Transcript June 15_Visual System

Visual System
I Spy With My Little Eye…

Electromagnetic energy enters the eye
in the form of light waves
http://faculty.washington.edu/chudler/eyetr.html
The Eye
http://webvision.umh.es/webvision/sretina.html
Enter Light



The amount of light entering the eye through the
cornea is controlled by the pupil.
Light then passes to the lens and is focused
through the movement of extraocular muscles.
Light lands on the retina at the back of the eye
http://contactlensdocs.com/ContactLensInformationCenter/BasicEyeAnatomy/tabid/122/Default.aspx
The Retina



Images are projected upside-down onto the
retina.
Photoreceptors on the retina convert the
light to electrical signals that the brain can
process.
Neural processing
then interprets the
objects in their
correct right-side-up
position.
Photoreceptors

2 Types of Photoreceptors:




Rods work well in dim light.
Cones work well in bright
light for color vision.
Fovea: Center of retina
which only contains cones
for acute vision
Signal with several
photosensitive chemicals




Rhodopsin
Retinal
Transducin
Opsin
http://health.howstuffworks.com/human-body/systems/eye/eye2.htm
From Retina to Brain

Axons merge at the
optic disk to leave the
eye and travel to the
brain via the optic
nerve.


The optic nerves cross
at the optic chiasm en
route to the brain

http://thebrain.mcgill.ca/flash/d/d_02/d_02_cr/d_02_cr_vis/d_02_cr_vis.html
Creates a “blind spot”:
no photoreceptors!
Thus, information from
left eye goes to right
side of the brain, and
vice versa.
Visual Processing in the Brain


Lateral Geniculate
Nucleus (LGN) of the
Thalamus
Consists of 6 layers,
each of which receives
input from only one eye



Magnocellular: Depth
vision (inner)
Parvocellular: Color &
detail vision (outer)
Information continues to
the Primary Visual
Cortex (V1)
http://mcb.berkeley.edu/courses/mcb64/cortex.html
Information Coding

Visual cells are specialized to handle a
specific type of information:






Light & Dark (Retina)
Color (Retina)
Orientation (Visual Cortex)
Movement (Visual Cortex)
Form or Shape (Visual Cortex)
The specialized areas that process these
types of information are often referred to as
“columns” or “blobs”. These can be charted
in a process called topographical mapping.
http://faculty.washington.edu/chudler/eyecol.html
Hubel & Wiesel


Created topographical
maps of the cat visual
cortex by placing an
electrode in the visual
cortex
By flashing light & dark
patterns, lines of
various orientations,
etc to the cat, they
were able to map
which neurons
responded to which
stimuli.
Hubel & Wiesel
Cow Eyeball Dissection Video
http://www.exploratorium.edu/learning_studio/cow_eye/step01.html
Stroop Test
Afterimages &
Complementary Colors
Dizzying Dots


Do you see dots
that appear at the
corners of the
squares?
What happens if
you stare at one
dot?
How about now?
Afterimages



Stare at the
yellow + in the
middle of the
blue field for 1530 seconds.
Now quickly
stare at a blank
white page.
What do you
see?
http://faculty.washington.edu/chudler/chvision.html
Can you put the fish in the fishbowl?


Stare at the yellow stripe in the middle of the fish
for 15-30 seconds.
Move your gaze to the fishbowl—it may help to
blink once or twice.
http://faculty.washington.edu/chudler/chvision.html
Seeing in the Dark Experiment
Photoreceptors & Color

Recall Photoreceptors in the Retina:




Rods: Dim light
Cones: Bright light, color & detail vision
So how do Cones process Color?
Both Rods and Cones use a pigment molecule





Opsin (a large protein)
Chromophore (a form of
Vitamin A that couples to
opsin)
When light hits the
chromophore, it changes shape
This change activates opsin
Ultimately, an electrical
signal is transmitted
http://www.pdn.cam.ac.uk/staff/harris/cell.jpg
3 Cones for Color

http://www.bio.miami.edu/dana/dox/photosynthesis.html
http://faculty.washington.edu/chudler/eyecol.html
Combined response
patterns of these 3 cone
types are responsible for
our perception of color.
Further Color Processing


Additional specialized
retinal cells called
ganglion cells enhance
the cone response
patterns to adjust for
differences in light
levels.
Information then
continues through the
LGN to V1.
http://faculty.washington.edu/chudler/eyecol.html
Something to Ponder…


We can see yellow-green. We can see
blue-green.
Why can’t we see red-green or blueyellow?
Opponent Process Theory

Color information is sorted into 3
different channels from the retina to V1.

