CH. 5: THE PERCEPTION OF COLOR

3 types of cone photoreceptors:

1. S-cones detect short wavelengths. 2. M-cones detect medium wavelengths. 3. L-cones detect long wavelengths. ⎫ More accurate to refer to them as "short," "medium," and "long" rather than "blue," "green," and "red," since they each respond to a variety of wavelengths.

Cone-opponent cell

A neuron whose output is based on a difference between sets of cones. o In LGN there are cone-opponent cells with center-surround organization.

Negative afterimage

An afterimage whose polarity is the opposite of the original stimulus. o Light stimuli produce dark negative afterimages. o Colors are complementary. Red produces green afterimages and blue produces yellow afterimages (and vice versa). o This is a way to see opponent colors in action.

The principle and problem of univariance

An infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. o Therefore, 1 type of photoreceptor cannot make color discriminations based on wavelength. *A single photoreceptor shows different responses to lights of different wavelengths but the same intensity

Rod monochromat

Has no cones of any type; truly color-blind and very visually impaired in bright light.

Achromatopsia

Inability to see color due to cortical damage/central nervous system/ CNS

Is there a particular place in the cortex specialized for color processing

Not clear- V1, V2, and V4 all involved in color perception, but not exclusively.

types of color space

RGB HSB

Color constancy

The tendency of a surface to appear the same color under a fairly wide range of illuminants. o To achieve color constancy, we must discount the illuminant and determine what the true color of a surface is regardless of how it appears.

Detection

Wavelengths of light must be detected in the first place. ⎫ Step 1: Detection—S, M, and L cones detect light.

Discrimination

We must be able to tell the difference between one wavelength (or mixture of wavelengths) and another. ⎫ Step 2: Discrimination—cone-opponent mechanisms discriminate wavelengths. ♣ [L - M] and [M - L] compute red vs. green. ♣ [L + M] - S and S - [L + M] compute blue vs. yellow.

Appearance

We want to assign perceived colors to lights and surfaces in the world and have those perceived colors be stable over time, regardless of different lighting conditions. ⎫ Step 3: Appearance—further recombination of the signals creates final color-opponent appearance.

the 3 steps to color perception

color detection, color discrimination, color appearance

Other animals have evolved a different system for color vision. Birds and some reptiles have _________ over each photoreceptor, which tunes them to different wavelengths.

colored oils

RGB Color Space

defined by the outputs of long, medium, and short wavelength lights (i.e., red, green, and blue).

Trichromacy/Young-Helmholtz theory

google explanation (idea that there are three receptors in the retina of the eye that are each sensitive to their own specific color. These three colors are red, green, and blue. Each of these color-sensitive cone cells has different sensitivities, with blue cones being the most sensitive and red cones being the least. The combination of these three colors can form any visible color in the color spectrum)

Lateral geniculate nucleus (LGN)

has cells that are maximally stimulated by spots of light. o Visual pathway stops in LGN on the way from retina to visual cortex. o LGN cells have receptive fields with center-surround organization.

Anomia

i. Inability to name objects or colors in spite of the ability to see and recognize them. Typically, due to brain damage.

Rods are sensitive to scotopic light levels.

o All rods contain the same photopigment molecule: rhodopsin. o All rods have the same sensitivity to various wavelengths of light. o Therefore, rods obey the principle of univariance and cannot sense differences in color. o Under scotopic conditions, only 1 type of rod photoreceptor is active, so that is why the world seems drained of color.

HSB color space

o Defined by hue, saturation, and brightness.

Illegal color combinations

o Illegal: We cannot have reddish green.

legal color combination

o Legal: We can have bluish green (cyan), or bluish red (purple).

how to tell the difference between lights of different wavelengths if 1 type of photoreceptor cannot make color discriminations based on wavelength

with all 3 cones Under photopic conditions, the S-, M-, and L-cones are all active.

according to our visual system, we can tell that luminance change _____ hue looks like a shadow and _____ hue looks less like a shadow

without with

Basic color terms

• Single words that describe colors and have meanings that are agreed upon by speakers of a language.

Hue cancellation experiments

⎫ Start with a color, such as bluish green. ⎫ The goal is to end up with pure blue. ⎫ Shine some red light to cancel out the green light. ⎫ Adjust the intensity of the red light until there is no sign of either green or red in the blue patch. We can use the hue cancellation paradigm to determine the wavelengths of unique hues. o Unique hue: Any of 4 colors that can be described with only a single color term: red, yellow, green, blue. o For instance, unique blue is a blue that has no red or green tint.