Dr.Niranjan Murthy HLAsst Prof of PhysiologySSMC, Tumkur

• Photopic vision or cone vision • Visible spectrum 380nm to 730nm• Color is a subjective experience

• Complement colors• Black is a positive sensation

Theories of color vision

Young-Helmholtz theory:Primary colors- Red, Green, & Blue3 types of cones with photopigments each

maximally sensitive to one of the primary colors- Erythrolabe, Chlorolabe, & Cyanolabe.

• Red color- L pigment- (723-647nm)• Green color- M pigment- (492-575nm)• Blue color- S pigment- (450-492nm)

Genetics of photopigments:• Rhodopsin gene- on chromosome 3• S pigment gene- on chromosome 7• M and L pigment genes- on short arm of X

chromosome• M and L opsins have 96% homology• All three have 41% homology• Mammals are dichromats & primates are

trichromats

Phototransduction

Incident light

Retinene1 is changed to All trans form

Change in opsin configuration

Transducin (Gt1) is activated

α Subunit separates

Rhodopsin Kinase inactivates metarhodopsin II within seconds

α subunit activatescGMP phosphodiesterase

Converts cGMP to 5’-GMP

Reduced cytoplasmic cGMP

Closure of leaky Na+ channels

Hyperpolarization

Ca2+ activates adenylyl cyclase which in turn increases cGMP & inhibits phosphodiesterase

Neural mechanisms• Absorption of photon always respond with same

electrical change• Cones respond preferentially to particular

wavelengths• Processing of color vision occurs in ganglion cells

and visual cortex

3 pathways of color vision:• Red-Green pathway: difference between L

and M cones {L-M}• Yellow-Blue pathway: difference between S

cone and sum of L and M cones { S-(L+M)}• Luminance pathway: sum of L and M cones

{M+L}

• M type and P type of ganglion cells• P cells are concerned with color vision and

spatial details• M cells are concerned with movement• The pathways project to deep portion of layer

4C and blobs of V1• From there, color information is projected to

V8.

Other theories• Single opponent theory• Double opponent theory

Tests for color vision• Pseudo-isochromatic chart test (Ishihara’s

plates)• Elridge Green lantern• Holmgren’s wool test

Color blindness

-anomaly: weakness-anopia: absence or loss-prot: red color-deter: green color-trit: blue color• Monochromat• Dichromat• Trichromat

• Protanomaly• Deutranomaly• Tritanomaly• Protanopia• Deutranopia

Red-Green color blindness

• Seen in 8% of males and 0.4% of females• X-linked recessive disorder• Females are carriers• Defect of red or green cones