at the same
time, locked to the peaks that occur at the same time in both ears.
The delay due to the longer pass lengths here, the axon lengths here, will cause
the spike to reach the MSO neuron at slightly different times on the two
dendrites. And then the MSO neuron will not see the
spikes at the same time and so it will not fire.
In summary we got here a circuit that a insensitive to the time of arrival, to
the, to the interval or time difference. It fires in this case, this neuron fires
when they. sounds arrives at the right ear first
then its the left ear later and it doesn't first when the sound reaches the
two ears at the same time. So this neuron can tell us that the sound
came from the right rather than from the center or, or obviously from the left.
This circuit requires extreme specialization of the bio physics of the
neuron that participate in it. The reason is that it needs to keep the
timing of the spikes extremely, precisely all the all the way from the auditory
nerve until they reach the MSO. And this is done by neuron in the
cochlear nucleus with the specialized not to do anything to the input for relying
it to the output. You learned with a done a lot about what
neurons can do. That did calculations, temporal
summation, partial summation and so on and so forth.
The neuron in the cochlear nucleus participate in this eh, in this silqeek
are designed so that they don't do anything of eh, eh, eh, any of these
things. The only thing that they do is to produce
a spike with very short, very small jitter for every spike that reaches them
from the auditory nerve. Secondly, the, the neurons that do the
coincidence detection in the medial superior olive also very special in terms
of, most of the structure and of the bio physical mechanisms.
And the reason that again, is that they needed a, this coincidence detection
process that they participate in, requires sensitivity to time differences
of a tens of micro seconds. You learned with [INAUDIBLE], that spike
wave is order of one millisecond, that's 1000th micro second.
So, we have spikes whose width is 100's of micro seconds, maybe 1000th micro
second. And with these extremely wide things, we
need to be able to discriminate into all time differences of a few tens of
microseconds. So we cannot allow our self any jitter,
we need a new one who's temple, eh, eh, temporal summation properties work with
time concerns that are extremely short. There are other problems, other issues
that these neurons have to solve as well. And in fact it turns out that some of
these a problems that a are due to the to, to, to the attempt to the
discriminate tens of micro seconds we say [INAUDIBLE] life will whose time
constants are much longer, some of these problems.
probably solved by using tricks that that have to do with the properties
[INAUDIBLE]. Ehh, the, this is as far as I want to go
into the description of the, eh, ITD interaural time difference all-time
difference calculation. But I want to remark that similar
mechanisms can be observed throughout the sensory systems.
Sensory systems have, each has its own specialized needs and specialized
calculations that needs to be done. I showed you here, the comp, the
comparison of timing between the two ears.
There's a parallel pathway that calculates the inter aural level
differences, the differences in the amplitude of the two sound that reaches,
reach the two ears. And in other sensory systems you find
other such eh, computational tasks that are done by neurons.
For example, the retina contains neurons that calculates motion, that they're
sensitive to motion and that they act to motion in one direction but not in other
directions. Eh, so this is a kind of calculation
that's done by the, by the retina. Eh, early processing of smells also has
its own share of eh, specialized eh, eh, calculations and needs to be done, and
all of these specialized needs are catered in different sensory systems by
different specialized circuits. So, for example, in the auditory system,
we have the specialized circuit in the auditory brain stem.
They're specialized in terms of the neurons that live there.
They're specialized in terms of the connectivity between them, and they'll
specialize in terms of the computations that they perform.
Similarly, in the retina, there are a lot of different [UNKNOWN], which are now
starting get eh, eh, eh, dissected out. And that perform all of these specialized
calculations that are specific to the visual system.
And similarly in the eh, eh, for the smell eh, sense, there's eh, a structure
called the olfactory bulb, which gets the input from the nose.
And eh, again, has a specialized circuitry that eh, performs the early
processing that's eh, that is specific the olfactory system.
And all of these, again, come with a lot of important fascinating details, and I
urge you to go and look for them. So this is what I wanted to discuss with
the spectral early processing of sensory information.