Chapter 9: perceiving colour

thus, out perception of an object's lightness is related not the amount of light that is reflected from the object, which can object,change depending on the illumination, but to the _________ of light reflected from the object, which remains the same no matter what the illumination

percentage.

colour constancy

perception of colours as relatively constant in spite of changing light sources

RESULTS observer not adapted

perception of green paper is shifted toward red [ the illusive reddish-green?]

RESULTS Observer adapted

perception of green paper is slightly shifted toward red - slightly reddish green ( appears more yellowish) partial colour constancy was shown in this condition

Monochromat[color blind]

person who needs only one wavelength to match any color

Dichromat

person who needs only two wavelengths to match any color [to their satisfaction -they are colorblind somehow according to the rest of us.]

chromatic adaptation

prolonged exposure to chromatic colour leads to receptors: - "adapting" when the stimulus colour selectively bleaches a specific cone pigment -decreasing in sensitivity to that colour

Most people find it easy to visualize a bluish-green or a reddish-yellow, but find it difficult (or impossible) to visualize a ________________or a _________________.

reddish green bluish-yellow

single opponent neurons are for colour ______________

regions

Achromatic colours are perceived as remaining relatively__________ when brightness changes

relatively

results of afterimage and simultaneous contrast demonstration original square Blue colour of outside afterimage- colour of inside afterimage-

outside-- yellow inside- blue

observer adapted

paper and observer in red light

baseline

paper and observer in white light

observer not adapted

paper illuminated by red light; observer by white [the illusive reddish-green?]

•Place a cup, on a white piece of paper on your desk. Then illuminate the cup at an angle to produce a shadow with a slightly fuzzy border. •The fuzzy border at the edge of the shadow is called the shadow's penumbra. •Now take a marker and draw a thick line so you can no longer see the penumbra. •What happens to your perception of the shadowed area inside the black line?

Apparently, the penumbra provides information to the visual system that the dark area next to the cup is a shadow, so the edge between the shadow and the paper is an illumination edge.•However, masking off the penumbra eliminates that information, so the area covered by the shadow is seen as a change in reflectance [i.e., the paper is darker].

Chromatic Adaptation Uchikawaet al. (1989) -Observers shown sheets of colored (GREEN) paper in three conditions: •Baseline-paper and observer in white light •Observernot adapted-paper illuminated by red light; observer by white [the illusive reddish-green?] •Observer adapted-paper and observer in red light RESULTS

Baseline -green paper is seen as green -Observer not adapted -perceptionof green paper is shifted toward red [the illusive reddish-green?] -Observer adapted -perception of green paper is slightlyshifted toward red slightly reddish-green •Partial color constancywas shown in this condition

Opponent-process mechanism proposed by Hering -Three mechanisms: - - -

Blue/ yellow red/ green white/ black

Proposed by Hering (1800s): -The relative sizes of the ____, ____, and_____ signals are not transmitted to the brain. -Instead, information about the _______between pairs of receptor signals is sent to the brain.

S M L difference

Perceptual Experiences are Creations of the Nervous System

The idea that the nervous system is responsible for the quality of our experience also holds for other senses. •Why do we perceive slow pressure changes as low pitches (like the sound of a tuba) and rapid pressure changes as high pitches (like a piccolo)? •Is there anything intrinsically "highpitched" about rapid pressure changes. We perceive some substances as "bitter" and others as "sweet," but where is the "bitterness" or "sweetness" in the molecular structure of the substances that enter the mouth? Again, the answer is that these perceptions are not in the molecular structures. They are created by the action of the molecular structures on the nervous system.

•Lightness perception under uneven illumination:

The illumination in three-dimensional scenes is usually uneven because of shadows cast by one object onto another or because one part of an object faces the light and another part faces away from the light. -For example, we need to determine whether the changes in appearance under the shadow are due to differences in the properties of different parts of the wall or to differences in illumination of parts of the wall. The perceptual system needs to distinguish between reflectance edges and illuminationedges.

The pigment that absorbs short-wavelength light enables the _____________to see short wavelengths that can't be detected by humans.

The pigment that absorbs short-wavelength light enables the honeybee to see short wavelengths that can't be detected by humans.

single opponent neurons

This M+L-neuron increases firing to medium wavelengths presented to the center of the receptive field and decreases firing to long wavelengths presented to the surround.•For: color within regions.

