The Visual Pathway
Parts of the Eye
PARTS OF THE EYE
Aqueous humor - clear watery fluid found in the anterior chamber of the eye;
maintains pressure and nourishes the cornea and lens
Vitreous humor - clear, jelly-like fluid found in the back portion of the eye:
maintains shape of the eye and attaches to the retina
Blind spot - small area of the retina where the optic nerve leaves the eye: any
image falling here will not be seen
Ciliary muscles - involuntary muscles that change the lens shape to allow
focusing images of objects at different distances
Cornea - transparent tissue covering the front of the eye: does not have blood
vessels; does have nerves
Cones -photoreceptors responsive to color and in bright conditions; used for
Rods - photoreceptors responsive in low light conditions; not useful for fine
Parts of the eye - continued
Fovea - central part of the macula that provides sharpest vision; contains only cones
Iris - circular band of muscles that controls the size of the pupil. The pigmentation of the
iris gives "color" to the eye. Blue eyes have the least amount of pigment; brown eyes
have the most
Lens - transparent tissue that bends light passing through the eye: to focus light, the lens
can change shape
Macula - small central area of the retina that provides vision for fine work and reading
Optic nerve - bundle of over one million axons from ganglion cells that carry visual
signals from the eye to the brain
Pupil - hole in the center of the eye where light passes through
Choroid - Thin tissue layer containing blood vessels, sandwiched between the sclera and
retina; also, because of the high melanocytes content, the choroid acts as a lightabsorbing layer.
Retina - layer of tissue on the back portion of the eye that contains cells responsive to
Sclera - tough, white outer covering of the eyeball; extraocular muscles attach here to
move the eye
Imagine that the colored
bar (half red, half blue)
is in front of your eyes.
The red part of the bar
will project to the nasal
part of your left retina
and the temporal
(lateral) part of your
right retina. The blue
part of the bar will
project to the nasal part
of your right retina and
the temportal (lateral)
part of your left retina.
Like many pathways in the nervous system,
right and left visual information cross to the
other side of the brain. This occurs in the
optic chiasm. After the optic chiasm,
information about the right visual field (blue)
is on the left side of the brain, and information
about the left visual field (red) in on the right
side. The pathways stay this way and all the
way up to the visual cortex.
Follow the blue and red
lines from the eyes to
see the flow of
information. From the
retina, the first synapse
is in the lateral
geniculate nucleus of
the thalamus. The next
synapse is made in
primary visual cortex in
the occiptal lobe.
The Visual Pathway
The Magic Cube
Look at the center cube. What side is the front?
Is the front as shown on the cube on the right side
or is the front as shown on the cube on the left
side or is there no front at all?
Which of the lines shown below is longer?
The lines are identical in length. We see them as
different because we have been taught to use angles
to tell us about size.
Black is the New White
Stare at the middle of the
picture with black
squares 15-30 seconds.
Are those really dots that
appear at the corners of
What happens if you focus
on a dot?
Now look at the middle of
the picture with the
Do you see dots again?
What color are they?
Figure Ground Illusions
• Do you see a vase or a face
in the figure below? This
type of picture was first
illustrated by psychologist
Edgar Rubin in 1915.
• Notice that it is very difficult
to see both the faces and the
vase at the same time.
• This may happen because
we tend to focus our
attention on only one part of
the image...either the faces
or the vase.
Stare at the + for about 15 seconds, then shift your gaze to the right
side of the image.
Do the lines on the right look straight? Are they?
Is the center circle on the right the same size as the center circle on
the left? For many people it appears that the circle that is
surrounded by the small circles is larger that the circle that is
surrounded by the larger circles.
This illusion shows that our brains judge size by comparing objects
to things in the surroundings
Which arc comes from the circle with the
It probably looks like arc C is part of the largest
circle. However, all the arcs are actually from the
SAME circle. Look at the same figure again however, this time I have blocked the right and left
sides of the larger two arcs. Each arc comes from a
circle of identical size.
Do you see a …
Cube, triangle, square and rectangle?
Filling the gap: with Subjective Contours,
your brain tries to fill in these pictures with
images that really are not there.
these two dimensional objects produce the illusion of
movement. Look at the center of these images to see
the circles of images rotate.
we see what makes sense