Dr. Mohanad R.Alwan

Black and White vision is adequate for most purposes.Black and White vision is adequate for most purposes.

Color vision is important in identifying ripeness,Color vision is important in identifying ripeness,counteracting camouflage...counteracting camouflage...

Humans, Old World monkeys and apes each haveHumans, Old World monkeys and apes each have3 types of cones (3 iodopsins) providing the3 types of cones (3 iodopsins) providing themost elaborate color vision in the animalmost elaborate color vision in the animalkingdom.kingdom.

COLOR VISION

Based on observation that any color of light can beBased on observation that any color of light can beattained by mixing various amounts of 3 colorsattained by mixing various amounts of 3 colorsof light. of light.

Proposed that humans have 3 kinds of photoreceptorsProposed that humans have 3 kinds of photoreceptorsthat work together to give the sensation of hue.that work together to give the sensation of hue.

lightslights

Photoreceptors: Trichromatic Theory of Color Vision

Due to in the color receptors (cones) in retina becoming "fatigued." When you then look a different background, the receptors that are tired do not work as well. Therefore, the information from all of the different color receptors is not in

balance. Therefore, you see the color "afterimages." You can see that you vision quickly returns to normal.

Photoreceptors: Trichromatic theory of color vision

AFTERIMAGE

• An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased (stop).

• One of the most common afterimages is the bright glow that seems to float before one's eyes after looking into a light source for a few seconds.

• The phenomenon of afterimages may be closely related to persistence of vision, which allows a rapid series of pictures to portray motion, which is the basis of animation and cinema.

Afterimage

• If the viewer stares at this image for 20-60 seconds and stares at a white object a negative afterimage will appear.

• Afterimages come in two forms, negative (inverted) and positive (retaining original color).

• The process behind positive afterimages is unknown, though thought to be related to neural adaptation. On the other hand, negative afterimages are a retinal phenomenon depend on rods and cones.

• Closing the eye can help achieve a better sense of the color in its own aspect.

Based on idea that some colors don’t blendBased on idea that some colors don’t blend(e.g. reddish green), and on negative afterimages(e.g. reddish green), and on negative afterimages

Trichromatic theory can’t explain these phenomena. Trichromatic theory can’t explain these phenomena.

lightslights

Based on observation of negative afterimagesBased on observation of negative afterimages..

Opponent Process Theory of Color Vision

Opponent Process

Opponent Process suggested that there are some color combinations that we never see, such as reddish-green or yellowish-blue.

Opponent-process theory suggests that color perception is controlled by the activity of two opponent systems; a blue-yellow mechanism and a red-green mechanism.

440 nm

530 nm

560 nm

NoteNote:: All cones respond to a rangeAll cones respond to a range of wavelengths, but their maximalof wavelengths, but their maximalresponse is at 440, 530, or 560 nm.response is at 440, 530, or 560 nm.This is determined by the type ofThis is determined by the type ofiodopsin in the cone.iodopsin in the cone.

3 types of cones

Each type of cone exhibits a peak sensitivity, butEach type of cone exhibits a peak sensitivity, but responds over a range of wavelengths.responds over a range of wavelengths.

3 types of cones

2 kinds of color sensitivity in ganglion cells2 kinds of color sensitivity in ganglion cellsred red opposes opposes greengreenblueblue opposes opposes yellow yellow

3 types of receptive fields with complementary colors. 3 types of receptive fields with complementary colors.

Blue on,Blue on,yellow offyellow off

Red on,Red on,green offgreen off

green on,green on,red offred off

Opponent Process Theory of Color Vision

440 530 560Red light “stimulates”

red cone

Red cone “stimulates”red/green ganglion cell

cones

signals redganglion cells

RETINAL COLOR CODING

440 530 560

green light “stimulates”green cone

green cone “inhibits”red/green ganglion cell

cones

signals greenganglion cells

RETINAL COLOR CODING

440 530 560

Red light “stimulates”red cone

Red cone “inhibits”green/red ganglion cell

cones

signals redganglion cells

RETINAL COLOR CODING

440 530 560

green light “stimulates”green cone

green cone “stimulates”green/red ganglion cell

cones

signals greenganglion cells

RETINAL COLOR CODING

440 530 560

blue light “stimulates”blue cone

blue cone “inhibits”yellow/blue ganglion cell

cones

signals blueganglion cells

RETINAL COLOR CODING

440 530 560

yellow light “stimulates”red and green cones

equally

Red and green inputs tored/green cell cancelred and green sum to

“inhibit” blue/yellow cells

cones

ganglion cells signals yellow

RETINAL COLOR CODING

440 530 560

Accordingly, we can seeAccordingly, we can see reddish-yellowreddish-yellow reddish-bluereddish-blue greenish-bluegreenish-blue

andand greenish-yellowgreenish-yellow

butbutwe cannot seewe cannot see reddish-greenreddish-green

oror bluish-yellowbluish-yellow

conescones

ganglion cellsganglion cells

orange

purple

turquoise

lime

RETINAL COLOR CODING

Processing in the Retinal Ganglion Cell

There are two different types of ganglion cells:

M (magnocellular) ganglion cells-input primarily from rods •constitute about 10 % of the ganglion cell population. •They are sensitive to the directions of visual motion and low contrasts (they saturate when the contrast is high)•They are not sensitive to colors of the lights. They only have black and white center-surround receptive fields.

P (parvocellular) ganglion cells- input primarily from cones•constitute about 70 % the ganglion cell population. •They are more sensitive to the form and fine details of the visual stimuli; •They respond poorly to low contrast but do not saturate at high contrasts; •They are sensitive to differences in the wavelength of light.

Visual PathwaysThe optic nerve has two principle branches

The optic nervesThe optic nerves join at the ventraljoin at the ventral aspect of theaspect of the brain to form thebrain to form the x-shaped opticx-shaped optic chiasm.chiasm.

Axons of retinalAxons of retinal ganglion cellsganglion cells form the opticform the optic nerves (cranialnerves (cranial nerves #2).nerves #2).

Visual Pathways

Visual Pathways•The optic chiasm is the cross-over point at which some of the axons move from one side of the head to the other •These are STILL axons of ganglion cells, but they are rearranged at the optic chiasm- now called optic tract

•Contralateral fibers provide information from the nasal retinas.•Ipsilateral fibers provide information from the temporal retinas.

•20% of the fibers are re-routed to the superior colliculus •80% of the fibers are re-routed to the lateral geniculate nucleus (LGN)

Visual Pathways : Lateral Geniculate Nucleus (LGN)

•Left-right, top-bottom organization from RG cells is maintained•The LGN contains 6 layers

•layers 1, 4, and 6 contain information from the contralateral fibers •layers 2, 3, and 5 contain information from the ipsilateral fibers

Receptive field properties: LGN cells have circular, center-surround receptive fields -- similar to those of Ganglion Cells.

•magnocellular/parvocellular distinction

•Topographically organized projection to V1(primary visual cortex)

Visual Pathways- Beyond the LGN: V1Information from both magnocellular and parvocellular layers sent to the primary visual cortex (a.k.a. the striate cortex, area 17,

V1).

•Area V1 (like the LGN) is layered and LGN inputs primarily to layer 4

•parvocellular input to a lower subdivision of layer 4 in V1•magnocellular input to an upper subdivision of layer 4 in V1

•Topographic representation and cortical magnification