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. Creates a “blind spot”:

no photoreceptors! The optic nerves cross

at the optic chiasm en route to the brain Thus, information from

left eye goes to right side of the brain, and vice versa.

http://thebrain.mcgill.ca/flash/d/d_02/d_02_cr/d_02_cr_vis/d_02_cr_vis.html

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

Do you see dots that appear at the corners of the squares?

What happens if you stare at one dot?

Dizzying Dots

How about now?

Afterimages

Stare at the yellow + in the middle of the blue field for 15-30 seconds.

Now quickly stare at a blank white page.

What do you see?

http://faculty.washington.edu/chudler/chvision.html

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.

Can you put the fish in the fishbowl?

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

Combined response patterns of these 3 cone types are responsible for our perception of color.

http://www.bio.miami.edu/dana/dox/photosynthesis.htmlhttp://faculty.washington.edu/chudler/eyecol.html

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 blue-yellow?

Opponent Process Theory

Color information is sorted into 3 different channels from the retina to V1. Red-Green: increased firing for red

decreased firing for green

Yellow-Blue Intensity

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 Eyes move inward as an object moves nearer Also a Binocular Cue

Divergence 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/artimageslibrary/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 byAntonio 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