VSP 3/8/11

• -John Dalton, chemist
• - called inherited color blindness = daltonism

almost always red-green

red defect

green defect

blue defect

92

99.6

red and green

photoreceptors

chromophore and opsin

opsin portion determines the photopigment's spectral absorption

L and M opsins are very similar

they are very similar and they are positioned next to one another on the x-chromosome

x-linked recessive (mother to son)

autosomal dominant

if 1 adult has a defective gene, then ~50% of their offspring will inherit the defective gene

result from gene duplication in the x-chromosome

can occur when there is unequal crossover during meiosis - extra copy of the L photopigment gene ended up on an ancestor's x chromosome

extra copy means more prone to mutation thus a different type of photopigment gene was formed from these mutations of extra L genes

when two homologous chromosomes cross over each other and exchange genetic material during meiosis

gene duplication is caused by unqeual crossover

because the two genes are in tandem on the chromosomes and are very similar, corssovers during meiosis are common

within the gene - can produce hybrid photopigments whose spectral sensitivity curve is slightly different from the normal L or M cone pigment

one daugter cell gets just the L cone gene -> baby is born deutranopic if male

the other daughter cell gets 2 copies of the M cone gene and one copy of the L cone gene -> normal trichromacy

alteration of L cone pigment -> protanopia or protanomaly

alternation of M cone pigment -> deuteranopia or deuteranomaly

depending on where the crossover occurs, the spectural sensitivity of the hybrid can be closer to the regular M cone spectrum of the L cone spectrum

by comparing stimulation of each photopigment spectrum

yellow has nearly equal stimulation of the red and green cones. hence the r-g different of the green dot is larger than the r-g difference of the yellow dot

three cone photopigments are present

the PEAK of the spectral sensitivity curve of ONE of the cone photopigment is shifted - shifting these closer makes it harder to distinguish wavelengths in the range of the visible spectrum

ony two cone photopigment types are present

the missing photopigment is believed to be replaced by a remaining photopigment (still same amount of cones, just don't have all three types)

a condition where the spectral sensitivity curve of the green photopigment is shifted twoards the red (to the right)

so each spot produces the same r-g stimualtion and thus looks the same

condition where the M photopigment is completely missing (no green at all!!)

since theres onyl 1 curve where the wavelength of the green and yellow dots are located, the person cannot compare stumlation so they see the color as the same.

1% of males - no green curve

they can still tell there's a difference b/c they can sense the difference in luminance of the wavelengths

red shifted towards green

they may not be able to distinguish btn 2 colors b/c there may not be a difference in the amount of red and green stimulation

1% of male

don't have red curve

wherever the green and blue curve DON"T cross, they see those as the same hue

1% male

• x - wavelength
• y- luminous efficiency

Deuteranope and trichromat will distinguish brightness of color about the same (their curves are almost overlapping)

protanope curve is shifted to the left! so they see things dimmer

• x-chromosome carries only a SINGLE visual pigment gene (usually they carry 2 remember?)
• they can either have green or red or a hybrid of the two
• these are the protanopes and deuteranopes
• normal trichromats use the overlap in sensitivity of M and L cones to distinguish colors in that wavelength range, these people can't so they see all apples as yellow

• they have two genes on the x-chromosomes but the genes are for similar photopigment
• they may also be considered anomalous trichromats since they have three sensitivity curve because they have a hybrid gene
• they have SEVERE r-g defect!!

• have two genes on the x-chromosomes but may have a hybrid gene that shifts one of their sensitivity curves closer to the other curve
• can be either mild, moderate or severe
• sever= effective dichromacy

they have the same symptoms as dichromat because their r-d curve overlap

• Any change in the output of a single class of photoreceptors is ambiguous with respect to stimulus intensity and/or wavelength
• you need more than one type of cone to distinguish clor and brightness!
• so the output of both 20 (even though one of them is radiance 40 and the other is 20) will be same brightness. so the cones would not be able to compare the hues (colors) of the stimuli because they only have one spectral sensitivity curve
• a single class of receptors cannot distinguish changes in wavelength from changes in light intensity

• achromatopsia
• only 1 FUNCTIONING retinal photopigment - changes ini wavelength are perceived as changes in birghtness (principle of univariance)
• "complete": only 1 receptor type
• "incomplete": rod + only 1 cone type
• very UNcommon

• (typical) rod monochromat - only got rods, no cones!
• Blue-cone monochromat (atypical or incomplete) - only have rods and blue cones!
• Cone monochromat

• no "color" perception
• they won't be able to see any of the figure or numbers on color testing plates
• can match ANY wavelenth with any other adjusting only brightness

• VERY few, if any, cones
• little or no photopic limb of dark adaptation
• little or no photopic ERG, normal scotopic ERG (cuz they still got functional rods)
• photophbia (they don't have much cones so they don't like bright light)
• poor VA (20/100 - 20/200)
• nystagmus common (cuz they don't have fovea so their eyes are constantly searching)
• little or no wavelength discrimination
• max light sensitivity near 507nm (scotopic wavelength :) )
• AUTOSOMAL RECESSIVE (.03%)
• no cones therefore can't see color and sharp edges (don't have high spatial frequency detection) -> low VA

• rod + s cones
• abnormal VA (20/60-20/80)
• photopic (and scotopic) adaptation and ERG present but abnormal
• photophobia
• mild nystagmus
• X-LINKED RECESSIVE (<.0001%)

• dark adaptation: approx normal (they have cone!)
• photopic ERG: approx normal
• photoic sensitivity: approx normal
• VA: approx normal
• wavelength discrimination: poor to none
• extrememly rare
• inheritance unknown...
• theyhave rods and either L or M cones, the brain just can't distinguish color

• got 2 cones!!
• can match any color with some mixture of only two suitable primary colors

dichromats have the same number of cone as normal trichromats, so the missing photopigment must be "replaced" by on of the remaining two

metamers for a dichromat plot on the CIE chromaticity diagram as "confusion lines" that converge to a "co-punctal point"

• a wavelength that is achromatic to a dichromat (they say it's colorless)
• each type of dichromat will have a different neutral point
• it's the opposite to the co-punctal point

• dichromats do pretty well at naming the colors of familiar objects
• they still know banana is yellow :)

• context
• brightness relative to other object present
• shape
• so you try to trick them with a green and red apple. well, the red apple will be darker than the green one for them so they will learn that the darker apple = red apple. clever!!

• co-punctal point is off the diagram
• neutral point is 498nm

• co-punctal point is on the bottom right of the diagram
• neutral point is 492nm

• yellow
• in the diagram that demonstrates desaturation: x-axis = wavelength; y-axis= log color purity (saturation)
• there are points where the deuteranope and protanope say it's 0.

• x-axis = wavelength
• y-axis = change in wavelength
• normal trichromat can distinguish changes in wavelegths really well across the visible spectrum
• protanopes and deuteranopes can only distinguish chnages in wavelengths where the blue curve overlaps with other curve they have so they can't distinguish changes in wavelengths for a lot of the visual spectrum
• there is a range (545-700nm) where everything loks the same color