Psych 130 Ch 9

The receptive field of a single-opponent neuron

+M-L neuron increases firing to medium wavelengths presented to the center of the receptive field and decreases firing to long wavelength present to the surround. Important for perceiving the color within regions.

Opsin

All visual pigments are made up of a larger protein component called opsin and a small light-sensitive component called retinal. Differences in the structure of the long opsin part of the pigments are responsible for the 3 different absorption spectra.

What does the addition of a third pigment do?

Although not necessary for creating color vision, provides additional ratios that allow additional discriminations of wavelengths across the visual spectrum.

Opponent neurons have called +B-Y neurons +S-ML, why?

Because +B corresponds to firing of the short-wavelength cone and -Y to combined activity of the medium-and long-wavelength cones. Similarly, +R -G are now caled +L -M neurons.

Why did British physicist Thomas Young say that Netwon's idea of size of vibration and each color wont work?

Because a particular place on the retina can't be capable of the large range of vibrations required.

Additive color mixture

Because mixing lights involves adding up the wavelengths of each light in the mixture, mixing lights is called an additive color mixture.

Why is the illumination in 3 dimensional scenes usually uneven?

Because of shadows cast by one object onto another or because one part of an object faces the light land the other part faces away from the light.

Subtractive color mixture

Because the blue and yellow blobs subtract all of the wavelengths except some that are associated with green, mixing paints is called subtractive color mixture.

Mixing of blue & yellow paint

Blob of blue paint: reflects all short, reflects some medium, absorbs all long. Blob of yellow paint: Absorbs all short, reflects some medium, reflects some long. Mixture of blue and yellow blobs: Absorbs all short, reflects some medium, absorbs all long.

What happens when you mix paint?

Both paints still absorb the same wavelengths they absorbed when alone (blue still absorbs long-wavelength light), so the only wavelengths reflected are those that are reflected by both paints in common. Ex: B/c medium wavelengths are reflected by blue and yellow, they mix and reflect green.

What does the color solid illustrate?

Both the multitude of different colors we perceive and the way they interact.

How is a wavelength in one field matched in a color matching experiment?

By adjusting the portions of the 3 different wavelengths in another field.

How is saturation depicted?

By placing more saturated colors toward the outer edge of the cylinder and more de-saturated colors towards the center of the cylinder.

How is value represented?

By the cylinder's height. Lighter colors up top, darker colors on bottom.

How are the colors of objects determined?

By the wavelengths of light that are reflected from the objects onto our eyes

Monochromatism

No functioning cones, so their vision has the characteristics of rod vision in both dim and light. They see only in shades of lightness (white, gray, and black). They're colorblind. Also have poor visual acuity and are so sensitive to bright lights they must wear sunglasses during the day.

What is an object's perceived color affected by?

Not only the observer's state of adaptation by also by the object's surroundings.

Principle of univariance

Once a photon of light is absorbed by a visual pigment molecule, the identity of the light's wavelength is lost. The receptor does not know the wavelength of light it has absorbed, only the total amount it has absorbed.

Counteract on Zeki's findings

Opponent neurons have been found in many areas of the cortex. This involved recording from single neurons in the monkey cortex. There is also evidence for areas in the human cortex that respond to both color and other visual qualities.

How is order depicted?

Order of colors in visible spectrum.

2 challenges in determining whether infants have color vision

Perception can vary on at least 2 dimensions: 1. its chromatic color 2. its brightness.

Opponent-process theory of color vision

Phenomenological observation proposed by Hering that color vision is caused by opposing physiological responses generated by blue and yellow, red and green, and black and white.

Transmission curves

Plots of the percentage of light transmitted at each wavelength- look similar to the reflectance curves but with percent transmission plotted on the vertical axis.

Isaac Newton in his Optiks

Presented the idea that color is not a property of wavelengths.

Chromactic adaptation

Prolonged exposure to chromactic color.

Semir Zeki

Proposed the idea that there is one area in the cortex specialized for processing information about color.

Another condition of brain damage in addition to achromatopsia or color blindness?

Prosopagnosia- the inability to recognize faces.

Most common types of dichromatism

Protanopia and deuteranopia, inherited on the X chromosome

Memory color

The effect on perception of prior knowledge of the typical colors of objects.

Keiji Uchikawa and coworkers

The idea that chromactic adaptation is responsible for color constancy.

