How does the human visual processing system works?
Now, I want to walk you through a model of what happens when
the light enters our eyes and how it is processed by the eye and the brain.
In order to do that,
we have to start from the eye itself.
I'm going to describe only the very essential parts of the eye.
Of course the eye is much more complex than what I'm going to describe.
So, light is coming from the world through either sources of
light or surfaces that are reflecting light coming from other sources.
So, at any given time what we see in front of us,
is the effect of light coming through our eyes.
Now, the eye as a number of structures.
First of all, there is the cornea,
which is basically a lens through which the light comes inside the eye.
We also have a pupil,
that is basically like a camera that can open or
shrink and according to how much light is coming in.
So gets larger and larger when the light is dimmer and
dimmer and it gets smaller and smaller when there is more light.
Now, when the light is reflected through the cornea,
it's actually reflected to the back part of the eye to a region that is called retina.
The retina is the place that is of the eye that is in contact with the optic nerve.
The retina again which is the back of the eye is full of photoreceptors.
What is a photoreceptor?
Well, it's kind of cell that
transforms signals coming from the eye to signals that go to the brain.
Now, let's look more closely at the content of the retina.
The retina is packed with these photoreceptors which you can think of like light sensors.
There are two main types of sensors,
there are rods and cones.
Rods and cones are complimentary.
Rods are necessary and they are active in
low light conditions whereas cones are active and necessary in normal light conditions.
Now, an interesting aspect of the retina is that
the resolution that humans have
in the surface of the retina changes according to the location.
More precisely, there is a small region of the retina that is called the fovea.
This is a region of the retina,
that has a much much higher resolution than the rest.
How large is the fovea?
Well, one way to figure out to get a sense of how large the size of the fovea is,
is to basically look at the size of your thumbnail in front of
you at arm's length and when you focus your attention on it,
the size of your thumbnail is roughly the size of the fovea in your eye.
Okay. You can try.
The resolution is enormous in this very small region.
Just to give you an example,
in a normal monitor,
there are lots of pixels and around 40 pixel per centimeter and the kind
of resolution that humans can have is about one tenth of one pixel so,
it's incredibly high in these specific region.
Of course, is not the same in the rest of the eye.
So, this is a diagram that shows you how the resolution
changes in different regions of the of the retina.
As you can see, resolution is incredibly high in the area around the fovea
but it decreases very very sharply as you move out of this central region of the eye.
