color vision by andrew j pakchoian psych 159 prof macleod

Color VisionColor Vision

By By

Andrew J PakchoianAndrew J Pakchoian

Psych 159 Psych 159

Prof MacleodProf Macleod

Road MapRoad Map

• Pigment Pigment anatomyanatomy

• Photoreceptor Photoreceptor anatomyanatomy

• Neurobiology of Neurobiology of color visioncolor vision

• Physiology of Physiology of color blindnesscolor blindness

Key TermsKey Terms

• MC layer magnocellularMC layer magnocellular

• PC layer parvocellularPC layer parvocellular

• LGN lateral geniculate nucleusLGN lateral geniculate nucleus

Pigment AnatomyPigment Anatomy

• 3 types of cones: 3 types of cones: short (S), middle short (S), middle (M), and long (L) (M), and long (L) wavelength wavelength sensitive.sensitive.

• (S): 430 nm = blue(S): 430 nm = blue

• (M): 530 nm = (M): 530 nm = greengreen

• (L): 560 nm = red(L): 560 nm = red

Pigment AnatomyPigment Anatomy

• Origin of pigmentsOrigin of pigments– Red/green from opsin gene on X-chromosome or sex Red/green from opsin gene on X-chromosome or sex

chromosome.chromosome.– Show very similar amino acid seqs. (96%)Show very similar amino acid seqs. (96%)– Blue on chromosome 7 and rhodopsin on chromosome 3 Blue on chromosome 7 and rhodopsin on chromosome 3

are very different.are very different.

Photoreceptor AnatomyPhotoreceptor Anatomy

• Cones more Cones more concentrated near concentrated near foveafovea– Adapts to a wide range Adapts to a wide range

of illumination colors of illumination colors and levels.and levels.

• Rods spread Rods spread throughout the retinathroughout the retina– Provide quick response Provide quick response

to changes in to changes in illumination.illumination.


• Measuring Measuring Wavelengths:Wavelengths:– Short wavelengths Short wavelengths

causes the green causes the green receptor to fire.receptor to fire.

– As the wavelength gets As the wavelength gets longer and closer to 580 longer and closer to 580 nm the red begins to nm the red begins to fire, surpassing the fire, surpassing the green.green.

– Get mix of wavelengths.Get mix of wavelengths.

• Therefore, color vision is the Therefore, color vision is the consequence of unequal consequence of unequal stimulation of the 3 types of stimulation of the 3 types of cones. In a specific ratio.cones. In a specific ratio.


• Example: if you Example: if you stimulate all 3 stimulate all 3 types of cones types of cones about equally the about equally the result is white or result is white or no color. no color.

Neurobiology of color Neurobiology of color visionvision

• Once again 3 types of Once again 3 types of cones: S, M, and L.cones: S, M, and L.

• Only 2 types of horizontal Only 2 types of horizontal cells: cells: – H1 which connects L and H1 which connects L and

M cells.M cells.– H2 which connects S H2 which connects S

with some L and M cells.with some L and M cells.

• The relation between The relation between high specificity of cone high specificity of cone connectivity and connectivity and chromatic processing is chromatic processing is unclear.unclear.

Neurobiology of color Neurobiology of color visionvision • Blue/Yellow Pathway:Blue/Yellow Pathway:

– 2 Systems:2 Systems:• Differentiate signal from S Differentiate signal from S

and the summed signal and the summed signal from L and M. = +S-(L+M)from L and M. = +S-(L+M)

• Receives input from S-Receives input from S-cone bipolars (ON) and L cone bipolars (ON) and L and M cone bipolars (OFF)and M cone bipolars (OFF)

• Second system = -S+Second system = -S+(L+M)(L+M)

• Input signals are unclear.Input signals are unclear.

– Conclusion: The ganglion Conclusion: The ganglion cells : Small bistratified cells : Small bistratified cell and small bodied inner cell and small bodied inner cell form blue/yellow cell form blue/yellow pathwaypathway..


• Red/Green Pathway:Red/Green Pathway:– Differentiate between Differentiate between

signals from the L signals from the L and M cones.and M cones.

– Path: Single L and M Path: Single L and M cones receive input cones receive input connect to single connect to single midget bipolar cells midget bipolar cells contact single contact single midget ganglion cells midget ganglion cells project to the PC project to the PC layer of LGN.layer of LGN.


• Red/Green Pathway Red/Green Pathway Cont’d:Cont’d:

• L and M cones have two forms: L and M cones have two forms: ON and OFF-centreON and OFF-centre

• ON-center midget cells for L and ON-center midget cells for L and M have identical arrays and same M have identical arrays and same with the OFF-centerwith the OFF-center

• There is no overlap between There is no overlap between dendritic trees of central midget dendritic trees of central midget ganglion cells which leads to the ganglion cells which leads to the exact 1:1 ratio of cones to exact 1:1 ratio of cones to ganglion cells.ganglion cells.

• More than 10 to 15 degrees in the More than 10 to 15 degrees in the periphery we do find some periphery we do find some overlap of dendritic trees in the overlap of dendritic trees in the midget bipolar cells, but not midget bipolar cells, but not ganglion. ganglion.


• Interesting facts:Interesting facts:– In the periphery of In the periphery of

the retina chromatic the retina chromatic sensitivity drastically sensitivity drastically decreases, but decreases, but responsiveness of PC responsiveness of PC cells is unchanged cells is unchanged Connectivity to Connectivity to midget bipolar cells is midget bipolar cells is not random. not random.

Physiology of color Physiology of color blindnessblindness

• Male dominant trait but Male dominant trait but females carry it.females carry it.

• Females have 2 X Females have 2 X chromosomes so trait is chromosomes so trait is normally not expressed.normally not expressed.

• Males have 1 X and 1 Y Males have 1 X and 1 Y chromosome which means chromosome which means recessive traits will show in recessive traits will show in phenotype.phenotype.

• As stated before, red and As stated before, red and green pigments originate on green pigments originate on the X chromosome.the X chromosome.

• 1 in 20 males suffers from 1 in 20 males suffers from some form of color blindness.some form of color blindness.

• Most common is red-green Most common is red-green color blindness which is color blindness which is caused by problem with M or L caused by problem with M or L cones.cones.

• However, they can still see red However, they can still see red and green, but have trouble and green, but have trouble with light or desaturated with light or desaturated colors.colors.

Physiology of color Physiology of color blindnessblindness

• A few types:A few types:– Anomalous Trichromacy: Anomalous Trichromacy:

have 3 photopigments, have 3 photopigments, but only from 2 groups.but only from 2 groups.• Most common is Most common is

deuteranomalous = 2 L deuteranomalous = 2 L photopigments. photopigments.

– Dichromacy: missing 1 Dichromacy: missing 1 group of photopigments.group of photopigments.

Which picture contains a red Which picture contains a red crayon?crayon?

ExampleExample

QuizQuiz

True or false: The dendritic True or false: The dendritic trees of midget ganglion cells trees of midget ganglion cells

overlap.overlap.

AnswerAnswer

FalseFalse

color vision by andrew j pakchoian psych 159 prof macleod

Documents