Color mixing idea

The idea that the color we perceive depends largely on the wavelengths of light that reach our eyes provides a way to explain what happens when we mix different colors together.

Hurvich and Jameson published a paper titled "An Opponent-Process Theory

This was in support of Hering's proposal. They noted that in the case of color blindness, colors often drop out in pairs, so people lose the ability to perceive yellow and blue or red and green.

How can the nervous system tell the difference between the 2 wavelengths, no matter what the light intensity is?

To add a second pigment.

Purpose of HUrvich and Jameson's paper

To go beyond evidence like the color circle and complementary afterimages to provide quantitative measurements of the strengths of blue-yellow and red-green components of the opponent mechanism.

Metamers

Two identical fields in a color-matching experiment. The reason they look alike is that they both result in the same pattern of response in the 3 cone receptors.

dichromats

Two types of cone pigment

Where does opponent-process theory take place?

Within opponent neurons within the lateral geniculate nucleus.

According to Trichomatic theory,

light of a particular wavelength stimulates each receptor mechanism to different degrees, and the pattern of activity in the 3 mechanisms results in the perception of a color. Each wavelength is therefore represented in the nervous system by its own pattern of activity in the 3 receptor mechanisms.

a+R-G

neuron that increases firing for wavelengths perceived as red and decreased firing for wavelengths perceived as green.

Our perception of lightness is related to

the % of light reflected from that object, which remains the same no matter what the illumination.

When is color vision present in babies

3-4 months

Color solid

3-dimensional color space. Demonstrates the relationship among hue, saturation, and value.

The insensity of light reaching the eye from an object depends on 2 things:

1. Illumination- total amount of light that is striking the object's surface 2. The object's reflectance- the proportion of this light that the object reflects into our eyes.

Why wasn't Hering's opponent mechanism widely accepted?

1. It's main competition, trichromatic theory, was championed by Helmholtz, who had great prestige in the scientific community; 2. Hering's phenomenological descriptions ex: canary yellow could not compete with Maxwell's quantitative color mixing data; and 3. there was no neural mechanism known at that time that could respond in opposite ways.

What was Zeki's finding based on?

1. Many neurons in a visual area called V4 respond to color. 2. Cerebral achromatopsia, a condition caused by damage to the brain. Person can perceive form and motion, but cannot see color. This is near or identical to areas identified as human color areas supports the idea of a specialized module for color perception.

Ways to determine color deficiency?

1. Using the color matching procedure to determine the minimum number of wavelengths needed to match any other wavelength in the spectrum. 2. Ishihara plates: Show a plate with the number 74 in green dots surrounded by more dots of different color. If you cannot see the 74, you may have a color deficiency.

What 2 developments that occurred in the 1950's made it Hering's theory

1. psychophysical experiments by Leo Hurvich and Dorthea Jameson that provided quantitative measurements of the strengths of each of the opponent mechanisms 2. Physiological demonstrations of opponent neural responses in the retina and lateral geniculate nucleus.

Trichromats

3 visual pigments

How do opponent neurons determine the difference between the receptor responses to different wavelengths?

A. The response of the +L-M neuron to a 500nm light is negative because the M receptor results in an inhibitory response that is larger than receptor L's excitatory response. This means that the action of the 500 nm light on this neuron will cause a decrease in any ongoing activity. B. The response to a 600 nm light is positive, so this wavelength causes an increase in the response of this neuron.

What happens when a colored lights are superimposed

All of the light that is reflected from the surface by each light when alone is also reflected when the light is superimposed.

Keiji Uchikawa experiment

All used with green paper basline- paper and observer illuminated by white light observer not adapted- paper illuminated by red light, observer by white light (observer is not chromactically adapted. Observer adapted to red- both paper and observer illuminated by red light (chromactically adpated)

Reflectance edge

An edge where the reflectance of 2 surfaces changes.

Hues

Another word for chromatic colors

Key finding from Maxwell's color-matching experiment

Any reference color could be matched provided that subjects were able to adjust the proportions of 3 wavelengths in the comparison field. Two wavelengths allowed subjects to match some, but not all, reference colors, and subjects never needed four wavelengths to match any reference color.

Wavelengths from 500-575

Appear green

Wavelengths from 590-620

Appear orange

Wavelengths from 620-700

Appear red

Color circle

Arranges perceptually similar colors next to each other around its perimeter. Color circle leaves out variations in a hue's saturation and value.