Color Processing in the Cortex •There is no single module for color perception cortical cells in , ___________________________, __________ and _________respond to some wavelengths or have opponent responses

V1, inferortemporal cortex v4

Intensity Relationships: The Ratio Principle

When the illumination is the same over the whole object, then lightness is determined by the ratio of: -reflectance of the object to the -reflectance of surrounding objects. •According to the ratio principle, as long as this ratio remains the same, the perceived lightness will remain the same.

Demonstration colour and the surrounding illuminate the green quadrant with tungsten light., and then look at the square through a small hole punched in a piece of paper so that all you see through the hole is part of the green area

When the surroundings are masked, most people perceive the green area to be slightly more yellow under the tungsten light than in daylight [in comparison to looking at an entire scene], which shows that color constancy works less well when an object is seen in isolation.

results of afterimage and simultaneous contrast demonstration original square red colour of outside afterimage- colour of inside afterimage-

outside- green inside- red

the ____________ information is the type of information sent by the opponent neurons to the brain

difference information

objects reflect _____________ wavelengths from these two sources ( tungsten, sunlight)

different

illumination edge

edgeis an edge where the lighting changes, as with the border between a and b, because area a is receiving more light than area b, which is in shadow

Trichromatic and Opponent-Process Theories Combined •Each theory describes physiological mechanisms in the visual system- Opponent-process theory

explains neural response for cells connected to the cones further in the brain

Trichromatic and Opponent-Process Theories Combined •Each theory describes physiological mechanisms in the visual system- trichromatic theory

explains the responses of the cones in the retina

There is no single module for color perception -Cortical cells in V1, inferotemporalcortex, and V4 respond to some wavelengths or have opponent responses - these cells usually respond to _____________ and _____________

forms orientations

Daltonism

from chemist John Dalton's descriptions of his abnormal color perceptions (1798)

RESULTS baseline

green paper is seen as green

sunlight

has approximately equal amounts of energy at all visible wavelengths

Tungsten

has more energy in the long-wavelengths (slightly yellower than the sun)

Orientation of surfaces[relative to the light source] provide information about

illumination and reflectance edges. ( Purves and Lotto's contrast illusion)

Habituation method-

infants are more likely to look at a novel stimulus One stimulus is presented repeatedly and the infant decreases the amount of time looking at the stimulus. -Once habituation has occurred, a new stimulus is presented. -If the infant can distinguish between the stimuli, dishabituationwill occur.

reflectance edge

is an edge where the reflectance of two surfaces changes: as between a and c, because it is made of different materials that reflect different amounts of light.

LGN

lateral geniculate nucleus is a relay center in the thalamus for the visual pathway. It receives a major sensory input from the retina

calculate colour constancy The actual light that is reflected from the sweater depends on both its reflectance curve and the illumination that reaches the sweater and is then reflected from it. To determine the wavelengths that are actually reflected from the sweater,

multiply the sweater's reflectance curve at each wavelength by the amount of illumination at each wavelength

anomalous trichromat

needs three wavelengths in different proportions than normal trichromat [M & L pigments are shifted closer together]

Perceptual Experiences are Creations of the Nervous System Physical energy in the environment does not have perceptual qualities. -Light waves are not "__________."

not coloured

This adaptation also causes ________colors to stand out, so yellow becomes more obvious in the ______ scene and the green stands out in the ___________scene.

novel yellow-lush green- arid

There are neurons in many areas that respond in an ______________________-increasing firing to wavelengths in one region of the spectrum and decreasing firing to neurons in another spectral region.

opponent way

results of afterimage and simultaneous contrast demonstration original square yellow colour of outside afterimage- colour of inside afterimage-

outside- blue inside- yellow

Bornstein et al. (1976)

-Infants were habituated to a 510-nm(green) light. -Then, either a 480-nm(blue) or 540-nm(green) light was presented. -Dishabituationonlyoccurred for the 480-nmlight. -Results of this experiment and others show that four-month-old infants categorize colors like adults do.

This principle operates when you walk into a room illuminated with yellowish tungsten light. The eye adapts to the long-wavelength-rich light, which decreases your eye's sensitivity to long wavelengths. •This decreased sensitivity causes the long-wavelength light reflected from objects to have ________effect than before adaptation, and this compensates for the greater amount of long-wavelength tungsten light that is reflected from everything in the room. •Because of this adaptation, the yellowish tungsten illumination has only a __________ on your perception of color.

