Transcript Color Vision

Color Vision
By
Andrew J Pakchoian
Psych 159
Prof Macleod
Road Map
• Pigment anatomy
• Photoreceptor
anatomy
• Neurobiology of
color vision
• Physiology of color
blindness
Key Terms
• MC layer
magnocellular
• PC layer
parvocellular
• LGN
lateral geniculate nucleus
Pigment Anatomy
• 3 types of cones:
•
•
•
short (S), middle (M),
and long (L)
wavelength sensitive.
(S): 430 nm = blue
(M): 530 nm = green
(L): 560 nm = red
Pigment Anatomy
• Origin of pigments
– Red/green from opsin gene on X-chromosome or sex
chromosome.
– Show very similar amino acid seqs. (96%)
– Blue on chromosome 7 and rhodopsin on chromosome 3 are
very different.
Photoreceptor Anatomy
• Cones more concentrated
near fovea
– Adapts to a wide range of
illumination colors and
levels.
• Rods spread throughout
the retina
– Provide quick response to
changes in illumination.
Photoreceptor Anatomy
• Measuring Wavelengths:
– Short wavelengths causes
the green receptor to fire.
– As the wavelength gets
longer and closer to 580
nm the red begins to fire,
surpassing the green.
– Get mix of wavelengths.
• Therefore, color vision is
the consequence of
unequal stimulation of
the 3 types of cones. In a
specific ratio.
Photoreceptor Anatomy
• Example: if you
stimulate all 3 types
of cones about
equally the result is
white or no color.
Neurobiology of color vision
• Once again 3 types of cones:
S, M, and L.
• Only 2 types of horizontal
cells:
– H1 which connects L and M
cells.
– H2 which connects S with
some L and M cells.
• The relation between high
specificity of cone
connectivity and chromatic
processing is unclear.
Neurobiology of color vision
• Blue/Yellow Pathway:
– 2 Systems:
• Differentiate signal from S
•
•
•
and the summed signal
from L and M. = +S-(L+M)
Receives input from S-cone
bipolars (ON) and L and M
cone bipolars (OFF)
Second system = -S+(L+M)
Input signals are unclear.
– Conclusion: The ganglion
cells : Small bistratified cell
and small bodied inner cell
form blue/yellow pathway.
Neurobiology of color vision
• Red/Green Pathway:
– Differentiate between
signals from the L and M
cones.
– Path: Single L and M
cones receive input
connect to single midget
bipolar cells
contact single midget
ganglion cells
project to the PC layer
of LGN.
Neurobiology of color vision
• Red/Green Pathway Cont’d:
• L and M cones have two forms: ON
•
•
•
and OFF-centre
ON-center midget cells for L and M
have identical arrays and same with
the OFF-center
There is no overlap between
dendritic trees of central midget
ganglion cells which leads to the
exact 1:1 ratio of cones to ganglion
cells.
More than 10 to 15 degrees in the
periphery we do find some overlap
of dendritic trees in the midget
bipolar cells, but not ganglion.
Neurobiology of color vision
• Interesting facts:
– In the periphery of the
retina chromatic
sensitivity drastically
decreases, but
responsiveness of PC
cells is unchanged
Connectivity to midget
bipolar cells is not
random.
Physiology of color blindness
• Male dominant trait but females
•
•
•
•
•
•
carry it.
Females have 2 X chromosomes
so trait is normally not expressed.
Males have 1 X and 1 Y
chromosome which means
recessive traits will show in
phenotype.
As stated before, red and green
pigments originate on the X
chromosome.
1 in 20 males suffers from some
form of color blindness.
Most common is red-green color
blindness which is caused by
problem with M or L cones.
However, they can still see red
and green, but have trouble with
light or desaturated colors.
Physiology of color blindness
• A few types:
– Anomalous Trichromacy: have
3 photopigments, but only
from 2 groups.
• Most common is
deuteranomalous = 2 L
photopigments.
– Dichromacy: missing 1 group
of photopigments.
Which picture contains a red crayon?
Example
Quiz
True or false: The dendritic trees of
midget ganglion cells overlap.
Answer
False