sens and percep chapter 5

400-700nm

The EM radiation that is visible to our nervous system

Reflect

to redirect something that strikes a surface - especially light sound or heat - usually back toward its point of origin

Absorb

To take up something—such as light, noise, or energy—and not transmit it at all.

Transmit

To convey something (e.g., light) from one place or thing to another.

Refract

To alter the course of (bend) a wave of energy that passes into something from another medium, as water does to light entering it from the air

Paper looks white

Light reflects all wavelengths equally

Paper looks red

Light selectively reflects long wavelengths

Grass looks green

Light selectively reflects medium wavelengths

Liquid looks red

Light selectively transmits long wavelengths

Reflectance curve

Plots the percentage of light an object reflects at each visible wavelength

Subtractive mixture

colors made by mixing paints

Additive Mixture

Colors made by superimposing lights

Spectral colors

Colors that appear in the visible spectrum

Nonspectral colors (magenta, brown)

Colors that do not appear in the visible spectrum because they are mixtures of other colors

Hue

the color that we perceive

Saturation

How much white/light has been added into a color

Value

Refers to light versus dark, how light versus how dark a color is

It if is a lighter color it has a higher value, if it is a darker color it has a lower value

Spectral sensitivity

the sensitivity of a cell to different wavelengths of electromagnetic radiation

Univariance

an infinite set of wavelength intensity combinations can produce exactly the same response from one type of photoreceptor

*one color cannot discriminate wavelength

Trichromacy

The color of any light is defined by the relationship of three numbers, the outputs of the S-, M- and L- cones

Opponent-process theory

the theory that opposing retinal processes (red-green, yellow-blue, white-black) enable color vision. For example, some cells are stimulated by green and inhibited by red; others are stimulated by red and inhibited by green

Evidence of trichromatic theory

Color matching: a 500nm test field can be matched by mixing 420, 560 and 640 nm lights in a comparison field

Three lights are enough to match any color

Physiology of trichromatic theory

S-, M- and L-cones respond differently to different wavelengths, following their spectral sensitivities

White light

Drives all three cone types

Blue light

Drives S cones

Green light

Drives M cones

Red light

Drives L cones

Yellow light

Drives M and L cones

Metamers

Different mixtures of wavelengths that look identical because they produce the same three cone responses and the visual system cannot tell them apart

Evolutionary advantages of color vision

Trichromats find food in natural settings faster and more accurately

Color helps judge which fruit is ripe and attracts mates through sexual attraction

Opponent-process theory evidence

Phenomenology: no color looks both yellowish and bluish, and none looks both reddish and greenish

The three opponent mechanisms

Red - Green

Blue - Yellow

Black - White

Negative afterimage

an afterimage whose polarity is opposite to the original, red produces green, yellow produces blue and light produces dark

Physiology of opponent-process theory

Bipolar opponent cells in the retina and opponent neurons in the LGN, including +B-Y and +R-G cell in the monkey LGN

Excitatory and inhibitory inputs from the three cone types build the red-green and blue-yellow mechanisms

How Trichromatic theory and Opponent-process theory fit together

light is first coded by three cone types(trichromatic), then its recoded by opponent neurons in the retina and LGN(opponent-process), then passed to the brain

Cytochrome oxidase

an enzyme present in blobs in the V1 that appear to respond to color

Cerebral achromatopsia

loss of color vision caused by damage to the cortex

Single-opponent cells

have receptive fields like retinal ganglion cells and may capture neighboring areas (L+ in the center and M- in the surround, best stimulus is in the center (L+))

double-opponent cells

are like simple cortical cells and may compare neighboring areas (L+M- in the center with L-M+ in the surround, the best stimulus is an edge between colors)

Illuminant

the light source

Spectral power distribution

the energy in the illuminant at each wavelength

Spectral reflectance function

The percentage of each wavelength surface reflects

Color constancy

a surface looks about the same color under a wide range of illuminants

what reaches the eye is surface reflectance X the illuminant, the same surface under two illuminants gives two very different sets of cone responses

the brain makes assumptions about both surface illuminant

surfaces tend to be broadband, the illuminants tend to be broadband with a smooth spectral curve, it uses these assumptions to infer the surfaces reflectance

what provides the constraints that makes color constancy possible?

implicit knowledge about the world