Hi everyone, welcome to 10 minute neuroscience.
In this installment, I’ll be covering the pathway of visual information through the
brain starting with the eye and ending with the visual cortex and surrounding areas.
This’ll be a very general overview, and I’ll be focusing more on the pathway of
visual information than on the processing of that information, but this should serve
as a good introduction to the way visual information travels from the eye through the brain.
Of course, vision begins with the eye, but the neural aspect of vision really starts
with the retina, which is the neural structure of the eye and is outlined in blue here.
Other components of the eye, however, help to create a focused image on the retina.
This is accomplished to a large degree through the actions of the cornea and the lens.
The cornea is a transparent layer at the front of the eye that lets light into the eye.
It also bends, or refracts, those light rays to direct the light onto the retina.
The lens also helps to direct light on the retina.
It has less refractive power than the cornea, but it has an advantage in that its shape
can be modified by muscles in the eye called ciliary muscles, and changing the shape of
the lens can help to maintain focus for objects that are closer or farther away.
As people get older, their lenses become less flexible and less capable of changing shape
to focus on nearby objects; this is why people tend to need reading glasses when they get
older.
The size of the pupil, the opening in the middle of the iris, which is the colored part
of your eye, can be adjusted to regulate the amount of light that reaches the retina.
For example, there are muscles in the iris that cause the pupil to dilate (or open up
more) in low-light situations, and constrict in an environment with higher levels of illumination
since it doesn’t need to let in as much light in that type of environment.
The retina is a neural structure, and it’s actually considered part of the central nervous
system.
Its main function is to detect light and use that light to produce electrical and chemical
signals that the rest of the nervous system can understand.
You can see here a close-up of a section of the retina, and although there are over 1,000
different types of neurons in the nervous system, there’s only 5 basic types of neurons
in the retina, and those neurons are situated in distinct layers.
So, even though there are hundreds of millions of neurons in the retina, compared to the
rest of the nervous system it’s relatively simple anatomically.
This has helped us to develop a better understanding of vision than of any other sensory system.
Surprisingly, the layer of the retina that’s at the very back of the eye is the layer that
contains photoreceptors, the cells responsible for converting light energy into electrochemical
signals, a process known as phototransduction.
The location of the photoreceptors is surprising because it seems counterintuitive to have
the light detecting cells at the very back of the retina, so light has to travel through
the eye and then through several layers of cells to reach the photoreceptors, but it’s
thought that their location is strategic in the sense that they’re next to a layer of
the retina called the pigment epithelium, and the cells of the pigment epithelium help
to maintain photoreceptor cells and keep them functioning properly.
There are two main types of photoreceptor cells: rods and cones, which are named for
their shape as you can see here.
These photoreceptor cells are the site where vision really begins; they each contain hundreds
of disks that are capable of absorbing photons of light and absorbing photons causes the
photoreceptors to change levels of neurotransmitter release in order to convey information about
a visual scene.
Rods and cones have different functional specializations; they’re involved in distinct aspects of
vision.
First off, cones enable us to see color, while rods don’t provide for color perception.
Rods are also very sensitive to light and have low spatial resolution, meaning they’re
not good at seeing details.
Cones, on the other hand, are not very sensitive to light and have high spatial resolution,
so they provide us with higher visual acuity.
In low light conditions, only rods are activated.
This makes it more difficult for us to make out details when there’s very little light
due to the poor spatial resolution of rods, and it also means that in dim light we can’t
perceive color.
When levels of illumination increase, eventually rods stop responding and fail to convey information.
Essentially their high sensitivity to light causes them to become overstimulated, or saturated
as its often called, in normal lighting situations like sunlight or even typical indoor lighting.
So in those conditions, cones are the dominant photoreceptor in determining how we see.
Surprisingly, even though cones mediate perception in typical light situations, we have far more
rods than cones in the retina.
There’s somewhere around 90 million rods and only about four and a half million cones.
However, there is one part of the retina, an area called the fovea, where there are
many more cones than rods.
In fact, at the very center of the fovea, which is called the foveola, there are no
rods at all.
Because of its high cone content the fovea is the part of our retina that has the capacity
for our highest acuity vision.
This causes us to unconsciously move our eyes so that important visual information lands
on our fovea, since this is the part of our eye most capable of discerning important details.
When photoreceptors absorb photons, it causes changes in the amount of neurotransmitters
these cells release, and this affects the activity of the next layer of cells: bipolar
cells.