Red-Green:

Yellow-Blue
Intensity

increased firing for red
decreased firing for green
Opponent Processes


Because information about red & green is traveling
in the same pathway through opponent firing
patterns, it is physiologically impossible to signal for
both colors at the same time.
But, yellow and green are on different channels,
allowing them to be processed simultaneously to
express yellow-green.
http://faculty.washington.edu/chudler/eyecol.html
Optical Illusions
Muller-Lyer Illusion
Which line is longer?
http://faculty.washington.edu/chudler/chvision.html
I See, You See…What?


What do you see?
Does your neighbor
see the same thing
as you?
http://faculty.washington.edu/chudler/chvision.html
Poggendorf Illusion

Is the line behind
the rectangles
connected? Or do
you see 3 separate
lines?
http://epsych.msstate.edu/descriptive/Vision/DepthValley/Poggendorf/pog02.html
Titchner Illusion
Which center circle is bigger?
http://faculty.washington.edu/chudler/chvision.html
Filling In
Do you see a
shape—a cube,
a triangle, a
square?
http://faculty.washington.edu/chudler/chvision.html
What do you see?
http://faculty.washington.edu/chudler/chvision.html
Is it moving? Try staring at the
center circle…
http://www.artlex.com/ArtLex/o/opticalillusion.html
Visual Techniques
& Brain Tricks
Perceiving Depth


If the retina is flat, how
do we see in
3-D?
Brain uses 3 types of
cues:



Ocular Motor Cues
Monocular Cues
Binocular Cues

Fusing of two slightly
different images from
your retinas
Ocular Motor Cues

Convergence



Divergence


Eyes move inward as an object moves nearer
Also a Binocular Cue
Eyes move outward as object moves farther
away
Accommodation


Lens & cornea adjust shape to focus an object
Closer image: Lens thickens
Monocular Cues

Kinetic Depth Effect


Timing of changes in
lights appears as
movement
Motion Parallax


In a train, the
movement of other
trains distorts your
sense of motion
Immobile stimuli appear
to be moving

Pictorial Cues








Occlusion
Relative Height
Shadowing & Shading
Relative Size
Familiar Size
Atmospheric
Perspective
Linear Perspective
Texture Gradient
Kinetic Depth Effect
Motion Parallax
Occlusion

If one object is occluding a second object, it is
assumed that the occluding object is “closer”
Imaginary
Landscape
by Bernardo
Bellotto
http://www.topartprint.com/artists/
Bernardo_Bellotto/art_prints_posters
/7024/Imaginary_Landscape.php
Relative Height

Objects located higher on the
y-axis are seen as higher
on visual field & thus
further away
The Gilded Cage
by Evelyn Pickering
De Morgan
http://www.1st-art-gallery.com/Evelyn-Pickering
-De-Morgan/Evelyn-Pickering-De-Morgan-oil
-paintings-2.html
Shadowing and Shading

Natural assumption is
that there must be a
light source and a third
dimension in order to
cast a shadow
Entering the Studio by
Raphael Soyer
http://www.flickr.com/photos/artimageslibr
ary/5378298822/
Relative Size


Object that is farther away will take up less retinal space,
and thus appear smaller.
Which building is
closer to you?
South Carolina
Landscape by
George Biddle
http://www.columbiamuseum.org/
exhibitions/artistseye/sneakpeek.php
Familiar Size

If an object whose size is familiar to
you is re-sized, you assume the object
has moved.
http://johndollin.blogspot.com/2010/08/why-do-objects-still-appear-in-3d-with.html
Atmospheric Perspective

When outside, things that are far away have an
increased amount of atmosphere between us and them,
making them appear blurred and bluish.
Landscape near Bologna
by Frans Koppelaar
http://en.wikipedia.org/wiki/File:Frans_Koppelaar
_-_Landscape_near_Bologna.jpg
Linear Perspective

“Parallel” lines are drawn as converging at a point
as they go off into the distance (farther away =
closer together)
View of the Molo by
Antonio Canaletto
http://www.lyons.co.uk/Canaletto/bightm/molo.htm
Texture Gradient

Close objects have distinct texture; far away objects
appear smoother
Paris Street: A
Rainy Day by
Gustave
Caillebotte
http://psych.hanover.edu/krantz
/art/texture.html
Binocular Vision Experiment
Visual Review
A & M United Methodist Church
Stained Glass Windows
Window A
Window B
Window C
Window D
Window E
Window F
Window G
Window H
Window H
Window I
Window J
Window K