-less small effect

There are three types of dichromatism: •Requiring only two wavelengthsto mach all other wavelengths (in their perception). - - -

-protanopia -deuteranopia -tritanopia

two types of opponent neurons in the cortex are: - -

-single opponent neurons - double-opponent neurons

The visual system's problem is that the amount of light reaching the eye from an object depends on two things:

1. illumination 2. object's reflectance

Objects that look gray reflect about ____________to______of the light ( depending on the shade of gray).

10 to 70%

objects that look black reflect about __________ of light

5%

object that look white, like paper in this book, reflect ________ to ___________ of the light.

80-95%

Ishihara plate

: a widely used test for color blindness that consists of a set of plates covered with colored dots which the test subject views in order to find a number composed of dots of one color which a person with various defects of color vision will confuse with surrounding dots of color.

Monochromats have

A very rare hereditary condition: 1 per 100,000 -True color-blindness -Poor visual acuity -Only rods: no functioning cones -Very sensitive eyes to bright light -Ability to perceive only in white, gray, and black tones

chromatic adaptation demonstration illuminate red area with a bright light from your desk lamp. With your left eye near the page and your right eye closed, look at the field with your left eye for about 30- 45 seconds. Then look at various coloured objects in your environment, first with your left eye and then with your right.

Adaptation to the red light selectively bleaches your long-wavelength cone pigment, which decreases your sensitivity to red light and causes you to see the reds and oranges viewed with your left (adapted) eye as less saturated and bright than those viewed with the right eye.

Opponent-ProcessTheory of Color Vision •Behavioral evidence:

Color after images and simultaneous color contrast show the opposing pairings-Types of color blindness are red/green and blue/yellow .

Trichromatic and Opponent-Process Theories Combined

Each theory describes physiological mechanisms in the visual system -The color-matching results, that three wavelengths are needed to match all other wavelengths, come from the cone receptors that are right at the beginning of the visual system, and the perceptual pairing of blue and yellow and red and green that we see in effects like afterimages and simultaneous contrast are created by the opponent neurons that come later in the visual system.

Possible Causes of Lightness Constancy

If we take the checkerboard outside into bright sunlight, where the intensity is 10,000 units, the white squares reflect 9,000 units of light, and the black squares reflect 900 units. •But even though the black squares when outside reflect 10 times more light than the white squares did when the checkerboard was inside, the black squares still look black. •Your perception is determined by the reflectance, not the amount of light reflected. [Not the whole story...

Information in shadows:

In order for lightness constancy to work, the visual system needs to be able 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. One thing the visual system may take into account is the shadow's meaningful shape. •We infer [see] that the shadow was cast by a tree, so we know it is the illumination that is changing, not the color of the bricks on the wall. •The shadow's contour looks like the tree's contour.

double opponent neurons

Most double-opponent neuronshave receptive fields with side-by-side regions (like simple cortical cells).•This example responds best to a medium-wavelength vertical bar presented to the left sideof the receptive field and to a long-wavelength vertical bar presented to the right sideof the receptive field.• For: boundaries between colors.

memory and colour

Past knowledge of an object's color can have an impact on color perception

Opponent-ProcessTheory of Color Vision

Proposed by Hering (1800s): -The relative size sof the S, M, and L signals are not transmitted to the brain. -Instead, information about the difference between pairs of receptor signals is sent to the brain. Color vision is caused by opposing responses generated by blueand yellow yellow, and by redand green. -Based on the results of phenomenological observations, in which stimuli were presented and observers described what they perceived.

Consider the perceptual difference between a summer & winter scene. •Based on calculations taking into account how this "greenness" and "yellowness" would affect the cone receptors, Webster (2011)determined that _______________

adaptation to the green in the lush scene would decrease the perception of green in that scene. Adaptation to the yellow of the arid scene would decrease the perception of yellow of the arid scene.

Tritanopia

affects .002%of males and .001%of females Probably missing the shortwavelength pigment -Individuals see short wavelengths as blue -Neutral point [perceiving gray] occurs at 570 nm -Above neutral point, they see red

Deuteranopia

affects 1% of males and 0.01% of females They are missing the medium wavelength pigment -Individuals see short-wavelengths as blue -Neutral point [perceiving gray] occurs at 498 nm -Above neutral point, they see yellow -[x chromosome]

Protanopia

affects 1% of males and 0.2% of females -They are missing the long wavelength pigment •Individuals see short-wavelengths as blue •Neutral point[perceiving gray] occurs at 492 nm• Above neutral point, they see yellow •[x chromosome]

Physiology of Opponent-Process •Researchers performing single-cell recordings found opponent neurons opponent neurons

are located in the retina and LGN - respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other