Ratio principle

As long as the ration between the object and its surroundings remain the same, the perceived lightness will also remain the same.

Cerebral Achromatopsia

Color blindness

Based on the finding that people with normal color vision need at least 3 wavelengths to match any other wavelength, Maxwell reasoned that

Color vision depends on 3 receptor mechanisms, each with different spectral sensitivities.

Connections between wavelength and color

Colors of light are associated with wavelengths on the visible spectrum. The colors of objects are associated with which wavelengths are reflected (for opaque objects) or which wavelengths are transmitted (for transparent objects) The colors that occur when we mix colors are also associated with which wavelengths are reflected into the eye.

Nonspectral colors

Colors that do not sppear on the spectrum because they are mixtures of other colors, such as magenta; a mixture of blue and red.

Wavelengths are...

Completely colorless

What have neurons with side-by-side receptive fields been used to provide evidence for?

Connection between color and form. These neurons canw fire to oriented bars even when the intensity of the side-by-side bars is adjusted so they appear equally bright. AKA these cells fire when the bar's form is determined only by differences in color. Thus, when you look at a colorful pic, the colors you see are not only "filling in" the objects and areas in the scene but may also be helping define the edges and shapes of these objects and areas.

HSV color solid

Cylindrical color solid whose 3 dimensions are hue, saturation, and value.

Saturation

Determined by the amount of white that has been added to a particular hue. When more white has been added, saturation decreases

Illumination edge

Edge where the lighting changes.

Reflectance curves

Ex: can plot the percentage of light reflected from lettuce and tomatoes at each wavelength in the visible spectrum. Red reflects long wavelengths and green reflects medium

Newton's thoughts on the prism colors

Newton thought that each part of the spectrum is defined b different physical properties and that these physical differences give ride to our perception of different colors. He thought that prisms separated differently colored light particles while others though the prisms separated light into differently colored waves.

Double-opponent nuerons

Have receptive fields with side-by-side regions. Important for perceiving the boundaries between different colors. Increases firing when a vertical medium-wavelength bar us presented to the left side of the receptive field and when a vertical long wavelength bar is presented to the right side of the receptive field.

Hering's finding about color wheel

In color circle, the colors all seem to fall into 4 groups that are defined by the amount of yellowness, blueness, greenness, and redness in each group.

Keiji findings

In the baseline condition, green paper is perceived as green. Observer not adapted condition, Observer sees paper's color as being shifted toward the red. Observer adapted to red condition- perception is shifted only slightly to red, so it appears more yellowish. Thus, the chromactic adaptation has created partial color consistency.

Where do trichomatic theory processes take place?

In the receptors in the retina.

Opponent neurons

In the retina and lateral geniculate nucleus that responded with an excitatory response to light from one part of the spectrum and with an inhibitory response light from another part.

a+B-Y neuron

Increased firing, above the spontaneous level, to wavelengths perceived as blue and decreased firing to wavelengths perceived as yellow.

What color is excluded from the spectral colors

Indigo because humans have a difficult time distinguishing it from blue and violet

How do we perceive colors relative to wavelength?

It depends on how our nervous system reacts to it.

Problem for the perceptual system

It has to take uneven illumination into account.

Actual light reflected from clothing depends on

Its reflectance curve and on the illumination that reaches the sweater and is then reflected from it. To determind the wavelengths that are reflected from the sweater, we multiply the sweater's relfectance curve at each wavelength by the amount of illumination at each wavelength.

Important result for Cristiana Cavina-Pratesi's experiment

Large yellow area that indicates how responding to color can occur in areas that respond to other visual qualities as well.

How do we determine what a dichromat perceives?

Locate a unilatral dichromat- someone with trichomatic vision in one eye and dichromatic vision in the other.

Complementary afterimages

Look at center of 4 squares (blue, yellow, green, red) for 30 seconds, then look at a white space. Observe the afterimage on the white background. Red and green switch places, blue and yellow switch places. The afterimage is the color on the opposite side of the color circle.

Dichromatism

Missing one of the 3 wavelengths

Protanopia

Missing the long-wavelength pigment. Perceive Short-wavelength as blue, and as the wavelength is increased, the blue becomes less and less saturated until they perceive gray.

Deuteranopia

Missing the medium-wavelength pigment. Perceives blue at short wavelengths, sees yellow at long wavelengths, and has a neutral point at about 498nm.

