Transcript Light reception

Light reception
• Photoreceptors
– Structure
– Function
• Eye evolution
• Neural connections
• Color perception
Rhodopsin
Opsin - 40 kD protein
bound to membrane of
photoreceptor cell
Rhodopsin - retinal
bound to opsin. Retinal
is derived from vitamin A
Rod
Photoreceptors
Cone
Ciliary
Rhabdomeric
Photoreceptor differences
Features
Ciliary
Membranes
discs
Rhodopsin recovery slow
Pigment density
high
Rhabdomeric
rolls
fast
low
Photoreceptor evolution
Evolution of eyes
Few annelids and starfish
Poor light sensitivity, no
need to focus
Best acuity,
vertebrates and
cephalopods
Camera vs compound eyes
Eye number variation
Cell circuitry in the retina
R = rod (1 pigment)
C = cone (often >1
Pigment, for color)
MB = midget bipolar cell
PB = parasol bipolar cell
AII = amacrine cell
Contrast enhancement
Lateral inhibition
Color detecting photoreceptors
• Different variant of retinal
– vitamin A1 = > retinal1, vitamin A2 => retinal2 shifts
absorption peak 25 nm
– Fish, some amphibians
• Change amino acid composition of opsin
– changes absorption peak from 350-620 nm
– Widespread, X-linked in primates
• Add colored oil droplet to the photoreceptor cell
– Birds, amphibians, lizards, snakes, turtles
Dichromat perception logic
Bipolar cell
Ganglion cell
Dark
Blue
Yellow
Bright
Dichromat perception
Wavelength discrimination
ability and spectral peaks of
two cone types. Best ability
is in between peaks.
Discrimination ability between
white light and monochromatic
light.
Found in most mammals, including
squirrels, cats, dogs, ungulates,
New World monkeys, some fish
Trichromat perception
Wavelength discrimination for humans, apes, Old World monkeys
Trichromat spectral response
LGN neurons of macaque
Red-green system
Yellow-blue system
White-black system
Hue space (2D-3D-4D)
Bees are also trichromats
Flowers reflect UV
Visible light image
UV light image
Human vs bee UV sensitivity
Perception of floral parts by a human eye vs a bee eye
Pigment sensitivity in fishes
Stomatopods can have 16
photoreceptor types
Permits high spectral acuity without a complex nervous system
http://www.mbl.edu/CASSLS/thomas_cronin.htm
The perfect eye
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Adjustable sensitivity
Good resolution
Excellent accomodation (focus)
Good spatial discrimination
High temporal resolution (fast pigment
recycling)
Light sensitivity and eye design
Round lens produces
smaller, but brighter
Image - galago
Owl
Deep sea fish
Spider - day
and night tapetum reflects
Resolution and eye design
• Improve resolution of camera eye by
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Decreasing diameter of photoreceptors
Increasing eye size
Increasing number of cones - area centralis
Reduce lens curvature - increase focal length, but
lets in less light
• Improve resolution of apposition eye by
– Increase eye radius
– Increase facet aperture size and decrease curvature
Accomodation and eye design
Birds and mammals adjust lens shape
Frogs adjust lens position
Nautilus pinhole eyes need no adjustment
Spatial discrimination
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Learn size of typical object
Use arallax by moving head
Use accomodation cues
Use binocular vision - requires
overlapping field of view for two
eyes
Spider eyes
Binocular vision
Temporal discrimination
• Cones have higher flicker fusion rats than
rods
– Humans = 16/sec
– all cone eyes = 100-150/sex
• Rhabdomeres have higher flicker fusion
rates than ciliary photoreceptors