Color vision is caused by opposing responses generated by ______and ___________, and by ________and ___________. -Based on the results of phenomenological observations, in which stimuli were presented and observers described what they perceived.

blue and yellow red and green

In other experiments, in which observers were shown patches of color and were asked to estimate the percentages of blue, green, yellow, and red in each patch, they rarely reported seeing:

blue or yellow or red and green at the same time ( just as it is hard to visualize them together)

Perceptual Experiences are Creations of the Nervous System Consider what happens to our perception of color as illumination decreases at dusk. Hues such as _____, ______, and ______become less distinct and eventually disappear altogether, until the spectrum, once lushly colored, has become a series of different shades of gray. •When only ________are active, there is no perception of color.

blue, green and red rods

Opponent-Process Theory of Color Vision Most people find it easy to visualize a _______________ or a __________, but find it difficult (or impossible) to visualize a reddish-green or a bluish-yellow.

bluish-green reddish yellow

double opponent neurons for:

boundaries between colours

When lightness constancy occurs, our perception of lightness is determined not by the intensity of the illumination hitting a an object, __________________________

but by the object's reflectance.

results of afterimage and simultaneous contrast demonstration original square green colour of outside afterimage- colour of inside afterimage-

colour of outside afterimage--> red colour of inside--> green

"All crimsons appear to me to consist chiefly of __________: but many of them seem to have a tinge of dark brown. I have seen specimens of crimson, claret, and mud, which were very nearly alike."

dark blue

Adaptation to the yellow of the arid scene would ________the perception of yellow of the arid scene.

decrease

Webster (2011)determined that adaptation to the green in the lush scene would _____the perception of green in that scene.

decrease

Opponent-process mechanism proposed by Hering -Three mechanisms: •blue/yellow, •red/green, and •• white white/black -Can you imagine a reddish green or bluish yellow? -These responses were originally believed to be the result of chemical reactions in the ____________.

retina

Anomalous Color Experience procedure

seeing if all wavelengths of light can be matched in perception by combinations of other wavelengths.

Memory & Color -Hansen et al. (2006) •Observers saw photographs of fruits with characteristic colors against a gray background.-They adjusted the color of:•the fruit •and a spot of light.-When the spotwas adjusted to physically match the background, the spot appeared gray.-But when this done for the fruits, they were still perceived as ___________________________ conclusion:

slightly coloured. conclusion: some top-down processing allows us to imagine some colour

Effect of surroundings Colour constancy-works best when an object is _______________

surrounded by many colours ( thus allowing a wide range " reading" of the entire light spectrum)

Penumbra

the fuzzy border at the edge of the shadow

This principle operates when you walk into a room illuminated with yellowishtungsten light. The eye adapts to

the long- wavelength-rich light, which decreases your eye's sensitivity to long (red)wavelengths.

The object's reflectance

the proportion of this light that the object reflects into our eyes

illumination

the total amount of light that is striking the object's surface

Double-opponent neurons -can fire to oriented colored bars even when the intensity of the side-by-side bars is adjusted so they appear to be equally bright. •In other words, __________________________________________________________________

these cells fire when the bar's form is determined only by differences in color.

Adaptation "___________ ____________"the dominant colors in a scene, so if we compare the perceived color of the lush and arid scenes, we see that the colors are more _______________than before the chromatic adaptation.

tones down similar

First, retinal receptors respond with different patterns to different wavelengths (____________theory). Then, neurons integrate the inhibitory and excitatory signals (_______________theory).

trichromatic theory opponent-process

Unilateral dichromat-

trichromatic vision in one eye and dichromatic in other [these people allow us to know what dichromatsare seeing]

cortical cells that respond to colour may also respond to ____________

white

Viewing a shaded corner a) illuminate the card so one side is illuminated and the other is in shadow. b) view the card through a small hole so the two sides of the corner are visible, as shown do we any any difference?

•(a) You can easily tell that both sides of the corner are made of the same white material but that the non-illuminated side is shadowed. I.e., you perceive an illumination edge. •(b) The illumination edge you perceive at first becomes transformed into an erroneous perception of a reflectance edge. I.e., you see the shadowed white paper as being gray paper. [Or do you?]

•You might be tempted to think bees see "blue"at 350 nm because humans see blue at the short-wavelength end of the spectrum, but ________________________________________

•You might be tempted to think bees see "blue"at 350 nm because humans see blue at the short-wavelength end of the spectrum, but you really have no way of knowing what the honeybee is seeing.