Anomalous trichromatism

Needs 3 wavelengths to match any wavelength, like a normal trichromat does. However, they mix these wavelengths in different proportions from a person w/ normal vision & are not as good at discriminating between wavelengths that are close together.

Value

Refers to the light-to-dark dimension of color. Value decreases as color becomes darker.

Monochromats

See shades of gray because they have one pigment.

Photons

Small packets of energy

Chromatic colors

Such as blue, green, and red occur when some wavelengths are reflected more than others, a process called selective reflection.

How do trichomatic theory and opponent-process theory work together?

The L-cone sends excitatory inout to a bipolar cell, whereas the M-cone send inhibitory input to the cell. This creates a +L-M (or +R-G) cell that responds with excitation to the long wavelengths that cause the L-cone to fire and with inhibition to the medium wavelengths that cause the M-cone to fire. the _S-ML (or +B-Y) cell also receives input from the cones. It receives an excitatory input from the S-cone and an inhibitory input from cell A, which sums the inputs from the M and L cones.

Partial color consistency

The perception of the object is shifted after adaptation, but not as much as when there was no adaptation. This means the eye can adjust its sensitivity to different wavelengths to keep color perception approximately constant as illumination changes.

Lightness

The perception of the shade of achromatic color

What happens when light is absorbed by the retinal part of the visual molecule?

The retinal changes shape. A process called isomerization.

Why must people with Monochromatism wear sunglasses in the day?

The rod system is not designed to function in bright light so it becomes overloaded in strong illumination, creating a perception of a glare.

How does lightness constancy work?

The visual system needs to take into account the uneven illumination created by shadows. It must determine that this change in illumination caused by a shadow is due to an illumination edge and not to a reflectance edge.

How can we perceive millions of colors when we can describe the visual spectrum in terms of 6 or 7 colors?

There are three perceptual dimensions of color, which together can create the large number of colors we can perceive.

portions of the color wheel

Upper-redness, lower-greenness, left-blueness, right-yellowness

Tritanopia

Very rare. Missing short-wavelength pigment. Sees the spectrum as blue at short wavelengths and red at long wavelengths with a neutral point at 570 nm.

Neutral point

Wavelength at which a protanope perceives gray. At wavelengths above this point, they perceive yellow, which becomes less intense at the long wavelength end of the spectrum.

Color constancy

We perceive the colors of objects as being relatively constant even under changing illumination.

Selective transmission

What creates transparency such a liquids, plastics, and glass. This term means that only some wavelengths pass through the object or substance. Ex: cranberry juice selectively transmits long wavelength light and appears red.

metamerism

When 2 physically different stimuli are perceptually identical.

Trichromactic theory

When Newton separated white light into its components to reveal the visible spectrum, he argued that each of the spectrum must stimulate the retina differently in order for us to perceive color. He proposed that "rays of light falling upon the bottom of the eye excite vibrations in the retina...which vibrations, being propagated along the solid fibres of the optick Nerves into the Brain, cause the sense of seeing." We know that it's electrical signals, not vibrations that are transmitted down the optic nerve from the brain. Became known as the Young-Helmholtz theory

When does color constancy work best?

When an object is surrounded by objects of many different colors, a situation that often occurs when viewing the environment

De-saturated

When hues become de-saturated, they can take on a faded or washed out appearance.

When does the visual pigment molecule isomerize?

When the molecule absorbs one photon of light. This isomerization activates the molecule and triggers the process that activates the visual receptor and leads to seeing the light.

Hering's proposal of 3 mechanisms

White+ Black- mechanism responds positively to white light and negatively to the absence of light. The red+ green- mechanism responds positively to red and negatively to green. The yellow+ blue- mechanism responds positively to yellow and negatively to blue. Thus, the more white a light appears, the less black it also appears; the more yellow a light appears, the less blue it appears.

Wavelengths from 450-490

appear blue

Wavelengths from 400-450

appear violet

Wavelengths from 575-590

appear yellow

A person with only one visual pigment

can match any wavelength and sees all wavelengths as shades of gray.

Isaac Newton's visible spectrum rule consisted of what colors?

red, orange, yellow, green, blue, indigo, and violet.

Hering's primary colors

red, yellow, green, blue and evolves into different colors as small amounts of the next primary colors are added.

Opponent pairs

red-green, blue-yellow & black-white as an achromatic pair

Achromatic colors

white, gray, and black. Occur when light is reflected flat and equally across a spectrum.