THE VISUAL SYSTEM: ESSENTIALS OF SIGHT Let there be light! The waves of electromagnetic radiation that constitute light have various characteristics: Amplitude (height) → brightness Wavelength (distance) → color/hue Purity → saturation (amount of whiteness in a color – less is more)

THE EYE Light is converted to neural impulses that are sent to the brain via the eye → light enters the eye through the cornea, a protective ‘window’ that bends light for focus → it passes through the pupil opening, which is surrounded and controlled by the iris (dilating/opening the pupil to let light in in darkness and vice versa)

THE EYE → behind the pupil is the lens, which focuses the incoming light rays by changing its curvature in a process called accommodation → the light gets focused as an upside-down image on the retina

THE EYE * Near- and farsightedness occur when this image falls short of/behind the retina respectively → nearsighted eyeballs are too long and farsighted ones are too short

THE RETINA The retina is the light-sensitive inner surface of the eye containing receptor cells and layers of neurons that begin the processing of visual information → where the optic nerve – the nerve that carries the neural impulses from the eye to the brain – enters the eye there are no receptors and thus there is a blind spot

THE RETINA Rods are elongated receptor cells located on the periphery of the retina, are extremely sensitive to dim light, but not to color or detail → rods allow us to detect black, white and gray and are necessary for peripheral and twilight/night vision To

THE RETINA Cones are located near the center of the retina (the fovea), have great sensitivity to bright light and detail, but not to dim light → cones are much fewer in number than rods, but allow us to perceive color in daylight

THE RETINA * Both cones and rods play a role in light/dark adaptation, the process by which your eyes become less/more sensitive to light in high/low illumination

VISION AND THE BRAIN Light hitting receptor cells triggers spark neural signals that travel along the axons of the twisted, rope-like ganglion cells of the optic nerve into the brain → the optic nerve travels to the thalamus, which relays the signal to the primary visual cortex in the occipital lobe

VISION AND THE BRAIN Once the message reaches the visual cortex, highly specialized neurons called feature detectors respond to specific features of the stimulus, such as shape, angle or movement → viewing different images will activate different areas of the brain (faces = lower temporal lobe)

VIEWING THE WORLD IN COLOR Remember, color is not a physical property of light; we perceive colors based on three properties of light: wavelength (color/hue), amplitude (brightness) and purity (saturation/amount of whiteness) → we can perceive at least a million varieties of color that occur simply as a result of mixing a few basic colors

VIEWING THE WORLD IN COLOR Subtractive color mixing removes wavelengths of light → mixing different paint colors is subtractive (blue, green, and red makes black) Additive color mixing superimposes lights, thus adding light → mixing different colors of light is additive (blue, green, and red makes white) tiv

THEORIES OF COLOR VISION Trichromatic theory of color vision holds that the retina contains three different color receptors (red, green, blue) which, when stimulated in combination, can produce the perception of any color → color blind people lack one of the receptors (usually red or green, maybe both) and thus have di- or monochromatic vision

THEORIES OF COLOR VISION The Opponent-Process theory of color vision holds that opposing retinal processes (red-green, yellow-blue, white- black) enable color vision → this theory adds yellow, which is necessary to describe many colors

THEORIES OF COLOR VISION → this theory also helps explain complementary afterimages

THEORIES OF COLOR VISION The two theories offer differing takes on the processes of color vision, but recent evidence suggest that the perception of color occurs in two stages, each incorporating one of the theories