Bipolar cells pass on signals about perceived light to the next layer of cells, which are
called ganglion cells, and these cells will carry the visual information to the brain.
We’ll talk more about that in a moment but I want to mention the other two major cell
types in the retina: horizontal cells and amacrine cells.
Horizontal cells have dendrites that spread horizontally and make contact with multiple
photoreceptor cells.
Horizontal cells modulate the function of photoreceptor cells to do things like enhance
contrast and adapt to changes in lighting conditions, among other things.
Amacrine cells make contact with bipolar cells, ganglion cells, and other amacrine cells,
and they also have a number of functions, but like horizontal cells they’re generally
thought to be involved with refining the visual signal through their ability to modify the
functions of other retinal cells.
So horizontal and amacrine cells are involved with very early processing of visual information.
But most of the visual processing occurs in the brain, so the information from the retina
has to be carried out of the eye and to the brain.
This is accomplished by the ganglion cells, whose axons leave the eye in a bundle at a
region called the optic disc.
Because the optic disc is the area where the ganglion cells leave the eye, and essentially
they need a place where they can exit the eye, there are no photoreceptors there.
So, this creates a small region where we don’t receive any visual information, or a blind
spot.
Amazingly, we don’t notice this blind spot in our visual scene because the brain fills
it in with information from other photoreceptors.
If you find it hard to believe that you’ve got a blind spot always present in your field
of vision, click on this link above for a quick and simple experiment that proves the
existence of your blind spot.
The ganglion cells leaving the eye form the optic nerve, which is one of our cranial nerves.
The optic nerve extends back to this region just below the hypothalamus called the optic
chiasm.
At the optic chiasm, about 60% of the axons from the optic nerve cross over to the other
side of the brain, while the rest stay on the side they originated on.
The fibers that are coming from the nasal part of the retina—the part closer to the
nose—cross over to the other side, or decussate, while the fibers from the temporal part of
the retina—the part closer to the temple–do not cross over.
The result of this is that all of the information from the right visual field ends up traveling
to the left side of the brain, and vice versa.
After the optic chiasm, the visual fibers are no longer called the optic nerve—they’re
now called the optic tract.
The optic tracts extend to multiple areas.
For example, some of the fibers go to an area in the brainstem called the pretectum, which
is involved in a number of visual functions such as the pupillary light reflex, which
causes your pupils to constrict when there’s greater illumination in your environment.
Other fibers go to a region of the hypothalamus called the suprachiasmatic nucleus; this nucleus
helps to maintain circadian or daily rhythms and uses information about light in the environment
to help to do that.
And still other fibers go to the superior colliculus, a region in the brainstem that
among other things helps to coordinate head and eye movements to focus on objects of interest
in the visual field.
Most of the fibers in the optic tracts, however, end in a nucleus in the thalamus called the
lateral geniculate nucleus, there’s one of these on each side of the brain here.
Here, the optic tracts synapse on neurons that leave the lateral geniculate nucleus
and extend toward the back of the brain as bundles of fibers called the optic radiations.
The optic radiations travel back to a region of cortex that surrounds a fissure called
the calcarine sulcus.
This small area of cortex that surrounds the calcarine sulcus is called the primary visual
cortex, or V1, it's represented by this striped area here.
There’s a collection of myelinated fibers that forms a white stripe here that can be
seen with the naked eye in anatomical brain sections of this region and because of this
striation or stripe, sometimes the primary visual cortex is called the striate cortex.
The primary visual cortex helps to make a visual image out of the information that has
been received by the retina.
To do that, there are neurons in the primary visual cortex that are activated preferentially
by different characteristics of a visual stimulus, such as orientation, movement, contrast, depth,
etc.
The primary visual cortex also communicates with a multitude of other visual areas that
surround it, including visual area 2, or V2, visual area 3 or V3, V4, V5, and V6.
Neurons in these other areas seem to be specialized to some degree for detecting specific aspects
of a visual scene.
Neurons in V5, for example, seem to be specialized for detecting movement.
These additional visual areas are recruited by V1 and they also begin to recruit other
areas of the brain to accomplish higher-level processing of visual information.
This enables our brain to move from identifying the basic aspects of a visual scene, such
as shape and contrast, to more complex tasks like object recognition, which can provide
the image in our brain with meaning based on previous experience.
So we just scratched the surface there but I hope that gave you a sense of the major
pathway that visual information travels in the brain.
Thanks for watching.