Psychology 1XX3 Notes – Vision II – Mar 1, 2010
Brain: place where visual perception all comes together is the brain.
Visual system: comprised of a set of assembly lines. Areas along the visual
pathways process parts of the visual input before sending those partially-
processed bits of information on to the next set of areas down the line for further
Visual Fields and Hemispheres:
Before reaching their respective hemispheres, the axons from the inner region of
each retina, that is the region of the retina closest to the nose, have to cross over to
the opposite hemisphere. The point at which the optic nerves from the inside of
each eye crosses over to the opposite hemisphere is called the optic chiasm.
Two Visual Pathways:
After the optic chiasm, the information from each visual field arrives in the
opposite hemisphere, at which point the optic nerve fibres split and travel along
Most of the retinal or ganglion cell axons travel along the main pathway and
synapse in the lateral geniculate nucleus (LGN), which is a part of the thalamus
that receives visual information.
After being processed here, the visual signals are sent to areas in the occipital lobe
that make up the primary visual cortex.
A smaller portion of the axons from the retinas takes a detour to an area in the
midbrain called the superior colliculus, after which information is sent upwards to
the thalamus and on to the occipital lobe or downward to structures in the
This smaller, secondary pathway seems to deal with coordinating visual input
with information coming in from other senses, as well as localizing objects in
space through head and eye movements. (See image below.) Main Pathway: Two Subdivisions
Within the main pathway are two subdivisions of specialization that are able to
process their specific information in parallel. The magnocellular pathway is
specialized to process movement information, while the parvocellular pathway
deals specifically with colour and form information.
Main Pathway: Lateral Geniculate Nucleus
The first stop for information that is sent from the retinal ganglion cells is the
Just like retinal ganglion cells have receptive fields that are made up of many
photoreceptors, LGN cells also have receptive fields that are made up of a
combination of many ganglion cells.
Information from many smaller bits are combined into one overall neural signal.
The LGN is made up of six layers.
Information from each eye projects to different layers of the LGN. Not only does
each layer of the LGN receive input from a specific eye, but each layer of the
LGN also receives input from a specific subpathway.
Movement information that is processed along the magnocellular runs to 2 of the
layers in the LGN, whereas information specific to the parvocellular pathway
goes to the other 4 layers.
Main Pathway: The Occipital Lobe
From the LGN, the visual information is sent to the occipital lobe for further
There are over 20 cortical areas that process visual information, but most of the
research done on visual processing has concentrated on area V1 of the occipital
lobe, otherwise known as the primary visual cortex.
Collectively, the visual processing areas in the occipital lobe outside of the striate
cortex are known as the extrastriate cortex. (See image below.)
Primary Visual Cortex:
Just as the receptive field of the LGN is made up of many ganglion cells, the
receptive field of a single V1 cell is a combination of the receptive fields of many
There is information from many sources being processed down into a single
target. Furthermore, the receptive fields from the retina are arranged in a topographical
map in the primary visual cortex, such that neighbouring locations in the retina
project to neighbouring locations in the visual cortex.
The primary visual cortex is made up of six layers, and the LGN projects directly
onto layer IV neurons from which information is carried to neurons in the other
The 6 layers of the primary visual cortex are organized into cortical columns that
are made up of about half mm squares of cortex that are perpendicular to the
Although the vast majority of neurons in the visual cortex can respond to visual
stimuli presented to either eye, most have a stronger response to one eye than the
other. information from each eye sent through the LGN projects more strongly
to some cortical neurons and less strongly to others.
The eye preference is maintained within the individual cortical column all
neurons within a given cortical column respond more strongly to input from the
To some extent then, information from each eye is still being processed separately
in the primary visual cortex, but the cortical neuron is also the first site of
Dorsal and Ventral Streams of Extrastriate Cortex: From the primary visual cortex, processed visual information, whether it is colour,
form, or movement, is sent on to the extrastriate cortex and gets separated into the
dorsal and ventral streams.
The dorsal stream is referred to as the “where pathway” because it processes
where objects are, including their depth and motion in the field.
The dorsal stream progresses from the extrastriate cortex to the parietal lobe.
In contrast, the ventral stream is referred to as the "what pathway” because it
processes what the object is, including its colour and form.
The ventral stream runs from the extrastriate cortex to the temporal lobe. We’ve
learned how our brain processes visual input. (See image in previous page.)
The Evolution of the Eye
Light Sensitive Patch:
Eyes could have started out as simple light- sensitive patches, like what jellyfish
and worms have today.
Curved Cup Eye:
At some point, some individuals may have developed a light-sensitive patch that
was formed into a slight depression, which would have allowed the direction of
light to be sensed, incurring a survival advantage. This is very much like the cup
eyes that today’s clams have.
Some individuals may later have adapted a crude lens, allowing them to process
visual input at different distances. From here, the lens could have successively
improved to allow better focusing and accommodation, such as a more transparent
lens or one with better curvature.
So the complex vertebrate eye evolved gradually, with new adaptations being
layered on top of old adaptations. Eye evolution is an example of what is called
cumulative selection, where small changes were made to the existing eye, and
then new small changes were made to the modified eye, and so on, thus gradually
increasing the sophistication of the eye. (See image below.) Animals in the Precambrian period likely only had crude light- sensors, if any at
all. Their tracks tended to be small and slow moving, like the motion of small
slugs or worms. b/c of poor mobility there was no need for detailed vision if
you see a predator you can’t escape anyway (See image below)
In contrast, the early Cambrian animals were larger and more mobile, and eyes
would have been useful for both hunting prey and escaping predators. This would
have led to an arms race between predators and prey to develop both vision and
locomotion skills, leading to adaptive selection for better and better eyes.
Different Eye Designs for Different Environments:
Eyes can vary a lot across different species according to what the species needs to
deal with in their daily lives: whether they live in an area with light or not,
whether their food tends to come from above or below, whether the movement,
colour, or shape of their prey is critical, and so on. The importance of the environment is clearly seen when comparing two closely
related species with different ecological demands.
See Image Above: These two species of crayfish are closely related genetically,
but differ in their habitat either open-water of cave-dwelling.
The cave-dwelling crayfish lives in an environment where no light exists and has
no need to form an image. For them, the biological costs of building and
maintaining eyes would far outweigh the benefits of having eyes, and so, they do
not have eyes.
On the other hand, the open water-dwelling crayfish would be expected to benefit
from the ability to form an image and so, have developed functioning eyes.
Different Kinds of Eyes:
Some animals don’t have eyes at all, and others have eyes that can only detect the
presence or absence of light but can’t form images.
These light instruments have evolved as solutions to the organism’s needs.
Image-forming eyes come in two quite different designs: Compound eyes and
simple eyes. (See image below.)
Compound eyes are found in anthropods, such as insects and crabs. The eyes of
these species are made up of an arrangement of individual tubular units called
ommatidia that each point in a slightly different direction to gather the light that
lays directly in front of it.
These eyes manage to form a single image by putting together many separate
signals from each ommatidium.
Compound eyes are very good at detecting movement, but only at close distances.
(See image below, left.) Simple eyes are found in vertebrates as well as molluscs, such as octopus and
squid. (See image on previous page, right.)
These are the types of eyes that we think of when we think of eyes; they have an
eyeball, lens, and retina.
The vertebrate eye can vary quite a bit in its design according to the environment
that the species lives in. In fact, the environment has played a role in determining
the shape and orientation of the pupil, the size of the eye, where the eyes are