Black and White vision is adequate for most purposes. Color vision is important in identifying ripeness, counteracting camouflage... Humans, Old World monkeys and apes each have 3 types of cones (3 iodopsins) providing the most elaborate color vision in the animal kingdom. COLOR VISION
Based on observation that any color of light can be attained by mixing various amounts of 3 colors of light. Proposed that humans have 3 kinds of photoreceptors that work together to give the sensation of hue. lights 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
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.
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.
Opponent Process Theory of Color Vision Based on idea that some colors don’t blend (e.g. reddish green), and on negative afterimages Trichromatic theory can’t explain these phenomena. lights Based on observation of negative afterimages .
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.
3 types of cones Note : All cones respond to a range of wavelengths, but their maximal response is at 440, 530, or 560 nm. This is determined by the type of iodopsin in the cone. 440 nm 530 nm 560 nm
3 types of cones
Each type of cone exhibits a peak sensitivity, but
responds over a range of wavelengths.
Opponent Process Theory of Color Vision
2 kinds of color sensitivity in ganglion cells
red opposes green
blue opposes yellow
3 types of receptive fields with complementary colors.
Blue on, yellow off Red on, green off green on, red off
440 530 560 Red light “stimulates” red cone Red cone “stimulates” red/green ganglion cell cones signals red ganglion cells RETINAL COLOR CODING
440 530 560 green light “stimulates” green cone green cone “inhibits” red/green ganglion cell cones signals green ganglion cells RETINAL COLOR CODING
440 530 560 Red light “stimulates” red cone Red cone “inhibits” green/red ganglion cell cones signals red ganglion cells RETINAL COLOR CODING
440 530 560 green light “stimulates” green cone green cone “stimulates” green/red ganglion cell cones signals green ganglion cells RETINAL COLOR CODING
440 530 560 blue light “stimulates” blue cone blue cone “inhibits” yellow/blue ganglion cell cones signals blue ganglion cells RETINAL COLOR CODING
440 530 560 yellow light “stimulates” red and green cones equally Red and green inputs to red/green cell cancel red and green sum to “ inhibit” blue/yellow cells cones ganglion cells signals yellow RETINAL COLOR CODING
440 530 560 Accordingly, we can see reddish-yellow reddish-blue greenish-blue and greenish-yellow but we cannot see reddish-green or bluish-yellow cones ganglion 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 Pathways The optic nerve has two principle branches
The optic nerves
join at the ventral
aspect of the
brain to form the
Axons of retinal
form the optic
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)