Physiology Color Vision
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Transcript of Physiology Color Vision
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